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This chapter deals with the most common method which has been used to produce a large proportion of the FRP vessels existing today. Single skin construction using the PLUG - MOULD -HULL sequence is the simplest and cheapest method to build a number of boats from the same mould.

8.1 Plugs and Moulds

The principles described for the construction of small plugs and moulds equally apply to large. In the case of a new vessel design construction of the plug calls for the traditional skills of a boat builder. Wood is the principle material for construction but foam or balsa can be used as well as other less common materials such as C-Flex. Though traditional skills are required, if carpenters have not built a plug before new demands will be made in terms of accuracy and quality of workmanship. This aspect is particularly relevant in developing countries where a conservative approach to new demands leads to slow progress and requires constant encouragement by an experienced technician.

Three aspects need particular attention for the construction of a good plug:

  1. Building the plug upside down. Even though the plug is virtually a wooden hull, for reasons of access and mould release it is better built in this position.

  2. Finishing the surface to a high standard. A mirror finish is often quoted and this is true in terms of sheen, smoothness to the touch and fairness of hull lines.

  3. Achieving the finish in FRP once the basic woodwork has been completed.

It needs to be made clear once again that the plug is NOT a boat but a means to building a mould ONLY. It will be used once and thrown away.

Figure 26

Figure 26 Mould construction

Figure 27

Figure 27 Polished surface of new mould

8.1.1 Hull plug and mould building

To begin, the frames of the plug are erected in an inverted position on a solid foundation. This is usually a heavy wooden frame laid on a level concrete floor. When all framing is erected, the skeleton can be strip planked or completed in any material judged suitable to achieve the shape of the vessel. At the deck edge which is in this case near the floor while the keel is some distance above it, a plywood flange is attached so that later during the moulding process, gelled FRP can be trimmed back to leave a solid laminate with a clean cut edge. It is usual to cover the plug with a layer of FRP as soon as the crude shape of the vessel is achieved to reduce distortion due to any shrinkage of the wood. This layer can then be filled where necessary with resin putty to remove the shallow indentations which will show up once the FRP skin is consolidated.

Likewise, any irregularity which spoils fairness can be removed by grinding or by hand sanding. This process of hand finishing is repeated until the foreman considers the plug smooth enough to receive a layer of hard tooling gelcoat. This especially formulated gelcoat is the beginning of the coating which will eventually give the polished surface from which the mould will be lifted. There is no shortcut at this stage, the more time that is spent hand sanding the plug with wet and dry abrasive paper, the better the plug will be. Further filling with resin putty and overspraying may be necessary before a satisfactory finish is achieved.

For a 10 m hull, five persons for a week is about the minimum time that is needed for the final preparation of the plug. This will include polishing with rubbing paste after the sequence of wet and dry paper has run its course. Polishing with a non-silicone wax to give 5–7 coats and a brilliant sheen precedes a coat of PVA release agent which is the final step before the mould itself is begun. A dust cover is useful in the final stages.

When the time comes to make the mould, the process is a reverse of plug building without the need for final finishing as this obtained by the faithful reproduction of the plug surface. Mould the thickness can be up to twice finished hull thickness. There is little point in using WR in a mould as the thickness provided by solid CSM layup gives the required dimensional stability. Once the mould has cured, reinforcing stiffening can be bonded to the outside so that external bracing is present to support the mould when released and to provide a framework for the attachment of wheels, etc., to allow it to be moved about the yard. Removal from the plug may be tortuous and cause minor damage to the mould. Any repair work should be carried out before the mould is prepared for service.

8.1.2 Deck and interior

Preparation of plugs and moulds for deck and interior follows similar methods. As the deck contains few compound curves but many flat panels and tight radii, some differences should be mentioned. Use should be made of sheets of hardboard, or plywood covered with formica to provide large flat areas of decks and bulkheads with a final surface which requires little hand finishing. Tight corners are generally made from resin putty and hand sanded to shape. Moulded - in textured surfaces are possible (non-skid) as are flanges and mounts for equipment. The most important factor to remember is to obtain good release angles on upright faces such as bulkheads as it will not otherwise be possible to release the deck from the mould.

