Fin Keel vs. Full Keel

[keelbolts]

Thank you Mr. Perry. I am clear on this now. What a great site this is, where I can learn about how a keel works from a designer of your stature. Thanks again.

[bob perry]

Keelbolts:
You would be surprised to find out how many times that question comes up when I talk to groups.

[hellosailor]

Bob Perry-
I would just like to thank you for the incredible balance of the I28. I've had the pleasure to spend a lot of time on one and never before met a boat (larger or smaller) that would simply hold course all by itself after being properly trimmed. We joke about "the figurehead on the helm" because the helmsman often never has to do anything. Even with some of the odd changes Islander made from your plans (like that offset mast support and head reconfiguration) the boat is incredibly sweet.
Not to hijack the thread--but are there any 36-40' sloops that you know of that have this same incredible balance?? Yours, or anyone elses?

Rayallan-
"Seems my bids are always out bid by $10.00 to $100.00 dollars even when I bit within the last minute. I think some people have a program to automatically bit or something. " Those are called sniping programs, and yes, they exist. But they cannot outbid you UNLESS you have set a lower maximum bid than they have. If you are willing to pay ten dollars more--then set your bid ten dollars higher. eBay only increments your bid up if there is someone bidding higher than you. If you bid $400 and someone else is willing to pay $450, they're going to win, with or without any special help.

[Hiracer]

Originally Posted by speedoo

if they truly were better to windward, why do all racing-designed boats sport deep fin keels with deep spade rudders? I think it is very well established that if you want a boat that points high, give it a deep fin keel with an appropriate shape, maybe a bulb at the bottom for additional stiffness, a spade rudder and a tall rig.

Bob will correct me if I'm wrong, but most of the lift is near the front edge of the keel. Deeper fin keels have longer front edges, and therefore more lift. Same as taller sails. Longer luff, better windward ability.

Plus, fin keels offer the ability to have more stability by virtue of deeper ballast, so more sail area can be carried. Two benefits from one design attribute. This is why Bob repeatedly says there is no real good substitute for draft.

[Dockhead]

Originally Posted by keelbolts

A full keel boat doesn't necessarily have more wetted surface. My boat, Favona, has a full keel with a 6' draft on a 24' waterline. A friend of mine had a C&C 32 with a swing keel and we used the same amount of bottom paint. Not a scientific study, granted, but useful to know. It's easy to see that a full keel boat would have more wetted surface than a fin keel if the only difference is the keel, but it's not so clear when comparing a narrow wine glass, full keel and a beamy fin keeler.

On the subject of windward ability, I can't imagine conditions in which a fin keel could out perform a full keel to windward. Running down wind - sure, but to windward - never. Going to windward is all about using resistance to lateral motion to create forward motion. It's, in a great part, about square footage of lateral surface. I have sailed fin keelers that pointed pretty high. The problem was they wouldn't go there. A fin keel, with its separate rudder isn't going to create lift. a keel hung rudder with a little weather helm does create a foil shape that provides some lift. I don't believe you get a lot of lift out of either one.

In the 20 some years I've owned Favona, I have never been beat to windward.

I think that for the same beam and LWL, a full keel must have more wetted surface -- much more -- than a fin keeler. It's a full keel because there's more keel down there; the fin keeler is cut away. Obviously one of those fat-butted French fin keel boats will gain back a lot of wetted area by virtue of hull shape. And old-fashioned full keelers often have short waterlines relative to LOA, which will reduce wetted area. But at the expense of hull speed, which is why modern boats tend to have less overhangs.

As to windward performance, I can't prove it scientifically. I've always heard that full keelers are worse to windward, besides being slower overall. And our own boat, with its modified full keel, is a real pig to windward compared to the fin keelers I have chartered.

[daedaluscan]

I think ebay will often encourage irrational behavior in the last few minutes of bidding. Your making a serious investment here so decide the MAXIMUM you want to pay and bid that. If you win-great; if not there will be another boat along and this one wasn't for you.

Personally I think at this budget you can find some great boats, but a lot of them aren't listed for sale. They are abandoned in a boatyard with a small fade for sale sign.

If you want to snipe, I have used Gixen with great success. It is free.

GIXEN - Free eBay Auction Sniper

Good luck.

[Cheechako]

Boat alert- Cape Dory 25, $1575 and 7 hours left. 2001 outboard motor, looks decent... 1977 Cape Dory 25' Used Sailboat - NJ:eBay Motors (item 110386812278 end time May-11-09 16:45:00 PDT)

[bob perry]

In short: Hi is right. it's all about leading edge whether we are talking sails or keels. It's about asect ratio. We can argue this and go right ahead, you will not convince me otherwise.

[Amgine]

Originally Posted by Dockhead

I think that for the same beam and LWL, a full keel must have more wetted surface -- much more -- than a fin keeler. It's a full keel because there's more keel down there; the fin keeler is cut away. Obviously one of those fat-butted French fin keel boats will gain back a lot of wetted area by virtue of hull shape. And old-fashioned full keelers often have short waterlines relative to LOA, which will reduce wetted area. But at the expense of hull speed, which is why modern boats tend to have less overhangs.

