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Why the Safe Leeward Works
"Arvel Gentry analyzes the
principles behind this tactic"
By Arvel Gentry
SAIL Magazine, September 1973
Interaction between sails on a boat has been confusing leeward position and the slot-effect of two sails on the
to many sailors. Not surprisingly, this confusion continues same boat. Everything is in fact consistent if proper
when one discusses what happens between sails located streamlines and flow solutions are used.
on two separate boats: one boat in the safe-leeward
Let's look at the complete flow field around a single
position. The old explanation of the slot effect between two boat (Figure 1). The dotted lines show the streamlines
sails on the same boat, and what we experience when the about the boat, and spaced along these lines are pairs of
sails are on two different boats, seemed to create a paradox. numbers. The top number is the local wind velocity at that
When the sails are on the same boat, the old theory had point in the flow. All these numbers are what I call relative
the jib forming a venturi of high speed air that helped the values for they include both the true wind velocity, and the
mainsail. But when the sails are on different boats in a close velocity created by the actual movement of the boat
safe leeward situation, somehow the boat behind and to through the water.
windward (like the mainsail) is hurt by the boat to leeward. The bottom number is the local flow angle. The plus
How can such sail interaction help in one case, and be a angle number represents an upwash flow direction from
hindrance in the other? the freestream direction, and a negative number is the
Previous articles in this series have totally disproved local downwash. In this example, the freestream
the old venturi slot-effect explanation, and we have seen conditions out in front of the boat were taken at 10 knots
how the jib influences the main, and how the main, in turn, and zero flow angle (apparent wind speed and angle). We
actually helps the jib. Exactly the same type of thing would have to go several boat lengths upstream before
happens between the sails on two different boats, and this these conditions are reached.
article will explore this situation. The air is significantly influenced for a considerable
Obviously, there is no paradox between the safe- distance all around the boat; to windward, to leeward, and
downstream. The effects are strongest near the boat and
decrease as you move away. Because small differences in
local wind speed or flow angle can make a considerable
difference in the performance of a boat, it comes as no
surprise that the safe-lee situation is in fact a dynamic
example of sail interaction.
In Figure 1, the favored safe-lee position is the flow field
area marked by the letter A. The flow conditions at this
point are a wind speed of 11.0 knots and a flow angle of
+5.7°. This means a boat in the safe-lee position has a wind
speed higher than freestream, and it sails in a wind-shift
corresponding to a lift (increased upwash), all created by
the boat to windward.
The safe-lee boat creates a downwash flow field that
produces both a heading wind-shift and a reduction in
wind speed on the aft boat, the flow field area marked B.
Here the wind speed is down to only 8.8 knots and the flow Figure 3. Aft windward boat.
angle is a header of 4.7°. Obviously, this is not a good place
A complete flow field about the two boats is shown in
Figure 2. Both boats have been placed at the same angle to
the freestream wind so we can judge the relative
interactions between the two sets of sails. Comparing the
single-boat flow field in Figure 1 with the flow field about
the two boats, we see that the windward boat W has much
less upwash than it does in Figure 1. The reason is that it is
suffering from the downwash created by boat L.
The shape of the streamlines downstream of boat W
have not changed very much, but boat L has a far greater
upwash in front of it than was the case in Figure 1. It is
experiencing a lifting wind shift that is created by boat W.
Figure 4. Safe-lee boat.
that with the safe-lee boat present, the aft windward boat
has much lower lee-side suction pressures and therefore
much less lift. We see that both the jib and mainsail of the
aft windward boat are seriously hurt by the safe-lee boat.
Equally important, however, are the effects the aft-
windward boat has on the safe-lee boat. The first effect we
have already seen; the aft boat creates increased wind
speed and a favorable wind shift that benefits the safe-lee
Figure 2 boat. You might ask whether the aft-windward boat
creates a higher speed wind flow region on the windward
How do these changes in the flow fields affect the side of the safe-lee boat, and if it does, shouldn't this
pressures on boat W's sails? The answer to this lies in the actually hurt the safe-lee boat?
pressure distribution plots shown in Figure 3. The dashes It is true that the effect does tend slightly to increase
represent the pressures on the sails when the aft- flow speeds on the windward side of the safe-lee boat over
windward boat is sailing alone, and the solid lines show what they would be if it were sailing all by itself. The
the pressures when the safe-lee boat is present. windward side of the safe-lee boat does, therefore, suffer a
The negative pressure coefficients represent pressures slight reduction in contributing to the drive of the sails on
less than the free-stream value (suction pressures) and the the safe-lee boat. Why, then, does the safe-lee boat still
larger the number, the higher the suction pressure on the leave the aft-windward boat behind?
lee side of the sail, and the higher the lift. Note, however, The explanation lies in what happens to velocities on
the lee side of the sails of the safe-lee boat. First, let's
assume the safe-lee boat has only one sail, and its leech is in
the high-speed flow region created by the aft-windward
boat. Because of this, the leech speeds on the safe lee boat
do not have to return to freestream conditions, as they
would have to with the mainsail of the aft windward boat.