8.2 Plug and Mould Details

A one piece mould lifted directly from the plug may be possible in the case of a small vessel but some complications arise if the design has tumblehome, a reverse transom, a deep narrow keel or is simply very large.

8.2.1 Water/air inlets

In the case of large vessels it may not be possible to overcome the vacuum between the mould and the moulding by the use of wedges alone. In areas which may be suspected of poor release, pipe inlets can be cast into the mould during plug/mould construction so that compressed air or mains water pressure can be introduced to encourage the separation of mould and hull. This is accomplished as follows: before the gelcoat which will form the surface of the mould is sprayed on to the plug, inlets are fixed to the plug with modelling clay. The gelcoat and laminate are laid up around it leaving the connector part of the inlet protruding through the finished laminate.

The release of the mould from plug is similar to the launch of a new boat. It is the day all the work is seen to come to fruition. It is likely to provide one or two problems during separation as well as needing some heavy lifting gear to roll the mould over once it is clear of the plug.

8.2.2 Split moulds

If the mould has acute release angles created by the hull form, then there is little alternative but to use a “split mould”. This is a mould which is in two or more pieces which allows easier access to areas foreseen as difficult to mould and to release mouldings where form would not permit release from a one piece mould.

A flange is needed to bolt together the two or more pieces of the mould. To create it, a pattern of the flange has to be made on the plug on each side of the joint line in a similar manner to the sheer flange previously described. This may cause some damage to the plug which must be repaired before the corresponding flange on the opposite side can be made. It is possible to use the first mould flange as the pattern for the second half; this will provide a very accurate joint and will allow bolt holes to be through drilled in situ before release.

8.2.3 Care and use of moulds

Preparing the mould for service has already been discussed and this forms the basis for care and use. Any purchaser of an FRP vessel will expect a first class vessel so the mould should be kept in a first class condition - AS NEW. Between mouldings it should be checked for damage and receive a coat of wax. Every five - ten cycles it should be thoroughly checked and any repairs carried out to wear on flange bolt holes, or damage caused by hammering during release. Wax can build up in some areas to form a thick layer which gives a dull gelcoat, this should be washed or polished off. The priority here is to maintain the inside surfaces as smooth and polished as possible.

During periods of long inactivity mould need to be stored undercover. Exposure to the sun and wind is not recommended as the shiny gelcoat surface will soon become dulled. Coating it with a protective layer is advisable. This could be a gelcoat and supporting surface tissue or a latex rubber paint which can be easily peeled off. Smaller moulds can be turned over or covered with a dust sheet.

Figure 28

Figure 28 Deck mould

Figure 29

Figure 29 Split mould

8.2.4 Variations to a standard product

It has been emphasized up to now that once the design has been chosen then the yard is “locked” into a standard vessel. This is true but variations can be made within reason. The hull itself can be made longer or shorter or have its topsides raised and lowered provided it is done so with the Naval Architect's agreement and that he has checked the stability of what is virtually a new design. The mould can be changed in length if there is a region of fairly constant section such as in a hard chine power boat where aft of midships little cross sectional change occurs. Topside changes require the cutting down of the moulding to give a lower sheerline or the attachment of an extension to the mould sheer to build them up. Changes to the form of round bilge vessels are not recommended and in general the standard hull should be used unaltered unless consequences are very thoroughly calculated by the designer.

The deck offers more possibilities. A one-off or custom made deck would have to be built in the traditional manner or in plywood and sheathed in FRP. It is not unusual for yards producing workboats to offer a standard hull with a range of engine and deck options to suit different tasks. This is an acceptable mode of operation but the yard now becomes a producer of individual vessels and complications arise trying to satisfy each customer's needs. At long distance from equipment suppliers this might prove difficult as well as missing the point of offering a standard cheaper vessel.

