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Taping Joints Part 2 — When to tape and what to use

A while back, I was asked by a builder of a W17, 'how light could he go on the taping of joints and seams?' as he wanted to race and weight was important to him. The question gave me reason to pause to really rethink that twister (conundrum), as a racer wants his boat as light as possible but is also likely to stress it more than most! But as I was facing the same question for my own boat, here's what I finally came up with and you can use it, if and as you feel it might apply to you. One thing that's important though. If you go to the minimum limits, you MUST use good materials and your workmanship must be up to the job also. If you're not confident of one or both of those, then I'd recommend to increase the minimum by about 20% to give yourself that little extra margin. The other thing I must underline, is that these weight saving suggestions, are NOT for a boat that is to go offshore—but rather for a day boat like the W17 that will either be racing around the buoys or making short day trips in semi-protected water.

For all the following suggestions, the factor 't', is the thickness of the plywood used at that particular joint or seam (in mm) and assumes it's a half-decent marine ply with 5 ply if over 5 mm.
Also note that as an engineer, you can expect my recommendations to have some base in science or physics, so as this will get mildly technical, you might want to have your calculator or thinking cap at hand ;‑). But nothing you can't figure or follow though.

(You can convert glass weight from oz/sq.yd to g/m² by multiplying by 33.8, say 34)

Let's ask first of all, are there any places where taping is NOT required at all?
Yes, I think so but it's also related to the fillet radius at the joint. If a bulkhead is present more as a support station for the building shape than for transferring loads, then simply a fillet radius of say 3 × t should be fine without taping. WEST Systems once conducted tests and found that with a fillet radius of 4 × t, such a joint was slightly stronger than the plywood—and that was without any glass covering. Depending on the actual angle between the joining surfaces, a radius of 4 × t means that the fillet will touch each ply, anywhere from about 2 × t for an open angle to 4 × t at 90 degrees.

If the skin to bulkhead is subject to flex, such as for the transom or a bulkhead to which the cross-beam is linked, then I would suggest to add glass tape of '8 × t' in width—that would assure that at least there was 2 × t landing of the tape on the plywood, clear of the fillet itself. That is the minimum I usually aim for. Such a joint also works for a longitudinal seam.

But with increasing skin thickness, the skin becomes so stiff that we need to look at the thickness of the glass also, as it's not only the connection to the skin that's a limit, it's also the ability of the joint to match the skin rigidity.
But what about the thickness or 'weight' of that glass? (Thickness is fairly proportional to cloth weight with each 10 oz (338 g/m2) giving about 0.015" in thickness. The relationship does vary somewhat with the cloth weave however.)

As (when all other things are equal), stiffness varies as the thickness squared (t²), then I recommend that the glass over the fillet, have a minimum weight (oz/sq.yd) relative to t² (in mm) in this case, [0.4 × t²] correlates well with actual experience, particularly for the range of ply thicknesses we are considering here.

If we now tabulate this for easy reference, it gives these results:

t (mm) t × 8 (mm) min glass
3 24 9 3.6
4 32 16 6.4
4.5 36 20 8
5 40 25 10
6 48 36 14.4
8 64 64 25.6

Now this table has about a ±15% tolerance to my experience but you can see how quickly the cloth needs pick up with thickness. Let's pass briefly through the table.
For a 3mm ply, most tapes will almost be overkill and you could certainly use a 1–1.5" tape of 4 oz to support a 12 mm radius fillet. For the 4.5mm ply specified for the W17 amas, you could use a 36mm tape (say 1.5") of 8 oz for the longl seams and any stressed bulkhead connections.
For the 6mm ply as typically used for the main hull of small boats, the 8 × t gives you a 2" tape and you'd get by with a 12oz tape in non stressed areas, but would need 2 × 8oz for highly loaded joints.
But for 8mm ply, you would need at LEAST 2 × 12 oz cloth tapes over the fillet, and with the previously suggested 8 × t in width, you would need two 64 mm wide tapes of 12 oz each. In this case, the intelligent thing to do would be to use two 50mm tapes but spread them open, so that they overlapped by about 35 mm, giving a total coverage of 65 mm but with 2 × 12 oz over the fillet and 1 × 12 oz as the tape tapered off into the ply. Neat and light—and also at the design minimum for this 8mm ply thickness.

This should help anyone figure out how light one can go without totally risking the structure. The same general principles would apply when taping the joints of a composite panel boat, with the joint designed to give a matching flexibility and strength for the skin.

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