Full or fin keel? - Page 15 - SailNet Community
Old 03-24-2012
Senior Member

Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,505
Thanks: 21
Thanked 112 Times in 95 Posts
Rep Power: 13

Re: Full or fin keel?

Nice boat

Regards

Paulo
PCP is offline

Old 03-24-2012
Senior Member

Join Date: Sep 2011
Location: Michigan
Posts: 318
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6

Re: Full or fin keel?

Quote:
Originally Posted by Jeff_H View Post
Bryce,

You apparently are mistaking simple 'moments' and simple 'inertia' for 'moments of inertia' the term that I used in the quote from my explanation.
No... I was also referring to moment of inertia. Like most, I use inertia to refer to the moment of inertia when dealing with rotary motion.

Quote:
When talking about dampening, the resistance to changes in rotation is not just the inertial mass of the object, but the resistive forces of the side force of water against the keel, and the side force of the wind against the sails,
Damping only refers to the velocity factor (angular). Inertia provides no damping. The equation below relates damping force and inertia force. Both are angular, the inertia is the moment of inertia around the axis of rotation.

Actual angular velocity may be a polynomial in velocity.

F = damping * d(theta)/dt + inertia * d^2(theta)/dt

The resistive forces you refer to is damping.

Last edited by BryceGTX; 03-24-2012 at 10:14 PM.
BryceGTX is offline
Old 03-24-2012
Senior Member

Join Date: Sep 2011
Location: Michigan
Posts: 318
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6

Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
There are other factors that contribute to dynamic stability and I don't think that one is the more important. Really important is everything that contributes to dissipate the wave energy in kinetic lateral movement and detrimental is everything that contributes to transform that energy in a rotational movement.
This completely ignores the most significant point that a properly designed rough water boat deals with rough water by reducing the heeling, not necessarily by dissipating energy better.

Quote:
Regarding inertia and roll moment of inertia, let’s consider two boats with the same positive area under the RM curve....
It is illogical to talk about a static stability curve when you put a boat on waves. Because a static stability curve only describes a boats RM on flat water. And of course moment of inertia only has significance with angular acceleration.. So a discussion of static curves and inertia is not appropriate.
Bryce

Last edited by BryceGTX; 03-24-2012 at 10:10 PM.
BryceGTX is offline
Old 03-24-2012
Senior Member

Join Date: Sep 2011
Location: Michigan
Posts: 318
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6

Re: Full or fin keel?

Quote:
Originally Posted by benjmin View Post
This Is Great!

All the smart people are posting info on full and fin keels in this thread.

Perhaps they are all still subscribed and could expand their info. I looked around a whole bunch but could not afford a full keel. I have yet to sail my boat because it is still being painted. I was wondering what to expect from what i believe are called 3/4 keel like the one on my boat? Is there a positive side to a keel like mine?

Documentation says my draft is 5'6"
36' LOA Vessel weighing 16,500lbs with a beam of 12'4"

I am hull #1 of 2 i believe. Created by Squadron Yachts of Bristol RI in 1981 in case anyone was curious where i got a boat they hadn't seen before.
The design of your boat has all the traits of an excellent rough water boat. Looking straight on from the bow, we see the deep v-hull design that provides much lower hull righting moment than the newer flat bottom boats. Lower hull righting moment means that this boat will heel considerably less on a wave than a flat bottom boat. The lower hull righting moment is compensated by a heavier keel. So in the end, your boat may have the same or more righting moment than the flat bottom boat.

The bow is lavishly raked along with hull sides to provide progressively increasing bouancy as the hull dives into a wave. Such a bow is well known to reduce the tendency of the bow to boring through the wave. Which is dangerous.

The deep v hull also provides a parting action to the water as the boat comes off a large wave dramatically reducing pounding when going into seas.

The long keel provides stability as has been pointed out. Interestingly enough you rudder is hung further aft, no doubt providing very good directional control. Not all boàts hang the rudder so far back.

The cost is that this boat may be a bit slower than the typical flat bottom boat. But your boat is going to be so much more comfortable in rough water. On the other hand the speed of this boat will be highly dependent on how much sail you put up. So you should not consider that this boat is inherently slow because of its hull design. Sail area will have a huge impact.
Bryce

Last edited by BryceGTX; 03-24-2012 at 11:35 PM.
BryceGTX is offline
Old 03-25-2012
Wish I never found SN!

Join Date: Jul 2006
Location: Presently in Brisbane, Australia
Posts: 2,114
Thanks: 9
Thanked 6 Times in 6 Posts
Rep Power: 11

Re: Full or fin keel?

The best keel and rudder set up are the ones that are under the boat you love and can afford. If all the boats were the same....blah blah blah.

Simon
Ericson 39B.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

I love my boat
S/V GOODONYA
Brisbane
present location Gold Coast

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

DELIVERY SKIPPER
Drinking Rum before 10am makes you a Pirate NOT an alcoholic
SimonV is offline
Old 03-25-2012
Multiman

Join Date: Jan 2009
Posts: 89
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 9

Re: Full or fin keel?

