How does purchase work with centerboard hoist?

Submitted by Phil Hertzler on Sat, 11/02/2013 - 12:29
Forums: 
Owning, Rigging, and Sailing the Flying Scot

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?
Thanks,
Phil Hertzler
FS 4879

sawyerspadre

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.
 
Phil

Phil Hertzler

Mon, 11/04/2013 - 23:00

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?