Re: centerboard: nacelle mounted(c.n.m.)
Presumably you are thinking of having the centreboard on the centre line of the nacelle. Then having it at approximately the centre of rotation of the hull as it pitches.
If that is so, it will have to be a long board. At least say 1.5 metres. That will mean some impressive pivot mechanism to support the side loads when it is down, since you won't have the luxury of having a large part of it supported by the hull in the nacelle. Conventional dagger boards are supported by the depth of the side hulls.
On my 26ft Heavenly Twin I have a central Outboard engine with a long leg. It is mounted in the aft part of the central nacelle which is just aft of the middle of the boat and is at the point of rotation of the boat. It is immersed up to say 10cm from the base of the engine. Nevertheless, in choppy conditions, I can get the prop over revving very near the surface. I can't see it to know if it breaks the surface. This would not do for a centre board if it is to be effective going to windward in a chop. So, as I said above, it would have to be big/long and the forces on the hinge would be considerable. My boat's nacelle is permanently just in the water. If you are considering this for a boat with a bridgedeck clearance, then this will make matters worse.
Overall, it seems to me like an interesting idea that may be too hard to engineer. But I don't know.
Nothing works on an old boat, except the skipper.