MONITORING SMALL VESSEL SEAWORTHINESS
(Ranger Hope version © 2008, contains edits of material courtesy
A vessel’s design is influenced by the following factors:
• Nature Of Service
• Area Of Operation
• Seaworthiness And Stability
• Survey Requirements
• Personnel Safety
• Construction Materials
• Commercial Requirements
A vessel is designed to perform a specific function such as carrying passengers, cargo or fish. It’s size, layout, accommodation, machinery and equipment are all related to the type of service it is meant to provide.
Generally, a vessel that is required to spend longer periods at sea without replenishment will have a correspondingly larger capacity to carry fuel and stores. Weather conditions also influence design. Frequent rough conditions may require the decks to be located sufficiently high above the water to keep them reasonably dry. The size of the vessel may have to be increased to cope with adverse conditions.
A vessel must be designed to ensure that it is capable of surviving the variety of weather and operating conditions likely to be encountered in its area of operations. This means that under normal operating conditions, the vessel should have enough stability to keep it upright and afloat.
Careful attention must be paid to the hull shape, the distribution of weights and the protection of hull openings.
A vessel’s structure should be able to withstand the stresses caused by:
• water pressure,
• weights on board such as cargo and machinery,
• action of wind and waves and
• operation of machinery
when carrying out its planned operations.
A small vessel must be designed, constructed and operated in accordance with the regulations contained in the Marine Acts of each State. Much of the detailed legislation is contained in the Uniform Shipping Laws Code (USL), published by the Australian Government Publishing Service. A vessel is surveyed by the marine authorities during the building stage, on completion of building, and then periodically throughout its life to ensure that it complies with the regulations.
Persons on board must have safe working areas, safe access to and from the working areas and safe accommodation. In the event of accidents and breakdowns at sea, necessary safety measures must be available as per statutory requirements.
Each type of building material has its own advantages and disadvantages as we shall see later in this section. The choice of building material will influence the carrying capacity, propulsion power requirements and construction method of the vessel.
Commercial requirements dictate that a vessel should be designed so as to keep the construction and operational costs at a level acceptable to its owner.
You should keep these requirements in mind when you study the remainder of this section.
In general, vessels subject to survey by a State Authority are covered by the provision of the Uniform Shipping Laws (USL) Code.
Most of these regulations use Measured Length as a reference.
Measured Length - It is the distance from the fore part of the hull to the after part of the hull, taken at the upperside of the uppermost weathertight deck, or, in the case of open vessels, at the height of the gunwale. Figures 1.1 and 1.2 show two examples.
Length Between Perpendiculars (LBP) (Fig. 1.3) - length from the forward perpendicular to the after perpendicular. The forward perpendicular is a vertical line drawn through the point where the load waterline cuts the stem. The after perpendicular is a vertical line drawn at the after end of the rudder post.
Length Overall (LOA) (Fig. 1.3) - the length from the extreme tip of the bow to the aftermost point of the stern.
Freeboard (Fig. 1.3) - the distance from deck to waterline.
Depth (Fig. 1.4) - the depth of a vessel is usually measured at the side and amidships, it is the distance from deck to keel.
Beam Or Breadth (Fig. 1.4) - is measured at the widest part of the vessel. It is the greatest width from one side of the vessel to the other.
A vessel in a normally loaded condition is said to be floating at its Load Waterline or Design Waterline (Fig. 1.3).
In that condition, the draft at which the vessel floats is called the Loaded Draft or sometimes the Service Draft. Draft is measured from the waterline to the deepest point of the vessel’s hull, usually the underside of the keel. (Fig. 1.4).
Some vessels have a keel parallel to the design waterline. Some have a keel at an angle when compared with the design waterline called declivity of keel.
The bow is the region at the front of a vessel. (Fig. 1.5).
When facing forward, the part of the vessel on the left hand is the port side, the part on the right hand is the starboard side. (Fig. 1.5).
The stern is the region at the back of a vessel. (Fig. 1.5).
A person moving towards the bow is said to be going forward. (Fig. 1.6).
A person moving towards the stem is said to going aft. (Fig. 1.6).
Amidships is the region in the middle of the length. Frequently this word defines the point at the middle of LBP.
