Sailing to Windward With the Gentry Tuft System - ArvelGentry.com

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Sailing to Windward With the Gentry Tuft System
Ranger 23 Newsletter
Back in 1973 I wrote an article for SAIL magazine
describing the leading edge separation bubble and the use
of a special set of yarns near the luff of the genoa as an aid
in sailing to windward. This article is included in the book,
The Best of Sail Trim, page 77. And, whether you race or
cruise, you will find that this system will improve your
sailing.
A recent conversation while at my sailmaker's loft has
prompted me to try reintroducing this idea to the sailing
community. I was describing my system of short telltales
(tufts) for a new genoa to my sailmaker when one of the
best local sailors, Scott Rohrer, came in and overheard part
of the conversation. Scott said that he had recently been
driving a big boat that had a telltale near the luff of the
genoa. He felt that the boat was going to windward great,
fast and pointing high, even though the telltale on the
leeward side was frequently twirling. The owner of the
boat argued that the telltale indicated that the sail was
stalled and that he should be pointing higher.
As I described the leading edge separation bubble idea
and my tuft system, Scott quickly saw that this was the
information he needed to settle the argument with the
boat owner. I had a copy of The Best of Sail Trim with me,
and Scott immediately made a copy of the article so that he
could use it to describe the idea to the boat owner.
The basic ideas behind my tuft system are illustrated in
the diagrams below. The regions where the flow is separated,
and therefore very unsteady, are shown in gray.
When all of the leeward tufts are lying down and the
first or second windward tuft is twirling or pointing
upward, the sail is about to luff.
When all of the tufts are streaming smoothly aft, you
are precisely on the wind and no separation bubble is
present.
If you head off slightly, the first leeward tuft will twirl
as it reacts to a small separation bubble. All the other tufts
will be lying down. The boat has good speed and pointing
ability.
Heading off a bit more will cause two or three of the
leeward tufts to twirl. The boat will have good acceleration
but the sail will be very near the full stall condition.
Heading off some more will cause all of the leeward
tufts to twirl and the sail will be completely stalled.
The rest of this article will explain why all of this
happens and how you can best use these ideas to improve
your windward sailing.
A. Near Luffing
B. On the wind
C. Speed vs. pointing
D. Acceleration
Near Stall
E. Stalled

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Some Telltale History
Somewhere back in time, a competitive sailor placed a
yarn telltale about two feet back from the luff. When the
leeward telltale twirled it indicated that the sail was stalled
and the boat slowed down. But why was it placed two feet
back from the luff? In Tom Whidden's book, The Art and
Science of Sails, page 196, he states that the telltales "should
be between 12 and 24 inches back from the luff--the bigger
the boat, the farther aft--to keep them away from the
confusing local turbulence, or form drag, of the headstay."
Well, this is not exactly correct. "...confusing local
turbulence", yes! If the leeward telltale was placed too
close to the luff, it would often twirl even though the boat
was still sailing fast. Confusing because of the "form drag
of the headstay", no! The fact is, if you could magically
replace the genoa with a curved sheet of aluminum
without a headstay, the flow near the leading edge would
still be "confusing." The cause of this confusion is the sail
leading edge separation bubble that I discovered and
described in the SAIL magazine article (March 1973).
After the publication of my tuft system article, grand
prix boats, maxis, and even America's Cup boats began to
use several telltales in a row instead of the single telltale.
They were placed about a foot apart, with the first one a
foot or more from the luff. With dacron sails it was usually
difficult to see the leeward telltales, and because of their
wide spacing, they could not really be used as I had
originally described. Instead, sailors started using the first
windward telltale as an indication of when the sail is about
to luff.
In addition to the widely spaced row of telltales that the
big boats used, they also usually had a round window with
a single pair of telltales. The leeward telltale in this window
told them when the sail had already stalled. They had
nothing to tell them where they were between the luffing
and stalled condition except the experience of the sailor.
However, my Tuft System tells you when the leeward
separation bubble is formed and how big it is. The size of
the bubble tells you when you are close to luffing, when
you are close to stalling the entire sail, and how rapidly
you are changing from one condition to the other. This
helps in both driving the boat and in coordinating sail trim
as the wind and sea conditions change.
The Leading Edge Separation Bubble
The leading edge separation bubble is best understood
by studying the tuft system diagrams shown on the first
page. These diagrams show only about the first 30 inches
of the leading edge of a genoa on a large boat, or about 16
inches on a boat the size of the Ranger 23. The airflow is
from the right to the left. Four short pieces of yarn or
spinnaker cloth, the tufts, are shown on each side of the
sail. Note that the first one is placed as close to the leading
edge as possible, and that the subsequent tufts are placed
head-to-tail. Each of the drawings show the sail at different
angles to the wind.
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Ranger 23 Newsletter
The thin lines with the arrow show the direction of the
air as it comes into the sail, and are what aerodynamicists
call stagnation streamlines. They represent the dividing
line between the air that flows on the lee side of the sail
from the air flowing on the windward side (the lower side
shown in these diagrams). The separated regions are
shown in gray. The angles shown in these diagrams and
the changes in the stagnation streamline have been
exaggerated for illustrative purposes, as has the thickness
of the separation bubbles.
