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More on the Slot Effect
"Arvel Gentry continues his discussion
of jib-mainsail interaction"
By Arvel Gentry
SAIL Magazine, August 1973
Last month we first studied the air flow around the This diversion of air to the lee side of the jib has a very
main alone; then we added a jib, and this gave us a good important effect. We know that the more air that flows on
picture of how the jib affects the mainsail. This month we the lee side of a sail, the greater its lifting force. Because we
will do just the opposite. First we will look at the airflow December 1999
have more air flowing to lee of the jib, it will travel at higher
about the jib alone, and then add the main. By doing this, speeds and the lee-side pressures will be lower.
we will see that the main actually helps the jib to become With lower lee-side pressures we have a larger pressure
the very efficient sail that it is. difference across the sail and more jib lift. This also can be
If you have been following this series, you should be an achieved by a higher angle of attack, or it can be
expert at reading streamline drawings and pressure accomplished at lower angles with the help of the mainsail.
distribution plots, so let's jump right in. Figure 1 shows the All this is illustrated in the jib pressure coefficient plot
streamlines about a typical jib-mainsail combination with in Figure 2. The negative pressure coefficients represent
the solid lines the streamlines when both sails are used, pressures lower than freestream (suction pressures and
and the dotted lines the streamlines when the jib is used high velocities); and the positive coefficients are higher
alone. than freestream (lower speeds).
First, note that the dotted stagnation streamline for the Note that the negative pressures on the jib are much
jib alone (Sjo) goes right into the leading edge of the jib. more negative when both the jib and mainsail are used.
However, when the main is also used, the stagnation The jib develops much more lift when it is operating in the
streamline shifts to the position Sj so that it starts much flow field influence of the mainsail. We would expect this to
lower (further to windward) and comes into the jib luff happen, however, for the jib is operating in the upwash
slightly on the windward edge. In this example the jib field of the main. In an actual situation afloat, we can make
could be pointed a little closer to the wind without luffing. use of this by sailing a bit closer to the wind. But this fact
From this we see that the mainsail shifts the jib alone does not account for the great efficiency attributed to
stagnation streamline to windward (a lifting wind shift) the jib in the presence of the main.
and allows the boat to sail closer to the wind. This If we examine pressures near the leech in Figure 2, we
increased upwash on the jib is caused by the fact that the will see a second reason why the jib is such an efficient sail.
circulation fields for the two sails (SAIL, July 1973) add Note that pressures near the leech are slightly on the
together to become stronger in the area in front of the jib. positive side (A) with a jib alone. This means the jib leech
This is not a new fact, but at least now we see exactly how velocity, without a main, is near the freestream value
this jib wind shift occurs. required to meet the Kutta condition on a single sail.
Another result is that the mainsail causes more air to be However, the jib leech pressures with a main present
diverted around to the lee side of the jib. You can see this by actually are negative which indicates leech velocities
comparing the levels of the two jib stagnation streamlines higher than freestream (point B). In this case, the velocity
well out in front of the sails. The stagnation streamline of at the leech of the jib is about 30% higher than freestream
the jib when both sails are set (SJ) is much further to speed. How has this happened?
I have mentioned before that airspeeds must be the
windward (lower) than it was without the main (Sjo). The
same on both sides of a single sail at the leech to satisfy the
distance between these two lines (L) represents additional Kutta condition. For a single sail, the leech speed turns out
air that the mainsail causes to flow on the lee side of the jib. to be close to freestream airspeed. When we have two sails,
Without the mainsail, this chunk of air would pass on the
windward side of the jib!
Figure 1 Figure 2
jib and mainsail, the airspeed at the leech of the last sail in
the line (the mainsail) also will satisfy this Kutta condition
and be near freestream values.
