SHIP KNOWLEDGE M5/M24

(Ranger Hope version  © 2008, contains edits of material courtesy of Col Tritton & A.N.T.A.  publications.)

 

 

Maintenance and Docking

 

Basic Principles of Stability

 

Longitudinal and Transverse Stability

 

Factors Affecting Stability

                                        

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Maintenance and Docking

                    

Corrosion and Deterioration     

Methods of Docking and Slipping           

Survey     

Planned Maintenance

Advantages of Planned Maintenance

Elements of a Planned Maintenance Program

                                                                                                                            

 

 

 

Corrosion and Deterioration

 

Deterioration of Timber

 

Fungal Attack

 

Breakdown of wood by fungi, commonly called rot or decay, can occur in timber whenever the moisture content rises above 20 to 25 percent. The fungi which cause decay spread by means of microscopic spores which are usually present in the air, so that any moist susceptible timber, even in almost completely sealed cavities, is subject to attack.

 

Warning signs of decay are:

           Paint or varnish failure

           A musty smell like mushrooms

           Fruiting bodies, like toadstools, spongy growths, or soft incrustations of various colours

           Mycelium, generally white threadlike growth, sometimes thick like cotton wool

           Any softening, cracking or other physical breakdown of the wood

 

 

Marine Insect Attack

 

Timber may be attacked by any of the following, depending upon conditions:

 

Termites and White Ants

 

           Subterranean types

           Tree dwelling type

           Dry wood type

 

All three of these varieties dislike the light and may be exterminated by the use of proprietary poisons.

 

Lyctus Borers

 

These only attack hardwoods which have sapwoods containing a high starch content.  Fortunately 33% of Australian hardwoods are immune from attack.  The attack becomes evident when an accumulation of fine flour dust appears on the surface of the timber. This borer may be exterminated by the use of proprietary poisons.


Marine Borers

 

           The pill bug - a crustacean

           The gribble - a crustacean

           The shipworm or toredo - a mollusc

 

The crustacean borers cause the typical “hour glass” type of wastage seen in neglected piles of wharves, etc. If allowed to go unchecked they are responsible for considerable damage to the underwater section of wooden vessels. Sometimes they are referred to as “putty borers”.

 

The toredo commences life as free swimming larvas which attach to submerged timber and immediately begin to bore. In Australian waters they may reach a length of up to 1 metre. They use the attached wood as habitation, the worm feeding on minute marine life in the surrounding water. For the owners of wooden vessels these borers are a constant worry. Prevention of attack from both forms of marine borer is possible by deep and total impregnation of the timber with creosote or proprietary preservatives. An alternative by costly procedure is metal sheathing.

 

 

Corrosion

 

Corrosion is the alteration and decomposition of metals or alloys by direct chemical attack or by persistent electrochemical reactions. Corrosion can be classified as:

 

1.         Chemical corrosion.

2.         Electrochemical corrosion.

 

 

Chemical Corrosion

 

This is the attack of metals by solutions of acids or alkalines which will chemically combine with the metal to form entirely new products. The material can be considered as being dissolved in the solution. Such attack is usually caused by spillage of liquids such as battery acids, galley refuse, or in toilet areas.

 

 

Electrochemical Corrosion

 

This is the most common type of corrosion. It is caused by very small electrical currents flowing between one metallic area to another. These electrical currents cause the material which is being corroded to change to a completely different substance; for example, steel changes to rust. Whether the corrosion takes place below the waterline, or above the waterline, the presence of both oxygen and an electrolyte (i.e. a conducting solution) play an important part. Saltwater is a liquid which encourages corrosion because it is an excellent conductor of electricity.

 

Corrosion is indicated by the presence of rust or wastage of a metal.

 

 

 

Preservation of Structures

 

Preservation of Timber

 

The following precautions will keep the risk of fungal and insect attack to a minimum.

 

           Ensure good ventilation throughout the boat, particularly when it is lying idle.

           Make sure rainwater cannot get in.

           Prevent condensation by ventilation. Where it is unavoidable e.g. on insides of windows, use water-repellent preservative on woodwork.

           Use a water soluble preservative in the bilge water. A cheap and effective one can be made by dissolving 0.65 Kg of borax and 0.45 Kg of boric acid in 4 litres of hot water. This mixture is non-corrosive and harmless to animals.

           Inspect the vessel’s timbers for decay regularly, at least every 6 months. If decay is found act at once, a few weeks in summer is enough for major damage to be done.

           Use a preservative from a variety of preservatives that have been developed for the successful treatment of timber for decay resistance.

           Use a proprietary poison for extermination of marine insects.

 

 

Preservation of Metals

 

There are two ways of preventing corrosion.

 

1.         By providing a piece of material which will corrode in preference to the vessel. Such a substance is usually found attached to the hull near the propeller or attached inside a tank, in the form of a sacrificial anode. When two metals in contact with each other result in one of the metals corroding, the metal which is preserved is called more “Noble” than the metal that corrodes.

 

            In such cases aluminium will corrode in preference to steel; steel will corrode in preference to brass; brass will corrode in preference to stainless steel. Different metals should not be used in close contact unless there is good insulation between them; for example, it is bad practice to connect a steel valve to an aluminium hull, without insulation. The aluminium may corrode around the steel.

 

            Lead, in contact with aluminium will cause rapid wasting of the aluminium. For this reason, lead based paints must never be used on aluminium hulls. Lead incidentally, is more noble than steel, but the problem is not nearly as noticeable.

 

 

2.         By coating the surface with a substance such as paint. Paint sticks closely to any surface to which it is applied and prevents corrosion. In order to ensure that the bond between the paint and the surface is good the surface must be properly prepared.

 

            In particular -

           Any cracked or flaking paint should be removed.

           The surface should be clean, dry and free from salt, oil, grease etc.

           Any corrosion should be removed.

           Any internal repairs to the surface should be completed.

 

It is beyond the scope of this learner’s guide to describe every type of paint there is, but some of the common types of paints are as follows:

 

Anti-Corrosive Paints - used on metal surfaces to prevent corrosion from occurring.

 

Heat Resistant Paints - either sprayed aluminium or aluminium/graphite pigments.

 

Fire Retardant Paints - the action of these paints is that as they burn, gasses are given off which blanket the flame and slow or stop the combustion reaction.

 

Anti-Fouling Paints - used on the hull to prevent the growth of marine organisms.

 

Barrier Paints - in the case of painting an underwater section with a new coat of anti-fouling, unless the old system is completely removed, it is essential that a coat of barrier paint is used between the old and the new coats of anti-fouling.

 

This is because the solvent in the new paint will react with the old and some of the poison will leach down through the old paint thereby reducing the amount available to come out of the new coat to seaward.

 

Likewise when using a ‘high performance’ 2 part paint over the top of a coat of conventional paint, the coats must be separated by a coat of barrier paint. The chemical reaction occurring in the HP paint will damage the underlying conventional paint.

 

Non Skid Paints - used on decks and steps to prevent slippage. Generally around door entrances, windlass area, boarding areas and on steel step ladders.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.1 Spray Painting Antifouling

 

Paints can be applied by brush, roller or spray gun. In all cases you should refer to the manufacturer’s instructions on the recommended procedure, materials and safety precautions. This information is usually available from the paint container itself.

 

 

 

 

 

 

Methods of Docking And Slipping

Careening

 

This method does not require a slipway or dry dock, so it is suitable for repairs in an isolated area or in an emergency. The only requirement is a tidal range greater then the vessel’s draught.

