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

Andy Champ wrote:
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
I think you are confusing PMofI with the pivot point. When you put the
rudder over the stern swings out far more than the bow swings in. There
is nothing to push the bow sideways. the stern gets pushed sideways.
The pivot point of the boat is well forward (when going ahead) and long
keels hinder the turn because of the lateral drag on the keel by the
rudder. That is why keels with the "Brewer bite" turn quicker and fin
keels turn quicker. But put a skeg in front of a spade rudder and the
lateral resistance goes up and the turn slows down.

PMoI is more critical in pitching motion in a seaway. That is why
racers like to keep the ends of the boats light.

Gaz