Full or fin keel? - Page 12 - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > Out There > Cruising & Liveaboard Forum
 Not a Member? 


Like Tree241Likes
Reply
 
LinkBack Thread Tools
  #111  
Old 03-21-2012
blt2ski's Avatar
Senior Member
 
Join Date: May 2005
Posts: 6,646
Thanks: 0
Thanked 18 Times in 17 Posts
Rep Power: 10
blt2ski will become famous soon enough
Re: Full or fin keel?

At the end of the day, it really only matters "IF" the boat design works for the type of sailing you wish to do, where you wish to do it etc. Be it a fin, full or something in between. if a one design, you will not have a lot of choice. If cruising, or ocean racing, look at the designs thru the years, in the past, lots of full keel's, more recent, fin's......

Not sure which is best. but from sailing my post fastnet IOR halfton, vs my step dads Bill Garden modern design plywood "Sea Bird" yawl. About the same wt, length, WL etc...... no way would I want to be sailing that thing in ANYTHING but calm water. It does not back up, does not go forward well either......then again, maybe it is the actual design of the boat!

At the end of the day, does it do what you want? if not, then it is the wrong design boat for you. If yes, then it is the right boat for you!

marty
__________________
She drives me boat,
I drives me dinghy!
Reply With Quote Share with Facebook
  #112  
Old 03-21-2012
skygazer's Avatar
Senior Member
 
Join Date: Sep 2011
Location: western Maine
Posts: 366
Thanks: 1
Thanked 2 Times in 2 Posts
Rep Power: 3
skygazer is on a distinguished road
Re: Full or fin keel?

Quote:
Originally Posted by blt2ski View Post
At the end of the day, it really only matters "IF" the boat design works for the type of sailing you wish to do, where you wish to do it etc. Be it a fin, full or something in between.
marty
But at the beginning of the day, I wish for more understanding, so I don't have to buy too many boats that are 'wrong' for me at the end of the day.
Reply With Quote Share with Facebook
  #113  
Old 03-21-2012
PCP's Avatar
PCP PCP is offline
Senior Member
 
Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,160
Thanks: 21
Thanked 95 Times in 79 Posts
Rep Power: 10
PCP will become famous soon enough
Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post
But at the beginning of the day, I wish for more understanding, so I don't have to buy too many boats that are 'wrong' for me at the end of the day.
There is only on answer for that. Get enough sailing experience (if you have not already) and try as many boats as you can, including heavy full keelers, medium fin keelers and light bulbed modern boats (a foil and a torpedo) and take your pick. You have also a big variety of hull shapes to chose.

Nothing will substitute personal experience to find the type of boat that will suit you. We all know that boats are compromises. Modern NA moved in a direction that they find that suits most tastes including their own, but that do not mean necessarily that your tastes will fit with the compromises they find better for a globally satisfying boat in what concerns seaworthiness, sailing performance, interior space and sea motion.

Regards

Paulo
Faster and mitiempo like this.
Reply With Quote Share with Facebook
  #114  
Old 03-21-2012
BreakAwayFL's Avatar
Senior Member
 
Join Date: Sep 2010
Location: Fort Lauderdale, FL
Posts: 207
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 4
BreakAwayFL is on a distinguished road
Re: Full or fin keel?

I was under the impression that a full keel also puts more weight lower in the water, which would make it more forgiving if you happen to have too much sail out...
__________________
Nelson Abreu
Serenity Now
Coronado 25
Insanity Later
Walker Bay Dingy

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Reply With Quote Share with Facebook
  #115  
Old 03-21-2012
Faster's Avatar
Just another Moderator
 
Join Date: Sep 2005
Location: New Westminster, BC
Posts: 14,604
Thanks: 67
Thanked 178 Times in 174 Posts
Rep Power: 10
Faster has a spectacular aura about Faster has a spectacular aura about Faster has a spectacular aura about
Re: Full or fin keel?

