Height of rig above waterline...

Bolger remarked about the Jessie Cooper design, " since the height of a rig a boat can stand has to be figured from the waterline regardless of her freeboard".......has left me with a few questions.

An assumption on my part is he is referring to righting moment, without specifically stating it?

Im debating a small "get home" rig for a small motorboat with a wheelhouse, that will push the rig "up". I am aware that trying to keep the centre of effort of the sail low will require a shorter mast. Having seen all manner of different rigs used as auxillary sail. I see little gain in going high on a shallow hull as it wont point well anyway.

But the "from the waterline" comment stood out......as i have not seen that before. Is it a common formula I have not come across?

 

Re righting moment, yes, this is going to play an important part.
If you have high freeboard, then (all else being equal) your down flooding angle of heel (when water starts to seep over the gunwhale) is going to be greater than if you have less freeboard.

Do you have any sketches that you can post of the small motor boat with the wheel house, and showing what types of rigs you are considering?
There is a lot of expertise on this forum which can probably offer you some good advice.

 

I do not have anything to hand. I have already done ballast/righting moment and stability calculations. I was more intrigued by the Bolger comment "from the waterline". I worked out sail carrying power from the righting moment, and the centre of gravity/bouyancy is not "at the waterline". I may be thinking too much about his comment, it may as well be from the deck, coachroof or wheelhouse, dependent on sail area and force moments. Maybe because the boat rotates around the waterline? Have not read any explanations by him for that comment.

 

"he is referring to righting moment, without specifically stating it?"

My impression is that yes

On the other hand the Aspect Ratio (Height squared / Area of the sails) is not so important, as its influence on the Aero Drag is lost so to speak among a lot of other things.

 

Beowulf (Alu, 79 LWL) beat in real time Mari Cha III (150 (?) LWL which I seem to remember was made of carbon fiber and in any case was designed to break speed records)

Certainly this race in the Caribbean was not upwind; but it is a nice illustration against the prevailing trend of high rigs and a reminder of the power of setting sails Low.

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147 LOA, Carbon Kevlar Sandwich
125 LWL

MARI-CHA III https://www.mari-cha.com/

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I think tall rigs are a waste unless provided with an appropriately deep keel to go with it. Exceptions being tall ships and old sail cargo vessels.
Thanks for the clarification; I will take Bolgers comment as meaning above righting moment.