Yet, the leech of the jib is in a high speed region of flow
created on the lee side of the mainsail; and detailed
calculations show that the air flowing around the jib
adjusts itself so that the Kutta condition is satisfied not at
freestream conditions, but at a speed that blends with the
high speed flow created by the mainsail in the region of the
This high speed region would be there even if the jib -60%
was not present (providing the flow on the main is not
separated), for airfoil shapes and angles used in my
research show that jib leech airspeed would be about the
same if the mainsail were used alone.
The Kutta condition on the jib must be satisfied in a
high-velocity region created by the combined flows of the
jib and mainsail (point B in Figure 2). The net result of this
is that the entire velocity distribution on the lee side of the jib
is increased by a considerable amount. These increased
velocities mean lower lee-surface pressures and a resulting +25%
increase in jib lift.
In other words, the mainsail actually helps the jib, not
only by giving it a lifting wind shift, but also by causing it to Figure 3
have much higher velocities on its lee side because of the 3B the jib has been moved five degrees closer to the
Kutta condition requirements at the jib leech. While the aft mainsail, and as you can see by the number at the left, this
windward side of the jib does lose some of its positive causes a 60% reduction in the amount of air that flows
pressure, this is a small price to pay for the large increases through the slot.
in the suction pressure on the lee side. The stagnation streamline for the jib now comes into
This phenomenon will be referred to as the dumping the sail on the lower, or windward, surface and the
velocity or bootstrap effect. The name bootstrap indicates stagnation streamline for the main comes into the sail on its
that the main is actually helping the jib in this unusual upper or lee side. This would cause higher pressures on
manner. the lee side of the main than on the windward side and the
What if a third sail were added forward and to lee of the main would luff (carry a large bubble). As soon as the sail
jib with its leech in the increased velocity region of the jib? changes its shape, our nicely calculated streamlines
This third sail would have an even higher leech velocity become invalid, for the entire flow field changes a bit in
and higher lee-side velocity distribution because of what response to the new shape of the sails.
the main is doing to the jib and what the jib, in turn, does But in Figure 3B we see what happens when we sheet
to the third sail. the jib in too close. The slot-flow air is reduced; the
The higher velocity flow that is forced to the lee side of pressures on the forward-lee side of the main become
the jib by the main has another important effect. These higher as the air flow becomes slower; and the mainsail
higher velocities all along the lee side of the jib mean that loses more and more of its driving potential until we reach
the boundary layer will be able to withstand more rapid the point where it luffs. Even in this luffing condition,
increases in pressure (stronger adverse pressure gradients) however, the main serves a useful purpose by causing an
without separating. The boat can be pointed at a higher upwash in front of the jib. And it still contributes
angle (with the jib stagnation streamline coming in slightly something, though less, to the bootstrap effect.
on the windward side) without the whole jib separating This, of course, assumes that only the forward part of
and stalling. the main is carrying a bubble and that the aft part of the sail
Now let's examine the effects of four different jib and is loaded up.
mainsail angles. Figure 3 shows four streamline drawings In Figure 3C, both the main and the jib are sheeted
with only the stagnation streamlines appearing so that you closer to the centerline of the boat by five degrees. This
can see clearly what happens to the slot flow as the sail causes a 30% reduction in the amount of slot air compared
angles are changed. Rather large sail angle changes of five to the sails with basic settings in Figure 3A. However, in
degrees were selected so that the overall effects would be Figure 3C, sheeting both the main and the jib in closer
easier to illustrate. causes even more upwash on the jib, and the stagnation
Figure 3A has the sails at the same setting used in the streamline comes into the jib even further back on the
jib-main flow study we already have discussed. In Figure windward side. Here the air is treated rather roughly as it
makes the sharp turn to get on the lee side and will reduction in velocities over the forward-lee part of the
probably separate and cause the jib to stall unless the boat mainsail rather than by a speed-up in the flow which is
is pointed closer to the wind. Both sails in short, have been the popular theory.