When moving along the length of a vessel a person is moving in a longitudinal direction.
When moving across a vessel a person is said to be moving in a transverse direction.
In some vessels the deck line curves forward and aft, this is called sheer. It aids water run off and contributes to reserve buoyancy. (Fig. 1.6).
Flat Of Bottom - in some vessels, the area of the hull near to the keel is flat, this is called Flat of Bottom. (Fig. 1.7).
Rise Of Floor (Deadrise) - if the bottom of a vessel rises from the centre line to the turn of bilge, there is said to be a Rise of Floor. (Fig. 1.7).
Bilge - the rounded part of the hull, where the side meets the bottom is called the bilge. A rounded bilge provides strength and reduces hull stresses. (Fig. 1.7).
Camber - if the deck of a vessel has an upward curve, it is said to have a camber. This helps water to run off the deck and reduces deck stresses. (Fig. 1.7).
Tumblehome - if the sides of a vessel, ‘fall-in’ towards the centre line as they rise to the deck-edge, the vessel is said to have tumblehome. This is not very common these days. (Fig. 1.7).
Flare - the outward flowing of the bow (sometimes called Flare) forces water outwards and away, promoting deck dryness and assisting the bow to lift over waves. (See Fig. 1.8).
Bow Rake is similar to flare in that it promotes deck dryness by forcing water forward when the bow strikes waves. See Fig. 1.9.
The main body of a vessel is called the hull.
Regardless of the material used in construction, the layout of the hull is similar in each case. Every vessel has a shell of material which keeps the vessel watertight. The shell is supported and obtains its strength from a series of internal stiffeners. In small vessels it is common for the main stiffeners to be in the form of frames, sometimes described as ribs.
If the main stiffeners of the hull run from side to side, the arrangement is called a transversely framed structure. If the main stiffeners of the hull run fore and aft, the arrangement is called a longitudinally framed structure.
Let us now look at some typical stiffening arrangements of metal and timber hulls.
The bottom structure of a vessel consists of a keel, with the flooring structure and side shell plating on either side. The keel is located at the centre line of the bottom structure and forms the ‘backbone’ of the vessel. Smaller vessels may be fitted with a bar keel, however on the majority of larger vessels the keel is of a flat plate construction. A longitudinal centre girder running along the ship’s centre line is fitted at right angles to the flat plate keel. This helps in resisting bending of the hull in a longitudinal direction. On some vessels with double bottoms, a ‘duct keel’ may be provided on the centre line. The duct keel forms an internal watertight passage that can be used for carrying the vessel’s pipework.
The bottom shell plating is stiffened by means of transverse or longitudinal frames. Additional strength is provided by fitting vertical plates to the bottom shell. Those fitted transversely are called floors and those fitted longitudinally are called side girders.
Figure 1.10 shows a “hard chine” (or Vee shaped) 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.
On some smaller vessels, a single bottom construction is employed as shown in figure 1.11. Figure 1.11 also shows the bottom construction of a “round bilge” type hull.
Figure 1.11: Single Bottom Construction
Double bottom construction is found on larger vessels. The double bottom space can be used to carry fuel, ballast and fresh water. In addition it provides an extra margin of safety, since in the event of bottom shell damage only the double bottom space may be flooded. The bottom structure is similar to that found in single bottom construction, but with an additional inner skin of plating. Figures 1.12 and 1.13 show transversely and longitudinally framed double bottom constructions.
Figure 1.12: Transversely Framed Double Bottom
Figure 1.13: Longitudinally Framed Double Bottom
Transverse Webs (built up frames) may be provided on some vessels to support longitudinal frames or where additional rigidity is required.
If it is necessary to enter a space, make sure that it is absolutely safe to do so and you comply with occupational health and safety requirements.
The bottom structure will contain additional fittings depending upon the purpose for which the space is being used. For example, if the space is used as part of a tank you are likely to find sounding pipes and striker plates, suction valves, strum boxes and bottom plugs. Other fittings may include speed and depth measuring devices.
Study figure 1.14 which shows typical timber chine hulls, and figure 1.15 which shows typical timber round bilge type hulls.
