Agree with infinity and zero seperation. Calculating the optimum is impossible as the hull influence changes with bow down/stern up changes, which are related to wind, waves, speed and point of sail. All we can do is experiment for the optimum and lift/lower rudders for the fine trim. The changes are not huge, so we need to be more advanced in terms of speed analysis/two boat testing before we know for sure.
I used the forward rudder for steering and the aft one for leeway reduction on Rare Bird which worked much better than the other way round. On El, the single rudder on the aft beam works well in normal trim, even better if I sit forward.
"The limit has not been reached" is based on the mono hull America's Cup boats which have two steerable fins supporting a long lead bulb and no rudder. The better the boat/crew shunts, the closer they can be together. Practical considerations outweigh ultimate performance at this stage of the development.
regards,
rob
in the limit of zero rudder separation all turning the rudder does is prevent (or magnify) leeway, much like a tacking centerboard.
in the limit of infinite separation the yaw-torque moment is so much bigger (due to the bigger lever arm) than the leeway-translation force that the latter essentially doesn't exist-- all turning the rudder(s) do(es) is turn the boat.
in the absence of being able to change the sail's center of effort, it seems to me there is on _optimum_ separation of rudders, in which the angle of attack needed to counter weather helm also _exactly_ counters sideslip.
is there an a priori way to calculate how to position the rudder(s) to achieve this balance? or is it more of an educated guess, build it, and see if it's close??
rob-- by saying we're not at the limit, do you mean that rudders could be spaced even closer than on your existing builds? to me that implies that there is too much sideslip for a given amount of weather helm (ie CoR,CoE balance). if so, one could partially lower the forward rudder to increase the weather helm. so long as that doesn't cause the aft rudder to stall.
ben arthur
weta #358
and dreaming of a big proa
rob said:
> Obviously, there is a limit, but I don't think we are there yet. The
> rudders do have to be bigger, but as they are doing the job of leeway
> prevention as well, this is not a big deal.
dennis said:
> > Is there a term for this front to back moment arm between rudders - it
> > needs a unique identifier - *Micha-arm* Anyway, I can imagine Micha's
> > description. As (*it) *gets narrower, the rudders have to compensate with
> > more angle of attack to get the same yawing moment. At some point, they'll
> > stall. OR you'd have to up-size them to get the same yawing moment. Either
> > way creates more drag. Do you feel you would have to up-size the rudders
> > because of the narrower moment arm?