8.2.5 Component assembly

One moulding which has not been discussed is the interior. In a workboat this may not be very large, possibly only comprising the accommodations areas. Depending on the vessel there may not be one at all as it could be fitted out entirely in wood. Another use for interior moulds is the wheelhouse. To use interior mouldings and to produce plugs and moulds for them requires the yard to have a sound market for the vessel over which to spread this extra development cost. However, the advantage of an interior moulding is that the whole unit can be fitted out on the shopfloor BEFORE it is placed into the hull.

It is not unusual for a 20 m vessel to have FRP cabins, toilets, galleys and stores fitted out, piped, wired and lowered into the hull while the hull is still in the mould. The advantage to a carpenter to work unrestricted by the confines of a hull is translated to a saving in production time and therefore a lower cost. Shorter overall production times are also achieved as the hull can be moulded at the same time as the deck and interior are undergoing fitting out operations which then require only assembly of the three large components to produce an almost finished vessel.

This is an over-simplification but the principle of using large FRP components fitted out by carpenters specializing in one task can enable the yard to achieve greater productivity.

Figure 30

Figure 30 Production line/component assembly

8.3 Construction Details

8.3.1 Keels and areas requiring extra stiffening

A keel is formed by laminating additional material along the centreline of the hull and is to extend forward to the stemhead and aft to the transom boundary, so forming a structural backbone. This backbone can be maintained at the midwidth throughout the length and be reduced in weight towards the ends, or can be constant in weight and reduced in width, or can be a combination of both.

In boats with deep keels, restricted access makes moulding difficult and split moulds are used. About the joint line a scarphed laminate should be made with succeeding layers 25 mm shorter than the preceeding layer. The laminate is completed when the mould halves are bolted together. Topsides are increased in thickness to form a strong sheer in powered vessels and further increased in local areas to take mast rigging or deck hauling equipment loads.

The boundary of the transom is to be increased to support and stiffen the sides, bottom and transom laminates. A typical boundary layup is formed by overlapping the side and bottom with the transom reinforcements, but as with the backbone the increase can also be achieved by the addition of strips of material laid around the boundary.

8.3.2 Framing and stiffening sections

The stiffener former is placed in position on the gelled laminate and the reinforcement is built up layer by layer as a continuous process. Frames are usually a solid or hollow core former covered with several layers of reinforcement forming a closed box or semi-circular section when combined with the hull laminate. These are known as “hat” and “half round”. The material of the former is chosen for light weight, workability, intertness, ability to withstand laminating pressure and economy. Solid cores are normally of foamed plastics of the desired profile and do not contribute to the strength. They are bedded on resin putty. Paper rope former is to be used only on small boats.

Figure 31

Figure 31 Typical lay-up along chine and transom boundaries

Hollow cores are plastic, cardboard or a single layer of FRP from a hat shape mould made by the yard itself which can be cut to allow a fit to hull curves. Frames can be tapered off in shape and weight at the upper and lower ends.

The strength and stiffness of the section can be varied by adjusting the section depth and retaining a constant lay-up or by increasing the lay-up where the stiffener depth cannot be increased. The lay-up can be increased by additional layers of the same material or by incorporating higher strength materials such as uni-directional or woven rovings on the face. The bonding of the section should be as neat as possible with good overlaps of basic and high tensile face material. The last layer of hull reinforcement can be timed to cover the framing to give a more finished appearance.

Figure 32

Figure 32 Methods of laying-up keel

Figure 33

Figure 33 Typical framing and stiffening section

8.3.3 Bulkheads

Bulkheads play a greater part in a plastic hull than they do in wood or metal boats. Apart from separating compartments, they are essential in providing transverse rigidity necessary for maintaining form in the comparatively flexible FRP hull. A collision bulkhead should be installed forward and machinery space bulkeads should be watertight. Other bulkheads may replace strongbeams and be required to support masts or deck loads. Exterior grade plywood is usually used for bulkheads and should be fitted while the hull is still in the mould. The plywood should have a roughened border to improve adhesion of the resin when the bulkhead is bonded into the hull. In small boats the thickness of the plywood is normally adequate for stiffness but some pillaring may need to be added in larger boats. All openings in bulkheads should be radiused to avoid local stress fractures. Separate sandwich construction bulkheads may be used but these will be more expensive. Bulkheads can be built as part of an interior moulding to gain a moulded gelcoat finish or resin painted (flowcoat) if the visible face shows the reinforcement such as when the core is made from plywood sheets.

