How does purchase work with centerboard hoist?
Submitted by Phil Hertzler on Sat, 11/02/2013 - 12:29
I'm curious as to how the purchase works on the centerboard hoist system. My physics/engineering is not good enough to work it out. Is the wheel generating mechanical advantage or just taking up the line while the purchase is generated elsewhere?
Sat, 11/02/2013 - 21:55
The centerboard drum adds mechanical advantage . I think it is 16 to 1, if I remember correctly. It also depends on if your line or cable to the CB is two to one or one to one.
Mon, 11/04/2013 - 23:00
f = R/r * F
I think I found the answer with the help of "Physics of Sailing" by John Kimball. My cable from the centerboard to the cylinder is 2:1 and I suppose the increased diameter of the drum that takes up the pennant provides the leverage. The equation is f = 2R/r * F where f is the force of the centerboard, r is the radius of the cylinder that winds the wire and R is the radius of the drum (minus the radius of the cylinder). If R = 8" and r = 1" (or any ratio thereof) than raising a centerboard weight of 96 pounds would require 6 pounds of pull on the pennant, increasing as turns are taken up on the drum. Sound reasonable?