This is a very interesting discussion, thanks to all who have contributed.
I suggested putting the rudders on the beams of BD and am very disappointed that they are not working. My suggestion was based on sailing Elementarry, initially with a rudder on each beam, and then just one rudder. El is a half size version of BD and with my weight (90 kgs) on the ww hull has approximately a 50/50 weight distribution between the hulls, which are 4 and 7.5 m long. This is a worse theoretical case for rounding up than BD. I sail El with a mainsail only rig and have sailed it with 2 on board making it theoretically much worse. My beam rudders are set 750mm inboard, about the same percentage as the new set up on BD.
Because my rudder was originally designed for a schooner rig, it is a bigger proportion of the sail area than BD's. 0.3 sqm and 11 sqm (2.7%) on EL vs 1.5 and 72 (2.1%) per rudder on BD . However, BD uses both so has 4.2%.
Rudder and balance related experience:
On all my boats I have found that once it is moving, the rudders have more than enough power to overcome any imbalance in the coe/clr. The only time there can be a problem is getting going after a shunt if the boat is starting from closer to the wind than 90 degrees. This is much reduced if the leech at the top of the main is not hooking to windward. Easiest way to prevent this is to apply topping lift to bend the mast and flatten the sail (big boat) or to loosen the battens and increase the luff downhaul (little boat).
We found on Rare Bird (heavier sister ship of BD) that locking the stern rudder and steering with the bow rudder was not only much less load on the wheel but allowed it to point much higher and reduce speed much more and still steer back on course. This was a big breakthrough in sailing the boat upwind. At least part of this was that the aft rudder would trap a lot of water between itself and the hull when bearing away which served as a brake. For this reason, I would not put the rudders too close to the hull.
On U I used a lee board with small rudders. It worked after a fashion, but there are some very weird load situations with these at high speed. They need a lot more restraining than the heavy, slow speed lee boards commonly used.
Rare Bird sails virtually level (see the video at http://www.youtube.
I appreciate that some of the above seems contrary to what Rudoph has found and am obviously perplexed as to why. Hopefully we can continue discussing and experimenting and sort out what is going on.
For what it is worth, on Solitarry I am using side of the hull mounted rudders (1% of sail area each) far enough away from the hull to allow 360 degree rotation, and plan to use both upwind in light air, and only one the rest of the time. They are mounted 2.5m either side of the mast, which is 17% of loa, compared to 25% on El and the new BD rudders and 31% on Rare Bird and the old BD. They are not on the beams as it only has one.
Regards,
Rob
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