Keel
information
A fin under a sailboat's hull providing weight for stability and lateral
resistance to leeway. It consists largely of ballast. Unlike centerboards,
most keels are fixed in place and not retractable (the exception is
the lifting keel). A keel boat is any boat with a keel. A keel-centerboarder
is a keel boat that also has a centerboard.
Article written by Ted Brewer
The purpose of a keel, fin, or centerboard is to provide resistance
to making leeway; in effect, to keep the yacht from sliding sideways
through the water due to wind pressure on the sails. Various shapes
of underwater plane have been in and out of style over the past 150
years.
The basic full-keel shape had the longest run, as
it was the standard for bluewater sailing craft from pre-Roman times
to the earliest days of yachting. The deep, full keel was supplemented
in the mid-1800s, for the shoalwater areas of Britain and North America,
by centerboard craft. These cover such working types as the sharpies,
Cape Cod catboats, and Chesapeake Bay oyster skiffs, to mention a
few.
The first truly modern keel yacht, with a cutaway
forefoot and highly raked rudder post, was designed by Capt. Nathanael
Herreshoff with his Gloriana design of 1891. But it did not catch
on for bluewater sailing. Until the late 1920s, the typical offshore
yacht, whether cruiser or ocean racer, resembled a sailing fishing
craft in the shape of its lateral plane: a long, full keel with deep
forefoot and fairly vertical sternpost.
Such a shape has the benefits of good directional
stability, ease of steering, and the ability to heave to in heavy
weather, all desirable traits for a boat. However, its faults may
include slowness in stays, excess wetted surface Ñ making it
slower in all types of air Ñ and an inefficient lateral plane
shape that has excess leeway, considering its relatively large area.
Typical small yachts of this type are seen today in the Colin Archer
types and the Tahiti ketch and its copies, while replicas of traditional
sailing craft such as Bristol Channel Cutters, Friendship sloops,
fishing and pilot schooners, and similar lovely vessels still appear
in our waters. Fortunately, many of these workboat types have been
developed to the point where the ills of the true full keel have been
greatly reduced. Then the result is a handsome cruiser that sails
quite well and attracts a great deal of attention wherever she drops
her hook.
The cutaway keel was revived for ocean racing by Olin
Stephens in the late 1920s, with his lovely yawl, Dorade, still sailing
and winning classic yacht races more than 70 years after her launching.
Her offshore racing successes finally proved that the full keel was
not essential to seaworthiness, and it definitely detracted from speed
and weatherliness. As a result of its improved performance and handiness,
the "modified full keel" form caught on quickly once Dorade
showed the way and became the standard for the next 35 years. This
type of lateral plane is still sailing in many popular older designs
such as the Albergs, the Folkboat, the Luders 33, the Whitby 42, and
even some newer yachts.
The modified full-keel form features generally good
handling and directional stability plus reduced wetted surface, compared
to her true full-keel sister. The yachts can perform well in all conditions
and, as they are generally of heavier displacement than contemporary
ballasted-fin boats, they do not give away much in light air, despite
the added wetted area. A yacht with a modified full keel can sail
right up with the best of them if she is given sail area commensurate
with her typically heavier displacement.
In my own work, I developed a modified full keel,
with the rudder set aft and vertically in the contemporary fashion,
in order to improve directional stability and handiness. Then, to
reduce wetted area, the lateral plane is substantially cut away ahead
of the rudder in what some have termed "the Brewer bite."
The Cabot 36 and Quickstep 24 of my design were early examples of
this form. The size of the cutout depends to a large degree on how
insistent my client is on having a "full keel," and I try
to make the cutout as large as I can decently get away with. I don't
claim to have originated the shape, though, as the late L. Francis
Herreshoff used a not dissimilar profile many years earlier in the
design of the lovely 57-foot ketch, Bounty.
Like all good things, the modified full keel was cut
away more and more for bluewater and inshore racers in an attempt
to reduce wetted area until, finally, some designers took it to extremes.
