Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/453



is scarcely necessary to point out that the peripteral theory set forth in the present work is capable of wider application than to the problems concerned in aerial flight.

The sailing boat, for example, offers a very promising field for the application of the peripteral principles of flight, and furnishes strong confirmation of the present theory. We may look upon the sailing boat, and especially the racing craft with its fin or deep keel, as an aerofoil combination in which the under-water and above-water reactions balance one another.

Laying on one side for subsequent consideration the part of the problem that relates to the heeling of the vessel and its stability, we may treat the matter in the first instance as if the under-water and above-water forces lie in one horizontal plane. Under these conditions the problem resolves itself into an aerofoil combination in which the aerofoil acting in the air (the sail spread) and that acting under water (the keel, fin, or dagger plate) mutually supply each other's reaction.

The result of this supposition is evidently that the minimum angle at which the boat can shape its course relatively to the wind is the sum of the under and above-water gliding angles.

If the boat had no body (hull), and the conditions of our supposition be complied with, this reasoning shows that the minimum angle of the course relatively to the apparent direction of the wind would be the sum of the γ for water and the γ for air, which is probably a degree or so less than 20 degrees, or rather less than two "points."