Longitudinal strength is provided by major structural members running fore and aft such as the keel, hog piece, stringers and hull planking. The shape of the transverse section is maintained by means of floor timbers, gussets and handing knees that tie the whole structure together.
Note how various parts are arranged to resist the stresses that we had identified earlier when considering design requirements. In this respect, the arrangement of structures is similar to a metal hull.
Figure 1.14: Typical Timber Chine Hulls
Figure 1.15: Typical Timber Round Bilge Type Hulls
The stem is the forward most part of the vessel’s hull. The stem is made up of a stem bar from the keel to the load water line and a stiffened plate structure up to the forecastle deck. The plate stem rakes well forward, providing an increased deck area and a readily collapsible region in the event of a collision. The side shell plating is flared out to increase the deck area further and to deflect sea water and spray away from the ship. Some vessels are fitted with a bulbous bow which is a protrusion below the waterline designed to increase the ship’s speed in ballast condition and to reduce pitching.
Figure 1.16: Fore end constructi
The type of stiffening given to the fore end of a steel constructed vessel can be seen in Fig. 1.17 which shows the bow section of a tug in the process of being converted to a tanker.
Figure 1.17: Fore End Construction
Let us turn our attention to the fore end structure of a timber vessel.
Dead Wood is the term generally applied to timber used in the build up of a forward or after end of the keel structure, providing solid material for the fastening of stern or sternpost and horn timbers.
Stem is the extreme forward member of the main framework of a vessel to which stringers and plank ends are attached.
Typical arrangements at the fore end of a timber vessel is shown in fig 1.18.
Figure 1.18: Typical Stem Assembly in Timber Vessel
Panting is the in-and-out movement of the shell plating that results from variations in water pressure as the vessel pitches in a seaway. Special structural arrangements are provided in the bow region to strengthen the shell plating against this action. These arrangements include:
• horizontal plates welded to the sides of the vessel (known as panting stringers)
• transverse beams extending from side to side (known as panting beams)
• partial bulkheads
On some vessels, panting beams are replaced by perforated flats. Perforated flats are flat plates, similar to decks, with round holes cut in them.
Figure 1.19: Panting arrangement
Pounding results from the heaving or pitching of the vessel, which causes the forward region to ‘slam’ down on the water. To resist pounding, the shell plating is increased in thickness, frame spacing is reduced, and additional side girders and solid plate floors are fitted in the forward region.
In a typical arrangement, the anchor chain leads from the anchor through the Hawse Pipe onto the forecastle deck where it passes through a Bow Stopper to the Windlass. The purpose of the bow stopper is to take the strain off the windlass when the vessel is riding to its anchor. From the windlass, the chain passes through a pipe known as the Spurling Pipe into the Chain Storage Space, commonly called a chain locker. The chain locker is provided with means to pump out any water
that collects there. The inboard end of the chain (known as the Bitter End) is secured to the chain locker, usually in such a way that the chain can be released from outside the chain locker.
The aft end of a vessel is designed to provide a smooth water flow into and away from the propeller. The overhanging structure at the aft end experiences large slamming forces, and the structure is therefore suitably stiffened. The shell plating at the after end terminates at the stern frame. In single screw vessels, the stern frame has a boss on the centre line for the tail shaft to pass through. The stern frames of multiscrew vessels are reduced in size as they do not have to support the tail-shaft and propeller. A single-screw aft end arrangement is shown in figure 1.6. On some vessels the support at the bottom of the rudder is eliminated. In such cases, the semi-balanced rudder is supported by a large horn, which in turn is attached to the vessel.
Figure 1.20: Aft end arrangement
Let us turn our attention to the aft end structure of a timber vessel.
Dead Wood is the term generally applied to timber used in the build up of a forward or after end of the keel structure, providing solid material for the fastening of stern or sternpost and horn timbers.
Stern Post is the vertical post ending the deadwood at the stern.
Typical arrangements at the aft end of a timber vessel is shown in fig 1.21.
Figure 1.21: Typical Deadwood Aft in a Timber Vessel
The weather deck, bottom, and side shell plating form a watertight envelope that provides the buoyancy to keep the vessel afloat. The shell plating is composed of many strakes (plates arranged in a fore and aft direction and welded together). The shell plating is subject to static water pressure, as well as the dynamic effects of pitching, rolling and wave action. Stiffening against the compressive forces of the sea is provided by transverse or longitudinal frames. Additional plate thickness is provided at the forward and aft ends of the vessel to withstand local stresses.