This reduced directional stability and produced craft that were almost
impossible to steer in breezy conditions, broaching with monotonous
regularity. I can recall working on the design of many short-keel
5.5-Meter yachts in the 1960s, and we always said they were three-man
boats with six-man spinnakers! It's hard to believe none of them were
knocked down and sunk, as they were extremely difficult to control
on a reach or run, and the hulls were pure leadmines, with 3,500 pounds
of ballast in their very short keel and only 1,000 pounds of wood
and rig above it!
Olin Stephen's genius began another fad in the mid
1950s, the keel-centerboard design. After Finisterre showed the way,
keel-centerboard yawls were built in sizes from 24-foot midget ocean
racers, to the largest offshore yachts, in order to take advantage
of favorable ratings under the CCA rule and emulate Finisterre's record
of wins. The keel-centerboard hull has gone out of fashion now, but
the type still has merit where a stable, beamy, shoal-draft yacht
is desired with little sacrifice of weatherliness or seaworthiness.
Indeed, the Bill Tripp-designed Block Island 40 and Bermuda 40 are
keel-centerboard ocean racers from the old school and have been in
production for more than 30 years now. These classic yachts have made
many long ocean voyages, including several world circumnavigations
and are first-class bluewater cruisers in every respect.
The fin shape is not new either, as ballasted fin
yachts were pioneered by Herreshoff at the turn of the century for
inshore racing. Then, due to excesses and bad design, the shape died
out, except for a few one-design classes, until Bill Lapworth dropped
a bomb on the ocean-racing scene in the mid-1960s with his Cal 40
design. The Cal 40s made believers out of many yachtsmen who could
not believe that a ballasted-fin/spade-rudder yacht was a serious
bluewater ocean racer. After wins in the Trans-Pac, many East Coast
races, and the 1966 Bermuda Race, it became evident that the fin was
here to stay for ocean-going and coastal cruising yachts. Please note
that I do not use the term "fin keel" anymore, as I feel
it is a misnomer. The keel is the structural backbone of the vessel,
and the fin hangs from it. Fish have both backbones and fins; so do
yachts.
A well-designed fin, in conjunction with a skeg-hung
rudder, can provide excellent directional stability, handiness, reduced
wetted area and improved weatherliness. The fin/spade rudder combination
reduces wetted surface even more. It may have a little (or a lot)
more sensitive helm than a fin/skeg rudder yacht, but it has one big
advantage over it and all other forms of lateral plane: it can be
steered in reverse under power. This can make life a great deal easier
in today's crowded marinas, as many have discovered.
These are some of the reasons that we see fins on
the great majority of our new yachts today; they are not simply a
fad. There are good fins and bad fins, of course, and it is not always
easy to tell them apart. The shape of fins over the years has been
limited only by the designer's imagination. Fins have been set at
every angle from the vertical to highly raked aft. They have been
deep and narrow, shoal and long, resembling a shark's fin or whale's
tail, or boxy fins similar to the original Cal 40 design.
A very deep, narrow fin can be a problem to haul on
a marine railway, so the cruising skipper should consider haulout
ease when boat shopping. A crane or travel lift is the best method
for hauling yachts with extreme fins, but may not always be available
in out-of-the-way areas. There is also the danger of damage to the
shaft or strut if slings are improperly positioned. Still, the major
problem of the high-aspect-ratio fin is structural strength, as it
can impose extreme loads at the point of attachment to the keel. Indeed,
some years ago I was an "expert witness" in a court case
concerning three men who drowned when their yacht sank as a result
of its fin tearing off when the vessel ran aground.
