Fin v. Full Keel

[keelbolts]

I was in another forum where they were discussing the fin keel's obvious advantage to windward over full keels. I thought I'd read somebody say that the sun was coming up in the west. I tossed in my 2 cents worth, but I left them confident that a fin keel is superior to windward and that I am terribly confused. I've been confused on too many occasions, but I've never been passed to windward. One of the forumites is even a well known designer of yachts. Much of my sailing has been on Favona and a friend's Anker-Jensen 10 meter and I'm wondering if they've distorted my view of things.

A fin keel is better to windward? What? I thought everybody knew a full keel is better to windward. What do you think?

[Hirilondë]

I don't think it is such a simple A vs B thing. But if you are simply comparing boat speed to windward I will take a Volvo 70 and beat any full keel boat from any year. I don't know that that makes it a comprehensive test. So much has been learned since the original full keel fore'n'aft rigged boats came about. The Renegade and the Cal 40 were the first 2 production fin keel boats built, and they kicked butt the first couple years after they came out. If I had to pick one as better to windward I would say fin keel, but just because a boat has one doesn't categorically mean it is better.

[Tim]

Generally speaking, fin keels produce more lift; ergo, a fin-keeled boat should point higher; ergo she is "better" to windward in that the closer one can sail to the wind, the more quickly one can achieve windward distance. Speed isn't considered in this generality.

In practice, we all know of myriad exceptions to this generalization, and also know that there is far more to what makes a boat good (or not) on any point of sail than its keel design. There is also more to windward (or any other) ability than mere pointing angle, though all other things being equal the boat that points higher will reach some arbitrary windward location more quickly and efficiently than a boat that points lower.

Take away from these sorts of academic discussions what you will. The more one tries to generalize, the more it ends up being like forcing oversize square pegs into round holes.

[keelbolts]

The lift thing was mentioned and I had to do a little research into nautical terms. I think of lift in terms of aircraft where lift is created by the shape of the airfoil as air moves over the foil from directly ahead. Lift, in nautical terms, is different. An aircraft wing creates lift, a sailboat fin keel is lifted as your hand is lifted when you stick it out your car window. Wouldn't a larger hand be lifted more?

I raced on a J29 that would point high, but not go there. I got us in trouble a couple of times, misjudging our ability to make a mark, until the owner learned I couldn't be trusted to make those decisions. I had to learn that the J29 wouldn't go where Favona would.

[keelbolts]

In fact, I remember reading, in an old book or magazine, where research had shown that a full keel with an attached rudder, with a little weather helm, actually created lift. Note, it wasn't lifted, it created lift. Think of the rudder as flaps on an airplane.

This is all very interesting to me. I swear I thought that a modified full keel's superiority to windward was common knowledge. I thought there was room for comparisons of turning radius, downwind speed, of agility, but not of windward ability.

[Case]

It entirely depends on the sailboat design itself. And to a large degree, depends on the sailing skills of the sailor himself/herself, quality of sails, bottom fairness or foulness.

But for the most part... fin keel sailboats tend to go upwind better than full keels. The foil shape is more efficient. Read up naval architecture more. Ask Bob Perry over at Cruising Anarchy.

Bob Perry often said this: Draft is King. He would always fight for more draft when doing a design for a client. Having sailing shallow and deep draft sailboats, I'll say this is mostly true with some exceptions.

In the end it truly depends on the sailboat design. I have read that some full keelers from the 1930s to 1950s often go to windward better than many fin keeled sailboats nowadays. But if you check the underwater profiles of those full keel sailboats, its a "short" full keel, not a truly full length keel.

- Case

[keelbolts]

Not arguing here. Learning.

If a high aspect fin keel is more efficient? Then why didn't the earlier, slower aircraft have delta wings? Because, at the speeds they were moving, a delta wing wouldn't create enough lift. And remember, I know everybody says this or that keel "creates more lift", but I have yet to see them create lift. They might be lifted, but not create lift. If you had a fin keel designed to create lift, unless you could change the shape of that keel back & forth, it could only create lift on one tack, and work against you on the other.

I know a high aspect ratio sail is better to windward, but remember it does have a shape that acts like an airfoil.

