Surface drive rudders pics

[Frosty]

Hers a pic of some rudders I made for my 44 foot 2x250HP cat.

A slightly less chopped version have been tested and are by far the best rudders I have made so far. They corner beautifully and don't allow the boat speed to drop as they did before.

I just kept changing them until I ended up with this Dolphin fin shape.

There used to be 2 legs on the rudder meaning 4 rudders in totall. Chopping one off made the most difference.

They are a wedge rudder using 6mm sharpened and tapered off to12mm.

The prop is placed as it is when fitted.

Any comments criticisms welcome.

[powerabout]

Nice
How did you work out the tiller arms to get the ackerman angles correct in a turn?

[Frosty]

Well you now I asked the forum about that years ago, with little response. Nothing on google could be found that boats even incorporated the Ackerman principle.

So I made an arm with multiple holes to take the ram, I guessed at a bit.

I don't think I have anything like a true Ackerman angle but its not Zero.

When these arms were last used cornering was acceptable.

Im not a mathematician I would'nt know where to start to work out anything like that.

Thank you for your reply and interest.

[powerabout]

I sail a few beach cats ( wife has raced them for years and sells them)
I've had a few converations with the guys that design them and they have done what you have done.
The probem for the beach cat is you can do a variety of turn radii and at different speed you get a different radius with the same tiller angle on your loaded rudder, not something that really happens with a car ( until you get tyre slip)
I know one builder that did a 2 boat side by side test of no ackerman versus various angles and tried to measure who came out of a tack faster. It was so close he didnt get any useful results?
So I think the answer is there is no answer?
I used to fit up a few outboard power cats in Australia ( Shark Cats ) and I did build in some ackerman on both mechanical and hydraulic steering as it was very obvious it needed it or you needed to be a gorilla to make a turn.

Maybe the test we need to do it with load cells on the tillers?

Cheers

[Frosty]

To be honest a full Ackerman principle adopted must be better for a cat of 5 or more meters beam,-- as the rudder angle at a full on corner would need to be very different from each other.

If not the inner( the slowest) would be less effective in its operation.

Its an interesting question which I have given a great deal of thought about but does not seem to interest this forum, as I say I asked this years ago.

Again ,--I woud'nt know how to work it out so I guessed it.

For a car the Ackerman principle is an easy affair as the line drawn between the center of the king pin through the steering ball joint should touch at the back axle.

How that would work in a boat I dont know.

My second wife needs the Ackerman principle to negotiate the turn from the couch to the pantry and we solved that problem with substantial toe-in. (looks good. Frosty)

[Fanie]

I agree, well done.

[Frosty]

Ive just painted the arms and they are going back in tomorrow. I have no idea about how to measure up an Ackerman or if its necessary on boats . I would have appreciated a bit of help or support with this but--- or just a kick about with some ideas.

I guess no one knows. No I don't know everything!

I would take a guess at the Ackerman angle on the rudder arm to be oh say 15 degrees off center rudder!! I suppose thats no where near enough but will discourage stalling a bit.

To go more would mean repositioning of the rudder ram mounts --sigh--

Ille just crack on,-- on my own then.

Ille just have another google before I mount them ,not going to be easy with the boat still in the water, then 2 foot 4 blade props to go back on.

[powerabout]

Frosty
Hard to work out as you need to get the radii of the what each aft hull is doing during during a turn
I think its all trial and error just by seeing if you are dragging the inside rudder in a turn. or not
Clearly some is better then none
You can induce some by setting the rudders up slightly toe'd out ( looking from the aft)
Best of luck
Cheers

[Frosty]

Thats what Akerman does it automatically compensates for differing radius of the hulls by the steering geometry. Its a nifty trick.

Its done by setting the rudder arm to an angle. that at straight ahead is not 90 degrees yet the ram will be working at 90 degrees , the result is a further movement of one of the rudders to the other that is exactly right for the cornering radius.

Cars would be screeching around corners at 10mph hour if it wasnt for Mr Ackerman.

Toe in or toe out is a different thing sir. Depending if your rams are pushing or pulling, toe in or toe out is a preload.
And toe in or toe out on your car depends on wether the track control arm is before or behind the wheels, ie compression or tension.

Pre loading by 2mm will mean they are in perfect alignment when running on the road.