MONITORING SMALL VESSEL SEAWORTHINESS
SHIP KNOWLEDGE FOR COXSWAINS
(Extracts courtesy of A.N.T.A. publications, Ranger Hope © 2008 www.splashmaritime.com.au)
A history of experiment by trial and error has resulted in vessels and their component parts being constructed to meet the forces and stress encountered at sea. Apart from rafts that float above the water, every vessel has a skin of material that keeps it watertight.The main body of a vessel being its shell (skin) and hull (supporting framework). This must be strengthened for the hogging, sagging, pounding, panting and racking stresses anticipated in the sea area of operations. As well as the hull, certain other areas experience extreme loads that require additional strengthening, typically achieved by the strategy of doubling (extra duty construction).
Timber vessels longitudinal structures
Regardless of the material used in construction and the host of differing local names of component parts, the fuction of hull structures are similar. Even in monocoque construction (one piece mouldings), the shell is supported by internal stiffeners. In all but the smallest of craft it is common for the main stiffener to run longitudinally as a backbone to the vessel, called a keel and keelson or hog. This is scarf jointed to a stem timber at the bow and to a sandwich of timbers at the stern called the deadwood. Together these form a landing for the planking shell which is rebated to the keel along the rabet line. This keel structure provides the necessary resistance to hogging and sagging. Figure 1 shows the keel of a typical timber vessel terminating in the stem timbers forward (bow) and stern post and transom (aft).
Fig. 1 Typical timber longitudinal section
This forward (bow) structure must be resistant against pounding and the number of frames and floors may be doubled for this purpose. Each side of the stem post may even be solid with closely packed frames called the knight's head. The natural hinge of keel to stem post is backed by a naturallly curved grown chock cut from a tree bough.
Fig. 2 Typical stem assembly (bow)
Likewise this aft area has closely packed floors around the stress point of the stern post and propeller shaft.
Fig. 3 Typical deadwood aft
Timber vessels transverse structures
The simplest of hull forms is the flat bottom or sharpie that is essentially a raft with side planks, While this type can take the ground, planes and is cheap to build its seakeeping abilities are limited. At the other end of the scale the round bottom type allows excellent sea keeping and strength to drive through a head sea. The chine and multichine type provides a compromise between the two.
Fig. 4 Timber hull forms
While the stem, keel and deadwood provide longitudinal strength it is the planking (timber) or shell plating (steel) that keeps the water out of the hull.The main stiffeners of the hull plank or plate called frames usually run from side to side, this arrangement being called a transversely framed structure. The frames are tied together with longitudinal stringers, the one at the uppermost deck edge being called a sheer clamp. The transverse frames are sometimes called sawn frames. However, if they are formed from a continuous soild timber they are called ribs (or timbers if steam bent into place).
Fig. 5 Typical hard chine hull
Figure 5 above shows a hard chine (or Vee ) hull. It is called hard because the topside meets the bottom at an angle as opposed to a “soft chine” hull where the topside meets the bottom in a curve. Figure 6 shows the bottom construction of a round bilge type hull.
Fig. 6 Typical round bilge type hull
In timber construction the bow frames are often doubled up to incease the resistance to the stresses of pitching and panting.
Similarly, additional floors at the forward end of the keel/keelson provide stregthening against pounding along the bow's forefoot.
Steel and aluminium vessels
Small steel and aluminium vessels typically use a main logitudinal structure of a minimal flat keel plate backed by an internal longitudinal keelson or girder.The keel forms the backbone of a vessel’s hull.This helps in resisting longitudinal bending of the hull against hogging and sagging. Beams support the deck. Brackets connecting frames and beams help resist the distortion of racking to the hull when the vessel rolls. Stiffening by side keelsons (girders) may be utilised, being welded to floors along the bottom sections.
Fig. 7 Typical chine construction
In the case where longitudinal side keelsons are not continuous (they cross over the floors) they are termed intercostal keelsons.
Fig. 8 Typical round bilge type hull
In larger vessels the keel may be of box section with its underside effectively flush with the external shell plating. The box section allows a tunnel for services and even shaft log. When the main stiffeners of a steel hull predominently run fore and aft, the arrangement is called a longitudinally framed structure and when they run side to side the arrangement is called transverse framing.
Fig. 9 Framing of a large steel vessel
Much stiffening is incorporated in the front end sections against the stresses of panting, pounding and panting.
