Boating and Sailing.pdf - Moja ladja

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Part 7: Sailing, Sailing …
across a flat or concave section because it has to travel a greater distance to the aft edge.
This creates a lower pressure on the convex side than on the concave side, according to
Bernoulli’s principle. (There’s that pesky ninth grade science again.) So, there’s less pressure
on the convex or leeward side of the sail than on the concave or windward side, and a
lifting force is exerted toward the convex side.
Actually, a sail is so thin that by this logic, the difference in speed between one side of a
sail and the other would be negligible, and there would be no lift. Also, even if there was a
big difference in distance traveled on the two sides of the sail, there is no law of physics
that says that the air has to take the same amount of time to travel to the back of the sail
on either side, and indeed wind tunnel demonstrations show that this condition is usually
not met. So this explanation of lift is all wrong on several counts, but what the heck, it’s
still the one they use at the very best sailing schools ….
Lift is created in sails when
air flows faster over the convex,
leeward side than over
the concave, windward side.
Increased pressure
Reduced pressure
Mast
Mainsail
This gives lift in an aircraft wing and thrust in a sailboat. But in a sailboat, no mechanical
power is required—you travel on the power of the flowing air alone.
A keel in the sailboat “stiffens” the boat, holding it from blowing sideways or even over on
its beam ends with the pressure of the sail. (A boat rolled on its beam ends is flipped up on
its side, on the verge of capsizing.) The keel acts on somewhat the same principle as the
sail—it’s a rounded “hydrofoil,” and when directed slightly to windward by the rudder, it
generates lift in that direction.
The force acting on the sail is balanced by appropriate steering with the force acting on
the keel, and the boat goes straight ahead.
Many larger sailboats have weighted keels that function as “ballast,” a righting force when
winds threaten to capsize the boat. Smaller boats depend on movement of the crew from
gunnel to gunnel to act as ballast.
Wind