 

The vessel is driven to a flat, cleared section of the beach or river bank and positioned parallel to the shore or bank, to give even support along its length as the water level falls and rises. The bank should not be too steep, and must be clear of obstructions. The vessel must fall up hill if flooding on the incoming tide is to be avoided. It may be positioned between poles driven into the bed or simply weighted to fall up hill. Hawser lines may be tied to solid sections of the vessel, e.g. the foot of a mast, and secured to points on-shore to help prevent the vessel from falling downhill. When the water level is low enough, shoring is installed on the downhill side to prevent rolling over.

 

 

Heaving Down

 

In this method, a vessel is heeled over, while afloat, by means of tackles set up between its masts and another ship, or shore attachments.

 

This method is not as successful as careening in exposing the hull, but since the vessel is afloat, there is little hull stress, and the dangers, through touching the bottom, or damage to the hull and the intakes, are minimal. It must be remembered that by heeling a vessel you increase its draft and you should be sure that there is sufficient under-keel clearance for the job.

 

 

The Graving Dock

 

The graving dock is excavated from the land and closed to the sea by means of a large watertight door or gate known as the “Caisson Gate”.

 

The edge of the dock bottom beneath the gate is referred to as the sill. The dock bottom has a very rigid construction and is usually made of reinforced concrete. The dock bottom always has a slight slope towards the sill to aid drainage. The sides of the dock are usually terraced to enable side shoring.

 

Along the centre line of the dock are blocks of concrete topped up with timber. They form the keel blocks. Two parallel rows of blocks on either side form the bilge blocks or side blocks. Depending on the size of the vessel and the shape of the underwater hull, the blocks are repositioned to suit the particular vessel.

 

On the sides of the dock at ground level are rails on which winches travel along the length of the dock. Wires from the winches are used, two on the forward beam and two on the after beam, to help position the vessel over the keel blocks when the vessel is brought in. Cranes are used for heavy lifting.

 

When the vessel is in position the lock gates are shut and pumping out commences. A diver may be employed to ensure that the vessel’s keel is in line with the keel blocks. As water is pumped out the diver keeps checking that the vessel is taking to the blocks as planned. Sometimes blocks are shifted so that maintenance can be done on a sea chest valve, drain plug, etc.

 

 

The Floating Dock

 

The basic structure of the floating dry dock is a very strong and rigid double walled “U”. The bottom is constructed very similar to the bottom structure of ships. The sides of the dock are vertical wing tanks. Keel blocks and bilge blocks are laid on top of the double bottomed structure. The whole dock forms a floating, watertight structure which can be submerged by flooding the double bottom and wing tanks.

 

The vessel to be dry docked is simply floated into the dock and positioned above the keel and bilge blocks by use of mooring lines. Shores are fitted to provide support and as the dock tanks are pumped out the dock rises until the pontoon deck is dry.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.2 Floating Dock

 

 

 

The Synchrolift

 

Operates along the same lines as the floating dock in that the vessel is floated in over a submerged platform and is then lifted clear of the water by raising the platform. The synchrolift however, is a land-based platform which is lowered into the water by a series of synchronised winches lining either side of the dock.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.3 Vessels on Synchrolift

 

 

Figure 1.3 shows a vessel on a synchrolift. Note the keel and bilge blocks. On the left of the picture, just clear of the bow you can see one of the lifting winches.

 

 

The Floating Cradle (Patent Slip)

 

One of the most common methods of removing a small vessel from the water involves the use of the patented slipway. This is basically a sloping, reinforced concrete runway which extends well below the low water mark. On the slip itself is built a set of railway tracks set well apart. Wheeled carriages run on these tracks and depending on the size of the vessel being dry docked, carriages can be linked together to form a single unit. Cradles are fitted onto these carriages with keel blocks on the centre line atop the carriage. The entire assembly is made up to suit the vessel being dry docked.

 

The vessel is manoeuvred onto the cradle under its own power and is secured with “springs”. As the vessel settles onto the cradle bed, wedges are inserted to keep the vessel upright. The entire assembly is slowly winched up the slipway. As the vessel takes to the keel blocks, securing beams are drawn tight and any shores, if required, are fitted. The vessel now secure in its cradle on the carriage is slowly winched out of the water.

 

 

The Travel Lift

 

A narrow dock is excavated and then opened to the sea. The vessel to be lifted manoeuvres slowly into the dock and secured temporarily with mooring lines while a mobile straddle carrier is positioned above the vessel. Broad slings which will eventually distribute the weight of the hull are then put in place. The weight is taken up by the slings. The moorings are released and the vessel is lifted clear of the water. The straddle crane, under its own power, carries the slung vessel to a suitable position in the shipyard, where it is lowered on blocks and shored and the slings removed.

 

The main advantage of this system is that many vessels can be docked at the same time and the slipping facility is not laid up for the duration of the vessel’s stay.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.4 Travel Lift

 

 

General Procedures For Docking And Slipping

 

Repair Lists

 

Prior to docking or slipping, a complete repair list of all work to be done while in dock should be made up. Several copies should be made so that all those directly involved in the work can monitor the progress being made and cross off the completed jobs.

 

 

Structural Features

 

When docking or slipping your vessel, the entire weight of the vessel will be supported at a few localised points, instead of uniformly over the hull, as is the case when the vessel is afloat. Most small vessels have sufficient strength to withstand these localised stresses without additional support. However, it should be remembered that external keel coolers, echo sounders, log and sonar transducers could be severely damaged if the bilge or keel supports came into contact with them. It is important that the dockmaster is supplied with up to date and accurate information regarding their location.

 

 

 

Stability Considerations

 

If you are using a patent slip for docking your vessel, then stability is not a major problem provided that the vessel is snugly secured in the cradle and the side support beams are drawn up tight before it is pulled clear of the water. The same is true of the travel lift.

 

If however, you are using a synchrolift, floating dock or graving dock, then you must be sure that your vessel has as much stability as possible. Tanks should preferably be empty so as to remove any free surface effect. The critical moment occurs just before the vessel settles on the keel blocks. Usually your vessel will be trimmed slightly by the stern. As the water level falls, the keel will touch the blocks at the stern first. This results in an upthrust on the stern which increases as the water level falls. This has the effect of reducing your vessel's GM by causing an apparent rise in the centre of gravity. If it did not have sufficient initial stability, it could topple off the blocks, with disastrous consequences. It’s happened before, make sure it never happens to you. Most shipbuilders will supply a recommended docking condition with the stability data for the ship. You should ensure that your stability condition is equal to, or better than the recommended condition.

 

All moveable weights should be secured, and all unnecessary weights on deck should be removed.

 

 

General Precautions In Dry Dock

 

           Transducers and impressed current anodes should be covered with grease and then masking tape.

           Remove drain (docking) plugs from all tanks that need to be drained.  Put them in a safe place and keep a written record of which plug goes where.  Ensure that plugs are all replaced prior to flooding the dock or entry into the water.

           Ensure that safe access is provided to and from the vessel.

           Ensure that fire safety precautions are adhered to.

           Ensure that all tanks, void spaces etc are opened, vented and ready for inspection by surveyors at the appropriate time.

           Ensure that all pollution control requirements are met.

 

 

In a dry dock the vessel may be unable to use its fire fighting system. Note the position of the fire hydrants ashore and the site of the dock supplied fire extinguishers. Keep a close watch on any hot work being done and stop any unsafe practices.

 

 

Undocking Checks

 

Ensure that:

           all docking plugs have been replaced

           all intake gills/grates have been replaced

           all transducers are uncovered and wiped clean

           all tanks are boxed up (manhole/inspection covers are replaced)

           anchors are secured

           all loose gear is secured

           new paint is dry to manufacturer’s specifications

           shore power supply is disconnected

           there is sufficient water depth to unslip

           sea cocks are open

 

 

 

Survey

 

The survey requirements of commercial vessels are laid down in the State Marine Acts and the USL Code. In this section, we will only deal with the requirements are per USL Code. Specifically, we will consider the surveys carried out for the issue of a “Certificate of Survey”.

