How do our Cadet
sails work?
And why is Ben so much faster than everyone else!
Why do we need main and jib sheets?
Sails generate a force by deflecting the wind from its normal direction, and it is this force which drives
our Cadet forward. The wind deflection creates a difference in air pressure between the two sides of the
sail, with the windward side of the sail as the high pressure side, and the leeward side of the sail as the low
pressure side.
On the windward side the air becomes lazy, and doesn’t want
to flow into the concave shape formed by the sail. Instead, it
takes a more direct route to the leech. This means that the air
right next to the sail slows down and results in a higher
pressure region on the surface of the sail.
The air that is going to flow on the leeward side does not
move any further to leeward than it has to to get past the sail.
The lee-side air passes very close to the forward part of the
sail, and because it is being squeezed through a narrow area,
the air accelerates, resulting in a low pressure region on the
surface of the sail.
Wind slows
High pressure
Wind accelerates
Low pressure
Interesting fact #1: due to the pressure difference, air that
might normally have gone to windward is effectively sucked
onto the leeward side, which in turn increases the speed of the
leeward side air.
Interesting fact #2: when the air separates at the mast, the air
travels faster on the leeward side, and reaches the leech more
quickly than the air on the windward side.
Interesting fact #3: because of this, the force created by the
low pressure side of the sail is much greater than the force
created by the high pressure side.
So if we create a force by
deflecting the wind, then it is easy
to see that the more we can deflect
the wind, the greater the driving
force.
Unfortunately, it isn’t quite as simple
as that. As the diagram on the left
shows, when our sail is inline with
the wind there is no deflection, and
as a result, no force. In this case, our
sail is behaving like a flag (called
“luffing”), with equal pressure on
each side, and probably flapping.
If we pull our sail in too far (known
as a high angle of attack) the wind is
deflected on the windward side of
the sail just fine, but the wind breaks
away from the leeward side of the
sail (called “stalling”) and the all
important leeward reduction in air
pressure does not happen. What
happens instead is turbulence and the
creation of drag, which only slows
our Cadet down.
Using telltales to help us “see” the airflow
On our Cadet, telltales are positioned on both sides of the
leading edge of the jib (called the luff). These small strips of
wool or ribbon show how the air is flowing past the luff and
indicate when you need to adjust the sail.
At the best sail trim, air flows smoothly past the luff on both
sides of the sail. The telltales then stream back horizontally,
as you can see in this photo. The red telltale is on the near
side of the jib (to port), and the two green telltales are
showing through from the other side of the sail (starboard).
This sail is in good trim because the telltales on both sides
are streaming straight back. With good air flow on both
sides, the sail’s shape generates maximum power.
It’s simple to trim the sail when the telltales show a problem.
All you need to do is change the sail’s angle of attack, either
to increase or decrease the amount of wind deflection, by
adjusting the sheet.
If the fluttering telltales are on the inside of the sail, as
shown in this photo, then the sail is luffing. Pull the sail in
to increase the angle of attack, and create more power from
the sail, so that the telltales are streaming back horizontally.
If you are close hauled, and the jib is already tight in, then
you need to bear away to increase the angle of attack and to
get the telltales streaming.
If the fluttering telltales are on the outside of the sail, then the sail has stalled. You need to decrease the
angle of attack by letting the sail out until they are streaming back horizontally. Again, if you are close
hauled, then you can decrease the angle of attack by pointing the boat up closer to the wind until the telltales
are streaming.
The sail can be trimmed using the telltales on most
points of sail—but not when running downwind.
When the boat is moving close to directly downwind,
the wind is pushing the sail rather than flowing by it
evenly on both sides.
The telltales then become useless for sail trimming
and may hang limply, as in this photo, or flutter.
Important lesson:
If we don’t pull our sails in enough, they luff and
don’t generate maximum power. If we pull our
sails in too much, they stall (much less obvious),
and don’t generate maximum power. In both
cases, our Cadet won’t go as fast as it could. Our
sails need to be sheeted just right, all of the time!
Why do we need a kicker?
A kicker is used to make sure the angle of attack for the top part of the mainsail is set correctly. Bear
with me while I explain why this is necessary and also important!
