Foam-sandwich Fiberglass Boat Construction

In general: Foam-sandwich construction in the following assumes the use of the pure polyvinyl-chloride rigid-elastic type of 5 to 6 lbs. per cubic foot density. Chemically blown cross-linked PVC foams as well as polyurethane foams do not have the same charateristics and will not necessarily function or react the same as pure PVC types. In foam-sandwich construction, the PVC foam is applied in sheet form over a male mold or form to make the hull of the boat. The FORM is a by-product and does not remain in the finished hull. The foam acts as a core material over which fiberglass material and resin are applied to both sides, thereby forming a rigid and strong sandwich-type of construction. See Useful Information & Suppliers for sources of foam core materials.

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Fig. 1 The form members are erected on a building form, accurately aligned, and braced to prevent movement. The traverse forms usually conform to the stations, and must be vertical, at right angles to the centerline, and centered. All members must be level about the centerline, and centered. All members must be level about the set-up level or reference plane. The form member edges often require beveling so that the longitudinal battens or ribbands will mate flat, and not just rest on corners. Note the bevels on each side of the stem form member. Several widely spaced battens are applied initially in order to determine the approximate positions for the balance of the battens onto the stem form. The keel batten is tapered forward so the adjacent battens will form a fair curve onto the stem form.

Fig. 2 The ribbands are located directly adjacent to the keel and sheer members to support the edges of the foam. The keel and sheer members protrude above the level of the ribbands to match the thickness of the foam. The ends of the battens along the stem form are trimmed off and then built up with wood laminations or other material to form a stern cap. This material is then faired and radiused to contour to be flush with the level of the foam. The foam is omitted in these areas along the stem, keel and sheer, and made of solid fiberglass laminate joining both the inner and outer skins once the hull is completed. The plywood strap shown between the stem and forward transverse form is used to prevent ribbands from "twisting" unfairly.

Fig. 3 Areas of the hull which are to be solid fiberglass laminate (i.e., keel, sheer, and stem) must be covered with a parting film or release agent in order to prevent the resin from forming a bond to the form members. Here, polyethylene sheeting is used, stapled in place. Optionally, the entire form could be covered with the material. Note that the form is high enough to allow access to the underside or interior which is necessary on all boats. After the ribbands have been applied and checked for fairness, and the parting film or release agent applied, the foam application can be made.

Fig. 4 The foam application can begin in most any area, however, the builder should make a check to determine the best usage of the material to minimize waste. The large sheets can be cut into virtually any shape or configuration to conform to a given area or contour. Here a panel is leaned against the form and tapped with a mallet. This transfers a mark to the panel so it can be cut and fitted in place. Note that the foam piece is resting on the edge of the sheer member. The foam can be easily cut and trimmed with a knife, hand saw, or power sabre saw.

Fig. 5 When the foam pieces have been cut and trimmed to size, they are held in place against the form for fastening. Here clamps are being used along with several nails around the edges into the solid wood form members. These nails will usually tear away from the foam with the form when it is removed, however, they should be kept to a minimum and located only around edges. In some cases, the nails are fitted with plywood washer blocks and used to hold the piece only temporarily while screws are driven from the back side through the battens to hold the foam in place, or for sewing the foam in place in lieu of using screws.

Fig. 6 Adjacent foam panels are fitted by using a small block as a marking gauge to determine the amount of material to trim away. Because the thickness of foam sheets may vary somewhat, the builder should check the sheets and use those that are adjacent to one another that are as close to the same thickness as possible in order to eliminate high and low points along seams. It is best to cut the panel somewhat oversize initially, especially if the panel must be heated in order to conform to any contoured areas.

Fig. 7 While the foam panels should fit as closely as possible, small gaps can be easily taken care of by adding scrap pieces of foam. However, mating edges of adjacent panels should be beveled or angled so that the fit is firm. This can be done by trimming with a knife as well as sanding the edge with coarse sandpaper. Along the sheer and keel members, the fit is not quite as critical, because these areas will be beveled along the inside once the form is removed and hull righted. However, well fitted joints are desirable and wide gaps should be avoided.

