Twin Rudders and CE lead???

[Rich_Hamer]

Hi

I'm looking for information regarding the affects of twin rudders on CE lead/Mast/rudder/rig positioning? If anyone here has any theories, rules of thumb or a hypothesis about twin rudders and the effect on balance I would love to hear them!

[Eric Sponberg]

Twin rudders are probably not going to affect the CE calculation all that much, compared to having just one rudder. If the boat has a rather large, long keel, the amount of rudder area, even with twin rudders, is not going to affect the position that much simply because the total rudder area is still relatively small in relation to the keel and the hull underbody. If you want to take total rudder area into account, do a simple area moment calculation with respect to some reference point, say station 0 or the front end of the waterline--the CLP position will change only slightly--and proceed accordingly with the standard rules of thumb. If the boat has a deep and narrow keel, the keel shape, area, and position actually have more influence with respect to CE, in my opinion, than the rudders. In this case, I ignore the rudders and pay attention only to the keel. That is, on a monohull with a deep narrow keel, I usually place the CE directly over the leading edge of the keel, or, if I have to cheat it one way or another, I will cheat the CE aft just a touch, toward more weather helm. Putting the CE forward of the leading edge of the keel results in more lee helm. On a multihull, I learned from other multihull designers a long time ago to place the CE directly over the center of the daggerboard. These rules of thumb have worked well for me.

I hope that helps. I am sure other are going to chime in with their own opinions.

Eric

[Rich_Hamer]

I’m working on a 34ft spirit of tradition design coming in at around 5950 lbs displacement and with a T-keel, I’m considering twin rudders as she is relatively narrow and I’d like here to track more like a long keeled boat and not be a pig to helm in a breeze.

My initial numbers are coming up with a rudder area of around 1.4-1.6% of SA which is approximately half of the keel area; however I’m not sure whether this will still apply with twin rudders? My thinking is that I could maintain the rudder area yet reduce the CE lead to around 6% of D.W.L, as when heeled the lee rudder would be more affective thus bringing the affective CLR aft.

[RHough]

For any given lead there is a yaw moment created. All the rudder system does is counter this yaw. If the yaw is into the wind (weather helm) the rudder must act to weather also. Thus both the keel and rudder are acting to reduce leeway.

The force required to keep the boat on course is a constant. Two rudders are like a biplane, they will be less effective if the total surface area is the same as a single rudder. They will need to have greater total surface area and more drag than a single rudder. On a "relatively narrow" boat there is no concern of centre rudder coming out of the water and ventilating as there is on a wide transom boat like an Open 60, so unless there is a draft concern a single rudder would be a better choice IMO.

5950 lbs for a 34ft boat is very light (D/L = 68), for a very light, narrow boat I assume the draft is deep to provide the righting moment to carry sail? Thus you should not have to worry about rudder draft or any of the other factors that make the extra weight and complexity of a twin rudder system worthy of consideration.