 

 

Definitions

 

Survey           a thorough examination performed by, or in the presence of a surveyor or an authorised person or society.

Inspection    a visual inspection performed by an approved person.

 

The Certificate of Survey is issued on completion of an Initial Survey. The surveyor submits a report, detailing the condition of the hull, machinery and equipment, and makes a written declaration of such condition.

 

 

Initial Survey

 

The main purpose of this survey is to ensure that the vessel will be able to perform the tasks for which it is intended.

 

All aspects of the vessel’s construction are examined to ensure that it meets the requirements of Section 5 of the USL Code. After the construction is complete, the Authority surveys the vessel once more and if satisfied, issues the owner with a “Certificate of Survey”.

 

The Certificate of Survey or its evidence (plasticised document or metal plate) should be displayed:

 

           near the steering position, except on passenger vessels, where the evidence should be displayed in such a position that it is readily visible to passengers, or if the Authority requires,

           in a position on board that it shall be visible from outside the vessel.

 

 

Periodic Surveys And Inspections

 

All vessels must under go ‘Periodic Surveys and Inspections’ to satisfy the Authority that the vessel continues to comply with all its laws and regulations.

 

The survey schedule for vessels of less than 35 metres in length is given in Section 14 of the USL Code and is reproduced below:

 

Annual Surveys

 

           Equipment.

           Running trial of each main engine and associated gearbox.

           Operational test of bilge pumps, bilge alarms and bilge valves

           Operation test of all valves in the fire main system.

           Operational test of all sea injection and overboard discharge valves and cocks.

           Operational test of main and emergency means of steering.

           Running trial of all machinery essential to the safe operation of the vessel.

           Inspection of all pipe arrangements.

           General examination of machinery installation and electrical installation.

           All safety and relief valves associated with the safe operation of the vessel to be set at the required working pressure.

           Pressure vessels, and associated mountings used for the generation of steam under pressure or the heating of water to a temperature exceeding 99 degrees Celsius

           Inspection of the liquefied petroleum gas installation.

           Inspection of cargo handling, fishing and trawling gear.

           Inspection of escapes from engine room and accommodation spaces.

           Inspection of personnel protection arrangements in machinery spaces.

           Inspection of casings, superstructures, skylights, hatchways, companionways, bulwarks and guard rails, ventilators and air pipes, together with all closing devices.

           Inspection of ground tackle (anchors and chains).

 

 

Two Yearly Surveys

 

           Hull externally and internally except in way of tanks forming part of the structure.

           Sea injection and overboard discharge valves and cocks.

           Inspection of propellers, rudders and under water fittings.

           Pressure vessel and associate mountings of an air pressure/salt water system having a working pressure of more than 275 kPa.

 

 

Four Yearly Surveys

 

           Each screw and tube shaft.

           Anchors and cables to range.

           Chain locker internally.

           Tanks forming part of the hull, other than oil tanks, internally.

           Void spaces internally.

           Compressed air pressure vessels having a working pressure of more than 275 kPa and associated mountings.

           Pressure vessel and associated mountings of an air pressure/fresh water system having a working pressure of more than 275 kPa.

           Cargo handling, Fishing and trawling gear.

           Insulation test of all electrical installations above 32V A.C. or D.C.

 

 

Eight Yearly Surveys

 

           Each rudder stock and rudder stock bearing

           Steering gear.

           Hull in way of removable ballast.

           Selected sections of internal structure in way of refrigerated space.

 

 

Twelve Yearly Surveys

 

           Fuel oil tanks internally

 

 


 

Planned Maintenance

 

The master is responsible for the seaworthiness of the vessel and must ensure that all national and international requirements regarding safety and pollution prevention are being complied with. Effective planning is required to ensure that the vessel, its machinery systems and its services are functioning correctly and being properly maintained, including dry-docking to maintain hull smoothness.

 

Planned maintenance is primarily concerned with reducing breakdowns and the associated costs. Planned maintenance is of two kinds:

 

Preventative maintenance is aimed at preventing failures or discovering a failure at an early stage.

Corrective maintenance is aimed at repairing failures that were expected, but were not prevented because they were not critical for safety or economy.

 

 

Advantages of Planned Maintenance

 

           Fewer breakdowns and repairs.

           Equipment operates efficiently at all times.

           Fewer hazards to the crew when working with well maintained equipment.

           Vessel complies with survey requirements at all times.

           No areas of the vessel or items of equipment are overlooked or neglected.

 

 

Elements of A Planned Maintenance Program

 

You can develop a basic maintenance program for your vessel by taking the following steps:

 

Step 1 Determine what items need to be maintained.

Step 2 Determine the type of maintenance tasks required on each item.

Step 3 Determine the frequency of carrying out particular maintenance jobs.

Step 4 Prepare a maintenance schedule.

Step 5 Develop operational and recording procedures.

 

You will need to consider the following issues in the planning process

 

           Is an item worth maintaining? What would be the real cost of failure to maintain that item?

           Equipment manufacturers instructions.

           Statutory survey requirements.

           Classification society requirements.

           Maximum length of survey cycle.

           Magnitude of maintenance task.

           Maintenance/inspection that can only be carried out when the vessel is out of water.

           Resources required.

           Recording.

           Length of voyages, routes and trades the vessel is involved in.

           Spare parts replacement.

 

 

The plan must be adaptable to various weather conditions and must be flexible enough to accommodate changes in vessel’s trade.

 

It is convenient to draw up a maintenance schedule by breaking down the plan into various ‘time phases’. Two suggested categories are:

 

(a)       Short-term maintenance.

(b)       Long-term maintenance.

 

Short-term maintenance may include weekly, fortnightly or monthly inspections and greasing routines. Long term maintenance will involve major overhauls and surveys. Remember too that some operational maintenance tasks will only be carried out as and when necessary.

 

The actual operation and documentation of the plan will vary from vessel to vessel. Many vessels use a card index system or computer program for this purpose. Usually, a job sheet is prepared for each job. The job sheet contains a description of the work and a list of relevant spare parts and references to drawings and instruction manuals. On completion of the job, relevant details are entered in the job sheet.

 


 

 

 

 

 

 

 

Basic Principles of Stability

 

          Principle of Flotation

          Archimedes’ Principle

          Relationship between Weight and Buoyancy

          Definitions

          Displacement

          Draft

          Deadweight

          Freeboard

          Loadlines

          Fresh Water Allowance (FWA)

          Tonnes per Centimetre Immersion (TPC)

          Trim

          Reserve Buoyancy

           

 

 

 

Principle Of Flotation

 

Archimedes’ Principle

 

Archimedes’ Principle states that when a body is wholly or partially immersed in a fluid it appears to suffer a loss in mass equal to the mass of the fluid it displaces.

 

 

Relative Density

 

The relationship between weight and volume is called density. It is defined as ‘mass per unit volume’. One metric tonne of fresh water has a volume of one cubic metre. Therefore it has a density of 1.000 tonnes/m3. Salt water on the other hand, is heavier. One cubic metre of salt water weighs 1.025 tonnes, and so salt water has a density of 1.025 tonnes/m3.

 

The relative density (or specific gravity) of a substance is defined as the ratio of the weight of the substance to the weight of an equal volume of fresh water. In other words, it is simply a comparison of the density of a substance with the density of fresh water.

 

 

This is a pure number and has no units. The R.D. of sea water is therefore 1.025.

