Does the longer keel have the wing also? If not you may find that the
weight of the wing actually compensates for lack of depth and the C.O.B
(Centre of Balance) is about the same. Although, as mentioned in other
posts, you will most likely find that the shallower keel has less pointing
abilty. It not usually enough to worry about if you arent racing.

Usually the draught keel version of a hull is ballasted more heavily than
the fin keel sister so that the righting moment is the same despite the
shorter lever between centre of bouyancy and centre of gravity. Draught keel
version is therefore also usually a bit slower (even on a reach) because
overall it is a heavier boat.

The higher centre of gravity of a draught keel hull can also sometimes make
for a slightly easier sea motion, because you are closer to the centre of
gravity of the boat than on a fin keeler.

Keith wrote:

In message , Andy
Champ writes
Pick it up again, and twirl. It'll be a lot easier to twirl now the
mass isn't so far from the CoG.

Things are much more complicated with hulls than masts, but the
principle is there.

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

Yes, for the same mass, but it also means that she will carry her
angular momentum better, e.g. will be easier to tack. Once you start
her spinning, she'll be more likely to carry on round and not end up
"in irons".

Ric,
Of the several boats I have know both versions of... The shallow draft
version is usually withing a couple of hundred pounds of the deep keel
sister !! either way. Most designers don't want them to float on
different marks.

They may make the keel the same in a side view, but also do things like
add psuedo-wings (Big Thick Short hunks of lead) or go to a Sheel sort
if profile.

I also have a very interesting bit to add here. My boat has an
adjustable keel (actually a 700# dagger board on tackle). We run at
1.5-2ft, reach at 3.5ft and point at 5.5ft. If we try to go to weather
with the board at less than maximum draft, I can make the sails work
just fine and the seeming angle between tacks will not change, but the
instruments tell me that my course over ground to heading deviation is
three to five degrees more (this kills the wvc or vmg) than it should be.

Matt Colie S2-7.9 "Bonne Ide'e"
Lifelong Waterman, Licensed Mariner and Perpetual Sailor


Ric wrote:

Usually the draught keel version of a hull is ballasted more heavily than
the fin keel sister so that the righting moment is the same despite the
shorter lever between centre of bouyancy and centre of gravity. Draught keel
version is therefore also usually a bit slower (even on a reach) because
overall it is a heavier boat.

The higher centre of gravity of a draught keel hull can also sometimes make
for a slightly easier sea motion, because you are closer to the centre of
gravity of the boat than on a fin keeler.

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

Shoal keels v. Fin keels
 

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