called Tumblehome. However, its actual definition is a bit more prosaic, meaning that the beam at the gunnels is less than the beam below the gunnels - and it's typical of cobles. I'm not sure what function or advantage it has though - possibly can get away with shorter oars as the beam narrows and the boat would be more stable when leaning over the gunnels hauling pots, etc. Anyway, the practical result when building is that the completed hull won't separate from the mould. Jordan Boats suggest that you wobble it about a bit and the mould should come away; 
Selway Fisher say split the moulds (now they tell me!). Seven of the nine moulds have this tumblehome so, having now cut them out, I'll cut a small removable section from each - held in place during the planking. Once this is removed, the two halves of the mould should draw in towards the centre, to release the hull. Can't go wrong! The alternative is to simply destroy the mould in order to remove it but with the risk of damage to the hull. (Voice from the future: No, the mould didn't lift out!)Another thing I've noticed is that the transom dimensions are for the aft face but, being raked at about 35 degrees, there will be a considerable bevel needed - I'd guess up to 25mm wider in places on a 18mm transom. So what size do I cut it out and, given that the bevel will vary from one plank to the next, what's the best way to form these? John Leather says some builders just make it well oversize and then hack it down to size as each plank is fitted (he also says that if you do this, you're a BAD person!).
I'm thinking that what I'll do is fit a false 6 or 9mm transom (as Thom Hill suggests), and then transfer the angles between this and each of the ribbands, onto the 18mm transom (the ribbands being the strips of timber that run the length of the hull at the plank positions to make the mould a rigid structure). I confess I've also worked out these angles with trig. but have doubts about my figures. We'll see . . . .
I'm constantly having to guard against a techy desire for precision of measurements, at which times it's refreshing to read about the old time boat builders. Some history of the coble is given by the Hemlington Nautical History Society. It seems that the last East Coast coble was built around 1932 by Angus Hopwood who built boats at the rear of his house in North Marine Road, Flamborough and that he dismantled an old coble on the beach, noted the parts, and then went home to become a famous coble builder. It's even rumoured that he didn't possess a ruler at all but measured the rough proportions of his boats using a smoked herring! But then, you can't believe anything these days!
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