 

 

Relationship Between Weight and Buoyancy

 

Suppose we have a body or block that measures 1 cubic metre and weighs 4000 kg. If we now lower the block into fresh water, it will displace 1 cubic metre of fresh water - which, as we now know, weighs 1000 kg. In other words, there is a force acting upwards of 1000 kg and a force acting downwards of 4000 kg: the resultant force has to be 3000 kg downwards. That is, the block will sink.

 

If we take the same 4000kg block and mould it into a hollow box with a volume of 5 cubic metres, and then place it in fresh water, it has sufficient volume to displace 5 cubic metres of fresh water. If the box were now completely submerged, it would experience an upward force of 5000 kg.

 

 

However, the downward force of the box is still only 4000 kg, thus the resultant force will be 1000 kg upwards. In this case the box will rise out of the water to a level where the forces are equal and opposite, that is, with 4 cubic metres under water, and 1 cubic metre still outside water.

 

Thus for a body to (just) float in water, its weight must be exactly balanced by the force of buoyancy. If the volume of the body is further increased, it will float with a certain amount outside the water.

 

 

 

 

Definitions

 

Displacement

 

When a vessel is floating in water, the whole of the weight of the vessel is supported by the buoyancy of the water. In order to provide that buoyancy the vessel sinks in the water, until the portion of the hull which is below the water surface pushes aside, i.e. ‘displaces’ a weight of water equal to the weight of the vessel.

 

This is the law of flotation; namely, a floating vessel displaces its own weight in water.

 

 

Draft

 

When a vessel is floating in water the distance from the underside of the hull to the water surface is called the draft. Numbers are painted at the forward and after ends of a vessel, so that the draught can be read off at any time. These numbers are referred to as draft marks.

 

When a vessel is fully loaded with fuel, fresh water, cargo, gear, crew, etc., it will float more deeply in the water than when it has less weight on board.

 

In this situation the vessel is said to float at load draft and is therefore at load displacement.

 

When a vessel has no weights on board, that is when it consists of only the hull, superstructure, accommodation and machinery it is said to float at light draft and to be at light displacement.

 

Deadweight

 

The difference between load displacement and light displacement is called deadweight. Things such as fuel, fresh water, crew, gear, cargo, fish, etc., are all items of deadweight.

 

 

 

Freeboard

 

At any draft the distance from the waterline to the deck is called the freeboard.

 

 

 

 

 

 

 

 

 

 

Figure 2.1 Lightship

 

 

 

 

 

 

 

 

 

 

Figure 2.2 Loaded

 

 

 

Loadlines

 

Most trading vessels are required by law to have marks on the sides, at amidships, which indicate the draft to which the vessel can be loaded. Section 7 of the USL Code deals with loadlines. Loadlines are not required to be marked on vessels of less than 24 metres in length but note that the definition of length (as given in Section 7 of the USL Code, for loadline purposes) is not the same as measured length. For most vessels the loadline looks like the one shown in Fig 2.3.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2.3 Loadlines

 

 

Figure 2.3 shows the typical loadlines for a vessel trading solely in Australia. The abbreviations are as follows:

 

TF       Tropical Fresh Water Mark

F          Fresh Water Mark

T          Tropical Mark

S         Summer (Plimsoll) Mark

W        Winter Mark

 

The tropical, summer and winter are the marks which must not be submerged when the vessel is trading in a designated tropic zone, summer zone or winter zone.

 

The names of the zones are only loosely related to the seasons of the year. It is possible to have summer zones in winter and vice versa.

 

Bad sea and weather conditions are associated with winter zones; better weather with summer zones, and good conditions with tropical zones. As a result, a greater freeboard is required for the bad weather zones than for the good weather zones.

 

 

A seasonal zone is one which changes its name according to different times of the year. (See Fig 2.4).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reproduced with the permission of the Australian Government Publishing Service.

 

Figure 2.4 Seasonal Zones

 

 

In all cases measurements are made to the tops of the lines. For example a vessel loaded to full draft in a Winter Zone will have a waterline as shown in Fig 2.5.

 

 

 

 

 

 

 

 

 

 

 

Figure 2.5

 

The letters on either side of the disc indicate the marine authority which is responsible for the survey of the vessel. In the Fig 2.3 CA is used, this means Commonwealth of Australia. A full list of Australian marine authority designations is:

 

CA      Commonwealth of Australia

 

QA      Queensland

 

VA       Victoria

 

TA       Tasmania

 

SA       South Australia

 

WA      Western Australia

 

NTA    Northern Territory

 

NA       New South Wales

 

Fresh Water Allowance (FWA)

 

When a vessel is floating in water, the underwater part of the hull displaces a quantity of water which is equal to the weight of the vessel. The hull actually displaces a volume of water measured in cubic metres, which is equal to the underwater volume of the hull. Each cubic metre of water has a weight, 1 000 tonne in the case of fresh water; 1.025 tonnes in the case of salt water. The hull must displace sufficient cubic metres of water to balance the weight of the vessel exactly. One cubic metre of sea water will balance 1.025 tonnes of weight therefore 100 cubic metres of sea water will balance 1.025 x 100 = 102.5 tonnes of weight.

 

Imagine a vessel, floating first in sea water and then in fresh water. It will need to displace more cubic metres of fresh water to balance its weight, than it would in sea water, because each cubic metre of sea water balances more weight than each cubic metre of fresh water. The number of cubic metres displaced determines the size of the underwater portion of the hull.

 

In sea water, the underwater portion of the hull will be smaller, that is the vessel will not sink as far as it will in fresh water, and the draft in sea water will be less than the draught in fresh water.

 

The difference between the two drafts is called the fresh water allowance (FWA).

 

FWA is measured as the distance between the top of the Summer (S) line and the top of the Fresh (F) line.

 

When loading a vessel which has a loadline, the appropriate loadline must not be submerged. For example, if a vessel is in a Summer Zone, the waterline will look as shown in Fig 2.6.

 

 

 

 

 

 

 

 

 

 

Figure 2.6

 

 

If this vessel is loading in a river then it will be allowed to load as shown in Fig 2.7.

 

 

 

 

 

 

 

 

 

 

 

Figure 2.7

 

 

When the vessel reaches sea water it will rise to the summer load line level. It is important to take advantage of the FWA because there will be a loss of cargo carried, and therefore a loss of revenue, if the vessel only loads up to the summer loadline level in freshwater.

When a vessel loads in a brackish waters harbour, the specific gravity of the dock water must be tested with a hydrometer. The amount that the summer load line can be immersed is then calculated as a percentage of the FWA. The example below shows a vessel with a FWA of 50cms loading in dockwater of SG 1005. This water is only four fifths fresh so the vessel can only use 40 cms of its 50cms FWA if it must float at the summer loadline out at sea.

 

 

SEAWATER DOCKWATER

SEAWATER – FRESHWATER            

=

1025 –1005 

1025 –1000

=

20

25

=

4

5

x

50

=

40

 

 

 

Tonnes Per Centimetre Immersion (TPC)

 

As weights are loaded on board a vessel, it will gradually sink lower in the water. The amount of weight which will sink the vessel 1 cm deeper in the water, that is, the weight which will increase the draft by 1 cm is called the tonnes per centimetre immersion (TPC).

 

 

 

Trim

 

As weights are loaded on board a vessel, the draft will increase, as the vessel sinks deeper in the water. If the weights are loaded towards the ends of the vessel, it will not sink evenly. If a weight is loaded forward, then the draft at the bow will increase more than the draft at the stern. Of course the overall draft will still increase. At any given time therefore, a vessel may have different drafts at the bow and stern.

 

The difference between the draft aft and the draft forward, is called the trim.