That the structure or fitting will prevent the passage of water through the structure or fitting in any ordinary sea conditions.
a) In relation to a fitting above deck, that it is so constructed as to resist effectively the passage of water under pressure, except for slight seepage.
b) In relation to the structure of the vessel, capable of preventing the passage of water in any direction if the head of pressure were up to the freeboard deck, which in your case would mean the main deck.
Hull Watertight Integrity
The steel plating in a metal vessel, the planking in a wooden one, or the FRP laminate, have as their primary purpose, the task of keeping the interior of the vessel free from water. In all types of vessel construction, a structural framework is built first to provide the strength. This, when combined with the external covering, forms the hull. In steel and aluminium ships, the hull is made watertight by welding the steel plates together and to the framework. Often the frame is built upside down and the shell plating is welded onto the inverted frame. The hull is then righted and the internal welds are completed. This procedure allows for a better weld and hence improved water tightness since all welds are 'downwelds'.
Vessels constructed of timber are not normally totally watertight but rely on seepage of water to swell the planking and thus make them watertight. FRP and ferro cement hulls are continuous with no joints and are inherently watertight, as is their deck/hull connection.
Fig. 10 Small vessel watertight spaces
All vessels, except the very smallest of craft, are subdivided internally into watertight compartments by means of vertical partitions called watertight bulkheads. In vessels which have a measured length of 16 metres or over, a special watertight bulkhead called a Collision Bulkhead is fitted near the bow. The number and placement of other watertight bulkheads is dependent upon the measured length of the vessel. Vessels 12.5 metres and over in measured length must have a watertight bulkhead at each end of the machinery space except where the machinery space is located at one end of the vessel.
Look at the profile plan of KFV Albatross in Fig11 below. The dotted lines rising vertically from the keel represent the watertight bulkheads. The first collision bulkhead is located at frame no. 4. It rises from the keel to the underside of the foredeck.
The second watertight bulkhead is located at frame no. 6. It rises from the keel to the underside of the main or freeboard deck. The space between the two bulkheads is called a cofferdam. A cofferdam is a void space that separates two tanks. Normally a cofferdam is only required to separate oil and water tanks.
The space enclosed between the second and third water tight bulkheads is the refrigerated hold. The engine room space is located between frames 25 and 35. You will notice that there is a watertight bulkhead at each end of the engine room, making a total of four watertight bulkheads in all. The bulkhead at the after end of the engine compartment is known as the after peak bulkhead.
Fig. 11 KFV Albatross
Openings in Watertight Bulkheads
Openings may be necessary in watertight bulkheads to allow the passage of pipes or electrical cables, and special arrangements are made to ensure that the watertight integrity of the bulkhead is maintained. All pipes passing through a watertight bulkhead must be flanged to the bulkhead and do not pass directly through it (see Fig. 12). The pipe on the left has a valve incorporated in it for filling the tank on the other side of the bulkhead. There is a spindle running up to the main deck from where this valve can be operated. The siting of the valve outside of the tank it is servicing reduces corrosion and maintenance.
Fig 12 Pipes passing through watertight bulkhead
Doors may also be necessary, in watertight bulkheads, to allow the vessel to continue its normal operation whilst at sea. These doors can be of either a sliding or hinged type and must be capable of operation from both sides of the bulkhead. (See Fig 13).
Fig 13 Internal Watertight Door
A trip switch is fitted to watertight openings which will sound the alarm if the door were opened while at sea. Doors providing access from the main deck to lower compartments must have sills, which serve the same purpose as hatchway coamings. The sill heights are the same as for hatch coamings. Access doors can be hinged and should be marked "THIS DOOR IS TO BE KEPT CLOSED AT SEA”.
Fig 14 Hull openings
Hull and Deck Openings
Access hatchways must have a raised coaming to reduce the amount of water that could enter the ship should a wave wash over the deck while the hatch was opened. The height of the coaming varies according to the ship’s length. A trip switch is fitted to watertight side door openings which will sound the alarm if the door were opened while at sea.
Fig 15 shows the hatchways on the fore deck of a vessel that provide access to compartments below the main deck.
Fig 16 Raised coaming and seals
When a hatchway is cut into the deck of a vessel, the corners are rounded to reduce stresses.
Fig 17 shows a cut away section of a hatchway coaming.
