Ackerman angle on cats

[Stellar1970Cat]

My old catamaran does not have built in Ackerman angle.
The the rudder arms point at same direction no matter what is the rudder angle.
I am familiar with Ackerman principle as I have been building tadpole type 3-wheel kick scooters.
Many cats have some Ackerman angle but mine does not.
The steering is with wheels at helm and cable looped to the tie rod.
Tie rod and rudder arms are fully exposed so it would be easy enough to modify.
I have currently a sliight toe in which helps the boat tracking straight.
I suspect my tacking performance and speed when turning would improve a little bit if I had proper Ackerman angle?
The picture attached shows part of the steering linkage.

[ggray]

I assume the tillers are straight and lined up with the rudder?

Is the pin connecting the tierod to the tillers right in the middle of the tiller? If it is on the inboard side of the tiller you would have a bit of Ackerman geometry.

It probabbly doesn't make a whole lot of difference, but if easy to effect, why not?

[Dave_S]

I have Ackerman and can only believe it is a good thing, at least it makes sense to me. I can tack with either the jib and no main or main and no jib, it is a lightweight boat and maybe it would do it anyway but I put it down to reduction in drag from the benifits of Ackerman steering.

[tamicatana]

Don’t think it’s all that for cruisy cats, but here ya go

http://www.swingcat.co.uk/how/bridge...n_steering.pdf

https://forums.sailinganarchy.com/in...prove-tacking/

[tkeithlu]

This is a question, not an opinion. It would appear from the first table in http://www.swingcat.co.uk/how/bridge...n_steering.pdf that once the turning radius passes two times the length of the boat that the Ackerman angle is reduced to a few degrees. Do cats make such small radius turns in coming about? Obviously they do in manuveuring to dock, in which case the inefficiency is immaterial, but how much advantage is there to setting up asymmetric rudders in the usual cruising scenario? Does the turbulance around the inside rudder significantly slow the boat in coming about?

Of course, if there is no added complexity to the setup, even a tiny advantage is just that and worthwhile.

[tamicatana]

The rudders are not assymmetric, the tiller arms are merely bent slightly inward. Google also 'tom speer ackerman' for Speer's paper on the subject

[tkeithlu]

Sorry. I didn't mean that the rudders were different shapes. I meant that the rudder angle is different for the two rudders during a turn - Ackermann angle.

[Dave_S]

Originally Posted by tamicatana

Don’t think it’s all that for cruisy cats, but here ya go

http://www.swingcat.co.uk/how/bridge...n_steering.pdf

https://forums.sailinganarchy.com/in...prove-tacking/

I can't open the first link, I get that stooooopid 404 error.

I'm not sure the second link has the right people commenting.

[vpbarkley]

To calculate Ackerman angle you need to know the location of the center of rotation. This is possible for terrestrial vehicles conditional on the wheels not slipping.

The center of rotation is not definable for a catamaran because it's in water and slips continually.

[Dave_S]

Originally Posted by vpbarkley

To calculate Ackerman angle you need to know the location of the center of rotation. This is possible for terrestrial vehicles conditional on the wheels not slipping.

The center of rotation is not definable for a catamaran because it's in water and slips continually.

Interesting working out how much you need.

A little bit too much is probably a good thing (or maybe better than not enough) and a bit of drag on the inside will help rotate the boat. Don't forget the inside has less speed through the water so it can live with a higher angle of attack as well.

What is everyone's thoughts?

Maybe pick 3 probable arcs. Wide, medium and tight then calculate the actual rudder angles to suit each side including a necessary AOA then add again something like 1, 2.5, and 4° extra for the differential in water speed? Of course it will depend on which arc radius you chose.

[Stellar1970Cat]

Originally Posted by ggray

I assume the tillers are straight and lined up with the rudder?



Is the pin connecting the tierod to the tillers right in the middle of the tiller? If it is on the inboard side of the tiller you would have a bit of Ackerman geometry.



It probabbly doesn't make a whole lot of difference, but if easy to effect, why not?

Yes they are straight with rudders. So no Ackerman angle but has got a slight toe in.
I am thinking of experimenting with current tie rod removed and making adjustable length tie rod with some plates with holes inwards from the tiller tubes. That way I can go back to original setup if not much difference?

I made a quick sketch on how I do the temporary trial setup.
Not in scale.
The tiller arms can not be angled or bent that much until they hit the davit frame.
So I was thinking of making triangular aluminium plates to change the angle and not hit the davit frames on either side.
Then using a wooden pole with holes on right distance. So the modified setup will be same length than original cross bar.
Attachment 247770

[Dave_S]

If you break the attachment from one rudder to the other and if they are free to move. If you move the outside one say 30° and let the inside one find its angle of least resistance you will have a 0° AOA to compare, you could also then have someone else move it 5° +/- to see the effectiveness of Ackerman for yourself.

[catsketcher]

Originally Posted by Dave_S

If you break the attachment from one rudder to the other and if they are free to move. If you move the outside one say 30° and let the inside one find its angle of least resistance you will have a 0° AOA to compare, you could also then have someone else move it 5° +/- to see the effectiveness of Ackerman for yourself.

I was going to suggest the same thing. Do manouvres with one tiller attachment undone and watch the amount of difference. Repeat with iside and outside rudders. I also agree that it is not quite the same as car steering as car steering does not have foils at an angle of attack. The slower inside rudder can, and should be, at a higher angle of attack than the fast outside one. I would prefer to inside one to have higher drag to help turn the cat. Also the rate of turn changes through a tack. The turn rate starts slow and increases as the boat slows, so Ackerman may be more useful at the slower parts of the tack.

I have Ackerman on all my cats and they all tack really well, but is also due to big rudders.

[Stellar1970Cat]

Originally Posted by Dave_S

If you break the attachment from one rudder to the other and if they are free to move. If you move the outside one say 30° and let the inside one find its angle of least resistance you will have a 0° AOA to compare, you could also then have someone else move it 5° +/- to see the effectiveness of Ackerman for yourself.

Absolutely stellar idea!
This would save me doing a temporary setup and I could go straight to making a final Ackerman angle setup.
I just need another crew member to try this so I can measure the angles on a calm day, without waves and current. Have to repeat this both ways turning port and starboard.