Annie Hill wrote:
David Tyler wrote:
David has found out. What will the finished thickness be? 4mm? If so, use 9mm ply for the deck. I don't want this boat to turn out to be cranky.
Why not: her designer and owner both are?!
I'd already thought about putting on lighter decks, David. The side decks are going to need beams - they'll be too flexible even with stringers, but I'll make those out of laminated (treated) Kahikatea, of which I have a good pile. The centre deck can most certainly be 9mm, with its pronounced camber. However, in 20 years, the teak will be well worn and I wonder if 9mm will be strong enough for the side decks by then. The extra weight of wood will be about 10 kilos at the very maximum. The alternative of glass, epoxy, primer, polyurethane and deck paint would also add a not inconsiderable weight. Are we worrying, in fact, about 5 kg?
Now I've recovered (most of) my equanimity.
The side decks are flat. So neither stringers nor transverse beams alone are going to make them stiff, if the total thickness of the deck structure is kept down to 48mm, as designed.
The design relies entirely on the inner skin of the deck, the headlining, for stiffness. That is, 5mm ply, glued to the underside of either stringers or transverse beams, and either 9mm ply + 4mm teak, or 12mm ply + glass glued to the top, forming an I-beam structure. Either will be satisfactory. If 12mm ply is used, then either deck beams or longitudinal stringers (but not both) can be used. However, in the case of 9mm ply + 4mm teak running fore and aft, only transverse deck beams should be considered. Forget deck stringers, and laminate full-width deck beams, 30mm deep.
I don't buy the argument about the teak being so worn after 20 years that 9mm ply won't be strong enough. If it were that worn, it would have to be replaced, or the deck would be a dangerous and unsightly mess.
I emphasise: the deck is the very worst place to add extra weight to a boat. Even more so on a shoal draught boat. Don't do it. I'm finding it difficult to resist editing this paragraph to be in 72pt bold red text, but so far, I'm succeeding...