No idea of the term. Not many boats use it. The AC guys try it every so often with spectacular success, sometimes.
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. These big rudders have to be very strong, but with carbon, pressure moulding and beam mounted rudders, this is not such a big deal. The advantages of not having daggerboards or keels far outweigh any disadvantages. Beats me why cats don't use them.
rob
Rob,Is there a term for this front to back moment arm between rudders - it needs a unique identifier - Micha-armAnyway, 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?
Dennis
From: Rob Denney <harryproa@gmail.com >Sent: Thu, July 29, 2010 8:07:07 AM
Subject: Re: [harryproa] Re: 60' Trailerable Proa
Maybe. The jury is still out on Blind date and what is happening.
The rudders on my Solitarry will be on the beams which are 5m apart on a 15m lee hull. I see no reason why they won't work, based on my little boat tests.
rob
On Thu, Jul 29, 2010 at 9:16 PM, Dennis Cox <dec720@att.net> wrote:
Micha,I caught the yaw issue, but having the separation wider (fore/aft) than the beams didn't cross my mind. I guess I assumed that some boats having it on the beams was good enough. As it is... my beams are only 24' (out of 60') apart. So, I'd have the issues you're describing. So either I have to have all that appendage down the hull with the resulting loads through the hull and trailering issues, or I have to splay the beams. Too many things were relying on the beams being parallel. This will take some serious head scratching.Thanks,Dennis
To: harryproa@yahoogrou ps.com.au
Sent: Wed, July 28, 2010 6:52:46 PM
Subject: Re: [harryproa] Re: 60' Trailerable Proa
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