Center of lateral resistance vs center of effort, where to place sails on cat-ketch?

[mawtty]

Just a note to DonCF.
The faster speed of a cat means that the apparent wind is more to the bow than on a slower mono hense lesser ability to sail close hauled.
On the CLR question you are correct about the helm increasing with heal but not because the CRL changes. it is because the Centre of effort is carried to leward causing a 'moment' .ie force to turn into the wind.
Please dont take offence. For years the healing weather helm question has been unfairly blammed on change in hull shape.

[dustman]

Originally Posted by mawtty

Just a note to DonCF.
The faster speed of a cat means that the apparent wind is more to the bow than on a slower mono hense lesser ability to sail close hauled.
On the CLR question you are correct about the helm increasing with heal but not because the CRL changes. it is because the Centre of effort is carried to leward causing a 'moment' .ie force to turn into the wind.
Please dont take offence. For years the healing weather helm question has been unfairly blammed on change in hull shape.

Are you saying it creates a longer lever arm around the CLR so the same force on a longer lever arm will cause a greater twisting force?


If that is true, doesn't it mean that a catamaran would see very little of this affect since it barely heels?

[Don C L]

Originally Posted by mawtty

Just a note to DonCF.
The faster speed of a cat means that the apparent wind is more to the bow than on a slower mono hense lesser ability to sail close hauled.
On the CLR question you are correct about the helm increasing with heal but not because the CRL changes. it is because the Centre of effort is carried to leward causing a 'moment' .ie force to turn into the wind.
Please dont take offence. For years the healing weather helm question has been unfairly blammed on change in hull shape.

Oh yes but I am thinking of VMG for a good cat with daggerboards.. sure it won't be going as fast as it could off the wind, but it's still going pretty dern fast. But I could certainly be wrong. I have only sailed Hobie 16s, and that was long ago. (I thought the America's Cup cats seemed to do pretty well upwind though!)
I had always thought that the weather helm was exclusively the result of the CE moving aft as you say. But then I read some stuff that challenged that, so now I am curious to hear from the naval architects on that one. I suspect it will end up a combination of both hull and CE movement.
I am always happy to be proven wrong on any of my odd assumptions

[Montanan]

Originally Posted by dustman

Are you saying it creates a longer lever arm around the CLR so the same force on a longer lever arm will cause a greater twisting force?


If that is true, doesn't it mean that a catamaran would see very little of this affect since it barely heels?

While a catamaran doesn't heel over, tilt, as much as a monohull, the effort is transferred similarly athwartship. It is just the effort is transferred more towards the leeward hull and the leeward keel, at least as to the influence of the water. The windward hull retains its stabilizing effect due to the force of gravity. The center of effort is located in three directions, fore and aft, port and starboard, and in depth. Get's rather complex. As to lever arm, the hulls of a catamaran are comparatively widely set around the center point of the boat so there is more leverable objects to work with than with a monohull [half a cat].

[tomtriad]

Ketch rig on a 30' light displacement catamaran? Why? The boat is small enough that a conventional sloop rig would have easily managed sails, with less rigging, weight, and complexity of structural requirements to support the mast(s).
Also, weight is the kiss of death for both stability and performance on a multihull and a ketch rig is bound to weigh more than a sloop rig.
If you are serious about building such a vessel I suggest you get some hands on experience in sailing a few 30' catamarans first. Maybe you will discover that you don't need 2 masts.

[tomtriad]

You could save yourself a lot of grief if you choose to build a known design such as one of Roger Woods'. Heres a link to the Sagitta which is seakindly, a good passage maker, and reasonably fast:
https://www.sailingcatamarans.com/in...ns/171-sagitta

[Scout 30]

Originally Posted by tomtriad

You could save yourself a lot of grief if you choose to build a known design such as one of Roger Woods'. Heres a link to the Sagitta which is seakindly, a good passage maker, and reasonably fast:
https://www.sailingcatamarans.com/in...ns/171-sagitta

Good advice. Added plus is the superior resale value.