The shell plating provides the greatest longitudinal strength of the vessel’s structure. At this stage it is useful to know that a vessel is subject to stresses that are primarily caused by:
• Differences of weight and buoyancy which occur at various points along its length
• Vessel’s motion in a seaway and the action of wind and waves.
Stresses are greatest in magnitude along the vessel’s length and result in bending of the hull. This results in “hogging” and “sagging” conditions as shown in figures 1.22 and 1.23.
Figure 1.22: Hogging Condition
Figure 1.23: Sagging Condition
The greatest bending stresses are experienced at the strake nearest to the weather deck. The area where this strake meets the deck plating is known as the “gunwale”.
Decks are stiffened in a manner determined by the framing system of the side shell. With transverse framing, transverse deck beams stiffen the deck plating, and are in turn supported by girders. For longitudinal framing, the frames supporting the deck run fore-and-aft and are in turn supported by transverse members.
The superstructure is the part of the vessel built above the freeboard deck and is the full width of the ship. The superstructure plays an important part in the protection of openings in the freeboard deck such as machinery space openings. In addition, the superstructure houses accommodation, stores and navigating spaces. Frames, plating, girders and brackets are used for the construction of superstructure in a similar manner to the hull.
The bridge front is vulnerable to the impact of green seas in heavy weather. Furthermore, the ends of the superstructure where it meets the deck are areas of high stress. Abrupt discontinuities are avoided by ensuring that shell plating forming part of the superstructure is well radiused at the ends towards the side shell. Where the superstructure is constructed from aluminium alloy, special insulating arrangements must be employed to avoid galvanic corrosion where the aluminium alloy joins the steel structure.
You may have noticed that our description of major structural components has not included bulkheads in any detail so far. This does not mean that bulkheads are not an important structural item. In fact, bulkheads, in particular watertight bulkheads are so important to control flooding that we will consider them separately under the heading of ‘Watertight Subdivision’.
Additional strengthening is provided on deck where concentrated loads are situated or likely to be placed. Localised loading caused by incorrect storage of high density cargoes can cause temporary or permanent distortion of the ship’s structure.
Point loading occurs when the mass of an object is distributed over an extremely small area. Point loading is a feature of container and Roll On-Roll Off cargoes. In the case of containers, the weight of the containers is transmitted through the relatively small area provided by its four corners. Similarly, the weight of a vehicle used in the handling of Roll On-Roll Off cargoes is transmitted to the deck through its wheels. Purpose built vessels have increased localised strengthening e.g. below container stacking points and beneath vehicle decks. Information regarding the loading limits over various parts of a vessel can be obtained from that vessel’s plans such as the “capacity plan”.
Vessels are built from a number of materials, each of which has advantages and disadvantages.
The major types of materials used in hull construction are:
3. Glass Reinforced Plastic (GRP)
2. Does not rust - almost maintenance free
3. Easy to work
2. Comparatively easily damaged
3. Low melting point - poor fire resistance
4. Corrodes rapidly in contact with other metals such as steel, copper, bronze etc.
2. One piece hull, no seams
3. Not easily damaged
4. Fire resistant
5. Easily repaired
1. Chips easily
2. Skilled construction required
1. Corrosion free
3. Comparatively easily repaired
4. Cheap - especially if part of a mass produced design
1. Comparatively easily damaged
2. Poor fire resistance
3. Skilled construction required
2. Comparatively easy to build
3. Not easily damaged
4. Good fire resistance
1. Corrodes readily
3. Not easy to work
4. Very magnetic. Affects the compass.
2. Easy to work
3. Corrosion resistant
4. Not easily damaged
A vessel’s hull is subdivided into a number of large watertight compartments by means of watertight bulkheads. The number, location, and heights of these bulkheads is governed by rules as laid out in Section 5C and 5D of the USL Code.
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.
Perhaps you are wondering why there is all this concern about watertight bulkheads? The purpose of the regulations is to try and prevent massive flooding and hence rapid sinking of the vessel should it be holed (bilged) below the waterline. Ideally the vessel should remain afloat if any one compartment were holed.
