On my recently restored alloy hull Delta with a 1600 x flow we took it up to Loch Lomond this summer to give it a work out and found two problems that I am hoping someone has come across before and has a solution. Anything above 3800 revs the boat is turning hard to port, and a lot of right hand down rudder is required to keep it straight.
Also at higher revs there is a marked vibration.
On checking the alignment with a fine string line from bow to stern and a vernier calliper the centre skeg leading edge is approx 2 mm to port. The "p" bracket supporting the prop was found to be 3 mm offset to port. Before I spend more time and money has anyone any thoughts/experience of this problem.
The alignment of the engine was thoroughly checked on mating up to the prop shaft, this item was brand new as the original had a 13 thou run out. The prop has a little old damage to the tips of the blades but has not been checked yet for balance/trueness.
I am thinking of removing the engine this winter and having the bottom end balanced as these old cross flows were always a bit harsh when revved, is it worth having the prop, shaft and coupling balanced also?
Any help is much appreciated.

Try looking at the rear edge of the rudder(vertical part). A frequent mod on slalom course boats is to dress one side of the rudder with a file. This makes the boat pull to one side, the driver counteracts this and consequently the boat is much easier to steer arrow straight as it takes any 'slop' out of the steering. Higher the speed the greater the pull. I couldn't drive a slalom boat without it. At least it might give you a way to correct it if you'd rather not have the pull, or at least lessen it. Sorry I cant remember which side you'd dress but Google should tell you!