Quote:
Originally Posted by BreakAwayFL View Post
I was under the impression that a full keel also puts more weight lower in the water, which would make it more forgiving if you happen to have too much sail out...
Way too many factors involved for that to be a true general statement. Some full keel boats have little draft and poor righting moment - others have adequate draft but use low density ballast like concrete.... and there are plenty of fin keel boats that are 'stiffer' than some full keelers.
PCP and mitiempo like this.
__________________
Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Reply With Quote Share with Facebook
  #116  
Old 03-21-2012
PCP's Avatar
PCP PCP is offline
Senior Member
 
Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,160
Thanks: 21
Thanked 95 Times in 79 Posts
Rep Power: 10
PCP will become famous soon enough
Re: Full or fin keel?

Quote:
Originally Posted by Faster View Post
...... and there are plenty of fin keel boats that are 'stiffer' than some full keelers.
And some that are stiffer than any full keeler.

Breakaway, Stiffness has not to do only with a low CG but also with form stability and that is particularly true with the stability that is used for sailing. Even in what regards final stability (over 60º) generaly a modern boat is more stiff (60 to 90º) than a full keeler.

It has to do with the shape of the stability curve. That is not true to all cases but generally an old full keeler (that are normally less beamy boats) has a more rounded curve with a smaller GZ (arm), a bigger AVS and less inverted stability, but between 60 and 90º the GZ curve values are remarkably smaller than on a modern boat. Its stability is mostly based on displacement, versus efficiency (bigger and better GZ curve) on a modern boat.

Regards

Paulo

Last edited by PCP; 03-21-2012 at 10:44 AM.
Reply With Quote Share with Facebook
  #117  
Old 03-21-2012
Jeff_H's Avatar
Moderator
 
Join Date: Feb 2000
Location: Annapolis, Md
Posts: 6,503
Thanks: 3
Thanked 82 Times in 63 Posts
Rep Power: 10
Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about Jeff_H has a spectacular aura about
Re: Full or fin keel?

Quote:
Originally Posted by BryceGTX View Post
There really is nothing wrong with what he says, but it is incomplete. It has been well documented in 100 year old yacht design books that full keels have the advantage in big water because the rotation of the water in the wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs.
Bryce
I do think that there is something wrong with his statement beyond being incomplete and that you are mistaken when you say that "the rotation of the water in a wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs."

Perhaps I can explain the basis of my comments and we might be able to reach agreement.

I will start with the first quote by Pvajko:
"The most important reason is that a full keel with its bigger surface area damps the rolling motion better."

Here is the problem with that statement, dampening (the ability of a boat to dynamically to resist rotational motion) is directly proportional to a moment of inertia the amount of which results from the resistive force of the rotation and the distance that resistive force is from the instanteous rotational axis. In calculating a dampening moment, the force is a linear factor, but distance from the center of that force to the instanteous rotational axis is to the third power.

So that when you talk about the amount of dampening moment generated by a specific keel or keel type, the amount of area of the keel is a certainly significant factor, but the distance between the center of its rotational resistance and the instanteous rotational axis can be even more significant.

So, if we talk about the fin keels in the era when 'Seaworthiness' was written, these keels had perhaps a quarter of the surface area of a full keel on a similar length boat (and here I am not talking about the boats with long overhangs, an extreme cut away forefoot and raked rudder posts which had little more area than fin keels with separate rudders).

In the era that Marchaj wrote his book, between the shape of the fin keel, and the vertical height of the instanteous roll axis on fin keel boats of that era, the distance between the center of its rotational resistance and the instanteous roll axis was similar between a fin keel boat and a full keel boat and so the greater area of a full keel meant that there was significantly more dampening generated which is what Marchaj concludes.

But in the years since, several things have changed. Modern fin keel boat have greater draft, and differently shaped keels so that a greater portion of their area is deeper in the water, and their hull forms are such that their roll centers are slightly higher. That combination means that there can easily be a several time greater lever arm between the center of rotational resistance and the instanteous roll axis. So if we think that a modern keel has perhaps 20% of the area of a full keel but 2 or 3 times greater lever arm taken to the third power (in other words something like 8 to 27 times more leverage) it is easy to see that a modern fin keel boat could easily develop much higher dampening moments and so have better dampening than a full keel boat, making Pvajko statement incorrect that "The most important reason is that a full keel with its bigger surface area damps the rolling motion better."


In terms of Pvajko statement: "While a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story."

