Sheathing

Another catch-up post – the sheathing was actually completed a few weeks ago. Maybe it has just taken me this long to recover from the trauma in order to write about it!

Iain doesn’t actually mention sheathing in the plans or notes, except to say that the ballast keel may be laid on a double layer of 10oz glass. However I know from my communications with him that sheathing is a good idea if the boat is made of okoume ply instead of a denser and more durable species. I instinctively prefer the idea anyway – I have seen plenty of marine ply self-destruct when exposed to the elements, so I find the idea of a great thick epoxy layer very reassuring. And this is really the intention of this kind if sheathing: the glass is there mainly to provide a robust method of holding a thicker layer of epoxy in place than would otherwise be possible. From what I gather, if all you want is more stiffness and/or strength, you would be better off adding more plywood rather than glass. (There is a very interesting chapter entitled “Believe it or not, wood is best” in Dave Gerr’s excellent book “The Nature of Boats“.)

Initially therefore had no real idea of what cloth to use. I got a steer from Robert Ayliffe (of Norwalk Island Sharpies) via Iain, who suggested 400gsm biaxial. The two problems with this were that a) I couldn’t find any, and b) I was concerned that it sounded a tad heavy. I also agonised over whether to follow Reuel Parker’s writings and use xynole. It is not available in the UK, and I could not discover whether the available alternatives were as good – or indeed if he is still a xynole fan. In the end I stuck with the safe and popular option of glass, and went for the next lightest cloth to 400gsm that I could find, which was 320gsm biaxial.

The way I laid it was as follows:

  1. Starting a generous 5 inches to one side of the centreline, on the bottom, the first width was draped over the opposite chine.
  2. This was trimmed approximately to the waterline (plus a bit more each end to make sure the chine was still generously spanned).
  3. A second width started a few inches inside the chine, on the bottom, and draped over the chine down to the sheer.
  4. This was trimmed to the sheer.
  5. Both these overlapping layers were wetted out at once, with peel ply on the 10 inch width that will eventually be occupied by the keel.
  6. This first side then received two further epoxy coats plus a microballoon/epoxy coat, each applied just as the previous layer gelled.
  7. After the first side had completely set and the peel ply was removed from the keel zone, the whole process was repeated on the other side (though without peel ply this time).

The idea was to make sure that the keel area ended up with two layers of glass, as well as the chines (which were already taped, so now have a total of 3 layers). As a bonus, the topsides will actually have a double layer below the waterline, but most of this second layer was really only there to make sure the chines had their double layer.

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First width, not yet trimmed to waterline

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Second width being laid

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Frist side wetted out – peel ply is just visible in keel area which must eventually receive a second layer

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For subsequent coats we put polythene on the floor – doh!

This entry was posted in Hull.

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