In message
, d parker
writes
"Mike" wrote in message
Looking specifically at a Moody 425 (1980's) there are two keel
configurations available: a standard fin keel at 1.88 m draft and a
shoal keel at 1.44 m draft which has a flared lower edge (foot?).


The shallower keel will usually be longer along the length of the boat. This
length adds to the self steering ability of a boat. However, it slows the
response time on the helm and will take a little longer to tack- a problem
if you are after the Americas cup or the tin plate offered at the local
twilight races.

The Flared foot (or wing) will dampen the wave motion somewhat.

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.

I always get confused in regards to 'centre of gravity' and 'polar
moment of inertia' when it comes to yachting.

As you seen to know what you're talking about - what's the difference?
In yachting terms ?
--
Keith replace nospam with ilf0rd

Keith wrote:
I always get confused in regards to 'centre of gravity' and 'polar
moment of inertia' when it comes to yachting.

As you seen to know what you're talking about - what's the difference?
In yachting terms ?

Centre of gravity (which is actually misnamed) is the point where you
could say the average mass of the boat is. Polar moment of inertia is
how far from that point the mass is. That's hard to explain - so wander
along to your local club, and borrow a laser mast for a minute. (Both bits)

Put them together, and pick them up. Find the balance point, so you can
hold them in one hand. You hand is very close to the centre of gravity.

Now twirl them around. It's hard to get them going, and hard to make
them stop. That's because the polar moment of inertia is large, the
long thin shape moves most of the mass a long way from the CoG.

Take them apart, then tie them together with a couple of bits of string
or something so you have one short fat bit. (I think you might actually
be able to put the top mast inside the lower upside down).

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.

HTH

Andy.

"Andy Champ" wrote in message
...
Keith wrote:
I always get confused in regards to 'centre of gravity' and 'polar
moment of inertia' when it comes to yachting.

As you seen to know what you're talking about - what's the difference?
In yachting terms ?

Centre of gravity (which is actually misnamed) is the point where you
could say the average mass of the boat is. Polar moment of inertia is
how far from that point the mass is. That's hard to explain - so wander
along to your local club, and borrow a laser mast for a minute. (Both
bits)

Put them together, and pick them up. Find the balance point, so you can
hold them in one hand. You hand is very close to the centre of gravity.

Now twirl them around. It's hard to get them going, and hard to make
them stop. That's because the polar moment of inertia is large, the
long thin shape moves most of the mass a long way from the CoG.

Take them apart, then tie them together with a couple of bits of string
or something so you have one short fat bit. (I think you might actually
be able to put the top mast inside the lower upside down).

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.

Brilliant explanation Andy, many thanks!
TonyB

"Andy Champ" wrote in message
...
Keith wrote:
I always get confused in regards to 'centre of gravity' and 'polar moment
of inertia' when it comes to yachting.

As you seen to know what you're talking about - what's the difference? In
yachting terms ?

Centre of gravity (which is actually misnamed) is the point where you
could say the average mass of the boat is. Polar moment of inertia is how
far from that point the mass is. That's hard to explain - so wander along
to your local club, and borrow a laser mast for a minute. (Both bits)

Put them together, and pick them up. Find the balance point, so you can
hold them in one hand. You hand is very close to the centre of gravity.

Now twirl them around. It's hard to get them going, and hard to make them
stop. That's because the polar moment of inertia is large, the long thin
shape moves most of the mass a long way from the CoG.

Take them apart, then tie them together with a couple of bits of string or
something so you have one short fat bit. (I think you might actually be
able to put the top mast inside the lower upside down).

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.

HTH

Andy.

My Hats off to that one.
Well done.

DP

In message , Andy
Champ writes
Keith wrote:
I always get confused in regards to 'centre of gravity' and 'polar
moment of inertia' when it comes to yachting.
As you seen to know what you're talking about - what's the
difference? In yachting terms ?

Centre of gravity (which is actually misnamed) is the point where you
could say the average mass of the boat is. Polar moment of inertia is
how far from that point the mass is. That's hard to explain - so
wander along to your local club, and borrow a laser mast for a minute.
(Both bits)

Put them together, and pick them up. Find the balance point, so you
can hold them in one hand. You hand is very close to the centre of
gravity.

Now twirl them around. It's hard to get them going, and hard to make
them stop. That's because the polar moment of inertia is large, the
long thin shape moves most of the mass a long way from the CoG.

Take them apart, then tie them together with a couple of bits of string
or something so you have one short fat bit. (I think you might
actually be able to put the top mast inside the lower upside down).

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.

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?
--
Keith replace nospam with ilf0rd

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".

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