Yep, pretty simple in theory. The bottom hull acts as the fulcrum. The mast float has to counter all the moments that are rotating the boat into an upside down position. This would be the weight of the rig, the weight of the top hull, and the wind force acting on the bottom of the boat.

So, multiply the weight of the rig (assume 100lb) times the horizontal distance of the rig's center of mass from the bottom hull (assume 17ft) plus the weight of the upper hull (assume 100lb) times it's horizontal distance from the bottom hull (say 1 foot) plus the wind force on the tramp (say 50 lb) times the vertical distance of the center of effort of this force from the bottom hull (say 4ft) and this gives you 2000 lbft. Since the mast float is located 34ft horizontally from the bottom hull, the required bouyancy is 2000lbft / 34ft = 59lb. At 8lb/gal water density (bouyancy), this would equate to about 7.5 gallons for the mast float.

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