 

Trim = Draft Aft - Draft Forward

 

 

If the draft aft is greater than the draft forward, as shown in Fig 2.8 the vessel is said to be trimmed by the stern. If the reverse is true the vessel is said to be trimmed by the head. This is shown in Fig 2.9.

 

 

 

 

 

 

 

 

 

 

 

Figure 2.8 Trim = Draught Forward - Draft Aft

 

 

 

 

 

 

 

 

 

 

 

Figure 2.9

 

 

It is usually desirable to have your vessel trimmed by the stern. This gives you increased reserve buoyancy forward, and the vessel will ride more comfortably over head seas. The rudder will be more responsive and generally the vessel will handle better. Excessive trim by the stern is not good. The vessel becomes over responsive and considerably less stable. It should be remembered that the stability calculation for the safe operation of all

 

 

 

vessels are based on the assumption that the vessel is on an even keel (equal drafts fore and aft).

 

 

 

Reserve Buoyancy

 

The amount of freeboard which a vessel has, is a measure of the amount of buoyancy which is left above the water line, to support the vessel in case of bad weather or damage, etc. This buoyancy is referred to as reserve buoyancy. Every vessel is designed to operate with a certain freeboard which provides for safety of vessel and crew. See Figs 2.6 and 2.7

 

 

 

 

 

 

 

 

 

Longitudinal and Transverse Stability  

   

 

          Transverse Stability

          Centres of Buoyancy and Gravity

          Heel

          Equilibrium

          List

          Stiff and Tender Vessels

          Roll Period

          Weight Distribution

          Longitudinal Stability        

          LCB and LCG

          LCF

          Simplified Stability Data

          Stability Booklet

          Safe Practices

 

 

 

 

 

 

Transverse Stability

 

Centres of Buoyancy And Gravity

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.1

 

 

Fig. 3.1 shows a transverse section through a vessel.

WL      represents the waterline at which the ship is floating.

K         is the keel.

B         is the position of the transverse centre of buoyancy usually just called the centre of buoyancy. This is the centre of the underwater volume of the vessel. It is the point through which the force of buoyancy supporting the vessel acts vertically upwards.

G         is the position of the transverse centre of gravity. It is the point through which all of the weight of the vessel including deadweight items can be considered to act vertically downwards.

 

When the vessel is upright, both the centre of buoyancy and the centre of gravity are on the centre line of the vessel.


Movement Of Centre Of Buoyancy

 

The centre of buoyancy is the centre of the underwater volume of the vessel. As the vessel sinks deeper in the water, the centre of buoyancy will rise higher as shown in Fig 3.2.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.2

 

When the vessel is floating at waterline W1L1, the centre of buoyancy is at B1. If the ship sinks to waterline W2L2 then the centre of buoyancy will rise to B2, still on the centre line.

 

 

Movement Of Centre Of Gravity

 

The centre of gravity can be imagined to be a point, through which all of the vessel’s weight acts vertically downward.

 

The centre of gravity of the vessel at light displacement is fixed by the arrangement of hull, superstructure, machinery, etc. The addition of deadweight items such as fuel, cargo, etc. causes the centre of gravity to move in various directions. Therefore the position of the centre of gravity is dependent upon the size of weights added to the vessel, and the position in which they are added. In other words, the final position of the centre of gravity is dependent upon the practices of the vessel operator.

 

 


The following three rules describe the movement of the centre of gravity of the vessel.

 

(1)       The centre of gravity moves towards an added weight. See Fig 3.3.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.3 - Weight Added

 

 

(2)       The centre of gravity moves away from a discharged weight. See Fig 3.4.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.4 Weight Removed

 

 


(3)       The centre of gravity moves parallel to the movement of a weight which is already on board. See Fig 3.5.

 

 

 

 

 

 

 

 

 

 

 

Figure 3.5 Shifting Weights

 

 

The size of the movement of the centre of gravity is directly dependent upon:

 

(1)       The size of the weight involved;

 

(2)       The distance between the centre of gravity of the vessel and the centre of gravity of the weight.

 

(3)       The displacement of the vessel.

 

 

Suspended Weights

 

When a weight is suspended, from a boom for example as shown in Fig 3.6, the effect is as though the weight were situated at the point of suspension, that is, the head of the boom. Usually, this is a long way from the centre of gravity of a vessel and therefore, a suspended weight may cause a large movement of the centre of gravity.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.6  Suspended Weight

 

 

Heel

 

Previous diagrams showed a transverse view of a vessel in an upright position, with the centre of gravity and the centre of buoyancy on the centre line. It was said that all of the weight of the vessel, and any associated deadweight items, can be assumed to act vertically downwards through the centre of gravity. It was also said that all of the buoyancy effect can be assumed to act vertically upwards through the centre of buoyancy; and, of course, when a vessel is floating the weight is exactly equal to the buoyancy.

 

When the vessel is moved away from the upright by some effect outside the vessel, e.g. a wave, the vessel is said to be heeled.

 

 

In Fig 3.7 the vessel has moved to an angle of heel as shown.

 

The vessel was originally floating at waterline WL, and after heeling is floating at waterline W1L1.

 

 

 

 

 

Wedge 1 has come out of the water, wedge 2 which is of equal volume, has gone into the water.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.7 Vessel heeled

 

 

The centre of buoyancy (B) is the centre of the underwater volume of the vessel. Because the underwater shape has changed, the centre of buoyancy moves to the centre of the new underwater shape, which is at B1.

 

Buoyancy acts vertically upwards through the centre of buoyancy as shown and cuts the centre line of the vessel at a point called the metacentre (M). The initial position of the metacentre is determined by the shape of the underwater portion of the hull.

 

Note that the position of G has not changed, because no weights have been moved, but weight still acts vertically downwards, through G as shown.

 

The distance from G to M is called the metacentric height.

 

 

Fig 3.8 shows an expanded view of the relationship between B, B1, G, Z and M.

 

The lines of action of weight and buoyancy are separated by a distance GZ, this is called the righting lever

 

Imagine that GZ was a solid lever fixed in the centre of the ship. The whole weight of the ship pushes down through G. The force of buoyancy (which is equal to the weight) pushes upwards through Z.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.8 Righting Lever

 

 

What happens to the lever GZ? In this case it experiences a force (turning moment) tending to rotate it in an anti-clockwise direction. This turning moment has the effect of rotating the whole ship is an anti-clockwise direction. (The lever is fixed remember). Now look at Fig 3.7 again. An anti-clockwise rotation would return the ship to the upright position. As that happened, B would once again be vertically under G, and the righting lever would disappear, since a righting lever only exists when B is not directly underneath G.

 


Equilibrium

 

Equilibrium is the term used to describe a vessel that is afloat. It is a word made up of two words namely equal, and balance. A vessel will float when the forces of weight and buoyancy are equal, and they balance - that is both B and G are in the same vertical line, and the vessel is not being acted on by an external force (a force other than buoyancy or weight).

 

 

Stable Equilibrium

 

A vessel which will tend to return to the upright after being heeled by an external force, is said to be in stable equilibrium. When G is below M the vessel is in stable equilibrium e.g. the situation shown in Fig 3.7.

 

Unstable Equilibrium

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.9 Unstable Equilibrium

 

 

If G is above M as shown in Fig 3.9 the ship is said to be in unstable equilibrium. It will not remain upright. It will heel to an angle called an angle of loll. At that angle of loll it will have ‘picked up’ stability and will return to the angle of loll if disturbed by an external force. If G is sufficiently far above M, then the angle of loll may be very large and the vessel may capsize.

 

Angle of loll is described in Section 4.

