THE MOULD LOFT What is a mould loft ? A large and spacious floor, whose surface is planed smooth and painted black, upon which the vessels lines are faired and laid off full size.

What is meant by lines ? The ships lines are those curves obtained when the hull is cut through in sections. Cuts horizontal and parallel to the water line give the water lines; vertical and parallel to the centre line give the bow and buttock lines; vertical and square to the centre line give the body lines (usually called the body plan). The curves given by the intersection of planes at an angle to the vertical centre line plane are called diagonals. Fig. 19 illustrates simply the various lines. What is fair curve ? It is a curved line which when viewed in a foreshortened length is a continuous curve with no local flat spots, hollows or humps.

How is a fair curve obtained ? By using a wooden batten and pinning round the spots through which the fair curve is required to pass and then sighting by eye along its length. Adjustments are made as necessary to produce the fair curve. This is then chalked or scrived in on the floor. During the fairing process of the hull lines, the curves are in the first {Fig. 19 } The lines defined instance chalked onto the floor because, before the fairing process is complete, many adjustments will have been made. Fig. 20 illustrates the use of wood battens. {Fig. 20} Fairing a frame using wood batten and steel floor pins

What is fairing the hull lines ? This is a tedious process. The sheer drawing consists of a profile, half-breadth plan and a section or body plan. These are chalked in on the mould loft floor. The profile and the body plan are generally laid-off on the same base line so that the several water lines from keel to upper deck level intersect the bow and stern lines and cross the body plan as well. Fairing is the process by which curved lines are made to run smoothly and regularly when they are laid-off. This work must be done carefully, because enlarging the drawing so many times magnifies any errors.

How are the vessels lines obtained ? The hull shape will be determined at the design stage and a sheer drawing prepared. The sheer drawing enables the lines to be faired approximately and the displacement and other stability calculations made. When the designer is satisfied with the data obtained the sheer drawing is

then passed to the mould loft for the hull to be laid-off full size (or at 1/10th scale if such facilities have been introduced), faired, and the frame positions added. Up to now the hull has been delineated by eleven equally spaced divisions, called ordinates, between the forward and after perpendiculars. This is sufficient for calculations to be made. When the hull has been finally faired on the mould loft floor the frame stations can be added and the body plan drawn out.

Does a mould loft have to be long enough to take the largest vessel likely to be built ? No. It is usual to lay-off at half, or quarter length. This has the advantage of sharpening the lazy curves and makes it easier to fair. Modern techniques are replacing the work of the mould loft. One-tenth scale lofting using special boards and magnifying glasses can give a high degree of accuracy and requires much less space. Some shipyards use computer centres for the full-size fairing of the hull. This eliminates loftwork and requires only a scrive board.

When the lines are faired how are they used ? The mould lofts faired-in ship lines are scrived in on the floor, so becoming permanent master lines while the vessel is building and perhaps for a ling while after. The lines are taken-off and a table of offsets prepared. The loftsman measures the fame lines from the centre line along the water lines, diagonals and bow and buttock lines. these dimensions are set out in tabulated form so that the hull shape can be obtained for whatever purpose it is required. The fore and after end shape is also included in the table of offsets. For a vessel building in another yard, the hull lines can be laid-off that yard from the table of offsets.

What other work is done by the mould loft ? Full size moulds are made and supplied for the manufacture of the structural components that require shaping to the hull lines. Other moulds are supplied for the rolling and dishing of the hull plating and for the supports necessary on the building berth for the hull during building. Full or half size mock-ups are made to enable more difficult problems to be resolved in the third dimension. Difficult plate developments are also dine on the floor.

Are moulds made for every item of the hull ? No. Much structural work can be moulded from drawings, where no changing curvature occurs. Where the changing form of the hull affects structural components, moulds may be required or information supplied on battens. Where framing has to be formed, then the body plan is laid-off complete, i.e., both port and starboard sides are laid-off and scrived in, generally close to the frame bending machines or smithery. Such a floor is called a scrive board. When the frames were bent hot, the smiths would make a set bar, to the shape of the frame and with this bar dogged down on the bending slab, bend the red hot frame to it, thus settings the frame to shape. {Fig. 21} Wooden mock-up of anchor recess and hawse pipe When the frame was cold it would be checked on the scrive board lines. Similar techniques are

required wherever the frames are bent.