Diagram A shows the sail as it is just starting to luff. The
stagnation streamline is coming into the leading edge
slightly on the lee (top) side of the sail. As the air on the
windward (lower) side of the sail tries to make the turn
around the leading edge it will separate and form a bubble,
shown in gray. The air inside the bubble is very unsteady,
and is made up of twists and swirls of very confused air.
Any tufts inside of the bubble will twirl wildly. The bubble
gets thinner toward the back end, and tufts downstream of
the bubble will not twirl but will lie smoothly along the
surface of the sail. Except for the fact that the sail is about to
luff, the windward bubble has only a small effect on the
performance of the sail. On a real genoa, with the headstay
swept back at an angle, the general direction of the
swirling air within the bubble will be upward. This is why
the usual single windward telltale will lift and twirl
somewhat upward when the sail is about to luff.
Diagram B shows the sail at a slightly higher angle of
attack where the stagnation streamline is coming into the
exact leading edge of the sail. Precisely at the point where
the stagnation streamline hits the leading edge, the air
actually comes to a complete stop (that's why we call it the
stagnation streamline). The airflow is smooth on both
sides of the sail, and in fact starts to accelerate immediately
as it splits to flow on both sides of the sail. As we know from
modern sail theory, the air on the leeward side accelerates
much more rapidly than the air on the windward side.
This leads to a lower pressure on the leeward side than on
the windward side, giving us the pressure difference and
lifting force necessary to make our boat go. Since the flow
on both sides of the sail right at the leading edge is accelerating
smoothly, no separation bubble is formed. The sail is
pointing precisely into the wind.
In Diagram C the angle of attack of the sail has been
again increased slightly. The stagnation streamline is
actually around on the windward side of the sail. As the air
flows around onto the leeward side, it separates in the
form of a small bubble with only the first tuft slightly
agitated or twirling. Aft of this short bubble the flow is
again reattached to the sail as shown by the fact that the
other tufts are lying down smoothly. The sail is not stalled.
For many boats this represents the best condition for
windward performance.
In Diagram D the angle of attack has been increased
even more by heading the boat off slightly. The bubble has

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grown in length as indicated by the fact that the first three
tufts are twirling. If the first tuft is very close to the leading
edge, the windward tuft may even twist around to the lee
side of the leading edge.
In Diagram E the angle of attack has been increased
even more. But now, the bubble has burst and extends
clear past the trailing edge of the airfoil. The whole sail has
stalled. Lift goes down and drag goes up. The boat slows
down and does not point well.
By now you should have the idea as to how the Gentry
Tuft System works. Not only does the system tell you how
close you are to luffing (all the leeward tufts lying down),
how close you are to stalling the sail (the first two or three
leeward tufts twirling), but how rapidly you are changing
between these two conditions. If you need to point high,
make all the leeward tufts lay down, with only the first one
occasionally twirling. For better speed, bear off slightly so
that the first leeward tuft is agitated. For more power,
briefly head off allowing the first, second, and maybe the
third tufts to twirl. The key in using this system is to watch
the tufts on the leeward side of the sail.
Communication between the helmsperson and the
genoa trimmer is obviously very important. If the trimmer
sees the first, second, then third tuft twirl in succession,
he/she must either let out on the sheet some, or ask the
helmsperson to head up to avoid stalling the genoa. If all of
the leeward tufts are lying down, the sail is about to luff.
The boat should be headed off slightly or the genoa
trimmed in some.
Experience with the system also has a side benefit in
that it provides more precise steering. It also allows the
helmsperson to spend more time watching for waves and
chop instead of watching for the sail to luff or watching a
single telltale to indicate a stall, and all the time wandering
back and forth between the luffing and stalled condition.
The standard single telltale only tells you when the sail is
stalled, and that's too late. With the tuft system you can
avoid stalling the sail because you can see the stall coming
before it happens.
Tuft System Installation
The actual length of the tufts and their placement on
the genoa will depend upon the size of the boat. On a 23
foot boat the tufts should be only 3 inches in length. Placed
head-to-tail, usually three leeward tufts will twirl just
before the sail will stall. On a 35-foot boat the tufts should
be about 6 inches in length. It may take some experimenting
to find the proper length and placement for the sails on
your boat. One approach is to install several rows of tufts
with different lengths and spacing. You might also attach a
regular telltale about four feet back on the sail to help in
calibrating the tuft system for the fully stalled condition.
Then go sailing to find out which row works as described
in this article, and then remove all but the best row of tufts.
The tufts on the port side of the sail should be red, those
on the starboard side green. On a lightweight Mylar genoa
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Ranger 23 Newsletter
you may be able to see the tufts without a window.
However, on some polyester-Mylar sails and on older
dacron sails, you will want to install an elongated window
in the sail so that all of the leeward tufts can be seen. The
front edge of the window should be right up against the
bolt rope or luff tape. The photo below shows the tuft
system installation on Kittiwake's heavy number 1
polyester-Mylar genoa.