[Ryan]

I know everybody says this or that keel "creates more lift", but I have yet to see them create lift. They might be lifted, but not create lift. If you had a fin keel designed to create lift, unless you could change the shape of that keel back & forth, it could only create lift on one tack, and work against you on the other.

I think one area of misunderstanding in the above quote is it seems that you are saying that a symmetrical foil cannot create lift. It can indeed and the angle of attack of the foil (among numerous other variables) helps determines this. Keep in mind that by the logic in the above quote, airplanes wouldn't be able to fly inverted, but they can.

[Bluenose]

Not arguing here. Learning.

If a high aspect fin keel is more efficient? Then why didn't the earlier, slower aircraft have delta wings? Because, at the speeds they were moving, a delta wing wouldn't create enough lift. And remember, I know everybody says this or that keel "creates more lift", but I have yet to see them create lift. They might be lifted, but not create lift. If you had a fin keel designed to create lift, unless you could change the shape of that keel back & forth, it could only create lift on one tack, and work against you on the other.

I know a high aspect ratio sail is better to windward, but remember it does have a shape that acts like an airfoil.

I'll bite and jump in. First, there is a difference between aspect ratio and delta wings. The aspect ratio (or slenderness ratio) is the wing span square divided by the wing area. A delta wing is a highly sweptback wing design that usually has a low aspect ratio. My understanding of delta or swept wings is that they evolved as a result of higher speed aircraft to reduce local supersonic effects of the airflow over the wing. It wasn't that they don't work at slow speeds, remember all these fast planes need to land, it is just that they were optimum for high speed aircraft (velocity equal to 80%+ of the speed of sound)

All keels create lift. If not all sailboats would be blown to windward as the vector of lift off the sails during windward work is mostly sideways. A deep narrow keel is a more efficient lifting device than our old full keels. Think airplanes again. Compare a small Cessna with a glider. Short stubby wings verses long, slender wings. The glider is all about lift at slow speeds.

With regards to shape, shape is only one of the factors that creates lift. The biggest factor is angle of attack. You could fly an airplane with a symmetric wing. It just wouldn't be optimum for normal flight but it would fly just as well right side up as upside down. Kind of the way a sailboat works tack to tack.

Probably more than anyone wanted to know.

Cheers, Bill

[Tim]

Remember: lift is what makes a sailboat go. Sailboats are literally "sucked" through the water (most notably upwind) by the combination of lift created by (yes, created by) the sails and the underbody's shape. It's a misnomer to think of boats as being pushed by the wind (at least upwind); rather, it's this lift, which is created by the underbody and sails, that sucks them forward--the same exact physical mechanism that causes an airplane to become airborne at a certain speed (and then hopefully stay there). This physical phenomenon is just turned on its side as far as boats go, that's all.

Even a piece of plywood can function as a rudimentary foil section if its angle of attack is varied from dead on; the slight angle creates the high and low pressure areas on opposite sides of the "foil" which are the definition of lift.

Lift is all, really. All sailboats create lift; it's not a secondary thing that occurs--it's what the boat does, physically how a boat works. Otherwise, they don't go anywhere.

Beyond this basic understanding, I am far over my head as far as the technical details go. Bill's explanation just above this post, which is based on his experience and knowledge in the aerospace industry, takes it to the next level if you need more.

But why would you? Time to go sailing. Just enjoy the lift, and enjoy the fact that you have a boat you love and which performs well. I happily left book learning behind over 20 years ago.

[Bluenose]

Time to go sailing.

If you say so.

Cheers, Bill

Laura, Tim says I have to go sailing.... Bye

[Tim]

I do say so. Consider it a standing order.

[Oscar]

You could fly an airplane with a symmetric wing.

Many are. The wings on modern airliners are almost completely symmetrical over a large portion of the wing, in the high speed regime where there is so much airflow that very little lift is needed, and most of it "impact" instead of Bernoulli.

(His law states that a gas that is accelerated decreases in pressure. Longer travel over the top of the wing, acceleration to meet up with the molecules traveling along the bottom of the wing, hence a negative pressure area on top of the wing. Impact lift is what you feel when you stick your hand out the car window and turn it to create an angle of attack)

In the slow speed regime the wing section is changed with leading edge devices (slats) and trailing edge devices (flaps) to create a wing which is very asymmetrical and delivers lots of lift, again at slow speed.