What are mock-ups ? For difficult parts of the hull such as the bow in way of the anchors, and the stern, it is necessary to make a wooden three-dimensional part of the hull. This is called a mock-up. The anchor hawse pipe and its position for a snug fit of the anchor is determined. Another form of mock-up is that of a whole or part of component, to enable the best arrangement of equipment to be worked out. Such a costly item is only used when the compartment is packed with equipment. Mock-ups are usually constructed in the mould loft. Fig.21 shows an anchor recess mock-up.

In what form are the moulds ? Where full-size moulds are used they are made of timber of thickness depending upon the size of the mould and the life expected, i.e., will they be required for more than one job ? They are constructed in the manner shown in Fig. 66. Not all moulding information is supplied in this manner. Battens may be used where large areas of structure can be moulded that are not affected by the hull lines.

What are the battens like ? They are made of good timber and of length and size dependent upon their use, and the information to be put on them. They may be varnished to preserve them if they are to be used for a long time. The information is clearly marked by cutting into the batten with a sharp scriber or marked in pencil before varnishing. Fig. 67 illustrates the keel batten, one of the permanent battens employed.

Are all battens made the same way ? Most battens supplied by the mould loft will be of plain timber, marked in pencil.

Are moulds provided for all the hull plating ? No. Most of the hull plating will be moulded at the site. Some moulds will be made for those plates that require some laying-off, and where they cannot be moulded on site. The sheer strake and deck stringer are examples of plates that require accurate moulds. These two plating areas delineate the designed form of the hull.

What is a half block model ? This is an accurate model of one half of the hull, upon which the scheme of hull plating is worked out. The model is painted matt white and smoothed so that the draughtsman can, after the plating scheme has been finalinsed, draw the plates, etc., in ink. The whole is then varnished. From this model the hull plating is ordered.

What are lifting battens ? The term lifting is derived from the operation of taking or lifting information off the mould loft floor. Lifting battens give heights of parts of the structure from fixed datums such as the USK, or other hull form distances from fixed points. Such information is essential for checking hull form.

What information other than moulds is obtained from the ships lines ? Frames are not normal to the hull but are kept in a plane at right angles to the centre line. Where the frames are riveted, the flange that is riveted to the hull has to be bent to an angle which varies along the frame from the keel to the upper deck. This angle is termed a bevel and the smith needs to know that bevel in order to bend the flange. The mould loft supply a bevel board which gives the angle at the various water lines along the frame length. The drawing office would use the loft lines for many features in order to prepare the drawings. Girthing around the hull lines to determine positions for fittings, etc., is easily done on the floor and allows dimensions to be given accurately. As the drawing work progress, close co-operation must exist between the draughtsmen and the loftsmen. Structural problems arising from the fitting out will necessitate additional loft work, where structure needs to be altered in some way. The draughtsman can use the lines on the floor to enable direct measurements to be made so that he can decide what to do. The loftsman will lay-off portions of the hull lines separately if the problem demands it. Thus the draughtsman can by trial and error try out various schemes without using the master lines.

What form are the moulds where plating has severe or sharp shape ? Such moulds are called cradle moulds, because they cradle the plate when it finally fits. The moulds are built up using frames which are tied together to prevent movement. One surface, usually the upper surface, is the required shape. Such moulds have limited life, especially if the platework is smithed. The hot material burns the timber each time it is tried on the mould.

What is tenth scale lofting ? This is the scaling down of the work on the mould loft to that of 1/10 scale. The technique requires new methods, and staff require training. The lines are faired at 1/10 scale on special materials in an air conditioned room.

What are the advantages of tenth scale lofting ? Less space is required, and materials are used more economically.

