twin rudder design

[bigbear69]

Hi everyone,
I'm doing up a 7.8 m trailer sailer and changing it around a little bit. I am thinking of having a twin rudder setup and was after some design advice. Was planning on having twin blades in cassettes so I can raise and lower each rudder if I need to and for trailering.
would like some recommendations on plan shape, area, foil type etc
I've been trying to find anything on a mini transit 6.5 twin rudder setup but not having much luck.
Any help much appreciated

 

[Doug Lord]

Twin rudders

I ran across this-their thinking may help: http://www.i550.org/forum/index.php?topic=9657.0

 

[PAR]

Foil shapes and to a lesser degree plan forms will be dictated by the needs of the boat. Simply put, the boat needs to have to potential to make a twin setup viable. Unless you're racing there's no reason for the added complexity and complication. You're not going to gain much in terms of speed or pointing ability either, but in a race format against other exactly the same boats, there will be a very modest advantage on a twin rudder setup.

It would be more helpful to know what boat this is going on (year, make and model).

 

[bigbear69]

twin rudders

Boat is a Harold evans designed 780 express. designed and built in NZ ages ago. Maybe 70's.
It is a hard chined plywood design. very light with bulb drop keel. 7.8 m long

I will post up some pics when I get the chance.
cheers

 

[bigbear69]

photos of boat

here are a few photos of the boat I'm working on. and what I'm thinking of doing to it. Basically just widening the transom each side to take the gunnels out to just under maximum beam which is 2.4m. prob build the extensions out of foam and shape to fit and glass over.
will make anew transom from ply and glass.
hang twin rudders on the first chine divide the chine angle. rudders will be approx. 15 deg.
adding around 300 L in ballast tanks in lockers under the comings.

I have already widened the cockpit and moved the cabin top forwards.

rudders will be in cassettes so I can raise and lower.
From info so far was going to shape a naca 0012 profile rudder not sure on the chord length maybe 300-400mm will make rudders longer than necessary then trial and error by pulling more rudder out of the water then chop the top of the rudder off when I have found the length that works.

well that's the plan anyway.

feel free to offer your thoughts and advice
very much appreciated

Boat is intended for a club day racer short handed or crew of 4-5

cheers

 

[Moggy]

Maybe something here...

http://www.dixdesign.com/didimini.htm

http://www.dixdesign.com/didiminiMk3.htm

http://www.dixdesign.com/didiminicruise.htm

http://www.dixdesign.com/didiminicruiseMk3.htm