Fig. 8 Where hull contours change rapidly, it may be necessary to use smaller panels laid somewhat on the diagonal, or it may be necessary to heat the panel initially. This small panel was bent to approximate contour and then heated before placement. Panels are fastened from inside the mold with screws or sewn on. In either case, two people, one working inside and other on the outside, are required to get a good application.

Fig. 9 If the final surface on the foam has any discontinuities between panels, it may be necessary to even out the surface with resin putty filler. Also, any gaps should be fitted with scrap slivers of foam. Minor dents or indentations of the foam can sometimes be corrected with a spot application of heat as the foam will often reform to its original surface. Otherwise, resin putty filler should be used to even out the surface, although such a procedure should be kept to a minimum. These areas should be be sanded smooth, taking care not to gouge into the adjoining foam areas. The hull surfaces should be as fair and smooth at this point as possible, for any defects will be directly transferred to the final hull surface where correction will be tedious. Wax must be applied to the form members in any areas where the parting film is inadvertently removed.

Fig. 10 The first layer of fiberglass material applied to the foam is usually some type of chopped strand mat. The mat pieces should be cut to rough oversize initially. Joints in adjacent mat pieces are normally butted, however, along areas such as the keel centerline, it is common for the mat on one side of the hull to overlap onto the mat of the other side for extra strength and fiberglass build-up. Where hull contours change rapidly, the mat may not conform to the surface without "bunching". Where this occurs, tear a V-shaped gore by hand thereby feathering the edges to keep from building up a double thickness of mat at these points.

Fig. 11 The pieces of mat are laid in position and folded back onto themselves to expose the underside surface. Resin is generously applied and then the mat rolled or folded back in place. In this manner, the mat has resin starting the saturation from the back side, thereby assuring thorough resin coverage.

Fig. 12 While these workers are using brushes for resin application, other tools such as paint rollers or larger brushes would be used on larger boats. The main point is to apply the resin as rapidly as possible, and the more workers available, the better this can be done. Mat rollers can be used to distribute the resin and even out the surface as much as possible before proceeding with the lay-up. Have plenty of resin dispensed and ready for catalyzing so the operation can be done continuously.

Fig. 13 Usually the Continue layer of fiberglass will be woven roving, or on smaller boats, a layer of cloth. Wetting out woven roving is more difficult than mat, and care should be taken to assure that the resin is sufficiently worked to completely saturate the material. If enough resin was applied to the mat underneath, this will aid in the saturation while more resin is applied to the surface.

Fig. 14 Squeegees are used to smooth out and saturate the woven roving or cloth. The resin is best worked from high areas to low areas pressing firmly onto the surface to eliminate air bubbles and entrapment, and to distribute the resin evenly. It is important to squeegee out all excessive resin to prevent a resin-rich laminate which will be brittle, lack strength, and have excessive weight. As with mat, joints between adjacent pieces of material should be butted and not lapped.

Fig. 15 Where the final hull surfaces are relatively fair and smooth, the surface can be resin coated to conceal any fiberglass material and then sanded to a smooth surface. Enough resin is applied so that the sanding will not cut into the fiberglass material, and then the surface can be painted. However, where the surface is not fair and smooth, a resin filler coat is best applied. On larger boats, it is customary to completely finish and paint the hull while upsidedown, while on smaller boats, this finish work can be done inside and outside as it is relatively easy to reinvert the hull.

Fig. 16 Cradles are made to support the hull once righted. These can be made from scrap lumber sawn to the contour of the hull section. Carpet strips are laid on the hull over wax paper or plastic sheeting at the cradle location and coated with resin together with layers of fiberglass mat. The cradles are then attached to the carpet strips with bonding angles of fiberglass mat. The form member can either be detached from the hull initially, or righted together with the hull, depending on the builder's desires and available equipment or manpower. Cradle members can either be placed separate from the hull to receive it, or placed on the hull and righted with it.