If you have a good sewing machine you can install the
window yourself. A Window Kit is available from Sailrite
for $6 (Toll free at 1-800-348-2769). The key is to sew the
window onto the sail first, then carefully cut away the
dacron sail material with a sharp X-acto knife without
nicking the plastipane. I installed one on Kittiwake's 130%
genoa. It was easy and works great.
For the Ranger 23, first make a cutting pattern on a
piece of paper. Draw a 5" x 11" rectangle and, with a
drawing compass, scribe a 5" diameter circle inside the
rectangle at each end. This rounded rectangle becomes the
template for cutting out the plastic window.
Next, decide where you want the window on the sail.
For a tall rig R-23, put it about 9 feet up the luff from the
tack. You may have to adjust this up or down some to keep
the window away from any sail panel seams. With a pencil
mark a reference point on the luff tape at the chosen
location.
With a protractor measure a 72 degree angle down
from the luff tape. This will be the centerline of the window
and will allow the tufts to flow nicely in the
streamwise direction when the sail is set.
Lay the sail out smooth and, using the double stickybacked
tape provided in the kit, attach the window to the
sail. The front edge of the window should be right up
against the edge of the luff tape.
Now, carefully sew a row of zig-zag stitches around the
outer edge of the plastic window. Turn the sail over and
draw a line on the dacron that is 0.5" inside of the outer
edge of the plastic window. This is your guideline for
cutting out the dacron material. I used a sharp X-acto knife
to cut out the dacron. I made an incision in the center, then
carefully continued the cut to the guideline and around

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the inside of the window. Next, sew a final zig-zag seam
around the inner edge of the window.
Tufts made of yarn can be treated with a water repellent
to help keep them from sticking to the window in wet
weather. Tufts made from spinnaker cloth are more
sensitive than yarn. However, they are more susceptible to
rain and may need to be replaced occasionally if they get
creased badly when the sail is folded.
I prefer using the spinnaker cloth. I get red (or orange)
and green scraps from my sailmaker for this purpose. To
cut the half-inch wide strips, I have used a sharp-tipped
soldering iron, or more recently a sailmaker's hot knife
(from Sailrite). I have cut two 6" x 10" pieces from poster
board and taped them together to form a sort of book. I
open up the book and lay a piece of spinnaker cloth over
one side. I then use the soldering iron or hot knife to cut
half-inch wide strips of spinnaker cloth. The heat from the
cutting will cause the cloth strips to stick to the poster
board and keep them nicely in place on the board until
they are used. When I sail on another boat, I always take
my little book with telltales along in my seabag!
The tufts can be attached to the sail window or to a
Mylar sail with a small square of clear plastic tape. On the
Ranger 23 they should be about 3.5" long. Use a piece of
spinnaker repair tape to fix the first tuft to the luff tape. In a
sail with a window, the first tuft should be about 4" long
with only the tail of the first tuft visible in the window.
Driving The Boat
Why should you use this tuft system? First, it's more
fun to sail to windward more efficiently. Every racing
sailor knows this. But what if you only day sail or cruise? I
have had a number of sailors tell me that they don't race;
they just enjoy being out on the water. Soon, however,
they tell me how this big boat was passing them last
weekend, and how they sheeted the sails in a bit, watched
the trim, and beat the other boat back to the marina. Sure,
they don't race!
Sailing a boat to windward is not really a black art, but
at the highest level, it requires a higher degree of concentration
than most people realize. It also requires close
coordination with your genoa trimmer. These facts
remain, regardless of the size of the boat you are sailing.
Each boat, however, has its own special characteristics, and
the Ranger 23 is no exception.
The keel on the R-23 is rather short compared to a lot of
newer designs. The boat is also heavier, but with the 1500
pounds of lead in the keel, it is rather stiff (the R-23 with the
short rig is really, really stiff!). We can carry a little more sail
area than the lighter boats as the wind increases.
Different wind and sea conditions require different
approaches to driving the R-23. In light wind the boat does
not point as high as the newer designs. Part of this is
design, but I am convinced that many people that I sail
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Ranger 23 Newsletter
against simply try to point too high. I just foot off for speed
and usually get to the weather mark before them. The R-23
does point very high when the wind increases and the
boatspeed gets up in the 4.8 to 5.3 knot range.
Sail trim is always an important factor in sailing to
windward, and must be adjusted depending upon the
objective at hand. A recent race on Kittiwake is a good
example. After rounding West Point we had a 4.5 mile port
tack beat down and across the Sound to the first mark,
Blakely Rock.
We knew that we would not lay Blakely Rock and
would have to take a short leg out on starboard before the
final port tack approach to the Rock. We had the genoa
trimmed to 5 inches off of the spreader. The chop wasn't
bad but the boatspeed would frequently drop from 5.0
down to 4.8, then accelerate back up again. Jeff Crouch was
on the genoa trim, and I asked him to let the genoa out to 6
inches off the spreader. We would try that setting for a
while. The boatspeed then began to stay rock steady on
5.0 knots. Good move! We did not point quite as high but
the steady 5.0 knot boatspeed made up for it. With a good
downwind run we corrected out first in our class.