Why is economy realised ?

For full advantages to be gleaned from this process, all the ship parts are drawn out at 1/10 scale. Depending upon the type of machines for cutting the steels, either 1/100 scale negatives are made of the 1/10 scale drawings, or the machine cuts direct from the drawings. In drawing out the parts, they can be nested much more neatly on the material. This process dispenses with the very expensive manufacture of wood moulds.

What type of cutting machine is used ? An automatic flame cutting machine. This machine will cut port and starboard components simultaneously from the drawings or negatives. More elaborate machines will cut from magnetic tape.

THE HULL What is the hull ? It is a structure designed to float in a seaway and remain watertight. It supports machinery and provides space for cargo and accommodation. It is strengthened in way of appendages such as rudder, propeller supports and masts, as well as being shaped to provide the least resistance to the water to ensure economic fuel consumption. Main structural features of a typical hull Typical details of bottom construction with continuous longitudinals and transverse frames fitted interscostlly (top). An alternative method uses continuous longitudinals and transverses (bottom). Here the transverse members, or solid floors, are slotted to allow the longitudinals to pass through.

What are the main components of the hull ? The main components are the framing or skeleton to which the platting or skin is attached. The backbone of the skeleton is the keel to which the frames or ribs are connected. Deck beams are fitted between the side frames across or athwart the hull and are fastened to it by brackets. The frames are shaped to the hull lines and the deck beams are given a slight curve or beam round. How are the various parts of the hull joined together ? Nowadays most hulls are welded. Riveted hulls are found in older ships.

What are the advantages of welded construction ? Welded hulls are easier to construct, with no rivets to leak, and are completely watertight.

Welding enables the designer to make greater use of complex sections, and has dramatically changed methods of the construction and detail design. With welded hulls egg-box connections help to maintain the ships form because the structural members are continuous when interlocked . Alternatively continuous longitudinals can pass through deep floors. Welded construction gives improved strength, due to joint symmetry, before crippling under stress Are there any other advantages ? Major economies are found in the construction and operation of the vessel. Welding gives a smooth hull which reduces resistance in the water, thus requiring less power for the same speed. Welding saves on weight of the structure by eliminating lapped plates and butt straps. The welded hull contributed to changes in shipbuilding practice, and has reduced time on the building berth by allowing prefabrication sections of the hull in the fabrication shops or other areas away from the berth.

Is the hull plating all the same thickness ? No. The thickness varies, being thicker around the keel, turn of bilge, upper deck sheer strake and in way of large openings. In a riveted ship, the keel was laid and the hull erected on the berth piece by piece, working from either side of the keel out to each and. Building methods Deck beams tied together the frames and the outer edges of the decks which held the form. Structure which assisted in holding the hull to form was worked into the hull as quickly as possible. A welded hull is erected piece by piece in the same manner, but the hull is broken down into smaller sections or units. Each unit is built in a sequence with more than one building at any one time. This enables welding to be done under cover in a fabrication shop. The units are then transported to the building site where they are fitted and welded together. This cuts down time on the berth and with careful planning maximum utilisation of the berths is achieved, allowing more ships to be built on the one berth. Fig. 4 shows the difference between the systems.

How important is the keel ? Ina riveted hull it formed the backbone from which all other members were generated and connected. The keel consisted of : the keel plate, vertical keel and gutter strake (inner keel plate). The keel plate was often of double thickness. The keel supports the weight of the vessel when in dry dock In welded hulls the keel is still an important members for in both types of construction it is the central member, taking a large proportion of the vessels weight when docked in a dry dock .

What is the bilge keel ? This is a fin fitted on the outside of the hull at the turn of bilge over approximately the middle third of the middle third of the length. This helps to reduce the rolling of the ship in a seaway. It is also used when in Single plate bilge keel dry dock to hold the bilge shore heads. Bilge keels may be of single plate construction, free flooding double plate, or watertight. Fig. 6 illustrates a single plate bilge keel.

