Getting The Most From Gusts - by Ben Morris
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Why do some yachts seem to accelerate in gusts and others
simply stagger along, screw up to windward and end up two or three boat
lengths behind in the blink of an eye?
To examine this issue I will describe the simplest situation
of a yacht sailing on a square reach in a gentle breeze that receives a gust
from the same direction. To examine this I need to talk about real
wind and apparent wind - sorry! Look at the graphic below.
The yacht is now sailing with the apparent wind slightly
forward with the sheets pulled in a little tighter than would be the case if
the wind was at right angles to the yacht.
If a gust from the same direction now reaches the yacht the
wind velocity is much greater than the yacht velocity and the apparent wind
will now move back almost to the original direction at right angles to the
yacht. So what happens?
If no changes are made immediately by the skipper then the
sail will stall at the leading edge as the wind will be unable to turn the
sharp corner around the back of the sail. This has three effects.
The first is that the forward drive of the sail will decrease slowing the
yacht. The second is that the heeling force on the sail will increase
causing the yacht to heel. The third is that the yacht will experience
a sudden increase in weather helm causing the yacht to head up into the wind
thus spilling the wind out of the sails and losing all drive and speed.
As well the skipper will probably attempt to compensate for the sudden
change in direction by moving the helm vigorously which in the conditions
will simple cause it to stall, create drag and not steer it at all.
Every action (or inaction) then causes the yacht to slow down!
So what does a smart skipper do? The skipper will
have seen the gust coming using indicators such as boats nearby suddenly
heeling, the water surface will become rougher in the direction of the wind
and their own boat beginning to heel. The skipper will immediately let
the sails out from the green position towards the red position or beyond and
steer the yacht slightly away from the wind. As the leading edge of
the sail is now in line with the wind direction, the wind will stay attached
to the sail and generate more forward drive with the greater wind velocity
but not generate too great a heeling force. The change in heading will place
the wind slightly behind the beam where the gust can now blow the yacht even
faster and even further reduce the heeling force. As the yacht goes
faster with less heeling, the rudder becomes much more effective and
steering is not a problem - in fact the rudder become far more sensitive and
will have to be treated gently! It hasn't finished yet because the
nature of gusts are that they gradually die away so the skipper will be
bringing the yacht back onto course and gradually pulling the sails back in
as the gusts fade and the apparent wind moves forwards again. It is often
during this last section that the yacht seems to accelerate and get that two
or three boat length advantage!
Of course there is always that gust that is too strong even
for the best laid plans so survival is the order of the day then. Let
the sail right out until the heeling is controlled, the steering becomes
effective and the yachts stays pointing in the right direction. Easing
the sails back in will enable you to determine how much drive you can
get yet still not end up on your side.
You need to practise these manoeuvres as they really do make
a significant difference! As soon as I see or hear skippers saying
that their rudder has lost grip in the gust or they can't control the yacht
in the gust I know they haven't done those two key things, ease the sails
and bear away from the wind gust
Even tho this discussion deals with yachts on a reach, the
same ideas apply equally well from beating to broad reaching. It's a
little different for a yacht on a square run because really there is no
where to run other than to keep going down wind. So how do you cope
with this?
Luckily as the yacht is running away from the wind, the speed
of the yacht is already subtracting speed from the wind so the apparent wind
is already less and corresponding gusts are also less strong. Like
before as the speed of the yacht increases the effectiveness of the rudder
increases so small movements of the helm can cause quite large direction
changes so use small movements on the rudder stick. How often have you
seen yachts going downwind in a gust swing wildly from port to starboard and
back again as the skipper pushes his helm from one extreme to the other?
Once the wind has reached a critical speed the nose of the yacht is pushed
deeper into the water increasing the drag, slowing the yacht and increasing
the apparent wind thus pushing the nose in deeper etc etc. How to cope
with this? One way is to swing the nose slightly from side to
side. The curve on the side of the hull near the stem will impart a
lifting force as it moves sideways so this may help. You can always
cry yeeaaaaahh! as you go screaming downwind to end in a submarine dive.
It won't help the yacht but it might make you feel better.
Remember gusts are there to help you - you must get the most
from the gusts, more than the other guy anyway.
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
