Annie wrote:
To be honest, my own opinion is that if the wood structure is strong enough, all it needs over it is some light glass cloth to provide a hard and thick surface of epoxy. However, David told me to put bi-axial cloth on the skegs (and rudders and bilgeboards) and Pete said to put it on the tabernacle. They are the experts, so I did as I was told, but I don't understand how or why this fibreglass adds any real strength to such large pieces of wood. I don't like using bi-axial cloth because I find it difficult completely to wet out, without there being at least some bubbles left in the matrix.
I hope that someone with more technical expertise will chip in and enlighten us both!
Which David said that? Fanshi's skegs and rudders would be mega stiff, without any sheathing. I will certainly have advocated thick glass sheathing on underwater surfaces, to provide maximum impact protection against the possibility of bouncing the hull and control surfaces on sharp rocks etc. Biaxial glass is quite thick, and would build up the laminate quickly and smoothly, but its fibre orientation properties are not required. Woven rovings would be just as good. (But having said that, it's much easier to persuade glass cloth to conform to sharp bends such as the leading edge of a skeg, if the fibres are at +/- 45˚).
I would put unidirectional glass on the bilgeboards, vertically oriented, as they are designed to be relatively lightly constructed for ease of operation, and more stiffness will be a good thing. For tabernacles, I would have thought that heavy glass would be for peace of mind as much as any technical reason. For thick decks, I would put on more than a thin glass sheath, but only to protect against dropped tools and the like. For thin decks, that thicker layer of glass becomes structural as well.
I have seen a demonstration of how 6mm plywood is made twice as strong by coating it with epoxy on both sides (at Hobart Wooden Boat Festival). Thin glass would have added more strength, but I haven't seen that demonstrated. It's a question of the relative thickness of the glass and the wood, I think. A thick piece of strong wood , as in a tabernacle, would not be further strengthened by adding epoxy and thin glass, but a thin hull of cedar strip planks absolutely depends on layers of glass on both sides, for both point impact resistance, and also overall stiffness and strength.