Twin Rudders.

[whaletail38]

I have designed a very beamy, dishy, water ballasted trailerable 9.0 m rocket that I'd like to fit twin rudders to. As a general rule, how far off the centreline and at what angle to vertical do these work best please?
Thanks for your time, Glenn.

[tspeer]

I'd mount them perpendicular to the hull at the point where the leeward rudder will be vertical with the hull at its designed heel angle. For an inland lakes scow, like my old M-16, this was 15 degrees. The scows also made the rudders short enough that the windward rudder was out of the water. But I'd go for a much deeper rudder.

[yipster]

ok i see 2 rudders as the waterline shifts sideways at a wide stern under heeling.
the keel however shifts from pointing longtitudal to downwind than i belive.
is that a compromise or a good keel line transformation for some reason?

[yipster]

hi all, was expecting a few quick reply’s believing it was a simple to explain behavior I did not see and later realized I was kicking AC boats.
already started thinking its one of many trade-offs but than I realized that a downwind aiming heeled keel gives positive hydrodynamic side force. remembered the “adjustable lift keel” (http://www.boatdesign.net/forums/showthread.php?t=5798&highlight=lifting+body ) thread where keel lift was discussed but did not fully comprehended the waterline/keel action.
http://rodgermartindesign.com/images/49/pic2.jpg
the rotating y-axis keel (http://rodgermartindesign.com/binfo_result.asp?ID=49) makes it even in longitude adjustable as does the trim tab on the Amazon racer. said was that the point is how to produce the required lift with the minimum drag. Only now I see and promise to read before thinking next time...