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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction > Full Keel Vs. Fin or Modified Full Keel
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Thread: Full Keel Vs. Fin or Modified Full Keel Reply to Thread
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Topic Review (Newest First)
11-16-2013 02:52 AM
vtoz7053
Re: Full Keel Vs. Fin or Modified Full Keel

wow, um im an old sailor, too much detail, thank you, but i am confused. um i have a ericson26, with a short mast, keel i think is a fin and im going to sail her in the pacific. mostly with the wind (downwind), im a liveaboard. I have sailed a homemade square jig from maine to FL and back on a 23' wodden sail. not too big on education. ill figure it out as i go. or die, so im tring to figure out how bad i am going to rock n roll out there. i know she wanders a lot sitting at anchor in the bay, at ruff seas, and the wakes from other lovely boaters are mild. i only travel downwind (i like it slow) .
so what am i looking at .. strap everything down n hold on, or kiss my but goodbye or my coffee cup might fall over???
11-15-2013 07:30 AM
Jeff_H
Re: Full Keel Vs. Fin or Modified Full Keel

I am not sure whether you are talking about rolling in a short chop taken abeam or larger breaking waves. In either case, it depends on the specific design of the boat, its loading, and the shape of the wave in question. In a chop abeam, there will be situations were each type would be superior but a long keel with cut away forefoot would generally be worse in all cases. Iin a large, breaking wave taken abeam, except a long keel with cut away forefoot, the reality is that each keel type has benefits and liabilities.

The case is often made that a full keel is safer in a roll situation since it is typically shallower than most other keel types and so experiences less wave surface sheer forces (the force that results from the difference in speed between the water on the surface of the wave and in the center of the wave). But full keels generally have a lot of side area and does not stall so that area tends to generate a lot more rolling force even if the wave sheer speed difference is smaller. Typically full keel boats have smaller form stability so they have less force trying to initially roll them into conformance with the wave face (good) but that same lack of form stability means that there is less force trying to right them in the trough (bad). Traditional full keel boats tend to have large roll moments of intertia which resists rolling the boat at the start of the incident (good), but which may tend to cause them to continue rolling at the trough (bad) and since the roll moment of interia often comes from heavy spars, that tendency gets amplified (bad).

Probably the best choice in a large breaking wave taken abeam would be a keel-centerboarder with the board up. It would have a shallow keel and so would experience the least wave surface sheer, and has a small side area so the combined side forces would be smaller as well.

The deeper fin typically has a lower vertical center of gravity and a smaller area working in its favor. But being deeper the fin keel is exposed to a greater speed difference between the wave face and bottom of the keel due to sheering of the wave face. Mitigating against that higher speed is the tendency of deep fins to stall at large angles of attack which by definition a wave abeam would be. Then there is role of roll moment of inertia. Fin keels especially with a bulb when combined with the typically taller rig on a modern design produces proportionately high roll moments of inertia which would initially minimize the induced roll angle.

Probably the worst option is a long keel with cut away forefoot and an attached rudder is the worst option being deeper than a full keel but unable to stall as easy as a deep fin, and with a higher vertical center of gravity. All bad...
11-15-2013 02:16 AM
vtoz7053
Re: Full Keel Vs. Fin or Modified Full Keel

can any one tell me what is better for rolling in ruff seas? full or fin? and why?
04-08-2012 11:58 AM
souljour2000
Re: Full Keel Vs. Fin or Modified Full Keel

Thanks Wandering ...I think she's real pretty...that's important to me with boats...but they also got to be sea-kindly and safe as you also said...I think she is neither at the moment to be honest but I see alot of potential...gonna take alot of work but it should be worth it...I feel lucky to have her...she's gonna be a looker when I am done with her hopefully...and also strong and seaworthy as I can make her with my modest income...most of it is just plain elbow grease and a few bucks here and there...rustoleum oil enamel laid on with a good quality brush after a coat with a roller is good enough for me....painting hull is gonna be harder but same as above...the bottom job is gonna set me back though with a boat this size...I need to start savin now... her mast and standing rigging is good...or at least what I've seen near the decks..
04-08-2012 08:18 AM
WanderingStar
Re: Full Keel Vs. Fin or Modified Full Keel

And she'll be safe and comfortable. And people will remark on her beauty.
04-08-2012 12:23 AM
souljour2000
Re: Full Keel Vs. Fin or Modified Full Keel

