I’ve had this problem with two boats. On the first the problem was a subtle warp in the
rudder, the worse spot was in line with the prop wash. The warp caused an “airplane wing”
lift effect pulling the
rudder to starboard which turned the
boat to starboard. It was more easily detected by holding a straightedge against the rudder. The warp did not affect performance under sail because without prop wash there wasn’t enough “lift” to displace the rudder. It was a brand
new boat and the manufacturer replaced the rudder - problem
solved.
The second
boat the turning problem was more subtle, but still annoying. No obvious cause ever detected. Again under
power only. The solution? - a small
bronze trim tab about 6-8” high by 3” chord (fore to aft), about 1/16” thick, bolted to the trailing edge of the rudder in line with the prop wash. You bend the tab in the direction that the rudder wants to turn. It takes a bit of experimentation to get the right amount of bend, but on my boat about 5 degrees was plenty. No effect under sail, just
power. I got the sheet
bronze on eBay from sites for jewelry makers. If I
recall correctly it came as a 8”x8” piece.
Good luck. Hope this helps.