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Building the W17 Main Hull — Part 3: Interior

As already mentioned in Part 2 on Painting, perhaps the most important thing concerning finishing the interior of a small boat like the W17, is to make certain that all crevices and sharp intersections are sealed and smoothed out with thickened epoxy.


Some of these joints (like the longitudinal chine joint and stressed bulkheads) will need a fairly large radius fillet that will then be covered with one or two fiberglass tapes. Such fillets (min. 12mm radius) can be made using lightweight fillers like microspheres, microballoons or colloidal silica, as the final joint strength will mostly come from the glass overlay. In the case of the chine, I suggest to use two tapes of at least 50 mm x 300 gsm overlapping them over the fillet by about 30 mm. The bulkheads in way of the cross beams can work ok with only one tape as there's less potential flex at that intersection. This is also true of the center girder to bottom joint.

Other bulkheads can get away without tape, but then the fillet needs to be large enough (min. 15mm radius) and the fill material needs to be stronger – using a mix of silica with a high density filler such as WEST #404, or if that's not readily available to you, mix in some short (3-4mm) cuttings of glass. While still soft, make sure you smooth out the concave shape of the final fillet with the rounded end of a 'tongue depressor' or something of similar shape. Applying the mix into tight corner spaces is somewhat of a challenge, but I find using one of those small, cheap glue brushes (WEST #803 or equal) is a good way to proceed. With the filler in place, then your tongue depressor can mould it to shape. Once you have a neat fillet, use another flat stick with its end sharpened like a chisel to skim off any excess of fillet that's been pushed to the side. It's really worth the time to clean up the whole adjacent area while the filler is soft, as effective sanding in this area, without also damaging the plywood surface, is difficult. Once done, it should look something like this.

After the joints are filled and taped where required, one needs to install a few beams to support the cockpit sole (floor). First check that the center girder is the same height as the side stringers and either grind it down if too high, or bond on a shim if too low. Playing with the stringers at this point will be too difficult and risk plywood damage.

I found the lightest way to support these short beams at the outboard end, is to first fit them snug to the slightly sloping stringer face and then hang the ends in glass strands.
First though, once the beams are slotted through the top of the center girder and fit the stringer, improve the end bond by drilling a couple of indentations into the beam ends. Just the tip of a 10-12mm drill will do. This will reserve a place for the bonding mix—a mix reinforced with short glass fibers in this case. Bond these in place now. The glass hangers are only added with the closing plywood, so more on this later. (If the beams in the photo look pretty thin to you, it's because they are not only of mahogany, but added strength comes from a layer of UNI fiberglass cloth across their underside.)

Cutting the cockpit ply to fit can be a challenge, but a little trick will help. Lay the ply to be used on the foredeck. Then draw a centerline on it and draw a line exactly square to that, every 300 mm (or 12"). Now place a small strip of wood ~1" x 4" (25 x 100  mm) down the center of the cockpit area. This piece also needs a centerline marked on it, as well as the matching cross lines every 300 mm from one end. With a square, you can now measure from the centerline of the small strip, out to the side skin. Then transfer that to the plywood on the foredeck. Once done for every 300 mm along its length, you'll have the exact shape for the cockpit plywood. Double check dimensions and then cut with confidence. No bevel is required at the exterior as we actually do NOT want it to fit too perfectly on top as some space is required for the epoxy filler and after the sole ply is laid in, this will also be rounded off to form the upper bonding fillet.

The cockpit ply should be sheathed on the underside for both moisture resistance and strength, but once you're satisfied that ALL the surfaces are well covered for the planned life of the boat, you can prepare to bond the cockpit floor in place. Cut out the slot for the daggerboard now, as you'll need that access to pass 2 clamps to make a good joint in that area.
Now we can get back to the glass hangers for the short beam ends and they are made like this. (The ends of these short beams were first drilled out and bonded to the stringer with thickened epoxy.) Just pull out some straight glass fibers from almost any cloth of 200 gr (6 oz) or heavier. Cut these about 8" (200 mm) in length and bunch enough of them together to make about 3 mm diam. Then wet the central 5" (125) with epoxy and pass under each beam end and then spread the dry ends and tape them to the side ply above where the floor will be. You can now see that, when ready, the floor will both tension and trap the fibers as it is laid in place. Then brush on thickened epoxy and lay the plywood in (after a dry-fit to first check it fits ok). It's easier if you use a few barbed or spiral bronze nails (25 mm) down the center but not a must if you're organized with enough weights or struts to press down firmly on the plywood. For the exterior edge, it's easy to press down on the plywood, using strips that are clamped to the side or gunwale.

