For an introduction, see the Kayarchy chapter Sea Kayak Construction Methods (5) Skin on Frame.
Rough timber for the mould(s)
A boatbuilding mould is a temporary, removable structure, usually made of rough wood. It is used to make sure that the cross-sectional shape is correct, and especially that your boat is symmetrical. You definitely want the left side of your boat to be an exact mirror-image of the right side.
If you use steam-bent ribs, you can define the cross-sectional shape of the middle part of the boat "by eye". In other words, you look at the deck structure you have already built and you visualise the shape you want for the hull. When you've estimated the shape of the ribs needed for that part of the boat, you try to bend symmetrical ribs using your own judgement and memory. Unless you have 50 years' experience, it's probably better to use at least one mould. (If you use sawn frames, these do the same job as moulds and they become a permanent part of the boat.)
A temporary mould can be a convex shape that sits inside the hull, where the crew of the finished boat will sit (a male mould like illustration A); or a concave shape that sits outside the hull (a female mould like illustration B). You cut notches in the mould so as to locate the stringers in exactly the right places. When you're cutting your notches, remember they must also allow you to remove the mould when the stringers have been fixed in position. Then you fit the stringers, attaching them temporarily to the mould(s) and permanently at the ends; then you bend the ribs to fit inside the stringers, and finally you lash the ribs to the stringers. Now you have your basket structure, and you don't need the mould(s) any more.
Two moulds is often enough for a skin-on-frame boat, and a really long, thin hull shape may need only one mould. The design in the photo at the top of the page has four moulds. That's because it's a short, fat boat and its ends are different shapes.
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