determining ballast req's

[Pernicious Atavist]

hi all! what a great way to pick so many minds--where would i be w/o this forum and you folks? thanks!

now, i'd like to know how to determine the needed lead for a wood centerboard or kick-up rudder. i can't aford to rely on mechanical hold-down devices since i sail in such unpredictable shoals--often at night. as with everything else, there MUST be a formula, right?

thanks--y'all are swell!

[mmd]

M(w) = ((V(b) x Rho) - M(b) x 1.1)

where:

M(w) = mass of ballast (lbs)
V(b) = volume of rudder (cubic feet)
Rho = mass density of water (lbs, 62.5 for fresh water, 64 for salt)
M(b) = mass of rudder (lbs)

[Venchka]

No formulae from me. Just a real world example.

Centerboard:
4'-2" x 1'-6" x 1" thick
6" diameter x 1" thick lead disk 2'-3 1/2" from the centerboard pivot bolt.
Weight of lead disk: 11.6 pounds

[Pernicious Atavist]

one of each--thanks, guys!

[Paul Scheuer]

Weigh the board. Float the bouyant board. Estimate the fraction of the board that is out of the water. That's the fraction of the weight of the board that you will need to add to make it sink. (approximately)

[Venchka]

One more thought-don't expect lead to hold the kick-up rudder blade down. Best to use uphaul and downhaul lines. I think. That's what my plans call for.

[JimConlin]

If you're trying to analyze the mechanics of the thing, think in terms of torque about the pivot, not whether the entire board would sink if unrestrained .

[Venchka]

Originally posted by JimConlin:
If you're trying to analyze the mechanics of the thing, think in terms of torque about the pivot, not whether the entire board would sink if unrestrained .
Originally posted by Venchka:

Centerboard:
4'-2" x 1'-6" x 1" thick
6" diameter x 1" thick lead disk 2'-3 1/2" from the centerboard pivot bolt.
Weight of lead disk: 11.6 poundsWeight of Lead X Length of Lever arm = 26.6 foot pounds of torque.