Ventilators and Ventilators must be a minimum height above the deck and must have some means of making them watertight. This may be metal flaps, or in smaller vessels, wooden plugs and canvas covers. Airpipes, where exposed, should be of substantial construction and if the diameter of the bore exceeds 30mm bore then the pipe should be provided with means of closing watertight. Other watertight opennings include:
Side Scuttles (portholes)
Airpipes- All portholes below the main deck should have hinged metal covers (deadlights) that can be closed watertight.
Scuppers, Inlets and Discharges
All sea inlets are to be fitted with valves of steel or material of equivalent strength attached direct to the hull or approved skin fittings (in case of non metal hulls).
Drainage arrangements from weather decks
Weather decks are to be provided with freeing ports, open rails or scuppers capable of rapidly clearing the deck of all water under all weather conditions.
of Watertight Integrity
Watertight integrity can be breached through any activity or happening that allows the ingress of water in unwanted areas or compartments of the vessel. Typical examples include:
Lack of maintenance to seals, (painting over seals), screw threads and other locking devices.
Damage caused by collision, grounding or heavy weather.
Leaving hatches, doors, vents etc open.
Blocked freeing ports or scuppers.
Cracks along welds in metal vessels or loss of caulking from planked seams in timber vessels.
It is obvious that for a vessel to float, water must be prevented from gaining entry into the hull. The vessel designer has to ensure that under normal use water will not enter the hull in sufficient quantities to sink it. The shipbuilder ensures that is of sound construction to meet these requirements. This is verified at the initial survey carried out by an Authority.
It is your responsibility to ensure that your vessel’s watertight and weathertight integrity is maintained throughout it’s period of service. This is ensured by periodic surveys carried out by the Authorities. In general terms, the survey requirements require the vessel to be watertight below the freeboard deck and weathertight above the freeboard deck. This means that the shell plating must be intact and the closures to all openings leading to the hull should be in efficient working order. No alterations should be done to any structure that would adversely affect the watertight integrity of the hull without the approval of the appropriate survey authority.
It is essential that you are thoroughly familiar with the locations and closing mechanisms of all openings on your vessel through which water may enter the hull. This way you will not neglect to maintain, test and check the efficiency of any of the closing arrangements.
• Check that all access openings at ends of enclosed structures are in good condition. All door clips, clamps, and hinges should be free and well greased. All gaskets and watertight seals should be crack free. Ensure that the doors open from both sides.
• Check all cargo hatches and access to holds for weather tightness.
• Seals should never be painted.
• Regularly inspect all machinery space openings on exposed decks.
• Check that any manholes and flush scuttles are capable of being made water-tight.
• Check that all ventilator openings are provided with efficient weathertight closing appliances and repair any defects.
• All air pipes of diameter exceeding 30mm bore, must be provided with permanently attached satisfactory means for closing the openings.
• Ensure that the non-return valves on overboard discharges are operating in a satisfactory manner.
• Check that all freeing ports are in a satisfactory condition, e.g. shutters are not jammed, hinges are free and that pins are of non-corroding material. Check that any securing appliances, if fitted, work correctly.
You can test the efficiency of closures by means of a simple “hose test” or by a “chalk test”. Practical activities 3 and 4 will lead you through the process of using these tests.
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 including scantling sizes as defined in the aproved plans. He/she makes a written declaration of such condition.
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 the National Standards for Commercial Vessels from AMSA or other regulatory authority. After the construction is complete, the Authority surveys the vessel once more and if satisfied, issues the operator with a "Certificate of Operation" and the vessel 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.
A typical survey schedule for vessels of less than 35 metres in length is given in National Standards for Commercial Vessels (AMSA):
Typical Annual Surveys
• 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).
• Out of water annual inspections are usually restricted to timber vessels, metal and GRP usually two years.
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
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.
• 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.
Deterioration Of Timber
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.
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.
• 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 larva 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 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.
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.
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.
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.
Fig. 18 Spray painting antifouling
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.
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.
Fig. 19 Floating dock
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.
Fig. 20 Synchrolift
Figure 20 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 an be docked at the same time and the slipping facility is not laid up for the duration of the vessel’s stay.
Fig. 21 Floating dock
General Procedures For Docking And Slipping
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.
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.
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 stem. As the water level falls, the keel will touch the blocks at the stem first. This results in an upthrust on the stem 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 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.
• 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