Remember, the price of lift is drag, so you want the minimum necessary to achieve your goal.

The lift of a keel is impact, and created as an equal and opposite reaction to DRIFT. There is some loss of efficiency, hence the result being some drift. Fins are narrow, with a fine entry and have little drag (wetted surface). So, they do their job with a relatively low penalty. The advantage of a long full keel is of course tracking. And, the ability to put a larger percentage of the boat's weight below the center of gravity, which provides for increased stability.

To do this with a fin would require a LONG fin, which creates a longer arm, and thus more torque = righting moment. We see this on racers. Of course we know that gunkholing with a 12 foot keel is a wee bit impractical.

Boat design is compromise.

[LazyGuy]

Here is a completely different take:

Fin keel boats are typically lighter than full keel boats and they can almost turn in their own length. In order to fully appreciate the advantage of a sailboat going to weather, you need to include tacks. Here, I am defining a tack differently than the rules of racing: it is the time from going at a speed in one direction to the time achieving equal speed on the other tack. The lighter boat tacks faster and gets back to speed faster. Therefore can get to the windward mark quicker. I saw a J24 give Neith, 52' Herreshoff a run for her money doing short tacks up the shore of long island sound. Later, the captain of Neith said she was having a blast saying that the J would make amazing gains on each tack. She appreciated the sailing abilities of that little J24.

Also, a fin keel boat with lower weight and significantly lower amount of wetted surface "should" be faster in lighter breezes where hull speed is not achieved. Granted, it is a HUGE "should" because in light air, the trim of the boat is almost as important as the trim of the sails. I race on an old full keel Pearson with a captain that puts the hyphen in anal - retentive, in breezes less than 10 knots, you need to tell him when you are going to move and why. Chances are, he will move someone else on the boat to counter your weight shift. His motto in light air "Slow and steady wins the race."

[Tim]

Note that wetted surface can be deceiving: not all fin-keel boats have less wetted surface than comparable full-keelers. Some do, of course, but broad, wide, flat canoe hulls on modern boats have surprising wetted surface when compared with narrow, slack-bilged CCA-types. And lighter isn't necessarily faster, either.

As always, there's just so much more than any one thing that makes a boat perform well. And even "performance" is a relative thing depending on what is trying to be accomplished.

In the end, the boat with the better sailor in charge will probably come out on top. This has been proven time and again by local stars in small racing fleets sailing what appear to be junky boats, yet still end up with the silver.

[Bluenose]

Note that wetted surface can be deceiving: not all fin-keel boats have less wetted surface than comparable full-keelers. Some do, of course, but broad, wide, flat canoe hulls on modern boats have surprising wetted surface when compared with narrow, slack-bilged CCA-types. And lighter isn't necessarily faster, either.

As always, there's just so much more than any one thing that makes a boat perform well. And even "performance" is a relative thing depending on what is trying to be accomplished.

In the end, the boat with the better sailor in charge will probably come out on top. This has been proven time and again by local stars in small racing fleets sailing what appear to be junky boats, yet still end up with the silver.

Yea, What Tim said.

[Oscar]

Drag aside, the amount of sail vs displacement is a good indicator of performance. On a mono hull, the more ballast, and the longer the arm (deep keel, full or fin) the more righting moment and thus the ability to carry more sail per pound = horsepower, and thus the ability to get to hull speed in less wind.

Lady Kay III weighed 26000 pounds and carried 700 sq feet of sail. Lady Kay IV weighs 2600 pounds and carries 500+ sq feet, 600+ with an off wind sail on the sprit..... The beam is 19 feet !!! which is where all of it's righting moment comes from, and it's a factor many times the righting moment of a mono hull. Then there is little or no wetted surface, and no weighty keel.

So, there is so much horsepower that, like a power boat, it can climb on it's (small) wake and plane...... 18+ knots is feasible.......

Of course my interior volume is a lot less than some 25 footers. The last boat was about living and comfort.....this one is about saving money (trailer) and doing some FUN sailing.

Again, it's all compromise.

[Capn_Tom]

Gentlemen don't sail to windward.

[Figment]

Gentlemen don't need to get back to work Monday morning.

[Capn_Tom]

Lucky bastards!