8.3.4 Decks and superstructures

It is standard practice to fit FRP decks and superstructures to production line craft. Custom made decks based on a standard moulding have been discussed. The moulding may be single skin or more usually sandwich construction. Ply is the favoured core for working boats where weight is less critical while allowing fastenings to be applied to the deck with little fear of crushing the core. Because of its relative inflexibility ply should be sawn to blocks of 150 × 200 mm and installed individually to achieve a void-free bond.

Figure 34

Figure 34 Typical bulkhead to hull connections

The deck laminate should be increased locally to take loads such as deck machinery or at hatch corners, etc., which may be subject to impact damage. A pad on the underside of the deck is recommended to spread the load of a through fastening.

As with other applications of sandwich mouldings, 60% of the laminate should be on the outside and 40% on the inside. Advantage can be taken of the gelcoat to mould in a non-skid surface, also to design-in mountings for deck equipment which may be cast in at the plug stage.

8.3.5 Deck to hull connection

The choice of type of connection is based on the following requirements:

  1. The joint should be as strong as the weaker of the two mouldings being joined.

  2. It must be easily moulded and simple to bond.

  3. It should allow sufficient tolerance that the joint need not be forced into place.

Normal loads produce shear and tension at the joint. Exterior bonding would be more suitable to resist these loads but interior bonding is used for reasons of appearance. Weight of the bonding reinforcement varies with designs but should be 25% more than the lesser of the two mouldings being joined. It may require extra layers in areas of possible impact or extra local loading.

If the deck is to be of wood construction it is normal practice to fit a beam shelf around the inside of the hull to carry the beam ends. This can be done by fitting an outwhale and the beam to the outwhale by through bolting in the conventional manner. Top of frames need to be of wood so that beams may be through fastened. A moulded in knuckle at beam height running the length of the hull can accommodate a beam shelf, and simplify the process if wooden decks are standard.

8.3.6 Fuel and water tanks

FRP tanks are popular in larger craft as they do not rust, corrode or contaminate the contained liquid. They are of relatively simple construction, space saving and may be integral or separate. Tanks should be arranged so that they are not positioned at the widest part of the boat, above shaft brackets and sterngear nor at the deck to hull connection. Fittings through fastened near the tanks should be designed to fail before the tank and not to fracture the tank.

Construction should be as simple as possible with adequate access to the tank's internals to guarantee good quality joints when the final panel is bonded in place. For a separate tank normal procedure is to mould the bottom and sides as one piece then to bond on a separate top. Integral tanks can be fabricated from panels as necessary. Before either type is closed off any internal work should be completed such as fitting baffles, drains or gauges. Access is via a hole cut in the top. This will serve as access during bonding and later as an inspection hole with a strong cover and well sealed.

If possible, special gelcoats should be purchased with increased resistance to fuels for the diesel tank and an odour and taste - free gelcoat for the water tank. If these two types of tanks are in a continuous run then a cofferdam should be installed by doubling the separation bulkhead and leaving a space between.

8.3.7 Connections and fastenings


Interlaminar bonds can be defined as the achivements of strong, solid connections between successive FRP layers and can be classified as follows:

  1. Primary bonds are those between successive layers of reinforcement laid and cured at the same time.

  2. Secondary bonds are those made between a cured laminate and successive lay-up in place.

  3. Bonded joints are those between two previously cured laminates by applying new mat and resin which when cured forms a solid connection.

The laminate of the hull, deck, tanks and other assemblies will always be either primary or secondary bonds depending on the size of the moulded unit and the phasing. The bonding-in of the interior mouldings should be secondary bonds and completed during the “gel” stage of the laminate which will be a maximum of 24 hours in a tropical climate. Any bonding area should be as large as possible and designed so that the resin bond is shear. Lapped joints are preferred. In butt and scarph joints the bond is in tension and the joint should be reinforced on one or both sides. Whatever type of joint is chosen, the bonding area should be rendered free from release agent, grease, dust and dirt and then roughened to expose glass fibres. A wipe with acetone will remove fine dust and make the resin tacky once more. Gelcoat should be totally removed.

