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Building a Narrow AMA has its Unique Problems

Being as there are now at least three new designs out there using the simple 'flat bottom, deep vee' shape that I recommend on my website for 'ease-of-build but efficiency through the water', I think it's worth discussing some of the construction issues for all concerned. Many of these points are covered in the detailed Build Manual for the W17 but as these tips might also help those building a Seaclipper 20 or a Strike 18, I'm pleased to share them for how they might apply.

If the closing bottom ply is totally flat and the sides are of 6mm or more, it can be possible to exactly pre-profile the edge of the closing bottom skin and then pull the sides to line up with that bottom panel. But it's not always that easy … and if the bottom is inclined, it would be nigh impossible to do it like that as any pressure would push the sides in.

Compared to building a main hull for a trimaran, the amas seem like a small job. In some ways that's true, but the slenderness of amas does introduce some special aspects that are worth addressing. With a beamy hull like a monohull, it's much easier for the plywood skin to give a fair shape over a few frames, but for a slender ama that will probably also be using thinner plywood, using the skin to get a fair shape is not going to work!
It really requires some good stringers that are formed from wood that is initially straight and true, as any kink in them will easily be followed by the thin ama skin, especially if there is minimal curvature over its length.

Once the skin has been laid, it's very common these days to close the bottom to the sides without a wood chine log—using simply ply-to-ply edge contact and then adding a good epoxy fillet on the interior with tapes inside and out. But making that key joint is not so easy with light ply and little stiffness from minimal curvature.

Actually, this side-to-bottom joint is one of the most important ones for this particular design form and needs to done properly to last. Here is what I suggest.

First, that free plywood edge will need a temporary stringer attached to it. I spring a light one on edge, around the exterior, and hot glue this in place with short tacks top and bottom . These 'plastic welds' will be about 1" long, and centered every 4". Perhaps shorter tacks at the top will be enough—it's the bottom ones that take load from the taping … see later on. Don't put more than you need as it's more work to remove them.

[Note: Make sure that the ply is first treated with a coat of epoxy though, or you risk to damage the surface when you come to remove the hot glue later. I remove it with the back of a sharp ½" chisel and it takes about 15 mins a 15' strip.]


Arrange this stringer about 1" down from the edge—even a little lower if the bottom might slope down over it, so that it is well clear. (This also gives more removal space.) This will stiffen the edge and permit it to be planed fair to receive the bottom ply.
Once the closing ply has been all prefitted, make a couple of marks on the panel to show exactly where is has to fit to be in the same position as the pre-fit. I'll sometimes leave a clamp on the stringer near amidships and then cut a small notch in the bottom ply to fit against the clamp screw so that it's located fore-and-aft.

To make this joint strong, you want to be bonding epoxy to epoxy … not epoxy to dry wood. But first, bevel off the side of all the near vertical surfaces (sides and bhds) so that you'll reduce the risk of pushing out all the epoxy from local pressure. Then, before bonding, you need to prep both the side ply edges and the bottom surface, by giving an initial coat of unthickened epoxy. Once cured, sand this off with a flat sanding block.

If you are working alone, then you'll need some way of not only locating the panel RIGHT the first time, but also arrange some means of keeping the flexible ends of the bottom panel from coming down on the sides before you're ready and in perfect position. If not, there's a good chance that the bottom panel will slide a little sideways and wipe all your carefully applied epoxy off the narrow side ply edge.

Here's where the added stringers come in handy. You can make a couple of these little stands to sit on them—one for each end—and they will not only stay in place but also locate the bottom ply and keep the ends elevated until you're ready to position them individually.
Now you're ready to apply thickened epoxy to both the underside edge of the bottom (do this first) and then also a raised bead over the edge of the side ply and bulkheads.

Once ready, place the panel over the location, align the center with the clamp or whatever mark you have, but rest the outer ends on your temporary little bridges.
Now, using professional, industrial strength duct tape, tape down the center of the panel in the marked location. Then work towards one end, moving the temporary bridge along as you go and make sure that the fore or aft tip of the panel lands in the right place. Put a tape over that spot and then go back and repeat the same process at the other end. With the sloped bottom on the W17, be careful that you do not pull down too hard with tape on the outboard side or, because of the angle, the side can be easily pushed inboard and cause an unfair chine line. A good stiff batten hot-glued in place will help to avoid this.

Once both the ends fit in place and the panel lies flat over the side ply throughout its length, start taping it down with short pieces of tape over the ply edge and down around the stringer—always working from amidships towards each end.

Once the panel is all closely fitted, there's usually enough space between the deck sheer and the floor or platform to stretch up inside with something rounded that you can drag along the inside corner to remove most of the excess epoxy that might have fallen there. Although you can use the standard 'tongue depressor', I have a preference to using the tip of a plastic spoon for this, as the spoon acts as a receptacle for the excess resin. You'll be able to do a final and perhaps larger fillet once the hull is turned over, but making that first pass while the epoxy is still wet, will save you a lot of time later.

Once the panel has cured overnight, you can remove the tape and clean off the panel sides. I use either a saw, plane or occasionally a grinder for the first part, depending on the overlap but then finish with a sander for the last part. Apply a coat of regular epoxy to the open edge grain of the bottom as soon as possible and prepare for overall sheathing.


Adding a good interior fillet (8–10 mm radius) and then bias tape inside and out, will complete the joint.

Good luck with your boatbuilding project!

mike

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