daver@hp-pcd.UUCP (daver) (08/14/85)
>Back to basic airfoil theory -- why does a sailboat sail upwind?
While the previous responses are reasonably accurate the real reason a sailboat
can go upwind is because of the keel, centerboard or daggerboard. When the
wind hits a sail at the optimum angle the force vector on the sail is at best
perpendicular to the wind. The reason the boat is able to sail closer to the
wind than this force is that the combination of the keel/centerboard/
daggerboard with the rudder/skeg produces a resistance to motion perpendicular
to the centerline of the boat. If you decompose the force vector on the sail
into components parallel to and perpendicular to the centerline of the boat or
board and then reduce the component perpendicular to the boat by the resistance
force created by the stuff hanging down below the boat, you find that the net
resultant force, which propels the boat, can be actually somewhat into the wind.
The resistance force created by the keel, etc. is also responsible for a boat
heeling, as you need two forces to create a torque (in steady state). On a
sailboard the weight of the sailor on the upwind edge of the board compensates
for the combination of the downwind force on the board at the mast step and the
upwind force at the daggerboard to prevent the board from heeling. Since the
distance between the two forces (the lever arm) is much shorter on a sailboard
than on a conventional sailboat, there is much less heeling tendency to
counteract.
Dave Rabinowitz
hplabs!hp-pcd!daver