3.8 What about keels?
Description
This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
3.8 What about keels?
Courtesy of Matt Pedersen:
(Definitions used in this discussion: length refers to the fore and
aft length of the keel, depth refers to how far the keel sticks into
the water, width is side/side width)
General discussion of Keels:
Keels help you sail in a straight line. They are also a great place to
put a bilge, bilge pump, and tankage. What you want is a keel that is
very narrow in width when going to weather, and a little fatter going
downwind. I don't know how to make my keel do this, but when I do
figure it out you'll be the first to know. Narrow width keels also
stall out (lose their lifting ability) at lower speeds when compared
to a fatter keel. This is a negative.
Longer keels are harder to knock off course than shorter keels. Longer
keels are harder to put back on course than shorter keels. Longer
keels have more wetted surface than shorter keels, which hurts light
air performance.
Deeper keels go to windward better than shallow keels. Deeper keels
get the ballast lower in the boat, which helps sail carrying
ability. Deeper keels find the bottom sooner than shallow keels.
About wing keels:
Winged keels have a lot more weight down low which dramatically
increases the stability they provide. The wings supposedly help
hydrodynamics. I don't think it's all that great. They do increase
draft a little going to weather (the wing hangs down lower as you
heel). I'm not real convinced that a wing keel when heeled and
slightly deeper, but with a right angle in it is more efficient at
getting lift than a standard fin. Wing keels are good at catching
kelp, or anything else floating in the water. They also stick in the
mud better, if that's what you want. To be fair they are a way to get
shoal draft and a little stiffness too.
Bulb Keels:
These are basically a keel with a big torpedo shaped blob of lead at the
bottom. They are not more efficient than a straight fin. They do get more
weight down low, which helps in sail carrying ability.
Scheel keels:
Scheel keels are kind of like bulbs at the bottom of the keel, but they look
cooler. They may have some hydrodynamic improvement over a straight
fin, I don't know. They get ballast way down low. It's interesting that
many designers use a Scheel keel instead of a wing keel, even though they
have to pay a royalty on it. That says something about how difficult it is
to design a truly good wing keel. By the way Henry Scheel designs great
looking boats.
Recent history of keel design:
Now if you look at the design of fin keels over the years, you will see a
great deal of theory being applied to get you the fastest shape possible.
Let's see, there was the swept back "Sharks fin" of the early seventies. It
looks fast, therefore it must be fast. They were "proven" to be slow, so
you don't see them much anymore. However, David Pedrick (who
designed Dennis Conner's Stars and Stripes) has resurrected them for the
latest Freedom boats. Gee, maybe they are fast after all.
Then there was the "Peterson" fin. Straight leading and trailing edges.
High aspect ratio. Still pretty fast, but it doesn't put most of its weight
down low, where it does the most good. But then the IOR rule really
didn't care about that.
Then there was the winged keel of the eighties. They are great on big
tubby meter boats with draft limited by some rule, and you want a lot
ofweight down low (like 60+% of the boat is ballast). You can do that by
either increasing the size of the bulb/blob at the bottom of the keel, or
you can spend thousands on tank testing your wings, get the weight down
low with them instead, and psych out your competition at the same time.
Today the latest theory has keels of the semi-elliptical form, where you
have the leading edge straight, and the trailing edge gently curved.
Except for some of Bruce Farr's designs, which have a gently curved
leading edge and straight aft edge. Wait a minute, that doesn't fit the
theory! Farr's boats don't seem to notice that they don't fit the latest
theory though. They just leave everybody else behind them and go to the
winners circle. They are using bulbs today instead of wings on the hottest
racing boats, to get more stability with less total weight...
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