The Right Moment – Part 2

Sail Rocket – now to find a place for all those provisions

re: The Right Moment – Mast Load Calculations

Hi Ted

That’s a great detailed answer and I hope you don’t mind answering what is probably an ignorant question of mine….and I hope my question is understandable.

I was wondering if there was any benefit to a catamaran in being able to move the mast over the upwind hull? From your statement;

“The restoring or righting moment is the product of the vessel weight multiplied by the horizontal distance between the centre of gravity and the leeward hull centre of buoyancy so the value is thus greater when the vessel is heavier.”

I assume that the only benefit would come from the centre of gravity moving slightly towards the windward hull due to the weight of the rig, not from any benefit in the change of (what I’ll call) the “centre of pressure”, meaning where the wind force is applied to the yacht.

Is this a correct assumption or is there an additional benefit to my idea?

Again, really appreciate you taking the time to respond….if you do! 🙂




Your assumption would be correct since the forces at work are rotational ‘moments’ which are not particular as to the geography of the pivot points as long as the axis are consistent. I can’t imagine any justifying aerodynamic benefit to a hypothetically shiftable spar/sailplan for a catamaran but I can imagine lots of major mechanical/engineering problems.

The proa – plenty of room for pigs and chickens

You may be familiar with the proa, which always sails with the main hull to leeward and the lighter outrigger hull to windward. This is done by reversing direction to tack, it being necessary for all the people, pigs and chickens on board to turn around and face the other way once the sail has been brought around to agree with the new course. This may theoretically work (sort of) with two hulls of the same size and I suppose your speculative rig could then be tried out, fixed to one of the hulls. The running lights would also have to be reversible I suppose…

The sailing speed record holder Sail Rocket which is entirely asymmetrical in configuration also comes to mind, but in this case, the boat only sails on one tack. This semi-flying arrangement with its peculiar balance of forces does not conform to any stability diagram I have presented and indeed may not actually be a catamaran as we have discussed. That aside, I think it is a great machine and great achievement. Now if we can just find a place for the provisions…