Keith wrote:
Thanks for that.

This suggests that a hull design that has a greater distance between
CofG and PMofI will be harder to turn quickly - is this correct?

PMofI isn't a "point", it's more like "average distance from the CoG".
It's a large PMofI that makes it hard to "start" turning. Once it's
going, it'll tend to keep turning.

Now I think of it, this was neatly demonstrated by the 60ft narrowboat
we rented on holiday this summer. You could turn it on the spot, by
putting the rudder hard over, going forward for a bit (rudder pushes
stern) then back for a bit (rudder has no effect, but this cancels out
the forwards motion). It would actually keep turning continuously.

I think one of the reasons why long-keel boats are slower to tack isn't
the change in PMofI. It's that in order to turn the front of the boat
is going one way sideways against the water, and the back is going the
other. If the boat is deep at the ends - which is true of a long keel,
but not a fin keel - it's hard to turn. The same effect is seen on most
cats, with long thin hulls.

Andy