Shoal keels v. Fin keels
 

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?).

Apart from the obvious advantage of less draft in shallow waters can
anybody comment on how the different keels perform during different
points of sailing and in different wave conditions?

Thanks for any help.

Mike

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?).

Apart from the obvious advantage of less draft in shallow waters can
anybody comment on how the different keels perform during different
points of sailing and in different wave conditions?

If the waters where you sail will allow the deeper keel, just go for
the deeper keel and don't waste your time with any other type.

If your local water is shallow, you could opt for the shoal draft keel
but your keel may well weigh more and your windward performance may
suffer as a result of increased leeway and your yacht may be less
stiff, meaning you will have to reef at slightly lower windspeeds.

A question you could ask yourself is: is the slightly improved
stiffness & performance worth 22cm of extra draft, or can you live with
less performance for a shallower mooring/marina berth/sailing at the
top of your harbour?

Artie

Mike,

Just a point of observation. I have a 49=B4 aluminium cutter, 2.2 meter
draft fin keel. She sails like a dream and points inside of 33 degrees
(apparent). Her sistership has the `shoal draft`, 1.8 meters with a
retractable keelboard to 3 meters. The second vessel doesn`t point as
high or hold as well (a little under 45 degrees). But then again, she
can do the Bahamas. We can`t.

"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?).

Apart from the obvious advantage of less draft in shallow waters can
anybody comment on how the different keels perform during different
points of sailing and in different wave conditions?

Thanks for any help.

Mike

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.

DP

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.

Just a point of observation. I have a 49´ aluminium cutter, 2.2 meter
draft fin keel. She sails like a dream and points inside of 33 degrees
(apparent). Her sistership has the `shoal draft`, 1.8 meters with a
retractable keelboard to 3 meters. The second vessel doesn`t point as
high or hold as well (a little under 45 degrees). But then again, she
can do the Bahamas. We can`t.

And my lift keel trailer-sailer can do the Atlantic as well. (As many of her
class have proven)

Why can't your boat get across to the Bahamas? No air conditioning?

Artie

"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