STEERING MOMENT. You also might want rudders on the leeward hull in order to enhance steerage. When Blind Date went from the standard rudders on the leeward hull, to beam-mounted rudders, it started rounding up into the wind. Just a meter or two on each end can be enough to go from laminar flow to turbulent flow, and that makes all the difference. The further apart those rudders are, the less rudder you'll need at speed in order to maintain a course.
Drag on rudders positioned to windward of the yaw axis could also result in the rounding-up moment you observed. This effect would be worse the farther to windward they are. The same effect (with an opposite sign, of course) would be observed for leeward-positioned rudders.
Micha
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