Thanks Marvin, that's very interesting.
Yes, it may well be due to 'prop walk', caused by the circulating cone of water coming from the prop., but I'm surprised it's so pronounced. I'm not all that experienced with owning boats - I've had two and sailed four or five others over about 40 years, but none has had any tendency to dramatically change course when under engine. The worst that's happened is that the boat slowly drifts off course, and normally the faster you are travelling, the more directionally stable is the boat.
But with my boat, there's a force of around 5kg (or 50N if you're being picky) on the tiller, so if you let go, even for a second, the boat immediately swings around by tens of degrees. At higher speeds the force is greater: at about 5kt I had to lean my body hard against the tiller to keep it straight, maybe 10kg. Had I let go, there would have been an instant course change of 90degrees! That was going up the channel through the moorings at Itchenor, so a moment of inattention could have caused a lot of damage!
Under sail, the boat behaves beautifully, with no steering problems at all.
At first I thought the prop must be out of alignment with the boat, but it looks OK, so it can't be more than slightly out, and not enough to cause this effect. If it's being caused by prop-walk, i.e. the top part of the cone of water coming from the prop hitting the stern and kicking it sideways, then it's a much larger effect than I've experienced on other boats.
I'm wondering if it's the wash from the prop which is hitting the forward part of the balanced rudder, thus pushing it sideways. Maybe the rudder is out of alignment...?
Yes, I'd be very interested to know the pitch of your prop, or indeed anyone else's...
All the best, Peter