Figure 35

Figure 35 Typical deck to hull connections

Figure 36

Figure 36 Tank manhole details

Metal Fasteners

Laminates can be satisfactorily fastened with bolts or screws. They should be corrosion resistant or resin coated if made of plain steel. Rivets are sometimes found as the hull to deck connector in very small boats. Bolts should be fitted with large washers under the head and nut but should not be used in a laminate of less than 450 g weight. As a general guide, bolt diameter should be the same as laminate thickness. Self tapping screws may be used if the load is not heavy and machine screws can also be used in a pre-threaded hole. They should have their long axis perpendicular to the reinforcement layers and should never be screwed into the end of a laminate. Not to be used when laminate weight is less than 450g.

Figure 37

Figure 37 Typical designs of engine bearers

Attachment of Metal Fittings

A metal fitting may be bolted on in the conventional manner or may be bonded on. Through bolting of the hull should be kept to a minimum and avoided where possible. For propellor brackets and other loaded fittings however, it is advisable. The holes should be just sufficient to accept the bolts which should be dipped in catalyzed resin to prevent leaks just before insertion. Copper alloy bolts will inhibit the resin cure. A mastic sealant should be used if the bolts are required to be removed later. Drilled and tapped metal plates can be moulded into the laminate to take heavy loads such as engine holding down bolts. The plates should be bevel edged and have a large area for load spreading. All rust should be removed from steel and keying can be improved by cutting perforations or scoring the surface. Watertightness should be ensured to prevent the formation of any corrosion which could break the resin to metal bond.

Deck fittings such as bollards and cleats should be bedded on a flexible sealing compound. The laminate should be increased by 25% and a wood back up-pad placed on the inside to spread the load. If the deck core is foam or balsa then compression tubes or a solid ply core should be inserted locally to prevent crushing.

8.3.8 Bonding to materials other than FRP

One of the sources of problems in an FRP vessel is delamination and the ingress of water due to material incompatibility, that is the bonding of FRP to materials other than FRP. Polyster resins are not good adhesives and an approach which achieves a good mechanical bond should be employed. That is where the resin has a physical grip or lock on the adjacent material.

Bonding to Wood

Debonding of FRP angles securing structural members such as bulkheads and frames is a common fault. Wood should be roughened, dry, dust free, unpainted and given as big a contact area as possible. Resin thinned with 10% styrene or 5% acetone will allow a keying coat to better penetrate a wood surface than a more thixotropic straight resin. Any wood which has been treated with preservative should not be used, but if unavoidable then a joint using screws or bolts should be substituted. Oily hardwoods such as teak and iroko should be degreased with acetone and the moisture content of all woods checked before use. Delamination through resin contamination or movement caused by wood shrinkage will render the joint useless. Epoxy resin has better adhesive qualities.

Bonding to Metal

Some metals accelerate gelling time, others slow it down. Copper or its alloys are to be avoided unless of small pieces, completely encapsulated and above the waterline. Brass or bronze fittings should receive a coat of epoxy resin before being overlaid with any structural polyester resin. Aluminium and steel are best etch primed before coating. All metals should be degreased before use, even finger marks or sweat from a hand can render the bond inffective. Roughened surfaces provide better keying. It is generally advisable to avoid metal to wet resin joints as a faultless bond can only be achieved under conditions of laboratory cleanliness which are not present in a boatyard. Sooner or later corrosion will appear on the metal fitting and cause delamination and leaks.

Other Materials

Glass, polythene, formica, aluminium and the like will not bond and can be used to mould flat panels of FRP. Cardboard, hardboard, cement, and canvas all have rough porous surfaces and can be expected to give some degree of bonding.

Foams used for cores have been discussed and each treated in its own way. Scored and scrim-backed cores are superior as they provide a grossly keyed surface which is the basis of a mechanical bond.

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