Should cat rudders be slightly off-set?

[YPSILANTI]

I have had my rudders repaired and re-fitted on my cat. A colleague suggested that they should be slightly off-set. What is the correct fitment, perfectly straight or slightly off-set and if so, which way?

[Sailmonkey]

It's a whole other ball of wax, but on my beach cat (Nacra F16) with transom mounted rudders I toe them in (trailing edge spaced greater than leading edge by 2mm).

Mind you, this may not apply to boats with the rudders below the hulls......fwiw.


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[Cotemar]

Maybe this rudder discussion will help.
.
http://www.cruisersforum.com/forums/...ml#post2253452

[YPSILANTI]

thanks so much to both of you.
The discussion was very helpful as well.

[clockwork orange]

I believe what you are refering to is Akerman geometry which is where the rudders are set up to be paralel to each other when going straight ahead but when tacking they both scribe different arcs. Obviously the inner hull is scribing a much tighter radius than the outer hull so if both rudders turn paralel to each other there is more drag than if each hull turns its correct radius. Automobiles have Akkerman geometry built into the steering because if they didnt the tires would scrub out in no time. Not all cats are set up this way but I believe most are. You can see it in its simplest form on beach cats, when the rudders are straight ahead both tillers are angled inward and connected by the tiller crossbar.

Steve.

[GordMay]

Originally Posted by clockwork orange

I believe what you are refering to is Akerman geometry which is where the rudders are set up to be paralel to each other when going straight ahead but when tacking they both scribe different arcs...

Thanks Steve.

A good explanation, and some additional links, here ➥ http://www.swingcat.co.uk/how/bridge...n_steering.pdf

[Cotemar]

They do not use Akerman geometry on Large Production Catamaran, because the boats mostly go straight.

When you have to turn a sharp corner or turn on a dime your rudders are still straight, because you use the engines to turn.

On a Small Catamaran Akerman geometry steering works well.

On a Large Production Catamaran it just adds DRAG.

[smj]

Originally Posted by Cotemar

They do not use Akerman geometry on Large Production Catamaran, because the boats mostly go straight.



When you have to turn a sharp corner or turn on a dime your rudders are still straight, because you use the engines to turn.



On a Small Catamaran Akerman geometry steering works well.



On a Large Production Catamaran it just adds DRAG.

How does it ad drag if the rudders are straight when going in a straight line?


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[Cotemar]

Originally Posted by smj

How does it ad drag if the rudders are straight when going in a straight line?

Correct, but a few here think that the Rudder should be angled.
For Akerman steering the rudders are straight and the tiller arms are toed in.

[boatman61]

On my Wharram Tiki's.. 21 & 26.. I had it so the rudders were set with a very slight turn to the opposite side..
P to S and S to P.
Found it great for holding a steady course on certain angles

[clockwork orange]

With akerman geometry the rudders are straight when sailing straight so have less drag than if they were toed in or out, but without it one rudder will always be scrubbing when tacking so creating more drag which is the last thing you want. I'm not sure how you would set it up on a wheel steered cat, I havn't given it much thought though. I would guess that performance cats would use akerman geometry but don't really know.

Steve.

[thinwater]

Originally Posted by clockwork orange

... I'm not sure how you would set it up on a wheel steered cat, I havn't given it much thought though....

Easy. You still use a cross arm, just as Cotemar illustrated, and then drive just one side with the steering gear, as you would a mono. The cross bar can be hidden inside the aft end of the bridge deck.

In this image I have just tied up the cross bar (disconnected) because I hit a submerged log and bent the port rudder a little. It still steers fine.

[clockwork orange]

Ah yes, that makes sense. I have just not sat down and thought it through. I have an old Gemini which has push pull cables to each rudder with no tiller arms so would require a complete redesign so a low priority but maybe one day.

Steve.