I might agree with you that this is in part a true statement. All keels generate more lift in flat water than they do in disturbed conditions, but since fin keels tend to stall out much more quickly than longer chord keels, they lose a larger percentage of their lift, in other words, "stormy weather with big seas is a whole different story" for all keels but especially for fin keels.

But here is where that statement is misleading, in the years since 'Seaworthiness' the better modern fin keel shapes and corss sections have been developed to perform across broader range of conditions while losing a smaller percentage of their performance advantage. The impact of better dampening, the endplate effect of the bulb, foil shapes which more quickly establish flow and respond to it, means that fin keel boats may lose some small amount of their advantage over full keels in heavy going, depending on the course relative to the waves(i.e.beating upwind), but the modern fin keels still retain a significant performance and motion comfort advantage over a traditional full keel of similar length and displacement.

This last sentence is where it gets tough to make an ‘apples to apples’ comparison. In a broad general sense, full keeled boats tend to be heavier for their length (I know this is a ‘duh statement) and have different hull forms than most modern fin keel boat. Because of that disparity it is easy to ascribe attributes to a full or fin keel which have nothing to do with the keel type and everything to do with the boat’s design as a system. But even taking that into account, the statement seems to imply that a boat with a full keel will out perform a fin keel boat in heavy conditions, and while that may be true for some fin keels vs. full keels, it is not a universally accurate statement.

Regarding your statement: “It has been well documented in 100 year old yacht design books that full keels have the advantage in big water because the rotation of the water in the wave causes the full keel boat to heel toward the wave rather than heel with the wave as with other keel designs.”

I personally don’t know of any 100 year old yacht design book that says anything like that, but when I go back and look at my earliest copy of Skene’s and Kunhardt, I find no reference of the sort so it might be helpful if you could provide a source for that. But even so, the idea that full keels heel toward a wave while fin keels rolls away flies in the face of what is known about the motion of boats in big waves.

What the science would suggest is that there are a number of factors which determine whether a boat heels into a big wave or away from the wave. First of all there is the rotational force. If you dissect the surface of a large wave, the water at the surface is moving faster than the water deeper in the wave nearer to the wave center. This progressive difference in speed between the surface and the center of the wave, means that the deeper the keel, the greater the sheer in the water speed acting on the boat trying to rotate the boat so that it heels away from the surface of the wave. Similarly, a keel with a greater side area will experience greater rotational force and so will have a greater tendency to heel away from the surface of the wave. But also, fin keels stall at very steep angles of attack, as might be experienced beam to on the side of big wave, thereby reducing the side force per unit area that the deeper keel may experience. This combination of factors means that in any specific case, either a fin keel or a full keel could experience the greater rotational force.

Resisting the roll force are stability and the roll moment of inertia. In the case of the fin keel vs. full keel discussion, modern fin keels, with their deeper drafts and densely concentrated ballast bulbs, generally generate much higher proportional stability than full keels. That was not the case at the time when ‘Seaworthiness’ was written but since modern designers have paid attention to the lessons of seaworthiness, and modern racing rules do not penalize stability as much as they did back then, it is true on the better modern fin keeled designs of today.

This greater stability means that a modern design would generate proportionately greater force to keep them upright and therefore greater force trying to heel the deck back toward the wave face.

The other factor, roll moment of inertia is similar to the discussion on dampening. The two factors impacting the amount of roll moment of inertia is weight and the distance between that weight and the instantaneous roll axis. While modern fin keeled boats tend to be lighter, they also tend to be deeper and taller so that due to their weigh distributions, they develop a disproportionately large roll moments of inertia.

In big waves, a large roll moment of inertia does two things, at the top of the wave, it delays the rotation of the boat relative to the rotational force. A good thing, but at the bottom of the wave, its greater stored kinetic energy, tends to cause it to get out of phase with angle of the wave face and continue to roll as the bottom of the wave flattens out so that there is a greater danger of dipping a spar in the water (never a good thing).

But to look at your statement fairly, we might also look at factors that have nothing to do with keel type. Modern designs tend to have greater form stability. Greater form stability tries to keep the waterline of the boat parallel to the wave face. At the top and middle of the wave, that would tend to roll the deck of the boat away from the face of the wave, the behavior that you describe in your quote. But that has nothing to do with the keel type. Two boats of equal form stability, similar draft and ballast stability, and roll moment of inertia would have the same angle of heel whether the boat had a full or fin keel.