The cruising skipper would do well to avoid yachts
with extreme fins, both for considerations of haulout ease and structural
strength. Fortunately, the heavier, deeper hull and generally shoaler
draft of the typical cruising yacht mean there is less height available
between the bottom of the hull and the point of maximum draft. So,
a longer, lower-aspect-ratio fin is the only solution. On the other
hand, the racing sailor will want a fin with an aspect ratio as high
as the draft rule will allow. Such a fin is more efficient per square
foot, so the area can be smaller and the wetted surface reduced. In
Aero-Hydrodynamics of Sailing, C.A. Marchaj recommends about 4 percent
of the sail area as a good guide for fin area, and I feel the cruiser
should err on the high side, as a small increase in resistance is
preferable to increased leeway. On the other hand, I have used as
low as 1.75 percent area with good results on an extreme racer with
a fin of 2.75 aspect ratio.
This "aspect ratio" is the ratio of the
span (depth) squared to the fin area; that is, my extreme fin had
an 11-foot span and 44 square feet of area, so its aspect ratio was
121/44, or 2.75. If it had a 4-foot span with 44 square feet of area,
not uncommon proportions for a cruising yacht, its aspect ratio would
be 16/44, or a low 0.3636.
The aspect ratio can also be described as the span
divided by the mean chord, the average fore-and-aft length of the
fin, and this gives the same result.
A large part of the resistance of a keel is created
by the vortices, similar to miniature whirlpools that form when the
water flows across the bottom of the keel from the high-pressure (leeward)
side to the low-pressure (windward) side. It requires energy to form
those vortices and that energy is then not available to propel the
boat forward. Obviously, the shorter the keel or fin tip, the smaller
and weaker those vortices will be, and that translates to reduced
resistance. This is one reason that racing yachts usually feature
high-aspect-ratio fins with short tip chords.
However, the formation of vortices can be greatly
reduced by using end plates, or wings, to change the flow direction
and eliminate crossflow. My own preference, for a fin of average span,
is for an end plate that is but a few inches wider than the maximum
width of the fin bottom. We tested an actual yacht with such an end
plate on one side only and noted a substantial improvement in performance
when she was heeled so that the end plate was on the leeward side.
Where the draft is shoal and the fin span is on the small side, then
a wider end plate, or even a wing, might prove beneficial. However,
a wide wing can be a structural weakness, particularly if the boat
goes hard aground and has to be towed off, or pounds on the rocks
for any length of time.
In the 1970s, I saw more than one very-high-aspect-ratio
fin with tremendous sweepback angle. This certainly gives an impression
of speed but, as Marchaj pointed out, tank tests have shown that the
sweepback angle can be related to the aspect ratio: the higher the
aspect ratio, the more vertical the fin should be. Indeed, the very-high-aspect-ratio
fin on my BOC racer was set absolutely plumb until a hard grounding
set the tip back a quarter inch or so, the result of taking a yacht
with a 13-foot draft through a channel dredged to 11 feet! Most cruising-yacht
fins are of low aspect ratio, of course, so should have substantial
sweepback, up to 57 degrees, with an aspect ratio of 0.5, according
to Marchaj. Although most designers try, it is unfortunate that obtaining
the perfect sweepback angle is secondary to locating the fin to balance
the sailplan, as well as fitting the ballast at the correct spot for
proper fore and aft trim. The taper ratio (tip chord length/root chord
length) also deserves consideration. Tests on one series of fins showed
that a fin with 0.32 taper ratio was 1 percent more efficient than
an untapered fin and had very slightly less resistance. This is a
small difference but cannot be ignored by the racing skipper. Again,
the reduction in drag may be due to reduced vortices from the shorter
tip chord. Marchaj also states that the taper ratio should be reduced
as the sweepback angle increases. However, the very-low-taper-ratio
fins may not be the best solution for a cruising yacht. The tip chord
should be long enough so the vessel can be hauled on a marine railway
with no major problems. Too, on a moderate-draft cruising yacht, a
short tip chord forces the ballast higher, so stability can suffer.
