Articles page 2
"You don't seem to be pointing very well". It evokes the same kind
of response that finger nails on a black board produce. The unsuspecting crew
member that mutters this all too obvious observation usually receives a less
than enthusiastic response.
Anyone can point a boat higher into the wind, but you usually loose speed. So
a more useful discourse has to involve something which I call
Performance is the result of the delicate relationship between apparent wind
angle, apparent wind speed, sail power, boat speed and ballast. Performance is
the greatest factor in determining the results of a sailboat race!
Lift and Drag
Every thing that affects your boat's performance can be expressed in terms of
lift and drag. Whenever lift is produced, drag is the natural consequence. They
go together hand-in-hand. Fortunately for sailors the ratio of lift to drag is
not constant. It is possible to increase lift faster than drag, and vice versa.
Understanding this relationship is useful in trouble-shooting performance
Keels create lift because they slide through the water, both forward and to
leeward. We call the leeward direction "leeway". It may not be
perceptible, but all keels and centerboards have some leeway while sailing
upwind. The orientation of your keel to this leeway determines the angle of
attack of your keel.
When you point your boat away from the wind the angle of attack of the keel
decreases. That means both lift and drag are reduced. Less drag enables the boat
to speed up. The increased speed forces more water over the keel, which
increases lift without significantly increasing drag. In this case, the ratio of
lift-to-drag changes. More lift and less drag enhances performance. For many, it
is counter intuitive to head down in order to gain progress to windward. This is
a common mistake many sailors make in light air.
There is a limit to how fast your boat will go, however. Drag increases
exponentially with boat speed. So there is a point depending on boat speed where
lower angles will not significantly decrease drag. At top speed, you will have
similar drag no matter what direction you point (unless you can plane). As you
approach this point, upwind performance declines the lower you sail. Some people
find themselves going very fast, but they can't hold their lane. By the time
they get to the windward mark they are back in the cheap seats.
If you turn towards the wind, the angle of attack of the keel increases.
Eventually the flow across the windward side of your keel starts to breakup, or
stall. A stalled foil quickly looses lift and dramatically increases drag. Boat
speed drops, which further provokes stall. Performance quickly degenerates at
too high an angle.
Most sailors have an intuitive notion of a "groove" while sailing on
the wind. Whether they understand the concept or not, a good helmsperson is
better at keeping the keel's angle of attack between the two extremes described
above. In light-to-moderate wind speeds, great sailors are able to keep their
keels closer to the "verge of stall", usually without ever letting
them stall. The verge of stall is usually the point where the maximum
lift-to-drag ratio is found.
Rudder angle is the primary indicator for monitoring the lift of your keel. A
mushy or leeward rudder angle is a symptom of a stalled keel, or a keel that is
not generating enough lift. Many people get so distracted during the race that
they forget to feel the rudder pressure. They then haveno way of knowing what
the underwater surfaces are doing. They inevitably end up outside the
"groove" for long periods of time.
If your boat has a wheel, it is important to have some indication of the
rudder angle. Most boats sail with an optimum rudder angle of about three
Sails provide power by creating lift and drag. We can alter the sail's
lift-to-drag ratio through sail controls.
In light air the goal is to maximize lift and minimize drag. Simply put, deeper
sails tend to create lift faster then than drag, but if they get too deep the
leeward aft edges start to stall. We monitor this stall with leech tell tails
and visual clues such as distance off the spreader, and top batten angle.
Increasing the angle of attack such as raising the traveler or sheeting the
Genoa increases lift faster than drag, but if you go too far, the wind will
strike the leading edge of the sail at too wide an angle and the leading edge
will start to stall. When the leading edge stalls, the entire leeward side of
the sail is vulnerable to stall. That is why most sailing handbooks suggest
sailing with the windward tell-tail just lifting. In light-to-moderate air, the
best sail trimmers are very good at keeping their sails at the verge of stall.
Lift generated by upwind sails is not directly oppostite of the lift generated
by the keel. The degree to which the lift is not directly opposed is vectored
into forward motion. As the wind increases, there comes a point where the
opposition of these forces create heel. Too much heel is disastrous to
performance for a multitude of reasons, but let it suffice that too much heel
increases drag and reduces lift in both keels and sails. It's nature's way of
equalizing the forces.
Centerboard boats can reduce heel by reducing their exposed underwater surfaces.
Keelboats do not have such flexibility. Heel has to be controlled through
minimizing the lift and drag that the sails produce. As it turns out, flat sails
reduce lift and drag fairly effectively. Decreasing the angle of attack reduces
both lift and drag. A flogging sail, however, creates almost no lift at all, but
it generates an large amount of drag. Great sail trimmers are able to flatten
their sails and maintain an appropriate angle of attack, but they rarely if ever
let their sails flog. Their goal is to minimize drag and maintain the
appropriate amount of lift suitable to the boat's ballast. It is better to have
a little too much heel and keep it consistent than it is to over heel and then
over flatten over and over again.
As it gets windier, the force of the sails continues to overcome the boat's
ballast. Instead of lift pushing the boat abeam to leeward, the drag on the
already flat sails becomes more influential. Drag wants to push the boat aft and
to leeward. This increases the angle of attack of the keel. Somehow the
excessive lift of the keel has to be relieved in order to control heel. Great
helmspeople can feather their keels at extreme angles of attack so that a
portion of the windward side of their keel becomes stalled. Too much stall and
speed will diminish. Not enough lift will be produced and the bow will drop off
to leeward. The sails will overpower the ballast. The boat will want to spin up
into the wind. The result is usually a series of minor broaches as the boat
cycles through successive stalls and overpowerments.
A Delicate Balance
Great sailors maintain a steady angle of heal in overpowering conditions because
they monitor how much of the keel is stalled and the distribution of power
across the sails. They concentrate on rudder angle and heel. If the keel wants
to lift too much they experience too much rudder angle. The boat will want to
climb into the wind. If they don't intervene by easing the main, the rudder will
stall and the broach cycle starts anew. If they ease the mainsail too far, the
boat is thrown out of balance and the jib pulls the bow to leeward. The cycle
starts again. If the keel begins to loose too much lift they sense it early
because the rudder suddenly feels mushy. They can ease the sails quickly and
head off to reattach the flow around the keel.
In extremely heavy air, great sailors are constantly adjusting their sails as
they attempt to delicately balance their sails over a partially stalled keel.
The groove becomes almost impossibly narrow because the immense forces transform
the rudder into an increasingly impotent tool. Teamwork is put to the test, as
changes in course become impossible without sail adjustments.
Low and Slow
If you ever find yourself low and slow there is a high probability that
something is stalled. The challenge is to find out which foil or foils is the
culprit, and find a way to reattach the flow.Performance Articles page 2
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