And lastly, at the bottom of the wave, the boat with greater form stability would generate more righting force, remaining in sync with the wave surface and so would be less likely to dip a deck or spar and keep rolling.

What all of this suggests is that the specifics of the boat design and the conditions will determine whether it heels relative to the wave surface, but that the use of a fin keel or full keel is but one minor factor.

Quote:
Originally Posted by BreakAwayFL
I was under the impression that a full keel also puts more weight lower in the water, which would make it more forgiving if you happen to have too much sail out...
Strictly speaking that is not always or even usually correct as it is written. While it is easier to keep the weight lower in a longer keel of an equal draft. But modern fin keels generally are deeper and have a bulb which makes it easier for them to carry their ballast with its vertical center lower than most full keels. But also there are a lot of factors that make a boat ‘forgiving’. A modern fin keel boats relatively greater stability, lighter helm loads, more forgiving rig and sail handling gear, and more easily driven hull form might work in its favor ‘forgivingness’ wise. The typically better directional stability and lower vertical center of effort work in the favor of a typical full keel boats ‘forgivingness’.

Respectfully,
Jeff
pvajko and outbound like this.
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies

Last edited by Jeff_H; 03-21-2012 at 01:20 PM.
Reply With Quote Share with Facebook
  #118  
Old 03-21-2012
Member
 
Join Date: Jun 2011
Location: San Diego
Posts: 90
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 4
Rozz is on a distinguished road
Re: Full or fin keel?

i love my full keel, i have raced many of my friends and true, i am slower... but i always win cause i dont have the hour and half around the kelp beds lol
also on that note, i like to free/scuba dive and having the option to go to there is nice too. have we mentioned the dumb crab pots? hehe
ok beat the not getting hung up on stuff... i am willing to sacrifice the little speed for ease of mind and comfort, she rides well in most weather. im happy... thats what matters right?
__________________
1967 Bristol 29 #35
Reply With Quote Share with Facebook
  #119  
Old 03-21-2012
Member
 
Join Date: Jun 2011
Location: San Diego
Posts: 90
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 4
Rozz is on a distinguished road
Re: Full or fin keel?

oh and forgot to mention, i havnt done to much with her worth talking about, but the p/o went all up and down the coast from Alaska to cabo and HI many times... so figure ill be ok
__________________
1967 Bristol 29 #35
Reply With Quote Share with Facebook
  #120  
Old 03-22-2012
Senior Member
 
Join Date: Sep 2011
Location: Michigan
Posts: 180
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 3
BryceGTX is on a distinguished road
Re: Full or fin keel?

Quote:
Originally Posted by PCP View Post
safely and fast
Any reasonable boat can cross the ocean. The difference between an IP and say a Jenneau is how you deal with a strong storm and rough water. That is what the discussion is about.

With an IP, you can simply heave to and you will be comfortable till the storm is over with. With a lightweight boat like a Jenneau, such a tactic could be suicidal. Such a boat must continue to move or be on a series drouge.

Racing boats are much the same as the Jenneau. Such boats have only hull moment. So they continue to move. However, race boats typically have a boat load of highly motivated sailors who better deal with fatigue. Contrast that with a couple on the cruising boat.

Now you can design any hull you wish, but it does not change the fundamental difference between these two types of boats. And it has nothing to do with what has happened in the last 30 years or the last 2000 years.
Bryce
Reply With Quote Share with Facebook
Reply

Tags
boat buying , newbee advice


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

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Full Keel Vs. Fin or Modified Full Keel AjariBonten Sailboat Design and Construction 51 11-16-2013 02:52 AM
Full Keel vs. Wing Keel small boats Kyhillbilly General Discussion (sailing related) 4 07-26-2011 11:51 AM
4 ft draft, full keel, 30' and over? birdlives Boat Review and Purchase Forum 6 09-17-2010 11:00 PM
Older Full Keel boats JerryO39 General Discussion (sailing related) 30 06-09-2010 09:06 AM
Bristol 32'' full keel johnblsc Boat Review and Purchase Forum 2 12-13-2002 05:37 AM


All times are GMT -4. The time now is 06:56 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012