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Re: Wood - Plywood - Epoxy bond, re-building my ketch Tonga
I didn't think you put new ply over the old deck. But sometime someone did, and when you changed the deck you respected the thickness you found, so now yo have a stronger deck.
The deck and hullsides do not have to be in some relation to eachother. Your vessel works like this: keel and deck are the flanges of an I-beam and resist up and down flexing. For this the hullsides only need to keep them apart. The hullsides resist torsional forces, hidrostatic pressure, puncture loads and so on. The stringers impart longitudinal stiffness and the frames and bulkheads transversal. They also divide the planking in panels and the smaller this panels are the thinner the outside skin can be, because stifness is provided for it. Case in point your vessel could use a 4mm skin with some semi-atached fiberglass. In a semi-monocoque like yours the skin also carries torsional loads, that's why you align veneers at 45° to the frames. The skin acts as a gusset or knee between stringers and frames. The fewer stringers and frames you have the thicker the skin must be because it carries more and more loads and needs to be stiffer to resist buckling, until you come to a true monocoque structure where there are no frames and stringers and only one or two bulkheads to keep the hullsides apart under the rigging stress because the skin carries all the loads.
The structure you have is proven to work with a 4mm skin. Increasing skin thickness does little. It provides more reserve in case stringers and frames break. You already have so many stringers that you sailed with a few rotten ones without problems. Thicker skin has better puncture resistance but your stringers are spaced so closely apart that it would matter only if a sharp pointy object like a piece of rebar would hit the hull exacltly between two stringers. For any other scenario your effective hull thickness is 50mm plus whatever skin you have. And if you smash your boat onto a piece of rebar 12 or 18mm plywood will not really make a difference considering the weight of the boat. Abrasion resistance is also increased but plywood is not great at that anyway. For this reasons I would stay at two layers of ply, it is already 3 times what was proven to work. It is a cost/benefit analysis after all, and a third layer does not bring much except increasing time and cost. Weight is not really consideration, the aded displacement will probably ofsett the aded weight. With carveel construction sometimes it even is a net positive because the timbers are not waterlogged anymore, but I suppose it's not your case.
If you do stay with 3 layers of plywood then you can arrange the first layer at 90° to the stringers and the next two at +/-45° for a complete isotropic sequence. This is just best practice derived from veneer use since plywood per se is regarded as an isotropic material, but the 90° layer will require less spiling so it is worth doing it.
Reread the ply-on-frame chapter from the Gougeons book, they also have rules that explain stringer and frame spacing.
Wishbone rigs were favoured when the boat was built because of the available materials. Today a boat like yours would probably be rigged as a sloop or cutter with a big roach square headed main, and be faster, pointing higher, and have improved handling. Just be glad that you don't have the wishbone anymore and have more sailarea.
I like the Melquiades design, but all Buehler boats are overbuildt. With metal and fiberglass boatbuilding what looks strong is probaly masively overbuildt. But that is often the case with amateur construction anyway. Not much of a problem on big boats who can carry the weight but smaller ones do struggle with it. On the other hand they do have inbuildt corrosion allowance with that plate thickness.
I didn't think you put new ply over the old deck. But sometime someone did, and when you changed the deck you respected the thickness you found, so now yo have a stronger deck.
The deck and hullsides do not have to be in some relation to eachother. Your vessel works like this: keel and deck are the flanges of an I-beam and resist up and down flexing. For this the hullsides only need to keep them apart. The hullsides resist torsional forces, hidrostatic pressure, puncture loads and so on. The stringers impart longitudinal stiffness and the frames and bulkheads transversal. They also divide the planking in panels and the smaller this panels are the thinner the outside skin can be, because stifness is provided for it. Case in point your vessel could use a 4mm skin with some semi-atached fiberglass. In a semi-monocoque like yours the skin also carries torsional loads, that's why you align veneers at 45° to the frames. The skin acts as a gusset or knee between stringers and frames. The fewer stringers and frames you have the thicker the skin must be because it carries more and more loads and needs to be stiffer to resist buckling, until you come to a true monocoque structure where there are no frames and stringers and only one or two bulkheads to keep the hullsides apart under the rigging stress because the skin carries all the loads.
The structure you have is proven to work with a 4mm skin. Increasing skin thickness does little. It provides more reserve in case stringers and frames break. You already have so many stringers that you sailed with a few rotten ones without problems. Thicker skin has better puncture resistance but your stringers are spaced so closely apart that it would matter only if a sharp pointy object like a piece of rebar would hit the hull exacltly between two stringers. For any other scenario your effective hull thickness is 50mm plus whatever skin you have. And if you smash your boat onto a piece of rebar 12 or 18mm plywood will not really make a difference considering the weight of the boat. Abrasion resistance is also increased but plywood is not great at that anyway. For this reasons I would stay at two layers of ply, it is already 3 times what was proven to work. It is a cost/benefit analysis after all, and a third layer does not bring much except increasing time and cost. Weight is not really consideration, the aded displacement will probably ofsett the aded weight. With carveel construction sometimes it even is a net positive because the timbers are not waterlogged anymore, but I suppose it's not your case.
If you do stay with 3 layers of plywood then you can arrange the first layer at 90° to the stringers and the next two at +/-45° for a complete isotropic sequence. This is just best practice derived from veneer use since plywood per se is regarded as an isotropic material, but the 90° layer will require less spiling so it is worth doing it.
Reread the ply-on-frame chapter from the Gougeons book, they also have rules that explain stringer and frame spacing.
Wishbone rigs were favoured when the boat was built because of the available materials. Today a boat like yours would probably be rigged as a sloop or cutter with a big roach square headed main, and be faster, pointing higher, and have improved handling. Just be glad that you don't have the wishbone anymore and have more sailarea.
I like the Melquiades design, but all Buehler boats are overbuildt. With metal and fiberglass boatbuilding what looks strong is probaly masively overbuildt. But that is often the case with amateur construction anyway. Not much of a problem on big boats who can carry the weight but smaller ones do struggle with it. On the other hand they do have inbuildt corrosion allowance with that plate thickness.
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