 

 


Neutral Equilibrium

 

If G and M coincide, as shown in Fig 3.10 then, theoretically the vessel will have no reason to remain upright. Also, if it is heeled, it will have no tendency either to heel further or to return to the upright.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.10 Neutral Equilibrium

 

 

List

 

In order that a vessel is able to float upright the centre of gravity and the centre of buoyancy must be on the centre line. The buoyancy and weight are then equal in size and are acting along the same straight line but in opposite directions. See Fig 3.11.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.11 No List

 


If weights are loaded or discharged or moved within the vessel G may move off the centre line.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.12 G Located Off Centre Line

 

 

The buoyancy and weight are not acting along the same straight line now, as shown in Fig 3.12. The vessel will tend to take up an angle of list as shown in Fig 3.13.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.13 Vessel Listed

 

As the vessel lists B will move until it is vertically below G at B1. Then buoyancy and weight will be acting along the same straight line and the vessel will come to rest at an angle of list. An angle of list may be removed by moving weights so that G moves back onto the centre line.

 

Stiff and Tender Vessels

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.14 Stable Equilibrium

 

 

Fig 3.14 shows a vessel in a stable condition. This is the only condition in which any vessel should be operated.

 

The way in which the vessel returned to the upright was illustrated in Fig 3.7 and 3.8. In general, the bigger the righting lever, the more stability a vessel will have.

 

The size of the righting lever is dependent upon the position of G.

 

 


In Fig 3.15 it can be clearly seen that G1Z1 is smaller that GZ and G2Z2 is bigger than GZ.

 

Also, G1M is smaller that GM, and G2M is bigger than GM. In fact the size of GM and GZ are related. The bigger the GM the bigger the GZ.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.15 GZ is determined by position of G

 

 

If a vessel has a lot of stability i.e. if it has a big GZ and GM then it will tend to roll quickly, and perhaps uncomfortably, it is said to be 'stiff'

 

If a vessel has a small measure of stability, i.e. if it has a small GZ and GM then it will roll slowly and easily, it is said to be tender.

 

 

 

Roll Period

 

All vessels have a natural roll period. When heeled by a wave, they will begin rolling. It is a bit like a pendulum set in motion - the period of roll remains the same, even though the angle to which the vessel rolls changes. The period of the roll is governed by two factors.

 

(i)         the GM

(ii)        the beam of the vessel

 

If the GM is large, the roll period is short. If the GM is small, the roll period is long.

 

A vessel’s roll period is a good indicator of stability. The roll period may be measured at sea, or in port by rolling the vessel artificially. On a small vessel you can conduct your own rolling test to obtain GM. To conduct such a test, the boat should be alongside, in smooth water, with no wind or tide, with slack moorings and adequate side and bottom clearance. All loose weights should be secured and all slack tanks pressed up. A roll is initiated, perhaps by pulling on a masthead line from the wharf, and when the vessel is rolling freely, several rolls are timed, and averaged.

 

An approximate value for the GM in metres may then be found from the formula:

 

 

where: B is the beam of the vessel in metres

            T is the time for one complete roll (port, starboard, port) in seconds.

 

Example: A vessel has a beam of 12 metres and a roll period of 18 seconds. What is the vessel’s GM?

 

 

            GM   =  (0.59)2

 

                     =  0.34 metres

 

Thus, roll period is a sensitive indicator of stability. You should time your vessel’s roll period on several occasions, and thereafter be aware of it. If the roll period seems longer, or the roll sluggish, investigate the stability immediately.

 

 

 

 

Weight Distribution

 

The distribution of deadweight items within a vessel is the responsibility of the operator. It is normal for vessels to be stable and upright in their lightship condition. Therefore, if a vessel is unstable or listed, after the addition of deadweight items, it can be corrected by the action of the operator.

 

Remember

 

(1)       G moves towards a loaded weight.

(2)       G moves away from a discharged weight.

(3)       G moves parallel to a shifted weight.

(4)       A suspended weight acts as though it is located at the point of suspension..

 

Also

 

(1)       Stability improves if G is lowered.

(2)       Stability gets worse if G is lifted.

(3)       Free surface effect makes stability worse.

 

Therefore stability is improved if

 

(1)       Weights already on board are lowered.

(2)       Weights are added low down.

(3)       High weights are removed.

(4)       Suspended weights are lowered.

(5)       Tanks are kept completely full or completely empty.

 

Stability is worsened if

 

(1)       Weights already on board are lifted higher.

(2)       Weights are added high up in the vessel.

(3)       Weights are removed from low down in the vessel.

(4)       Weights are lifted on booms etc.

(5)     Many tanks have free surfaces.

 

Calculating Loading and Unloading of weights:

 

The amount that the centre of gravity of a vessel is shifted by the loading of a weight can be calculated by the formula:

 

Distance from VCG    x      weight added

Weight of vessel          +      weight added

=

Change in metres of

Vertical Centre of Gravity

The amount that the centre of gravity of a vessel is shifted by the unloading of a weight can be calculated by the formula:

 

Distance from VCG    x      weight added

Weight of vessel           -      weight added

=

Change in metres of

Vertical Centre of Gravity

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Longitudinal Stability

 

Parameters such as centre of gravity and centre of buoyancy have been used in describing transverse stability, so far. They can also be used to describe longitudinal stability.

 

LCB and LCG

 

 

 

 

 

 

 

 

 

 

 

Figure 3.16 LCB Forward of LCG - Vessel trims by the stern

 

 

In Fig 3.16 LCB is the longitudinal centre of buoyancy. This is the longitudinal centre of the underwater volume, and is the point through which all the buoyancy can be said to act vertically upwards.

 

LCG is the longitudinal centre of gravity. This is the point through which all of the weight of the vessel can be said to act vertically downwards.

 

If the position of LCG and LCB are as shown in Fig 3.16 then the actions of buoyancy and weight will cause the vessel to rotate as shown by the arrow. The stern will sink deeper, the bow will rise higher. LCB is the longitudinal centre of all underwater volume. As the vessel rotates, the shape of the underwater volume will change and LCB will move to the new centre.

 

When LCG and LCB are in the same vertical line, the rotation will stop, the vessel will be trimmed by the stern as shown in Fig 3.17.

 

 

 

 

 

 

 

 

 

 

Figure 3.17 LCG and LCB in same vertical line - no trimming moment

 

 

If the vessel had started with LCB aft of LCG as shown in Fig 3.18 then the rotation would cause a trim by the bow.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3.18 LCB aft of LCG - vessel trims by the head

 

 

 

LCF

 

Vessels are not generally symmetrical fore and aft, therefore a vessel does not necessarily rotate about amidships when it trims. In fact, it rotates about a point called the longitudinal centre of flotation (LCF).

 

LCF is the centre of the shape of the waterline at which the vessel is floating. See Fig 3.19.

 

 

 

 

 

 

 

 

Figure 3.19

 

 

Calculating Loading and Unloading of weights:

 

The amount that the centre of gravity of a vessel is shifted by the loading of a weight can be calculated by the formula:

 

Distance from LCG    x      weight added

Weight of vessel          +      weight added

=

Change in metres of

Long. Centre of Gravity

 

The amount that the centre of gravity of a vessel is shifted by the unloading of a weight can be calculated by the formula:

 

 

 

Distance from LCG    x      weight added

Weight of vessel           -      weight added

=

Change in metres of

Long. Centre of Gravity

 

 

 

 

 

 

 

 

 

 

 

 

Simplified Stability Data

 

Much of the information discussed in earlier sections of these notes can be found in the Simplified Stability Information Booklet that may be provided on your vessel.

 

 

Stability Booklet

 

The booklet is set out in an approved format and contains the following information.

