If the problem is weather helm, raking the mast aft would make it worse, if anything you should rake it forward.

But I don't think that will help either. I think you are probably right in suspecting imbalance of the hull when it is heeled - not at all an uncommon thing. Many hull types are renowned for it. The only immediate solution to that is to not press the boat into excessive heeling - that is: reduce sail.

I had the same "problem" with my Pelorus. Pushed hard she would develop excessive weather helm. I could reduce sail easily by roller-reefing the headsail - the opposite of what "centre-of-effort" theory would suggest - and guess what, the weather helm went away. Why? Because she was heeling less. Another problem with the Pelorus, I suspect, was poorly cut sails, with "hooking" in the leech - you could look at that, but if the problem only arises when the boat is pushed too much into heeling, then it suggests it is inherently part of the hull shape.

I dare say Arne is correct in suggesting a larger rudder, though introducing larger rudder forces into the equation means introducing more drag too, I would have thought. But you could think about that, because a larger rudder, in a way, does alter the underwater hull profile and shift its geometric centre of area aft (like adding an aft centreboard). I dare say Arne has already explained it better.

(I just saw David W's post - he explains it I think, although I question the analogy with ancient junks - they had hull shapes different from yours). I still think you are over-canvassed. If reefing down leaves you without enough drive, maybe you need a more efficient sail - maybe give David T's sail a try?

Anyway, don't try to solve it by raking the mast aft, in my humble opinion that would make things worse, if anything.

PS if it were a dinghy you could reduce weather helm by shifting your weight aft. So your idea was on another possible right track, but not by tilting the mast back. It may be that because of the weight of the rig being so far forward, your boat is trimmed down a bit by the bow. It wouldn't hurt to try sitting right aft and even shifting a passenger or anything else movable, a little further aft when you are having the problem. It wouldn't cost anything to try that theory. Don't give up - it looks like a neat little boat that should sail really well. I still think it is a matter of getting something to work efficiently within the sail area limits of the boat. Plenty of boats sail better upright, they don't all like to be "sailed on their ear".

Excess weather helm, in this case purely from heeling too much.

And another un-scientific thought:

Let's say the root cause of my boat's problems is the hull shape, especially the narrow and not-so-buoyant bow. This is why the excess weather helm turns into a problem only when heeling too much (which isn't that much). If this is true, the main cause is not sail balance.

What if I try to put some aft mast rake? This would take the mast weight and maybe some pull on the boat more to the aft and hep the bow from diving. Or would it?

And a more radical idea: moving the mast aft, 50-100 cm and using 20-25% sail balance.

Am I just trying to fix something that can't be fixed?

On reflection, the sail would need to be bundled with the hinged battens and yard, and transport to Finland might be difficult. It seems to make more sense for me to sell to someone in the UK who can come to inspect it to see what they're taking on, then put onto a roofrack to transport home. 

Jami wrote:

I just can't get away the feeling that I need more drive with a smaller sail area, especially at the reefed end, to control the heeling (and the hull issue).

Yes, indeed you do. I like to think of this in automotive terms. Before WW2, there were huge sports cars with engine cylinders the size of buckets. Nowadays, there are sports cars with highly developed engines a fraction of the size that could easily out-pace them. Now, substitute an oversized cambered panel sail for the first example, and my current wingsail for the second...

How come the jib option is not favoured? Is it just for the sake of simplicity or something else?

I just can't get away the feeling that I need more drive with a smaller sail area, especially at the reefed end, to control the heeling (and the hull issue).