Akermann Steering

[Figgy]

I'm basically blowing up Tornado plans to 150% for a fast 30' cat. Among other things to consider, I was thinking about the steering. A quick search brought up a spread in Professional BoatBuilder about Akermann Steering. A quick quote:
"When both rudders go over at the same angle, the outboardrudder is actually too far over and stalling somewhat (excess drag), while the inboard rudder isn't over far enough (insufficient steering response)."
Basically I'll make it so I have a 10 deg. in-angle. Is this something worth considering in my design? If I apply this for a cat, would I even notice? Do A-cats use this type of steerage now?
I could not post the picture, it's a PNG file and my version of Photoshop sucks.
Thanks for your help. Opinions always welcome!

[BillyDoc]

Figgy,

I don't know how much difference it would make, but it seems to me that the linkage for an Ackermann-principle dual rudder system is simple enough that it is worth trying. I posted an image for automobile Ackermann below (from: http://en.wikipedia.org/wiki/Ackerma...ring_geometry). To apply this to a boat it seems to me you could use the center of lateral hull resistance as the equivalent of the point shown on the diagram centered on the differential and arrange the linkages to the rudders as shown.

This is just a guess on my part, however!

Bill

[Corpus Skipper]

There was a good article on this in the last issue of Professional Boatbuilder. Don't know how much difference it would make in a cat, they weren't mentioned in the article.

[Figgy]

Corpus Skiper- Pro Boatbuilder is where I got the idea from. I never really gave the steering too much thought untill I saw it. The idea came when they mentioned twin-screws boats on pg.104. I just thought it might be applied in my case.
BillyDoc- Thats a good idea! I wasnt really sure it was an idea worth consideration. The more I thought about it, the more it made sense. I'm curious if its worth the time and effort....it doesnt matter, I'm going to do it anyway
Thanks!!

[SailDesign]

Figgy,
WE used to use this on BOC-style boats with twin rudders. In the "dead ahead" position, aim the tiller arms at the boat's pivot point (guess at it, it'll be close to the keel root). That usually does it.
Also, be aware of the flow from the transom. Do not just aim both rudders dead ahead and forget it - check where they have least drag (hint - it will probably mean you have to toe them OUT)
Steve

[Figgy]

Originally Posted by SailDesign

check where they have least drag (hint - it will probably mean you have to toe them OUT)

In the dead ahead position on a 30' cat, why wouldnt that induce drag? I see how that would work for turning, but it seems to come at a price. How well did it work for you? Did you notice a difference in performance?

[SailDesign]

If you're talking about a cat, then you are not going to have to worry about toe-in - I was talking monohulls

Steve

[SailDesign]

Figgy - the tiller ARMS should be aimed at the pivot point, when the RUDDERS themselves are aimed dead ahead.
Steve