What is a bar keel ? A bar keel is a thick vertical section which is fitted as shown in Fig.7 (a) with the outer bottom plating (garboard strakes) abutting to it, i.e., let into the hull, or to the underside of the outer bottom plating under the inner vertical keel, as in . Bar keels

What is a longitudinal ? It is any continuous fore and aft structure, contributing to hull strength, such as deep girders contained within the double bottom (where fitted) and of similar construction to the vertical keel, oil or watertight where they form a compartment boundary. Other longitudinal may be fabricated units and not so deep as a double bottom longitudinal, or simply an RSJ. In welded construction where deep frames are used in conjunction with a longitudinal system of stiffening the longitudinals are more closely spaced, of lesser size \(scantling) and are called stringers.

Does any other structure contribute to longitudinal strength ? Any major fore and aft structure contributes in some way. Long deck structures may be designed to contribute, internal fore and aft bulkheads will add to hull strength as do main machinery seatings, which may align with the longitudinal or the longitudinal may be deflected slightly to align with the seatings.

What happens where a longitudinal meets a main watertight bulkhead ? Deep longitudinal fit close to the bulkhead and are bracketed to it. Bulkhead stiffeners are arranged to align with longitudinals so that the connection between them transmits loads into the main structure. Where RJS longitudinals are ended either side of the bulkhead, the ends are not left square but are shaped by removing a semi-circular portion. This reduces the stiffness of the longitudinal and introduces a degree of flexibility to prevent the longitudinal end from piercing the outer bottom plating if subjected to severe loadings such as grounding. Substantial bracketing is required in order to fix the longitudinal end and provide continuity of strength at this weak spot. Where a longitudinal pierces the bulkhead and therefore is continuous, the watertightness is

maintained by fitting collar plates around the longitudinal and welding to the bulkhead.

How are longitudinals ended ? A brupt endings (discontinuities) must be avoided as these form notches and in regions of high stress could cause cracking. Where longitudinals are ended or run into smaller longitudinals they must be tapered over a reasonable length, such as four or five frames spaces. This allows the stresses to flow gently into the other structure. (see chapter 4)

What is a double bottom ? A double bottom is exactly that. In large ships provide some safeguard against sinking if the outer bottom is damaged, an inner bottom is fitted. The cellular construction that arises from such a construction is termed the double bottom. Its depth arises from the strength considerations in determining the longitudinal and keel dimensions. The compartments so formed are used for carrying fuel etc.

What is the extent of the double bottom ? Usually it is fitted over the main machinery spaces. In large ships the double bottom may extend across and up the sides for some way but in merchant ships it is usually ended just before the turn of bilge by a margin plate fitted to meet the hull at right angles.

What is an intercostal ? It is a main structural component that is not continuous, but fitted between other continuous main structure. Such structure must be accurately fitted and substantially connected. As a rule, inside machinery spaces, longitudinals are continuous and transverse frames intercostal. Outside machinery spaces transverse framing is continuous and longitudinals intercostal.

What is floor plate ? It is a solid part of transverse framing in double bottoms, also the connecting bracket for side frames to the margin plate or longitudinal structure.

Why are the holes cut in the structure ? Simply to save weight so that the vessel can carry as much fuel and cargo as possible. To effect this, lightening holes are cut. Some of these holes are large and the edges of the holes have a stiffener welded around them. Lightning holes must be positioned to have the least effect on strength. The edge of the hole should be smooth to avoid possible cracking in service.

What is a carling ? A carling is an intercostal stiffener fitted between main structure to provide local additional strength.

Where transverse frames cannot be fitted, how is the hull framed ? Special frames called cant frames are fitted normal (at right angles) to the hull. Fig. 8 shows a typical arrangements and is generally found in vessels with a cruiser stern. Flat or transom sterns eliminate this type of frame. What is a cruiser stern ? This is a stern which gives a curved profile in plan, elevation and section. The term was derived from warships and was typical of cruiser stern design illustrates a typical cruiser stern. A typical cruiser stern What arrangements are made where main structure is pierced for hatches, hawse pipes, inlets, discharges etc.? The loss of plating area is compensated for. In riveted vessels this done by fitting doubling plates and additional stiffening. In welded hulls the plating is increased in thickness. (Doubling plates are not good welding practice, being an inefficient way of increasing strength and seriously affecting the fatigue life of welded structures).