I am looking forward to seeing what my (new) old CCA battle-wagon can do with her sails and some wind....'66 Morgan-designed Columbia 40.....low freeboard, more than adequate cabin room/storage, narrow sea-slicing 10.5 foot beam, slow-dispensing reserve bouyancy at the ends with her overhangs ...3/4 full keel..she's 18,200 lbs gross disp. with 8,400 lbs lead...and a 9-foot centerboard that raises to give 4 and a half draft....diesel is slung low under salon sole...under tow the other day in 15 mph winds in mid-river she rode like the Rock of Gibraltar...Lot's of work to do on her mind you....she's old and there's lots to replace/upgrade...decks need re-enforcemnt and corework needs to be finished...hull-to-deck joint needs further inspection but no sign of leaks yet...should be the same joint as on my '66 Columbia 29...same factory..
Overall...I ain't got time on no Open 40's or open 60's certainly...but we'll take chances with what we gots...and with a still respectable theoretical speed 7.17 knots ...so I am a knot slower than other 40-footers of alot newer design...but If I never go anywhere I aint gonna spill my drink at anchor until it starts blowin into the 40 knot range she's so solid...and when I do go somewhere...crew fatigue is going to be less...especially if that crew is just me...or one another person.
03-27-2012 08:46 AM
keforion
Re: Full Keel Vs. Fin or Modified Full Keel

One point that I have not seen in this discussion regarding keel hung rudders is that when close-hauled and with just the right touch of weather helm, the keel-rudder system is now an asymmetric hydro-foil giving lift to windward even if the hull is going straight ahead through the water. The modern fin-keeler with separate spade or skeg-hung rudder does not have this advantage and in order to generate lift from the fin-keel the entire boat, fin-keel and hull, has to go a bit sideways through the water.

As to performance, I sailed an IOD, with what might be the classic example of what Jeff H calls the "worst of all possible worlds," and it went to weather like a witch. I plotted courses over the ground in many passages of an upwind course and the average angle between legs was 78 degrees. Note that this is not "tacking angle" or "pointing angle" but the angle between the actual courses on successive Port and Starboard tacks made by the boat over the ground. Generally no tidal current and averaged over many different days and conditions.

Does anyone have actual data on similar performance with one of the modern designs?

Incidentally, the IOD with a 25' lwl would also overhaul Cal 40's downwind as well.
08-12-2009 02:24 PM
Yteam
Point taken

Hi again Jeff,

First the british tax system ... this can be summed up in a few words. Squeeze them like lemons. I could probably use all my tax demands as sufficient ballast if I had kept them all!

Second, I am not committed to my crazy idea as yet, just wanted to kick it around and see if anyone else had done anything like it.

Third - dismasted several times? You are not really Sinbad the Sailor in disguise are you? Only joking my friend.

Fourth and last. Great idea. Why dont I launch her as she is without the mast and rigging and drive around for a bit and see what she handles like. Why couldn't I think of that.

Cheers buddy
Y-team
08-12-2009 01:59 PM
Jeff_H I have been on several dismasted sailboats over the years. The most graphic memory was when my folkboat lost its rig. We could not get around even crawling on the deck. I ultimately had to lie on my belly and move about holding on to stay aboard. A fully rigged folkboat has about as sweet a motion as any 25 footer that I have ever encountered.

I have also heard of bigger boats converted to trawler yachts but again the owners have done pretty extensive mods (bilge keels) to make the manageable. Properly closing up the cut offs or bolt holes, fabricating a shortened rudder etc will involve quite a bit of epoxy and buying a used powerboat has to be extremely less work than making the mod's even if you had to depend of family volunteers to scout out a worthwhile candidate.

I realize that an older boat with blown out sails have very little value, but the same boat modified as you suggest has negative value since someone will need to chainsaw it and take it to the dump when you are done with her. I don't know British tax laws but in the States you can also donate a boat for tax credits and that may permit you to buy a small power boat.

If you absolutely have your heart set on doing this. I would suggest that you have one of your volunteer family helpers launch the boat without the rig and see how bad that is.

Jeff
08-12-2009 01:37 PM
Yteam
Never claimed to have any sense

Hi Jeff and many thanks for your reply which I appreciate.

I know this doesnt seem sensible at first glance and yes it is a British Hunter, one of the originals which took part in the single handed transatlantic yacht race and later developed into the Europa. Designed I think by Oliver Lee.

Basically the costs for cutting the keel down to a skeg, and modifying the trailer would be minimal as I would do this with family help. I had assumed that I would have to add internal bottom ballast to dampen the movement. The advantages of then having a vessel which I have spent time and money on over the years and I could still use as a powered weekender without mooring costs appeals to me.

The sails are the weak point in any sales pitch I make, I have kinda sailed them to destruction and I have also advertised her but have had no luck selling her on.

In a nutshell the costs to me for doing this modification would be nil and I would be using her on inland waterways and lochs once the draught had been reduced. (No more big waves to contend with)
Also I would not have the expense of buying another boat at my time of life.

Do you think the movement would be so bad? I had heard about a number of yachts being used successfully as powered boats after being demasted by some of your hurricanes.
Thanks again
Y-team
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