Best rudder foil shape

I am enquiring as to the best rudder shape for a 60 foot semi displacement boat, running twin 620 hp engines. The rudders are free hanging directly behind the props, supported of course by bearings etc etc inside the boat.

Looking at NACA shapes, it appears 0020 area is appropriate, but I would like some advice from those that know. I am concerned about how much lead area would be introduced if I use lower NACA numbers, and its effect on steering hydraulics etc etc.

Kind regards.

 

Unfortunately, its not quite that simple. I've used the 00xx series sections before and you sometimes find that the section is sometimes driven by required plate thickness, and stock dimensions rather than an optimum "lifting" surface.

 

And the foil is only part of the game.

The planform is also important.

See http://www.becker-marine-systems.com/03_products_content/03_products_pdf/becker_tlksr.pdf

I have see I do not remember where, a rudder with a continuous varying twist along the heigh, following the direction of the rotational flow behind the propeller.

The cord was also varying, giving the rudder kind of sigma form. Long chord top and bottom, small chord at middle height.

It was on a navy ship.

 

NACA 0020 should be a good section for your operating regime. It will continue to perform at high angles of attack so give large steering force for it area.

You might be able to come up with something better if you spend a lot of time at it. There is much discussion on this thread:
http://www.boatdesign.net/forums/showthread.php?t=18688

The most efficient planform for a foil in undisturbed flow is half an ellipse with the upper edge close to the line of the hull. With twin 620HP I doubt that the efficiency of the planform will be off great concern.

Rick W.

 

Have look at some Professional Boatbuilder articles. The Maclear Thistle rudder in this article might be of interest.

http://www.proboat-digital.com/proboat/200608/?pg=63

 

FCFC, Hey the Becker rudder is certainly a different way of doing things, very interesting.

Others should folloy the web site, it is a logical thought.

 

I would use a 00 section. I find it the best section for rudders as it provides lift through higher angles of attack without stalling. It is also a well balanced foil shape for the rudder application. You will get good balance without to much foil being forward of the stock. It's been a while but I think 15% of chord is a good start.

Good luck.