Ackerman Steering Set-up Tips for a Racing Cat

I'd appreciate any comments on the proper set up of an Ackerman steering compensation system for a racing cat. Thanks........

 

See your post above.

 

OK, now it's below! LOL

 

Doug
Know some racing guys that tested ackerman and non ackerman and couldnt measure the difference around a race course.
Hard to believe but I guess in the big picture maybe so?

 

Ackerman

Thanks-I've heard that twice so far....... I just talked to Matt McDonald(Falcon Marine) and he's going to get back to me.

 

I have thought of this before too, on a road vehicle all 4 wheels make different dia radious in a turn so you have to make the inner wheel make a tighter radious than the outter wheel to prevent scrubing the tires.

On a catamaran, or any boat with a rudder, you are just created a turning moment with the rudder at an angle of attack to the flow stream. If one of two rudders was slightly higher angle of attach in a turn than the other, there is not much ill effect since you turn just as well. in fact you can have one rudder in the middle, or just one on one or the other hull, and turning (yaw) authority is not affected much.

however, it might be a slight advantage to have the inside rudder turn more than the outside. the extra drag on the inside rudder would assist in the yaw, and you would not be fighting the drag on the outside rudder. But there is no geometry I think that would be advantageous.

It might be interesting for someone like Leo to set up a computer program to see what combination of asymmetric angle of attack on twin cat rudders would give the least amount of induced drag in a turn. The various beneficial effects may just cancel. Or it may take some kind of cam or 4 bar linkage to create enough of an angle difference to be noticeable. It might even be beneficial to just have the rudders swing out only, so the starboard rudder is only operates for starboard turns, and port rudder for port turns.

to get "ackerman" on the rudders, what you need to achieve it is to have the tillers "toe-out" (with the rudders at "zero" toe), and than tie them together with a connecting rod. when you steer it pushes the inboard rudder into a higher angle of attack relative to the outboard one.

 

Search at Anarchy found this thread.

http://forums.sailinganarchy.com/index.php?showtopic=157241&hl=ackerman

 

Ackerman

Thanks Red- good info.

 

Hey Doug,
you can find the angle difference geometrically. Of course, assuming a lot of things are as simple as put on paper.

Draw your cat or whatever boat in plan view (XoY), now figure out what is the real pivot point when the boat is turning, you will have to make some assumptions there or even test it in real world.

Once you draw that pivot point just rotate the rudders until their centerlines are tangent to a circle who's center is that turning pivot point. You will find that the 2 rudders have different turning angles in order to be tangent to those 2 circles corresponding to those 2 rudders.

Now, depending on the speed of the boat, the actual pivot point that the boat revolves around when turning, is different for different speeds, so you will have to average something. Obviously it is different also depending on how sharp you want to make the turn. if you start to draw up different scenarios than you can average something out and see if it makes any sense in real world to bother with it.

 

Thanks, Florin!

 

Doug, apologies if I missed this in this thread - the following link to Tom Speer's "Ackerman" paper will help if you haven't seen it

http://hem.bredband.net/b262106/Boat/TSpeer_acker.pdf

Cheers,
TomH

 

Ackerman

Thanks for trying ,Tom-but red's link at SA(amazingly) had the link and some other good stuff. The cool thing is that, as I learn more about it, it seems there may be an advantage to changing the setting for different speeds/conditions-and each rudder will be hooked to a separate servo so I'll be able to program them for the conditions!