OK, the boat looks to be a pretty normal hull with sterndrive power. What HP are you running? Why no power steering, as I had thought most/all sterndrives had it?
There is a fix used on high performance hulls. The kind of hulls where the engine is high enough on the transom, so the anode Trim Tab is out of the water at high speed. The fix is to add a Torque Tab. Torque Tab is a small wedge of material that is attached to the skeg. It will counteract most of the steering torque, at all times. They are avail and adjustable at many places on the web.
I made one for my speedboat out of one of those zinc anodes that people bolt to their trim tabs or hull, because I had one laying around. The anode shape is roughly a top 1 inch slice of a volleyball.
I cut a wedge out of it, and fastened it to the skeg of my speedboat. It took some trial and error, cutting and filing the wedge down, until it behaved correctly. So at high speed, when the antiventilation plate and trim tab are out of the water, the torque tab is on it's own.
