Sailing Catamaran steering Geometry

[lyndon]

I need information on the geometry for the setup of the rudders in relation to each other and the design principals to make the inner rudder turn more than the outer rudder.

[tspeer]

Look up "Ackermann steering geometry" for cars. It's essentially the same problem for catamarans.

The easiest way to achieve it is to use tillers to control the two rudders, linked together with a bar connecting the ends of the tillers. If you angle the tillers toward the centerline, the inside rudder will be turned more than the outside rudder.

How much to angle the tillers depends on what you are trying to achieve. The tighter the turning radius, the more Ackermann angle to use. As a quick approximation, draw lines perpendicular to the keels/boards at their center of lateral resistance (say, the quarter chord) and perpendicular to the plane of each rudder and board. You want all three lines to meet at the center of the turning radius. This sets the amount of differential deflection of the rudders.

I've done a more complete analysis of the effects of Ackermann geometry on the turning of catamarans, including the effect of the leeway angle on boards and rudders. If you're really into the math, I can dig it up.

I think most owners have found 10 - 15 degrees of toe-in to the tillers to be adequate.

[sorenfdk]

Tom: If you can dig up your analysis of the Akerman geometry, I would be most interested!

Best regards,
Søren Flening

[lyndon]

Thanks TSPEER.
I understand the principal and would appreciate your further information on this. We have a problem in that the people manufacturing a 40ft cat for us dont seem to understand this principal and I am trying to gather some information to put to them. If you have or know where i can find info refering to Catamarans this would als be greatly appreciated.

[lyndon]

Hi Tom,
Thanks for your reply. These details would be a great help. Please email me if possible at callen@cqe.com.au

Thanks a lot,

Lyndon