M_Boats: M-15 balance

I’m wondering about the rudder balance on my M-15. Under power when I let go of the tiller the boat immediately falls off with the helm going hard over and the boat spinning in circles. It makes singlehanding very difficult when you can’t let go of the tiller for even one second to raise or drop sail or grab something out of the cabin. I have a line for securing the tiller, but even centered the boat doesn’t want to hold a course and is always trying to fall off one direction or the other. Factory kickup rudder. Any thoughts or observations from M-15 owners? (And I have to say this every time...No 17’s please! They are completely different boats!) Rusty on Vanilla www.rustyknorr.weebly.com