 

1.         The vessels name, official number, port of registry, gross and net tonnages, dimensions, operating displacement, deadweight and draught.

 

2.         A profile view of the vessel showing and naming all compartments, including tanks.

 

3.         The capacity and the centre of gravity, vertical and longitudinal, of all spaces used to carry fish, water, fuel, stores etc.

 

4.         Tank calibrations for every tank holding 2 tonnes or over, plus the free surface effect of every tank.

 

5.         Information about the following hydrostatic particulars.

 

          (a)          Displacement in salt and fresh water.

          (b)          K M

          (c)          T P C

          (d)          L C B

          (e)          L C G

          (f)           Trim information

 

These values are recorded for various draughts.

 

6.         Sample loading conditions such as Lightship, Loaded departure from port, worst operating condition, etc.

 

7.         Guidance notes and warnings dealing with such things as recommended distributions for fuel, water, cargo etc., recommended operating procedures and warnings about dangerous practices.

 

The purpose of this information is to let the master of the vessel know under what conditions the vessel will have sufficient stability. If you operate a vessel so that its condition is better than the worst condition that is still safe, then you will know that your vessel has sufficient stability for normal conditions.

 

 

Safe Practices

 

The following rules for safer stability are taken from MV “Twosuch” simplified stability booklet.

 

The sheets which follow are general comments to enlarge on good seamanship and house-keeping and issued only to enable the operators to use the stability data to best advantage.

 

Tank Usage and Slack Tanks

 

(1)       Tanks which are not in use, must at all times be full and pressed up, or empty where possible. Remember that slack tanks create free surface and the effect of slack tanks results in actual and often large reductions of stability.

 

Free surface effect is explained in Section 4 of this learner’s guide.

 

(2)       When manipulating tank contents by pumping from one tank to another, make every effort to maintain level trim. Develop a system of tank usage which keeps the trim of the vessel from becoming excessive. Remember that the calculations for stability are accurate only within a small range of trim.

 

(3)       Transference of fuel or fresh water and the ballasting of tanks should only be carried out in favourable weather conditions.

 

(4)       There is in this book a recommended sequence for the use of liquids in tanks, departure from which may be dangerous. These recommendations should be followed unless there are specific reasons at the time for not doing so.

 

(5)       Occasionally, conditions of loading and tank manipulations can lead to trim by the bow. This can be avoided by coordinating the operations; in other words, the effects of loading can be offset by correctly manipulating the contents of the tanks.

 

(6)       Excessive trim by the bow can lead to difficulties in handling the vessel and may result in poor seakeeping.

 

 

Water On Decks

 

Large amounts of water on decks raise the centre of gravity of the vessel and drastically reduce its stability.

 

 

(1)       Shipping large amounts of water should be avoided by good seamanship.

 

(2)       It is essential to allow quick drainage for any water on decks by keeping the freeing ports uncluttered and free from obstructions at all times.

 

 

Free Surface Effects

 

The effect of Free Surface of liquids is to raise the Vertical Centre of Gravity, therefore reducing stability.

 

(1)       On Deck

 

            Do not allow water to accumulate on main deck or upper deck.

 

(2)       In Tanks

 

            The number of slack tanks at any one time should be kept to a minimum. To restrict the amount of Free Surface, it may be necessary to transfer liquids between tanks, bearing in mind the trim required and the weather conditions at the time.

 

 

Effect of Wind and Waves

 

High speed wind and gusts can cause a considerable angle of heel, especially for vessels with large superstructures, thus reducing the range of stability. The situation can become serious, particularly in heavy and confused seas.

 

In heavy weather, make sure that all manoeuvres are carried out in accordance with the best practice of seamanship.

 

 

Weathertight Integrity

 

In severe weather, it is the responsibility of the Master and Crew to ensure that all hull, deck and superstructure openings are closed and watertight as far as is practicable. In emergency conditions, all openings must be closed, particularly weathertight doors, hatches and ventilation trunks and only opened at the Master’s discretion.

 

 

General Comments

 

(1)       Always determine the cause of a list or a change of trim of the vessel.

 

(2)       Heavy rolling of the vessel should be regarded as a potential hazard. Oblique seas, particularly from astern, reduce the average stability below that calculated on the sheets in this book.

 

            The most undesirable condition occurs when a wave crest is amidships and when running before a high following or quartering sea.

 

            Rolling becomes more violent when seas approach the vessel at about 15 degrees aft the beam.

 

(3)       Fish or cargo shall be properly secured against shifting which could cause dangerous trim or heel of the vessel.

 


 

 

 

Factors Affecting Stability

 

          Suspended Weights

          Use of Ship’s Gear

          Loads on Fishing Gear

          Free Surface Effect

          Practical Aspects of Stability

          Water on Deck

          Bilging

          Structural Changes

          Angle of Loll

 

 

 

Suspended Weights

 

 

 

When a weight is lifted by a crane or derrick, the centre of gravity of the weight will be immediately transferred to the point the weight is suspended from (the head of the crane or the end of the derrick or boom). This occurs the instant the weight is lifted and from that point on the centre of gravity will not change further no matter how high the weight is lifted.

 

 

 

 

 

 

 

 

 

 

Figure 4.1

 

 

 

 

We will now consider the sequence of events that occur when a vessel lying port side to a wharf discharges a heavy weight from the starboard lower hold by means of the vessel’s crane.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 4.2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.3


 

Lifting

 

As soon as the weight is clear of the deck and is being borne by the crane head, the centre of gravity of the weight appears to move from its original position, to the crane head (g to g1). In Fig 4.3 G the original position of the vessel’s centre of gravity, moves upward to G1 parallel to gg1. The centres of gravity will remain at G1 and g1 during the whole of the time the weight is being raised.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.4

 

 

As the crane begins to swing the centre of gravity of the weight will remain at the head of the crane (g1). The vessel’s centre of gravity (G1) will begin to move out towards G2, parallel to the movement of weight and the vessel will begin to list. (Fig 4.4)

 

Lowering

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.5


 

 

The crane has now swung over to plumb the wharf and the boom is lowered. The crane head has moved from g1 to g2 and since the weight is suspended from the crane head, its centre of gravity will have also moved from g1 to g2. The vessel’s centre of gravity has also moved parallel to the weight, from G1 to G2. Maximum list will be experienced at this point. (Fig 4.5)

 

 

Landing

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.6

 

 

The wire is now lowered and the weight is landed on the wharf. It is in effect being discharged from the crane head and the vessel’s centre of gravity will move from G2 to G3 in a direction directly away from g2. G3 is therefore the final position of the vessel’s centre of gravity. The net effect of discharging the weight is a shift of the vessel’s centre of gravity from G to G3, directly away from the centre of gravity of the weight finally discharged (g1). (Fig 4.6)

 

 It should be clear now that a vessel must have adequate stability before suspending weights from its derrick or crane. If the shift in the CG of the vessel is large enough to make it unstable, the vessel will take up an angle of loll. The angle of loll will be increased further due to the list caused by the suspended weight. In extreme cases, the vessel may even capsize.

 

Loads on Fishing Gear

 

When towing trawls or other fishing gear, the force exerted by the tow will be felt at the point of suspension, as shown in diagram Fig 4.7. This is the equivalent of a weight acting at the point of suspension. If the point is high above the deck, such as occurs when towing from a boom end, then the movement of G1 towards the point of suspension may be large. This can have a detrimental effect on stability. The same situation applies when gear is being lowered or lifted on board, using booms or powerblocks. If a vessel has good stability these operations should present no problems. If stability is poor, then steps should be made to improve stability.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.6

 

 

If gear becomes foul when towing, there will be two effects:

 

1.         Dynamic effect - the vessel will heel over because it will be still trying to move ahead.

 

2.         Static effect - as long as there is any strain on the gear, the circumstances will be the same as described above, i.e. the vessel will heel. The angle of heel will be less than that caused by the dynamic effect.