This is the same situation that was discussed last month
when a jib came under the influence of the mainsail.
The airflow on the lee side of the sail of the safe-lee boat
does not return to freestream speed at the leech. Instead, it
is at a much higher velocity. This higher velocity on the lee
side occurs because the Kutta condition on the safe-lee
boat must be satisfied in a high-speed region created by the
aft-windward boat. This means the entire velocity
distribution on the lee-side of the sail of the safe-lee boat is
higher than it would be without the aft-windward boat.
These increased velocities and, therefore, reduced
pressures on the lee side of the sail of the safe-lee boat will
more than offset the loss in drive on the windward side of
the sail. This is the "dumping-velocity" or "bootstrap" effect
described last month.
Only this time the sails are on different boats, and the
aft-windward boat actually helps the safe lee boat by
increasing its air velocity and angle, and by maintaining
increased velocities all along the lee surface of the safe-lee
If the safe-lee boat has a mainsail and jib, there is a
"double bootstrap" effect. The aft-windward boat helps the
mainsail of the safe-lee boat. This in turn, helps the jib of Figure 5
the safe-lee boat even more. The proof of all this is shown made through the sails of both boats at the same height.
in the pressure distribution plot in Figure 4. On a real boat, there are different airfoil shapes and sizes
These plots show that the pressures at the leech of both that run from the deck to the highest point on the sails.
the main and the jib are more negative (higher airspeeds) This means the relative influence of the effects just
when the aft boat is present, and the lee sides of both sails discussed will vary at different heights above the deck. The
have higher negative pressures (higher velocities) all along effect is still there at every level; it's just the relative
their surfaces. There also is a slight reduction in windward magnitude that changes.
side positive pressures. However, this loss in lift is more Look back at Figure 1. You will see that, although there
than compensated for by significant increases in the lee are large differences in speed and flow angle in the flow
suction pressures. field about the boat, there are no sudden changes in short
How does this situation actually affect the two boats? distances. If this is so, why do we sometimes experience a
First, a boat to windward and aft of the safe-lee boat has a sudden change in our own boat when we cross behind
lower windspeed than the safe-lee boat; and it also sails in another? One explanation may be that the sails on the
a header. Because he will have to bear off from his original other boat have large regions of separated flow, which
course to keep his sails at the same angle to the local wind, cause a wide unstable wake behind the boat. This is
his speed-made-good to windward goes down. particularly noticeable when you pass a boat on a reach.
The safe-lee boat, however, can point higher because of Another reason lies in the fact that a sail is three-
the increased upwash caused by the aft windward boat, dimensional. It has a foot and a head that create a trailing
and a higher local wind speed. Although he does suffer a tip-vortex system just as airplanes do. The tip-vortex
slight loss in lift contributed by the windward side of his system is illustrated in Figure 5, though I want to
sails, this is more than compensated for by increased emphasize that this figure is just a sketch of what is really a
velocities and suction forces on the lee-side of his sails. The very complex phenomenon. The sketch shows a swirling
result is that he can point higher and go faster than the aft- vortex of air that is shed off the top and bottom of the sail.
windward boat and, in fact, points higher and goes faster Each sail has its own tip vortex system, and those flowing
than he would if he were merely sailing alone! from the jib will interact and possibly merge with those
All my analysis assumes the sails on both boats are from the main.
trimmed to perfection and have no flow separation. The Each vortex is caused by high pressure air on the
analysis uses two-dimensional airfoils; that is, a cut is windward side of the sail, either at the top or bottom,
trying to flow around and on to the lower pressure lee-
side. Although this twisting is present to a certain extent all
along the leech, its major influence is felt at the ends of the
sail. If a boat passes nearby, it will experience a sudden flow
change as its sails pass through these trailing vortex wakes.
Interestingly, in the center part of the sail flow speeds and
angles are not affected too much by these vortex systems
and remain about as shown in Figures 1 and 2.