What are the structural arrangements in way of large openings such as cargo holds ? Deep fore and after (longitudinal) girders are arranged to form the two sides of the hatch opening. These are supported by pillars at each corner. The deck opening is cut with smooth radius corners and is made thicker at the corners. The edge of the hatch opening is fitted with a deep coaming which is bracketed to the deep girder. Intermediate beams are similarly bracketed to the deep girder. Why are pillars fitted ? To provide support to the deep fore and after girders that support the beams and in particular under heavy machinery or in way of large openings.

Do pillars need to be hollow tubes ? Not necessarily. They may be simply RSJs, RSHs or fabricated hollow square tubes. (RSJ = Rolled Steel Joist ; RSH = Rolled Hollow Section.) How are the pillars secured ? At the heads and heels by brackets to the hull structure. The securing must be capable of taking not only compressive but also tensile forces which may occur when the vessel is subjected to

severe pitching. What is a bulkhead ? Any athwartship partition fitted to form a compartment. Such a partition may be watertight or nonwatertight. For watertight subdivision of the hull such bulkheads are of substantial construction and contribute to hull strength. Is there a specific number of transverse bulkheads ? For passenger ships, yes. The number depends on the length, type of service and the number of passengers to be carried. In cargo ships the general requirements are not statutory though there is a requirement to fit a collision bulkhead some distance abaft the stem. Where else should special thought be given to fitting watertight structure ? A vulnerable part of the ship is the stern, so watertight bulkheads should be fitted to limit flooding should there be breakage in the stern tube. The propeller shaft is also enclosed in a watertight tunnel. It is usual to fit watertight bulkheads at either end of the machinery spaces and intermediate bulkheads between cargo holds. Thus even in a cargo ship the hull is subdivided giving some protection should flooding occur. What is a stern tube ? This also called the shipbuilders tube. It is a tube through which propeller shaft passes from inside the hull to the outside. To effect watertight, engineers glands are fitted at either end of the tube. During construction accurate fitting of this item is required to ensure accurate alignment (see Chapter 9).

What are A brackets ? They are supports for the extreme end of the propeller shaft, fitted integrally with the hull structure and so called because their shape is like a letter A. Not all vessels are so fitted. A brackets are usually fitted in naval vessels. What is the alternative to a brackets ? In most merchant ships the propeller shaft is totally enclosed right up to the propeller. The framing for the structure consists of a part of the hull frames. These are called spectacle plates because of their resemblance to spectacle frames.

How is the rudder attached to the hull ? When at sea the rudder is subjected to bending forces which have to be resisted. These are taken at the rudder stock and at the lower end of the rudder at the pintle. The stock is that part of the rudder which is fitted the rudder cross head. The upper bearing and gland are housed in a casting or fabrication which form an integral part of the hull structure, will special stiffening and bracketing arrangements. The hull structure is therefore designed to take the rudder weight and the sea forces to which it is subjected.

Why are some ships fitted with a bulbous bow ? Many large ships are fitted with a bulbous bow to improve the seagoing characteristics, giving smoother water flow and less resistance to forward motion. Generally ships have a soft nose stem which may extent from keel to upper deck or may consist of a solid bar up to the water line and he soft nose above that. Fig.10 shows different types of bows, ranging from that fitted in small craft to large tankers. Is the bow strengthened in any way ? Yes, to prevent panting which results from the variation of the water pressure when the vessel is in a seaway. Frames are increased in size, solid floors are fitted to every frame, and additional stringers added.