It is not necessary that you should have a detailed knowledge of these rules, but you should be aware of some of the requirements so that you can determine if your vessel is in fact complying with the law.
For example in cargo vessels of less than 65 metres in length, four watertight bulkheads are required if the machinery space is amidships, but only three are required if the machinery space is aft. The way this is determined is as follows:
• There must be a collision bulkhead at the forward end of the vessel.
• There must be an after peak bulkhead which encloses the stem tube to contain any leakage where the propeller shafts pierce the hull.
• There must be watertight bulkheads at both ends of the machinery space.
This clearly implies that for a vessel with machinery amidships the minimum number of watertight bulkheads is four, whilst for a vessel with machinery aft the minimum number of watertight bulkheads is three.
Look at the profile plan of KFV Albatross in fig 2.1. 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.
On vessels of less then 25 metres measured length, a stepped collision bulkhead may be fitted if the rake of stem exceeds 15°.
In case a vessel gets holed (bilged) below the waterline, watertight bulkheads will restrict the flooding to the bilged compartment and prevent the water from going into other parts of the vessel.
As per statutory requirements:
• The collision bulkhead must extend to the uppermost continuous deck.
• The aft peak bulkhead may terminate at the first deck above the load waterline, provided that there is a watertight deck all the way aft of the bulkhead.
• All other bulkheads should extend to the uppermost continuous deck - usually the main deck. Provision may be made to allow the bulkheads to extend to a lower deck provided that it is above the water line.
Wherever possible, piping and ventilation trunks should not pass through 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. Where this cannot be avoided, the following measures must be adopted.
• Pipes must be flanged to the bulkhead, not pass through it.
• Ventilation trunks must be fitted with a watertight shutter.
• If the collision bulkhead is to be pierced by a pipe, a valve must be fitted at the bulkhead. This valve should normally be kept closed and may be operated by remote control and/or an extended spindle.
As discussed, all pipes passing through a watertight bulkhead must be flanged to the bulkhead and do not pass directly through it (see Fig. 2.2). 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.
Figure 2.2: Pipes Passing Through Watertight Bulkhead
In some cases it is unavoidable that a larger hole is required in a watertight bulkhead; for example, between the engine room and the propeller shaft tunnel. In this case, the opening must be fitted with a watertight door which complies with the following basic rules.
• The opening must be framed, and thicker plating used around the opening.
• The opening must be as small as possible.
• Below the load waterline, doors must be of the sliding type
• Sliding doors must be capable of operating with a 15° list.
• Doors must be able to be operated both remotely and locally.
• Hydraulically-operated doors must be fitted with local alarms at each door to warn of closing/opening by remote control.
• Doors are tested by hose testing, except for passenger vessels where the doors are required to be tested under a head of water, equal to the height of the bulkhead in which the door is to be fitted. Hose testing simply means that a jet of water is directed at the door in the closed position, at a specified pressure, and the door sealing arrangement is checked for leaks.
• Above the load waterline, hinged doors are permitted, provided that the hinges are made of suitable non-corrosive material.
Figure 2.3: Internal Watertight Door
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 2.3).
The collision bulkhead, as the forepeak bulkhead, and the aft peak bulkhead are tested for watertightness by filling the peaks with water to the level of the load waterline. Where the bulkheads form the boundaries of deep tanks, they are tested by filling them with water up to the top of the airpipe. Double bottom tanks are tested to a head sufficient to give maximum pressure that can be experienced in service. Tanks are normally tested every five years.
Other watertight bulkheads of compartments not forming the boundaries of liquid-carrying spaces are simply tested by hose testing.
A ship is nothing more than a water tight container or storage compartment with its own means of propulsion. Its purpose is to load and carry cargo, whether the cargo is passengers, fish, or a host of other commodities. Each type of ship is specialised for the trade in which it will operate. One of the most important factors of design is to ensure that the water in which your vessel floats, does not enter the hull and cause progressive flooding. We call this characteristic of a vessel its watertight integrity.
(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.
Weathertight means that the structure or fitting will prevent the passage of water through the structure or fitting in any ordinary sea conditions.
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 it 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 its 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.