trimmed in too tightly for the boat angle being sailed. 5. Much less air goes between the headstay and the mast
In Figure 3D the jib is at its original basic setting but the when the jib is placed in the flow with the main. The
main has been sheeted in five degrees to open up the slot. circulations of the main and the jib tend to oppose and
This causes a 20% increase in the amount of air that flows cancel each other in the area between the two sails, and
through the slot, and the stagnation streamlines for both more air is therefore forced over the lee side of the jib.
the main and the jib have moved slightly around to the 6. As the jib is sheeted in closer to the main, there is a
windward leading edge of the sails. continuing decrease in suction pressure on the lee side
These comments do not pretend to show how sails of the main. When pressures both to the windward and
should be trimmed for maximum speed; they are included leeward side of the mainsail become equal, there no
only to show the general effects as the sail angles are longer is the pressure difference across the sail
changed. All these results, however, clearly indicate that necessary to maintain the airfoil shape, and the sail
the amount of air that flows through the slot between the begins to luff.
jib and main will vary depending upon the relative
sheeting angles of the two sails. The angles also have a The major effects of the mainsail on the jib are:
direct influence over the way in which the stagnation 1. The upwash flow ahead of the mainsail causes the
streamline comes into the sails. And this, of course, affects stagnation point on the jib to be shifted around toward
the pressure distributions and determines whether or not the windward side of the sail, and the boat can be
the lee-side boundary layers will separate and the sails pointed closer to the wind without the jib stalling or
All my comments to date have primarily applied to a 2. The leech of the jib is in a highspeed flow region created
jib-mainsail combination having considerable sail overlap. by the mainsail. The leech velocity on the jib is, therefore,
A similar situation would have occurred if the sails had less higher than if the jib alone were used.
overlap. The only difference would have been in the 3. Because of the higher leech velocity, velocities along the
relative magnitude of the observed effect. With less entire lee surface of the jib are greatly increased when
overlap, a jib would not have such a strong effect on the both the jib and main are used, and this contributes to
mainsail, and the mainsail lee-side suction pressures the high practical efficiency of a jib.
would not have been reduced as much. 4. The higher lee-surface velocities on the jib mean the jib
The downwash effect of the jib on the main also would can be operated at higher angles of attack before the jib
have been smaller. With less overlap, the leech velocity on lee-side flow will separate and stall.
the jib would have been a bit lower and the bootstrap effect 5. Because of all this, proper trim and shape of the mainsail
not quite so strong. significantly affect the efficiency of the overlapping jib.
Of course all these comments also would apply to a Anything that causes a velocity reduction in the region
single sail combination with varying sail shape and of the leech of the jib (such as some separation on the aft
overlap from the deck to the top of the mast. Nevertheless, part of the main) results in a lower driving force
the basic conclusions still would be the same. contributed by the jib.
I now would like to classify the major jib-mainsail 6. The trim of the main significantly affects the pointing
interaction effects. Although they are for the most part ability of the boat, for it directly influences the upwash
interdependent, I will segregate them into the effect of the that approaches the luff of the jib.
jib on the mainsail, and the effect of the mainsail on the jib. 7. The mast in front of the mainsail always has been blamed
for making the main less efficient than a jib. From my
The major effects of a jib on the mainsail are: studies, I believe this is only part of the answer. Another,
1. The jib causes the stagnation point on the mainsail to and probably equally important, factor is the increased
shift around toward the leading edge of the mast (the velocity on the jib and the fact that its Kutta condition
header effect). must be satisfied in a local high speed flow region that is
2. As a result, the peak suction velocities on the forward created by the mainsail.
leeside of the main are greatly reduced. Since the peak
suction velocities are reduced, the recovery adverse
pressure gradients also are reduced.
3. Because of reduced pressure gradients on the mainsail,
the possibility of the boundary layer separating and the
airfoil stalling is reduced.
4. A mainsail can be operated efficiently at higher angles of
attack without flow separation and stalling than would
be the case with just a mainsail alone. This is caused by a