Does any other part of the structure requires special stiffening to withstand varying water pressure ? If the vessel is subjected to violent pitching the fore end over the keel region slams into the sea. To prevent any structural damage the framing is made stronger and the hull plating thickness increased. There may also be a need to fit additional stiffening over the midships area of a flat bottom for the same reason. What is the sheer strake ? This is the uppermost strake of side plating which meets the upper deck.

Why is the sheer strake of importance ? Because when the vessel is subjected to bending the forces alternative from tension to compression (see Chapter 4) and the sheer strake is subjected to maximum compressive and tensile stresses. Hence it plays an important part in contributing to the strength of the hull. The upper edge which is contoured to the sheer line must be smooth and contain no notches.

How important is the gunwale ? It is important because it connects the sheer strake to the outer plate of the deck : the deck stringer plate. This connection is at the point of change of stress between the deck and ships side. It is at this point that bending and racking stresses coincide. The gunwale connection must be so designed as to accommodate such stresses throughout the life of the ship. In riveted ships such a connection presented little or no problem because rivet holes were smooth, contained no notches and would stop a crack if one occurred. Welded gunwales require greater care to eliminate crack starters caused by built-in notches and poor welding details. Yes, they are a cheap and efficient way of connecting two structural members that must remain static relative to each other without movement, i.e., without rotation. Bracketless connections have been designed, but they require greater care in fit-up and the design is such that they are more expensive than the normal flanged bracket.

luanchingWhat is launching ? It is the transferring of the completed hull from dry land to the water. This may be done by floating the ship out of a dock in which she has been built, or launching from the slipway or building berth from which the vessel slides down specially built launching ways. What is sideways launching ? The launching of the ship sideways on to the water. It is necessary where the water is not wide enough to allow lengthways launch.

How is the slope of inclination of the slipway determined ? The slope is related to the weight of the vessel and the sliding friction of the launch cradle and the grease. When the vessel is on an inclined plane, the weight acts vertically and a component of that weight acts down the slipway (Fig. 72). {Fig. 72} Components of weight on slipway That component must be sufficient to overcome the initial friction. Over movement has started then the effect of the friction is reduced and the vessel accelerates as she slides down the ways. Thus the angle of inclination must be sufficient to allow sliding to occur.

How is the angle calculated ? By knowing the starting acceleration of the ship which is determined by experiments with a model which stimulates the launch conditions. From the data obtained and using the laws of dynamic friction, the coefficient of friction is determined. When the acceleration is zero, at the start of the launch, the coefficient is equal to the tangent of the angle of the inclined plane, so if the coefficient of friction is known then the angle is easily determined.

What other calculations are involved ? The mains ones are : 1. Launch weight, position of longitudinal centre of gravity (LCG) and height of centre of gravity (CG) above the keel

2. 3. 4. 5. 6. 7.

Stability of the ship when afloat Position of the ship down the ways when the stern lifts Pressure on the fore poppet when the stern lifts Variation of pressure on the grease during the launch Tipping moment about the after end of the ways Distance between the forefoot of the ship and the slip floor at the worst position.

What is the launch cradle ? The launch cradle comprises the sliding way which is a continuous length of timber, made up of short lengths scraped or connected together by link plates and through bolts, positioned either side of the centre line upon which the supports, called poppets and stopping up are fitted. The whole is connected together to prevent any relative movement. Fig. 73 shows the usual arrangements.

What are the poppets and stopping up ? They are the supports fitted between the hull and sliding way. The poppets are vertical timbers, the stopping up is the horizontally fitted support. These are necessary where the distance between the hull and the sliding ways is small, over the amidships of the hull.

What prevents the poppets from movement in the fore and aft direction ? They are braced with steel stripes of plate, called dagger plates, secured with coach screws. How is the weight transferred from the building blocks to the launch cradle ? When constructing the cradle, allowance is made between the poppets and stopping up for wedges to be fitted. A day or so before launch (in the case of large vessels this may be weeks) these wedges are driven home in unison, forcing the poppets and stopping up under the hull to lift it. This operation is called setting up. The middle line blocks can then be removed.