Quote:
Originally Posted by mitiempo View Post
Boats price (when new) by the pound, assuming similar level of equipment.

So lets compare boats by their displacement.

Westsail 32 displacement 19,528 lbs

Beneteau first 42 displacement 18,600 lbs

Pounds /inch immersion

Westsail 32 1064 lbs

Beneteau First 42 1712 lbs

Isn't that a fair comparison?

Carl's Sail Calculator is a useful comparison tool as well.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
mdi is offline
Old 03-25-2012
Senior Member

Join Date: Sep 2011
Location: western Maine
Posts: 583
Thanks: 25
Thanked 16 Times in 14 Posts
Rep Power: 6

Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
The design of your boat has all the traits of an excellent rough water boat. Looking straight on from the bow, we see the deep v-hull design that provides much lower hull righting moment than the newer flat bottom boats. Lower hull righting moment means that this boat will heel considerably less on a wave than a flat bottom boat. The lower hull righting moment is compensated by a heavier keel. So in the end, your boat may have the same or more righting moment than the flat bottom boat.

The bow is lavishly raked along with hull sides to provide progressively increasing bouancy as the hull dives into a wave. Such a bow is well known to reduce the tendency of the bow to boring through the wave. Which is dangerous.

The deep v hull also provides a parting action to the water as the boat comes off a large wave dramatically reducing pounding when going into seas.

The long keel provides stability as has been pointed out. Interestingly enough you rudder is hung further aft, no doubt providing very good directional control. Not all boàts hang the rudder so far back.

The cost is that this boat may be a bit slower than the typical flat bottom boat. But your boat is going to be so much more comfortable in rough water. On the other hand the speed of this boat will be highly dependent on how much sail you put up. So you should not consider that this boat is inherently slow because of its hull design. Sail area will have a huge impact.
Bryce
Bryce, nice discussion of the attributes of that beautiful looking (and clean!) boat. Thank you. The combination of the long keel, but with an aft rudder, is interesting.

And just when I was worrying that you were here to argue rather than debate - I'm sure you have seen many threads destroyed that way.

Quote:
Originally Posted by SimonV View Post
The best keel and rudder set up are the ones that are under the boat you love and can afford. If all the boats were the same....blah blah blah.
True, but when not sailing, thinking about and shopping for sailboats is enjoyable. "Thinking and shopping" requires insights into how different types of boats may behave in differing conditions.

This is an excellent thread, with lots of good discussion.
skygazer is offline
Old 03-25-2012
Senior Member

Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,505
Thanks: 21
Thanked 112 Times in 95 Posts
Rep Power: 13

Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
This completely ignores the most significant point that a properly designed rough water boat deals with rough water by reducing the heeling, not necessarily by dissipating energy better.
....
There is nothing that come close to reducing heeling as beam in a boat. Tank testings showed a direct relations between beam and the size of the breaking wave needed to capsize the boat.

Regards

Paulo
PCP is offline
Old 03-25-2012
Senior Member

Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,505
Thanks: 21
Thanked 112 Times in 95 Posts
Rep Power: 13

Re: Full or fin keel?

Hey, I was just talking about this and we have here just a demonstration of what I was saying about inertia and the importance of returning fast to its feet after a capsize, to reduce the chances to be hit by a second wave when the boat is laying down:

Quote:
Originally Posted by G1000 View Post
Great images. Those are really big breaking waves

Outstanding the speed that this boats can return to its feet after being capsized, even with a lot of sail out That was a double hit, if the boat had a lot of inertia (a lot of weight for the same RM) and was still capsized when it was hit by the second wave, the story could be other. We can also see clearly the boat going sideways and rotating dissipating with movement the wave energy (look at the clouds) otherwise than with a rolling movement.

Regards

Paulo

Last edited by PCP; 03-25-2012 at 11:08 AM.
PCP is offline
Old 03-26-2012
Senior Member

Join Date: Sep 2011
Location: Michigan
Posts: 318
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 6

Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
There is nothing that come close to reducing heeling as beam in a boat. Tank testings showed a direct relations between beam and the size of the breaking wave needed to capsize the boat.

Regards

Paulo
You are correct that beam width is the best way to reduce heeling angle due to winds. Because it increases the hull righting moment. And I have said this earlier on this thread. However, excellent rough water boats are built with moderate beams with a more v-shaped hull design which actually reduces the effects of beam and such boats have significant mass in the keel.

No doubt, to meet some race boat criteria, someone has produced the results you describe. Your video shows exactly the problem with wide beam racing boats that have little mass righting moment. That is they heel with the seas. Such boats are easily the worst rough water boat anyone could design.

Here is a blurb from a yacht design book from the 1800s that pretty much summarizes what we know even to this day. Vessels with wide beams are most affected by wave motion. I think this is pretty much common knowledge.
Bryce
Attached Thumbnails

Last edited by BryceGTX; 04-10-2012 at 10:47 PM.
BryceGTX is offline

Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
OR

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

 Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)