The center can be braced by clamping beams across the gunwale and then using short verticals down to a temporary length of timber laid over the ply floor.
If all goes well, it will look similar to these photos.

Now you can fit in the cross beam supports and the extra side verticals of ply, that will be bonded and taped to the sides. Note the 'spring clamp' from a scrap plywood strip, that will hold the bottom of the verticals firmly against the side until cured. The center part of the cross beam support can now be cut out to save weight, as once the main beams are in place, it becomes redundant.
Add the 3 beams required. These can be built light by using mostly cedar but reinforced along the lower edge with either mahogany or UNI fiberglass. 'U' pieces of plywood give support at the sides, with the aft ones being larger to also take the mainsheet track attachment (see plans).

Another important job is the poured epoxy to solidify the bow on each side of the stem and then the installation of the forestay bulkhead. Because the main hull is too bulky to sit on its nose to pour epoxy around the stem (as feasible for the amas), you'll need to either use a foam 'dam' to limit where the thickened liquid migrates to, or use thicker filler material that does not run. I prefer the former as it gets deeper into the small crevices.

For the forestay bulkhead, make a cardboard template first, placing it at 15° to the vertical. This bulkhead fits under short inner gunwales installed now to specifically hold it in place. This sketch shows the glass requirement and stainless steel chainplate. The bottom of the double ply is rounded off so that there is no hard corner against the lower sides, should there be an impact below the stringer at the bow. (Guidance dimensions are given in the latest addendum to the Build Manual, that all plan buyers should now have.)

Now is the time to prepare for a hatch in the forward deck if you elect to have one. I was surprised when I first saw the size of the forward space on an actual boat—it was bigger than expected. So I decided to add a hatch of 23" x 13", large enough to lower an icebox through it, that will then rest on a removable lightweight shelf that itself, rests on the stringers. This hatchway will require additional fore and aft members between the beams, again made with cedar and mahogany for the bottom third.
Here's a photo showing one of the two forward reinforced hatch beams. The ends are carried in plywood hangers.

Here you can also see the hull cross plank that runs under the main aka crossbeam, after being cut out to lighten it. At the sides where the aka is eventually through-bolted, there are 2 layers of 19mm pine plus a 12mm plywood, that are bonded between the full bulkhead and a partial one. (Although latest plans call for a short fore & aft piece bonded high-up to the side ply, I am experimenting on this boat, to see if this can be left off.)

In the same photo, also note the foam added to the forward part of the bottom in the forward storage area. This is first bonded to the bottom and then sealed with a layer of glass cloth over it, and by not having foam for after 6" (150 mm), a small 'bilge well' is created that will serve to keep anything sitting on the foam bottom, much drier.

Before adding the deck ply, lay a straight-edge across the gunwales to make sure all is flat and fair. As for the cockpit sole, the deck ply will also require glass over its underside for added stiffness and strength.

Fitting the Decks

Plane or grind off any raised areas of beams, so that all is flush before you lay the plywood on. Prepare any wood scraps you will need under clamps to hold it in place without dents or ugly nails and be sure you have a lot of small clamps at hand. Then apply a good layer of thickened epoxy to all the contact surfaces and in particular, make sure there is good contact over all the bulkheads and both gunwales. Also note here, the slightly lighter option for reinforcing the side ply at the stern.

The FINAL part of the Main Hull will be covered in Part 4—and show how the cockpit seats are created and fitted.

See Main Hull — Part 1: Construction

See Main Hull — Part 2: Painting

See Main Hull — Part 4: Beams & Cockpit

Read more Construction Tips & Techniques.

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