Fig. 17 Righting the form together with the hull will assure that the hull will not distort. However, if the form is detached before righting, care must be taken to assure that the hull will not flex or change shape. This is especially critical along the sheer area, and this area should be reinforced with temporary outer sheer members if righting is done after the form is detached from the hull. When the form is left in the hull, it is easy to level-up and align the righted hull for the balance of the construction.

Fig. 18 Before removing the form from the hull, be sure that all reference marks such as those noting bulkhead locations, sole levels, centerline, etc., have been marked to the inside of the hull surfaces as applicable. With the form in the hull right side up, it can then be lifted from the hull intact. Of course, on larger boats this will require some mechanical hoisting equipment, or the form members can be dismantled piece by piece. Spreaders spaced as required to hold the sheer in position are advised on larger designs.

Fig. 19 With the hull righted and the form removed, the interior can be prepared for the inner laminate. Edges of the foam where the hull will have solid fiberglass must be beveled and the corner radiused so the fiberglass materials will not lift and cause air bubbles. This can be done with a knife, rasp, and by sanding. The bare fiberglass surfaces (as along the keel and sheer in the photo) should be sanded and preferably wiped with solvent also in order to assure a positive bond.

Fig. 20 Inside corners such as shown in the photo at the transom area should be fitted with cant strips or a resin putty filler fillet so the fiberglass material will conform to the corner without lifting from the surface. The resin putty filler is made into a trowelable consistency and formed with any circular object such as a plastic lid. Surfacing agent added to the resin or the use of finishing resin will make the filleted area easier to sand.

Fig. 21 As with the hull exterior, the fiberglass material adjacent to the foam is usually chopped strand mat, and it is applied in virtually the same manner. In the boat shown, a generous overlap is being made along the keel centerline. Pieces are initially cut to rough oversize and set aside to be ready for use.

Fig. 22 When applying the inner laminate, take care to keep excessive resin from flowing into the lower hull areas. Wrinkles in the mat at this point are not of much consequence because the squeegeeing to take place on the successive layers of woven roving, or cloth in the case of the boat in the photo, together with the use of a mat roller will remove most wrikled areas. A good, even coat of resin should be strived for at this point which will make saturation of the Continue lamination easier.

Fig. 23 Successive laminations are applied similarly to the exterior laminate. Take care to check the plans to determine the layers of material required as well as their extremities as this is critical in the area along the keel and stem, for example. White spots denote areas of the laminate which are not completely saturated and more work and resin will be required in these places.

Fig. 24 Resin will tend to flow to the bottom of the hull and the builder should take care to work this resin up to the topsides. Once the laminate is completely wetted out, any excess resin should be squeegeed up and out of the hull and disposed of. No final finishing of the interior is required, and the balance of the construction can take place.

Fig. 25 These workers are using nails fitted with plywood washer blocks to hold the foam in position. Screws are then driven from the underside or the foam sheets can be sewn onto the form members. After this, the nails are removed. Note the solid members along the keel centerline where the foam is omitted. This will be solid fiberglass laminate without any foam.

Fig. 26 The sequence in which the foam is applied usually matters little. Here the foam is being applied in relatively large pieces as these can conform readily to the hull contours. Where contours change rapidly, or on smaller boats, the pieces can be cut smaller and/or heated to be flexible.

Fig. 27 Righting a foam sandwich hull should be done with care as the hull will be quite flexible until the inner laminate is applied. If the hull is removed from the mold initially and then righted, the sheer area should be braced to prevent movement, and the hull should be set into a cradle which will adequately support the hull.

Fig. 28 After the hull is righted, the inner laminate can be applied to the foam hull. Note the solid area along the centerline keel where the foam was omitted. Sheer spreaders are used to brace the flexible sheer area until the inner laminate is completed and at least some of the interior structure is installed.