W22 Building Header Building the W22 Trimaran

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June 24, 2014

W22 Build INDEX

Building the W22—Article #9

Sheathing the Main Hull

In the last article, the gunwale extension was completed and this foam-cored main hull made ready to be sheathed. (Plywood is the option for these upper parts). In order to finish sheathing the interior with biax & bi-directional cloth, it’s convenient to have the main hull supported in two frames that easily permit the hull to be rolled from side to side for better interior access (see photo-1). After all the joints have epoxy fillets where bulkheads join the shell, 2 layers of biax cloth can be laid in – see manual for cloth weight details etc. (Plywood, will generally only need one layer).


Before starting with the exterior, the joint at the top of the main side panel will need a fairly large radius fillet where it mates with the gunwale extension. Say about 20mm radius – so this will permit the fiberglass to lay in place without wrinkles. See photo-5.

The vertical gunwale panels will each have two areas that require extra glass reinforcement in way of where the beam sockets will be installed. See photo-4.


For this foam-cored hull, the exterior was covered with two layers of biaxial cloth and then a final layer of standard bi-directional cloth that’s rolled over the whole area. Initially, clamp one layer of biax to the top of the gunwale edge while it’s fitted and prepared, ready for resin. Then roll this back and precoat the foam with epoxy. After laying the cloth over the whole side (from the knuckle to the gunwale top), the cloth is then carefully pushed in towards the hull and made to lay tight against the 20mm radiused fillet noted (white) on photo-5. The 2nd layer goes faster and the final layer will take very little extra resin, as the resin is worked up through the layers with a squeegee. Use an epoxy with a very slow hardener will give maximum working time, and avoid doing this on a very hot, humid day. Roll with a fairly short (75-100mm) roller, to work resin up through all the layers.

This job took 2 persons about 3 hours per side – with one side per day to permit sufficient curing. Once the epoxy has been well worked in, a layer of peel ply is laid over the whole area and worked flat to the surface to absorb as much excess resin as possible.

In Pic-7 below, we see the main hull inclined to make ‘applying the glass skin to the side’, that much easier. Of course, there are more layers of fiberglass applied when a foam core is used, compared to the stiffer (but heavier) plywood option that typically has one sheathing layer of regular ‘boat cloth’.


Pic-8 shows the first attempt to check beam alignment from aka to the main hull. Good to check this as soon as possible, as it’s far easier to correct any misalignments at this stage, than when the deck is all installed. Pic-9, shows a temporary cross beam placed to mark the boat’s centerline, as the final location of the amas must be measured from that centerline.

In the pics below, we can see the sleeve for the forward beam (aka), being supported (in the case of a foam core boat) by foam blocks. This particular boat owner, built his own CF aka beams and they were made somewhat larger than needed. Better too big than too small – but the extra size meant that the beam sockets needed to be bigger and this removed much of the outer gunwale strake. Fortunately, CF could be added to compensate and the deck itself will contribute important strength in that area.


Some idea of the cockpit size is given by Pic-11, with the cockpit self-draining to the stern, out through the large scupper in the transom. Once the beams are in place, the amas can be positioned correctly in height and trim, before also checking the toe-in of the amas as called for in the plans. See Pic-12.

Article #10 will show the completion of the cockpit and closure of the deck in that area.

See W22 Build INDEX for earlier articles.

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