How does purchase work with centerboard hoist?

Hello All,

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?


Phil Hertzler

FS 4879



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.



Phil Scheetz

FS 4086

Fleet 163, Nockamixon Sail Club

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?

Phil Hertzler

FS 4879