Multihull CLP vs COE

Looked through some previous threads and the jury seems still out of a definitive way to position center / dagger boards for and aft.

To use a dumb quote (or a quote from a man I have little respect for) which explains the situation well:

"We know there are known knowns: there are things we know we know. We also know there are known unknowns: that is to say we know there are things we know we don't know. But there are also unknown unknowns — the ones we don't know we don't know." —Defense Secretary Donald Rumsfeld, Defense Department briefing, Fe. 12, 2002

The location of the CLR is +/- 2% of water line of the COE of the sails - basically in line - ok what goes into CLR calculations?

Many designers reduce the significance of the rudder in the CLR by 1/2 to 1/3 this would be to avoid stall is it reasonable to reduce rudder area by 1/3?


Many designers do not use the Hull CLR at all as a round hull cannot produce much lift compared to aerofoil. Is it reasonable to eliminate underwater profile of say a catamaran from CLR?

Thanks
Alex

 

Multihulls are long and skinny, so the rules of thumb for monohulls don't quite apply. The lateral resistance of the hulls is considerable. I know that is not a precise term, but it really depends on the design. Most multihulls have slab sides which should be considered.

 

There is nothing unknown about this subject, so I'm at a loss as to why you think there may be some smoke and mirrors? Some small inaccuracies aside.

The CLR of the hull is simply the buttock plane area centroid. It has a low lift coefficient and varies depending on the shape of the hull- hence the small inaccuracies.

The boards and rudders have a higher lift coefficient, but the net moments can accurately be summed with the hull CLR to get the overall CLR. Better still, if the boards are placed inline with the hull centroid, then no further calculation is required, the center of both is still the center, only the magnitude can change not its location. The sail plan can be designed to line up with this, and everything is balanced.

The rudders should be included in the CLR, as they will contribute regardless of whether they *should* or shouldn't... They don't have a choice in the matter ... They are generally made of a thicker foil section and /or lower aspect ratio compared to the boards to avoid stalling.

 

Arekisir, From the knowledge collected from all my multihull designer friends, the solution is quit simple: Place the center of area of the daggerboard directly underneath the center of the sail area. That provides the best sailing balance for multihulls. You do not need to bother about the underwater profile area of the hull or the rudders--they are what they are. The daggerboards provide the lion's share of the lift, and that is all that the sails care about. To repeat: Place the center of area of the daggerboard directly underneath the center of the sail area.

Eric

 

"Place the center of area of the dagger board directly underneath the center of the sail area. "

Thanks Eric

Final quote from Sir Robert Muldoon, Prime Minister of New Zealand from 1975-1984 - New Zealanders moving to Australia “raised the IQ of both countries”.