How a Sail Works
convert the energy in the wind into forward movement of the
boat. They do this in the same way as wings of an aeroplane
provide uplift. The wind causes the sail to take a wing
The wind passes around the sail and because the distance
is greater on the leeward side of the sail, the wind must
travel faster. This means the pressure on the leeward
side is lower than the pressure on the windward
This pressure difference results in a force acting perpendicular
to the boom in the downwind direction. This force
can be resolved as a force on the boat pushing it forward
and a force on the boat pushing it sideways. The sideways
force results in drift
but is (for the most part) avoided by using
A sail has a particular angle to the wind at which it is
most efficient. This is usually 45 degrees, which means
that boats can travel as close as 45 degrees to the wind.
It is across wind that boats travel fastest, anywhere between 60 and 120 degrees from the direction of the wind. When moving in a downwind direction the sail no longer acts
like a wing, but more like a parachute, catching the wind
and moving the boat along with the wind.
What does a Keel do?
The keel or centerboard keeps the boat from being pushed
sideways by the wind. The resistance from the hull and the
keel translate the hydrodynamic lifting force to forward motion.
If you did not have a keel or centerboard the boat would simply
be blown across the water like a balloon blows across the water. However, with a well-shaped
hydrodynamic keel, the boat will sail mainly forward while sliding
slightly to leeward (away from the direction the wind is blowing).
The force works on the keel like a thumb pressing on an orange seed.
If you put an orange seed on a hard surface, like a table and place your thumb
on the right point and push down, it will 'squirt' forward. The keel, a similar shape to the orange seed, does the same
thing when a sideways force is placed on it and against the pressure of the water.
HOW TO SAIL -An overview
The windward side of anything is the side from which the wind
is coming from. The leeward side is the side in shelter.
Also used as directions, e.g."another boat to windward".
Steerage : The ability
to control the direction of your boat. You need to have
some forward movement for the rudder
to have any effect on the direction of the boat. Therefore,
adequate steerage is really adequate speed for steering.
Points of Sail : The
direction of the boat with respect to the wind. The
point of sail determines the sail and centerboard positions.
See a better
Tack : As in "starboard
tack" and "port tack". When you're sailing
across the wind, the wind will be coming over one side of
the boat. If it comes over the starboard side, it's
a starboard tack. When running (with the wind coming
over the stern) your tack is the side that the sail isn't
on. Very important for right-of-way rules. See
note on the word "tack"below.
A Note on the Word "Tack"
: There are at least four meanings for the word "tack".
- Tack: The side of the boat which the wind is coming
over. A starboard
tack or a port tack.
- Tack: A corner of a sail. The lower corner of the luff,
the leading edge.
- Tack: To turn the boat into the wind so that you change
from one tack to another (e.g. change from
starboard tack to port
- Tack: To get upwind you sail as close as you can to
the wind (you're on a beat) and then keep changing direction
and sailing close to the wind until you get to where you
want to go. You make a zig zag course. This
is known as tacking - you tack upwind.
Spinnaker : A large
parachute type sail at the bow. Used only when on a
Genoa : A large overlapping
foresail which takes the place of the jib. Can be furled without
taking it down to reduce sail.
Beat : A point
of sail . Also called "Close-Hauled".
When a boat is going as close to the wind as it can
it is on a 'beat' or sailing to windward.
Run : A point
of sail . Heading downwind. Boats
travel faster on a run than on any other point of sail.
Sideways Drift :
When a boat is crossing the wind at any angle it will experience
a force transverse to the direction of travel. This
force tends to push the boat sideways. This is sideways drift.
Abeam : Across the boat.