As to windward performance, I can't prove it scientifically. I've always heard that full keelers are worse to windward, besides being slower overall. And our own boat, with its modified full keel, is a real pig to windward compared to the fin keelers I have chartered.

Well, not must... I mean, a fin keeler can have an infinite amount of area in its keel if it's long enough. But generally you're right. On the other hand, the flatter the hull's shape - like many of those fat-butted french hulls - you also reduce wetted surface area. So the combination is often used. Who needs bilges? That's why some such designs are called sleds.

Which brings up why some full-keel hulls seem to outperform fin keels to windward. When there is much of a chop the full keeler with moderate or even slack bilges may not need to reef as early as the flatter lighter boat, and thus maintain a somewhat higher average speed in those conditions. The lighter, flatter, harder-bilged boat will have better accelleration, but its shape may also slam more and without the mass will decellerate more in such conditions. Similarly, heavy boats sometimes report better performance in fitful ghosting conditions as their mass lets them coast longer between puffs.

Waterline length isn't always a good predictor of speed, either, even within one or the other keel type. There's a video online of a Cape Dory 25D overtaking an Island Packet 29 upwind in 15-20 knots, and the CD has a waterline length of about 18' - 7 less than the IP.

But in average conditions, always bet on the fin keeler to get to windward first, all other things being equal.

[FloridaWriter]

Originally Posted by Rayallyn

I have been looking for a boat to cruise the east coast, islands and gulf. I have cruised years back and every boat I owned was a full keeled boat plus an excellent sea boat. My question is how does a fin keel compare to a full keel? I believe the fin keel boat would take more of a pounding.
I can't afford a large boat so I have been looking for a 24 to 27 footer. I really like the old Bristols and Columbias. The Bristol a bit more. I am single and will sail the vessel single handed. ( I don't do well in crowds anyway) (PTSD) but with the economy the way it is and knowing that after this summer my freelance photojournalism and news photography work will most likely dry up, I will need to purchase this summer or most likely move under a bridge for my next home.
I can find plenty of deals on fin boats but not so much on full keel boats. Hell, if I could hit the lottery with enough money I buy a 150ft. Barquentine and hire a crew but that's not going to happen. Well back to question what advantage/disadvantage is there in fin keel

You would not go wrong with an old Bristol. The Bristol 27 is an Alberg design, with a cutaway full keel and keel-attached rudder. The Bristol 29 is a Halsey Herreshoff deisgn also with a cutaway full keel and attached rudder. Both are excellent sea boats, and both (especially the B29) are very fast, and point very high. I own a B29 and it tracks in the groove as you would expect a full keeled boat to do, but turns on a dime. The B29 is very easy to singlehand; I have singlehanded my boat for the past 20+ years.

Good luck with your search.

[Cheechako]

Then there is the infamous 26' Thunderbird with a flat piece of steel plate for a keel shape. Won a lot of races for a long time in their class..... Which begs the question: on a 100 mile voyage how much less time would a boat with a NACA foil shape make the trip than one without? Curious, is it significant?

[hellosailor]

"is it significant?"

Define significant. If you are cruising with all the time in the world, it won't matter.
If you are racing, boats that have been out for 48 hours often finish within 30 seconds of the trophy positions...that's what, about one six-thousandth of one percent? Which otherwise might be called "not even worth a rounding error".

If full keels were in any way faster than fin keels, they'd have been used in the America's Cup, the Volvo race, and other no-cost-limit venues. Unless all the high-priced designers didn't do their homework.<G>

[svHyLyte]

Cheechako--With all due regard, the keel of the T-Bird was anything but a "flat plate". It was/still is a foil shaped iron casting similar in some respects to a schleel keel in that it is somewhat thinner at the root than at the tip. The yacht was designed for relatively light airs of Seattle but has been known to endure some terrific bashing in big winds and seas and can keep going when other yachts have struck their canvas. For areas like San Francisco, one can add a 500# lead "shoe" to the bottom of the keel to make the boat somewhat stiffer but many have been raced for a long time in heavy airs without shoes. Bob Perry once famously described the yacht as a boat without any bad habits.

FWIW...

s/v HyLyte

[keelbolts]

Remember that those loooong overhangs were there primarily to cheat the rules. Some of the later J-boats must have had LWLs equal to about half of their LODs. Ratings were based, among other things, on waterline length. Those boats were designed to sail on their ears & when you put the toe rail in the water, you LWL doubled allowing for a substantially higher theoretical hull speed.

If you're going racing, perhaps a fin keel boat is the way to go. If you're cruising and safety, simplicity, and comfort are more important than downwind speed, then you want a modified full keel boat with a keel hung rudder and a tiller. I can tell you from experience, when you're out there and the barometer is dropping like a rock, you'll be glad you've got something under you. And when it hits the fan your body will thank you with fewer bruises.

[bob perry]

Hylite is correct.
The T bird keel is not a flat plate. It may not be the ultimate in keel foils but it is far from a flat plate. Ben Seaborn was one of the very first designers to use a chord and percentage of chord breakdown to describe his keel geometry. I think he must have had some friends at Boeing.