Is setting up necessary ? For small vessels it may be sufficient to harden up the wedges and then split out the

capping block. An alternative to this is the use of a specially designed steel capping block which is filled with sand. For release, a plug is removed and the sand allowed to run out.

What governs the height of the launching ways above the slip floor ? When the stern lifts, the hull tilts or pivots about the fore poppets. Fig. 74 shows the effect of this, which is to cause the forefoot to move closer to the slip floor. The launching ways must be of sufficient height for the forefoot in the worst condition to clear the slip floor. {Fig. 74} Clearance of forefoot. The minimum clearance occurs when the vessel is just fully waterborne, hence the launch way height h is critical and must be more than C

How are the ways positioned and aligned ? Methods will vary form yard to yard. Chapter 9 refers to the level boards set up for leveling the building blocks and later for checking the USK. In a similar manner level boards are erected over the launch ways (once they have been put in an approximate position). These boards can be fitted to small shores which are wedged up under the hull. The ways are then lined up, levelled, and make up timber fitted underneath and secured. How is the launch cradle kept on the ways ? How is the launch cradle kept on the ways ? A stop is fitted along the length of the ground ways. This called the ribband. It may be fitted on top of the ways or along the side. Fig. 75 shows both methods. The ribband is shored for added strength in case of sideways pressure during the launch. {Fig. 75} Ribbands two methods. Shoring of the ribbands is essential especially from the position of the forepoppet at stern lift position to the completely waterborne position down the ways What greases are used for launching ? In Britain usually Tallene or tallow compounds, or a proprietory product such as Basekote petroleum grease (Esso) for the ground ways and a slip coat mixture of Tallene and sperm oil, or the proprietory equivalent, Slidekote (Esso).

How are they used ? The ground or standing ways are coated by pouring molten grease onto the surface and allowing it to harden. The depth will depend upon the weight of the vessel or past

experience, usually from 3.2 to 12.7 mm for Tallene grease and from 3.2 to 6.4 mm for the Basekote grease. A coating is also applied to the underside of the sliding ways. The slipcoat is applied liberally to both the ground and sliding ways. This is the lubricant which effects the launch. How is the vessel released for launch ? The launch cradle is held in the final moments by a series of levers called the trigger. The levers reduce by simple moments the forces acting upon the holding wire or rope to manageable proportions. When the wire or rope is cut, the trigger is released and the vessel is free to move. An older method is to allow a weight to fall knocking the restraining shores away. This knock the shores away. The weight is called the dog weight and the shores the dog shores.

Are any special arrangements required inside the hull ? The internal structure is fortified to prevent damage on setting up, and when the weight comes on the fore poppet when the sterns lifts. Fig. 76 indicates the method employed. The internal shoring is fitted behind the poppets or stopping up, onto solid structure. Such structure should be checked to make sure it is of sufficient strength to take the loading. The wedging of these shores should be overdone or structural deformation may occur. {Fig. 76} Poppets and stopping up that are not backed by main ships structure must have reinforcement fitted to prevent structure distortion Are any other precautions required ? This depends upon the size and type of ship. Certainly the hull must be checked for watertightness. All hatches not required to be open should be closed. Emergency lighting should be available, a portable pump or pumps placed on board, the rudder gagged to prevent movement if it is fitted, and the propeller free to move independently of the propulsion machinery. Some method of arresting the vessel after launch will also be required. Where there is sufficient depth of water then release of an anchor may be sufficient. Where the conditions are such that the river width requires the arrest of the vessels movement in a specified distance, then drag chains would be used.

How is the launch cradle removed after launch ? Usually the cradle is made so that after launch it breaks up and floats clear of the ship. The parts are then dismantled as required. Where poppet housing brackets are attached to the hull these are removed when the vessel is dry docked. Another method of removing the launch cradle is to place the vessel in dock, lower the water until the cradle is just off the dock bottom or on specially prepared blocks, secure the cradle to the dock bottom, release it from the ship, float the ship up and remove it from the dock. The dock is then pumped dry and the cradle can be dismantled.