The steel plating in a metal vessel, the planking in a wooden one, or the GRP 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’.
GRP and ferro cement hulls are continuous with no joints and are inherently watertight, as is their deck/hull connection.
Vessels constructed of timber are not normally totally watertight but rely on seepage of water to swell the planking and thus make them watertight.
There are numerous openings in the hull. These include overboard suctions and discharges for ballast and cooling water systems, stern tube, rudder trunk, thruster units, speed log probes, side scuttles (port holes), cargo access doors on the bow, stem or sides, etc. Openings on the main deck include hatchways, airpipes, watertight doors, ventilators, spurling pipe etc.
You are already aware of the subdivision requirements of vessels and the requirements to maintain watertight integrity when there are openings in watertight bulkheads. Next, we will look at the closing arrangements and statutory requirements of some of the openings.
Weathertight doors are fitted on deck. They provide access to stores, holds and accommodation. They are usually hinged, and are secured by twist dogs which tighten against wedges. A rubber seal (packing) in a channel around the rim of the door provides the watertightness. All hinged weathertight doors must be marked on each side in bold permanent lettering with the words THIS DOOR TO BE KEPT CLOSED AND SECURED.
Figure 2.4 shows the hatchways on the fore deck of a vessel that provide access to compartments below the main deck.
Figure 2.4: Access Hatchways On Fore Deck
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.
Figure 2.5: Hatch Coaming
Figure 2.5 shows a cut away section of a hatchway coaming. When a hatchway is cut into the deck of a vessel, the corners are rounded to reduce stresses. The square coaming is welded on top of the opening and a reinforced lid is made to fit. (See Fig. 2.6).
Figure 2.6: Raised Coaming
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.
Figure 2.7: Weathertight door on Main deck
Figure 2.8: External Weathertight Door
Traditionally, cargo hatches were closed by hatch boards or steel pontoon covers and made weathertight by using tarpaulins. Steel hatch covers on modern vessels generally consist of single-piece-lift-off pontoons, hinged pontoon sections which fold to the open position, rolling covers, and covers which stow on a drum. It is beyond the scope of this guide to examine each type, however in most cases the covers are made weathertight by a gasket resting against a hatch coaming steel bar (compression bar). Figure 2.9 shows the location of compression bar on the hatch coaming. Figure 2.10 shows how the connection between the hatch pontoon and hatch coaming is made watertight.
Figure 2.9: Cargo Hatch Coaming
Figure 2.10: Hatch Cover and Coaming Joint
The joints between adjacent hatch panels are made weathertight in a similar manner. The compression bar is attached to one panel and bears on the gasket set into the adjoining panel.
The hatch panels are held in place by cleats. It is important to realise that the purpose of these cleats is not to achieve watertightness by physical compression of the gaskets, but to restrict the movement of the hatch panels due to rolling and pitching of the vessel.
Roll On-Roll Off vessels may be fitted with bow, stern and side doors and ramps. These doors are gasketted and cleated. Alarms must be fitted to these doors to warn the person on bridge if these doors are not secured.
Must have hinged metal covers (deadlights) that can be closed watertight.
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.
Most of the openings described above would employ gaskets to provide a watertight seal between the rim of the opening and its covering. It is a relatively simple task to check the efficiency of these closures by means of a ‘hose test’ or a ‘chalk test’.
In Fig. 2.11 the loading hatch in the side of the hull is bolted and secured while at sea. An alarm system is fitted which will sound on the bridge if the door is opened.
Opening In The
Figure 2.12: Watertight Door Open Alarm Switch
Figure 2.12 shows a closer view of the trip switch which will sound the alarm if the side door were opened while at sea.
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).
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.
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, 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.
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
Timber may be attacked by any of the following, depending upon conditions:
• 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.
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.
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.
Figure 3.1: Spay 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 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 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 3.2: 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.
Figure 3.3: Vessels on Synchrolift
Figure 3.3 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.
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.
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.
Figure 3.4: Travel Lift
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.
• 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.
• 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
In Section 1, we briefly looked at the survey requirements of commercial vessels. These requirements 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”.
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.
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.
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:
• 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).
• 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.
• 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.
• 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.
• 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.
• 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.
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.