Another consideration in the fin profile is whether
the tip chord is sloped down aft or parallel to the waterline. The
parallel tip chord makes good sense. It allows the ballast to be lower
for added stability, it eases blocking up the boat when hauling and,
fortunately, tests have shown that it is also superior to the sloped
tip chord in other ways. Having the aft edge of the tip chord deeper
than the leading edge has no practical effect on aspect ratio, and
such a fin develops less lift and more drag than one with a parallel
tip.
The National Advisory Committee for Aeronautics (NACA)
tested a large variety of streamlined shapes for lift and resistance
and the information on these is available in a book, Theory of Wing
Sections, by Abbot and Von Doenhoff. These are the shapes that designers
refer to when they say their new magic fin has an NACA section. Generally,
the shape selected will be similar to NACA 0010-34 or 0010-64 series.
The leading edge will be elliptical, as a blunted nose increases resistance
while a pointed leading edge promotes stalling. The maximum width
will be about 40 to 50 percent aft, and the shape will be streamlined
to a fairly sharp (but not razor-sharp) trailing edge. The thickness
ratio will be 0.8 to 0.12 of the chord length, although this may be
increased to 0.15 to 0.16 at the tip chord. There are advantages to
having an increase in thickness ratio at the tip chord, including
being able to fit the ballast lower. This need not mean that the fin
is bulbed, though. For example, a fin that is 8 feet long at the root
and 5 feet long at the tip may have a 0.10 thickness (0.8 feet) at
the root and 0.15 thickness (0.75 feet) at the tip. The fin is still
slightly thinner at the bottom than at the top, but the thickness
ratio has increased.
It is not uncommon to see fins wider than 10 to 12
percent of their length, as the designer may need to fatten the fin
in order to locate the ballast in the correct spot for proper trim.
Very shoal-draft boats may require fatter keels or fins in order to
get the ballast as low as possible for stability. Still, extra width
does increase resistance so there is a tradeoff; added stability increases
performance while a thicker fin reduces performance. Thirty-five years
ago, when I worked for Bill Luders, we tank-tested dozens of 5.5-Meter
models. These very short-keeled 30-foot sloops had a minimum keel
width of 4 inches under the rule, and whenever we tried a model with
a wider keel in order to get the ballast lower, we found that overall
performance suffered.
We also tested a number of bulb keels on the 5.5 models
but they never proved out in the tank, either, although several different
shapes were tried. Then, in the late 1970s, I tank-tested the model
of the new Morgan 38 at Stevens Institute, first with a fairly fat
NACA fin in order to maintain the desired 5-foot draft, and then with
a patented bulb fin that we let its designer draw up, with no stipulation
on draft. The bulb saved only 2 inches of draft but showed so poorly
against the NACA fin that the 38 was put into production with the
more conventional shape.
The tip shape, viewed from ahead, may be flat, round,
elliptical, or bulbed. Tests show that the flat, squared-off tip develops
a bit more lift to windward and that the round or elliptical tip has
less drag on a run. The differences are slight but, today, I favor
the squared-off tip with an end plate for yachts of average draft.
A vee tip was tried in the 1960s on a few yachts, but never became
popular. Bulbs and wings, often in combination, are fairly common
on contemporary production boats. Usually they are an attempt to produce
a very shoal-draft yacht for use in waters where the bottom is close
to the top and, in those cases, they may make sense.
There is a never-ending variety of fin shapes and,
to be honest, I'm not sure which is best. Generally, I prefer a fin
similar to the old Cal 40, a little shorter perhaps, and fitted with
an end plate. Such a fin provides a desirable combination of good
performance, ease of haulout, and structural strength, all very important
factors for the cruising skipper.
Ted Brewer is one of North America's best-known yacht
designers, having worked on the America's Cup boats, American Eagle
and Weatherly, as well as boats that won the Olympics, the Gold Cup,
and dozens of celebrated ocean races. He also is the man who designed
scores of good old boats . . . the ones still sailing after all these
years.
Excerpts taken from article taken from Good Old Boat
magazine: Volume 3, Number 4, July/August 2000. Written by Ted Brewer
Keel Reviews:
(send your review HERE)
