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Making the main beams for the W17 — Part 1

Since November 2010, there's been a new build system for the main beams, in order to make them lighter without any loss of strength. (All registered plan owners have been sent this new design.) Though this requires care to the details, there's nothing difficult here.

First let me say that a few readers have already commented on the apparent strength of these beams and perhaps even wondered why. Cross beams must be designed to match the overall beam and the buoyancy of their amas and a boat as stable and powerful as the W17, needs pretty solid beams to achieve that. Small tris with flexible, lightweight beams can work if loads are reduced accordingly, but such boats should be limited to either small rigs, small amas, narrow beam or to sailing the boat in protected waters. If the main beam cannot take the full load of totally submersing the ama plus a reasonable safety factor, then I would see this as a major limitation against any serious high wind sailing. Even for the W17 that meets this criteria, I would consider adding a backup waterstay to the main (forward) beam if I were planning on frequently sailing 'at the limit' in very extreme conditions, especially if going offshore. After all, there's hardly a working part of ANY sailboat that can 'survive everything' without some jury-rigged backup 'get home' plan. Just plain good seamanship.

As one can note from previously posted photos, each beam is made up of one main central box and two curved end parts that are connected by rugged hinges, so that the amas can be folded-in for compact trailing. So let's first look at the new central beam structure in some detail.
Although the final covering will be of glass over plywood, we first need to build the top and bottom 'flange' of the box that will provide the timber needed to take the tension and compression loads that these parts will work under. In order to reduce warping and compensate for any local weakness in one piece of wood, each flange is built from 3 pieces; an inner solid plank with two outer rails. The given scantlings (see plans) have been designed around the use of red cedar, as this gives acceptable strength for the lightest weight. But if available, clear Sitka spruce is even stronger so could be used 10% thinner to help offset its somewhat greater weight.

While waiting to use your wood, it's a smart idea to clamp several pieces together while it continues to dry out and adjust to your climate, so that warping is kept to a minimum. At least by using 3 pieces for each flange, one can usually correct any mild warp by slightly over-curving it in the reverse direction while bonding on the side rails. So before bonding, sight along the edges of each piece to decide if any part needs correcting and if so, plan how best to brace the assembly while bonding, to help the final assembly to be as straight as possible.

With the pieces planed up and ready to use, it's a good idea to lay the 2 side rails together and apply epoxy to them both at the same time. Also good when assembling, to block up the central piece along its length, so that one can feel under the edge to make sure the side rails are properly centered on the central part.

Above is a top flange receiving its two side rails. A 6–8" clamp located about every 9" makes a good bond but do not over-tighten. Make sure glue is just coming out along its length but then stop. Clean off the excess before it hardens.

Once cured, clean off what will be the external surface of each flange and then mark out where you can cut out the excess, as defined on the plans. These cut-outs can be left until after the sides are in place if preferred.

The most critical area of all the beam parts are the ends, as this is where the hinge loads will be concentrated. Plywood hinge plates help gain cross grain strength. These can be simply bonded to the interior surface, but the exterior ones need to be recessed in so that the final surface is flush. Although the use of plywood (laid across grain), is also fine here, another perhaps easier option, is to lay in a layer of heavy glass woven roving or bias cloth as the slight recess for this can be achieved by a light surface grinding.
(A 25oz roving needs about 0.8 mm (132")—the bias cloth, slightly less. Adding a thin layer of chopped mat between the wood and the roving, will generally improve the bond.)

The top and bottom beam 'flanges' are spaced apart by 2 or 3 thick blocks of end-grain cedar and small corner spacers at the ends. The blocks give high compression resistance in way of the attachment bolts and can be made up from smaller parts bonded together. (see photo)

The inside of the flange needs to be coated with at least 2 coats of epoxy before assembly with the blocks etc. For the after beam, there's a suggestion in the manual to pass the traveler control lines through the beam near amidships. One way to do this, is to create a little pocket between the sides of the beam, so that sheaves can be slipped in later, to run the control lines. (Here's one example of such a box. In this case, the sheaves have also been angled about 2.5 degrees, to give an ideal feed to the line—though not absolutely needed for such a small misalignment.)

Here's the main internal structure of the beam all bonded and now ready for the side ply strips. First these need to be sheathed with a 45/45 bias cloth. Although this is available on the market, you can also use standard 200 g/m² (6oz) and cut the required cloth strips at 45 degrees. This is quite economical actually, as for bias cloth, it's not required that it be continuous all the length of the beam and the ends can just be butted. Prior to cutting, use a straight edge to draw lines across the cloth and do not pull it as you cut, or it will quickly appear to get narrower. Then roll the strips on to small tubes like empty paper-towel rolls cut to make two.

While experienced builders will no doubt have their own tricks and preferences for handling this bias sheathing, here are a few I can share.

With the cloth strips all ready at hand, mix about 40 g of resin for each ply strip. (With the MAS pumps, that's about 4 strokes each of resin and hardener; less with the larger WEST pumps). Now, it doesn't matter how long you've been doing this, there will STILL be times when you forget how many strokes of hardener you've added, so here's a tip:
Ideally, I'd be mixing larger quantities and so use calibrated small cups to pour each component in one shot—see: Measuring Epoxy.
But with the pumps, I now add one stroke of each, back and forth. But I do not count 1-1, 2-2, 3-3 anymore, it's too easy to forget which number it was for! So as I go back-and forth between containers, I now count 1, 2, 3, 4, 5, 6 … so that if I stop on any EVEN number, I know I've got a correct mix.

Once well mixed, pour a dribble of resin (about 8–10 mm wide) down one side of the panel and then back up the other. That will give enough resin to quickly spread over the whole panel with a cheap synthetic brush. (I often cut the bristles 1 cm shorter for a firmer effect.)
To avoid handling the cloth and getting resin over it too soon, just pass a stick through the short roll and hold it just over the plywood. With the other hand, lay the cloth down into the resin and use your thumb and first finger in 'an opening movement', to keep the cloth from becoming too narrow. (It's the same action as you use on an iPad or iPhone touchscreen, for zooming in ;-) See photo at left.

Very often, when doing a relatively small sheathing job like 1 or 2 of these panels, it's not economical to use a roller and then have to scrap it; so here's another tip that works well. Tear off a strip of good quality kitchen wax paper about 1" wider than the surface being coated Then lay this on the wet cloth and use a dry roller over the top of it. Then peel off the wax paper, by rolling it back over itself, keeping the corner you're holding as low as possible to prevent any lifting, and move it along and then roll again. This goes faster than my typing and is something I've found very useful for many surfaces. It works particularly well for bias cloth though, as it does not try to stretch or move the cloth along the rolling direction as when rolling direct does, so the cloth stays right where you laid it. The big bonus is that your roller stays clean and dry and by moving the wax paper along with some resin on its surface, it helps to distribute that more evenly too.

Once cured enough, the panels can be bonded to the sides of the wood core—but we'll cover final finishing of the beam in Part 2 of this article.


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