Hi Antoine
That all makes perfect sense; the epoxy/glass sheathing is almost 35 years old, and some previous leaks may have contributed to detaching it from the plywood. I think the fact that she has been of of the water for quite a long time will also have created stresses between the sheathing and the ply with the wide range of temperatures she will have experienced. I think your plan is the best approach. Concerning the rudder, I doubt that an aerofoil shape will make much difference at very low speeds; the hull design has a surprisingly good direction 'tracking' ability i.e. it travels very strongly in a straight line. This is hugely beneficial in most circumstances. I doubt if any rudder will be effective at very low speed, whatever the shape or size - and if it's too big you will always travel at low speeds! The self steering was fitted by Alan Burns after my ownership, so I have no experience of its faults; it looked alarmingly chunky and crude to me. With respect to the cockpit locker, the problem is that the horrible big tiller that was fitted to the massively overbuilt rudder by Alan Burns has compromised the access; if you make the tiller hinge upwards that would help. Having said that, the aft locker is not a good place to store anything much in such a long, narrow, light boat and it has very limited capacity; the forward cockpit locker, accessed via the hatch in the bridge deck, is a much more useful place to put outboard, fenders etc. Concerning the hinged battens, I'm much of the same opinion as you; a cambered sail a la Kverneland would be a better and more seamanlike alternative. Water Bear had solid spruce battens when I owned her, with a slightly smaller sail area, and was a great boat to sail except in very light winds, where her high wetted-surface area created too much drag for the small sail. My best wishes for a successful outcome of your work!
Gavin