 

All strain should be taken off the gear as quickly as possible by stopping the engines and if possible, slacking away on the trawl winches. If necessary, stability should be improved before action is taken to free the gear.

Further information about Loads on Fishing Gear can be obtained from the Trim and Stability Booklet.

 

Free Surface Effect

 

All liquids in partially filled tanks have a free surface, which is free to slop backwards and forward with the motion of the ship. This free surface effect can cause a serious stability problem if the movement of the liquid is not contained. You might like to conduct a simple, practical experiment to demonstrate F.S.E. for yourself:

 

 

(a)       Take a flat tray with raised sides and partially fill it with water. (A flat baking pan will work.)

 

(b)       Now hold it level, supported by the palms of your hands, held horizontal at arms length and at shoulder height.

 

(c)        Now gently raise your right hand a few centimetres.

 

As the water runs to the left of the tray/pan you will feel a marked increase in weight, tending to push your left hand down further and so aggravate the condition.

 

This is Free Surface Effect (F.S.E.). A ship reacts in the same way. It first rolls slightly to a small angle of heel as a result of the wave forces. The internal forces of the shifting water in slack tanks then increase the list further as the liquid flows to the low side. If this F.S.E. causes the vessel to list so that its deck edge is immersed below the waterline, it could well capsize. Fig 4.8 shows a vessel with a partially filled tank. Free surface effect reduces the size of GM. Therefore the size of GZ is reduced, and consequently the ability of the vessel to return to the upright position is reduced.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.8

 

 

Free surface effect is at a maximum in tanks which extend right across the breadth of the vessel. By partitioning the tank longitudinally, the flow of liquids to the low side when the ship is heeled can be restricted. It is not removed completely, but the F.S.E. can be reduced to acceptable limits. Obviously, correct loading and ballasting of the ship is also important, but this is an operational consideration and not a design one. Practically all tanks, with the exception of the fore peak ballast tank, are longitudinally subdivided for this reason.

 

Tank subdivision is effected by a continuous watertight divider extending in a fore and aft direction to each end of the tank and vertically from the inner bottom of the tank to the underside of the tank top.

 

Fore peak tanks are usually narrow and do not present a very large free surface problem. For this reason, it is unusual to find any longitudinal subdivision in them.

 

Where tanks are not longitudinally divided by a watertight divider, there are usually longitudinal wash bulkheads which act as baffle plates. While these do not stop the sideways motion of fluids in the tank, they are designed to retard the flow so that the heeling force created by the free surface effect is out of phase with the rolling of the vessel. This tends to damp the vessel's rolling instead of aggravating it, which can be quite beneficial.

 

The depth or quality of the liquid in the tank does not affect the free surface to any great degree. Free surface area is the main factor. Only a completely empty or completely full tank will have zero free surface.

 

 

 

 

Practical Aspects of Stability

 

Water on Deck

 

If water is shipped on board, then the effect is three fold. Firstly a weight is added high up in the vessel, thus reducing stability. Secondly, that water has a free surface effect, which will further reduce stability. Thirdly, the added weight causes the vessel to sink further in the water, thereby reducing freeboard, and reducing seaworthiness. Freeing ports are provided on deck, so that the water shipped on board can be cleared rapidly. These freeing ports should never be blocked.

 

 

Bilging

 

You may recall from Section 2 that reserve buoyancy is the volume of watertight hull areas above the waterline. As weight is added to a vessel and it sinks in the water, the volume of space above the waterline decreases. When this space (reserve buoyancy), is gone the vessel will sink.

 

If part of the engine room or the vessel’s hold is above the waterline, then providing that they are enclosed they will contribute to the vessel’s reserve buoyancy. Hence, the reason that all watertight doors are to be kept closed (except for access), at all times.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.9

 

 

It is necessary to have a certain reserve buoyancy as, when in a seaway with the ends or middle unsupported, the vessel will sink down to displace the same volume as it does in smooth water. This could result in the vessel foundering.

 

If a vessel is damaged, and water can enter a compartment which was previously watertight, the compartment is said to have been bilged. When a compartment is bilged the buoyancy provided by the underwater volume of that compartment is lost, as is the reserve buoyancy of the enclosed volume above it. Before bilging, the reserve buoyancy was the entire enclosed volume above the original waterline. After bilging it is the enclosed volume above the new intact water plane area.

 

If this compartment is to one side of the centre line then the vessel will take up an angle of list. Depending upon the location of the compartment, the vessel may also trim by the bow or stern. In any case, draught will increase, freeboard and therefore reserve buoyancy will decrease and the effect is always to reduce stability.

 

In case of flooding, the biggest danger is the loss of watertight integrity and the subsequent loss of internal buoyancy from the damaged areas. Your immediate action in this case should always be to close all watertight doors through the vessel to prevent further loss of buoyancy. It may be possible in some cases to bring the damaged area out of water deballasting the vessel or providing a list on the opposite side to the damage.

 

 

 

Structural Changes

 

If a vessel is changed structurally, for example if a new wheelhouse is added or if an extra mast or winch is installed, the effect on stability is exactly the same as though these items were added weights. Because structural changes are usually complex and old material is often taken off the vessel as well as adding new material it is a survey requirement that all of the vessel’s stability is reworked after structural changes have taken place.

 

 

Angle of Loll

 

The term loll describes the state of a vessel which is unstable when in an upright position and therefore floats at an angle to one side or the other. If disturbed by some external force, caused by wind or waves, the vessel may lurch to the same angle of loll on the opposite side. Loll is quite different from list, being caused by different circumstances and requiring different counter measures to correct it and it is therefore most important that the mariners should be able to distinguish between the two.

 

Fig 4.10 shows how an unstable vessel takes up an angle of loll. Note that M is not on the centre line when the vessel is in the lolled position.

 

To correct for loll the following procedure should be observed.

 

First verify that it is loll and not list. Lists are caused by shifting of cargo or uneven distribution of fuel, water or cargo. If none of your cargo has shifted and your fuel and water tanks are more or less even on both sides, then you should suspect that your vessel has loll. You must lower the centre of gravity. There are two options open to you

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4.10

 

 

.

 

(i)         You can take ballast. If you do so, (and if your vessel has ballast tanks that you can fill) then you should begin by pressing up tanks on the low side first. This will initially make your angle of loll worse because you are adding weight on the side to which the vessel is leaning and you are introducing a free surface (if you are ballasting on an empty tank). This is still safer than ballasting the high side first, because that could cause the vessel to flop-over to the other side, and possibly capsize. By introducing ballast you lower the centre of gravity. If you are pressing up half-filled tanks, you are still lowering the CG and removing the free surface. The only negative effect of adding ballast is that it will increase your draft, reduce your freeboard and reserve buoyancy wit the result that your vessel will ship water at a much smaller angle of heel.

 

(ii)        The second option open to you is to remove the cause. A loll does not suddenly occur. It is a result of decreasing stability which is caused by the progressive raising of the centre of gravity of the vessel. This can only occur if you are loading weights on deck, and using fuel or water from low down in the hull (where most tanks are located anyway). You would have felt the vessel becoming progressively more tender and the roll period, and angle of roll steadily increasing. You may have been catching a load of fish - your brine tanks full and a large load of fish on deck. Too much weight high up. In these circumstances you may have to jettison cargo. This may be a painful decision, but the cargo is no use to you when your vessel is upside down!