When you stand still, you can feel the wind on your face and can use this feeling to tell where the wind is
coming from and how strong the wind is. The wind that you feel is known as the “true wind”. This wind is
not what our Cadet feels, because it is moving.
Imagine that instead of standing still, you are riding a bike really fast. The wind that you feel will be
coming from right in front of you, and the faster you go, the stronger this wind will feel. This wind is
known as the “apparent wind”, and is made up of both the true wind, and the wind that you “create” because
you are moving.
The next part to understand is that the faster you move forward, the more
the apparent wind will appear to come from in front of you, and the
stronger it will feel. Conversely, as you slow down, the more the wind
will appear to come from its true direction and the weaker it will feel.
Changes in the true wind speed, or direction, will also change the apparent
wind we feel.
Interesting fact #1: our Cadet sails in its apparent wind, not the true wind
Interesting fact #2: as our boat accelerates, its apparent wind moves
forward, reducing the angle of attack of our sails, and means that we need
to sheet in slightly.
Interesting fact #3: if we sail into a calm patch, it can feel like we have
gone head to wind, even if we were originally on a reach.
Now we all know that, in a river, the water
flows faster in the middle than at the sides.
This slowing of the water at the sides is caused
by the friction between the river bank and the
moving water. The layer of slow moving water
is known as the “boundary layer”. With wind,
it is just the same! The wind nearest the water
is moving more slowly than the wind at the top
of our mast, and the difference can be quite
significant. What does this mean to our
apparent wind?
Well, as we can see from the pictures above
and on the left, the true wind increases as we
move up the height of the mast, and the boat
speed remains the same.
The result is that at the top of the mast, the
apparent wind is both stronger and coming
more from the side of the boat than it is at
deck level. This difference in direction
means that we need to set our sail differently
at the top of the mast, than we do at boom
level. We need a different angle of attack.
Here is where the kicking strap comes in!
We know now that the apparent wind at the top of the mast is both stronger, and comes more from the the
side of the boat than the wind at boom level. What this means is that we need to to let the sail out more at
the top of the mast so that the sail has a constant angle of attack from the top to the bottom of the sail. This
variation in angle of attack along the height of a sail is known as “twist”.
If we didn’t allow this to happen, then we would find that when we set the telltales so that the sail was
trimmed correctly at boom level, then the top of the sail would not be trimmed correctly. It would be either
luffing or stalling. Compare the two pictures of the same boat, on the same tack, and in the same conditions.
The only difference is how much kicker has been applied.
Slightly too much kicker results in virtually no
“twist” in the sail. This means that when the
sail is set correctly at boom level, the top of
the sail will be stalling.
Not enough kicker results in too much twist in
the sail. This means that when the sail is set
correctly at boom level, the top of the sail will
be luffing.
So how much kicker is just enough? Well we need our telltales again. Cadet mainsails have telltales on the
trailing edge (“the leech”), often attached to each of the batten pockets. Although they are different to
leading edge telltales, they can still tell us what is happening to the air flow over the sail. When the sail is
stalling, a leech telltale will flick around the leeward side of the sail. This is due to the turbulent air that is
created on the leeward side when the sail is stalled. If the telltale is streaming constantly, it means that
either the airflow off the back of the sail is smooth, or the telltale is simply acting as a flag because the sail
is luffing.
Bear in mind that we create maximum power when we deflect the air the
most we can without stalling the sail. So we should be setting the sail so
that the leech telltale is streaming most of the time, but very occasionally
stalling. This is an indication that the sail is set at its maximum possible
angle of attack.
Use the mainsheet to set the sail so that the bottom telltale is correct, and
then look at the top telltale. If the sail is stalling too often, ease the kicker.
If the sail is not stalling at all, then apply more kicker. As a rule of thumb,
if the top batten is parallel with the boom, the kicker is set about right.
As the wind strength changes, either increasing or decreasing, the mainsail will twist more, or less, with the
same amount of kicker applied. As a rule, when you need to move your weight in the boat because of
changes in wind speed, you should also check and, if necessary, adjust your kicker.
One final thought for you is what to do in very strong winds. If you have too much power, and you are
struggling to keep the boat upright, easing the kicker a little will allow the top of the mainsail to stall, reducing the power (and heeling effect) produced by the mainsail, and making your Cadet more controllable.
There is another way to reduce mainsail power which should be used first though, so read on!