Rudders on Catamarans - toe in, out, neither?

[geoffr]

We have just removed the rudders off our Prout Snowgoose as they are getting new interprotect and antifowling along with fixing the tiller arm to tie bar connection.

When I put them back on should they toe inwards? outwards or be symmetrical? (I think they were toed slighly inwards when I took them off).

I'm pretty sure outwards is a bad idea and inwards might be...

[Cotemar]

They should be parallel.

When centered, they should measure the same dimension on the leading edge , from rudder to rudder as on the trailing edges rudder to rudder.

No toe in or out

[a64pilot]

Toed in or out would just increase drag?

[donradcliffe]

Check out the Hobie tuning guides. After decades of racing, they all call for some toe in.

[a64pilot]

Originally Posted by donradcliffe

Check out the Hobie tuning guides. After decades of racing, they all call for some toe in.

I assume a lot of that racing is done with one rudder in the air?
Hobie's used to have asymmetrical hulls, I assume they still do?

[thinwater]

Originally Posted by donradcliffe

Check out the Hobie tuning guides. After decades of racing, they all call for some toe in.

I get that, it is a good point (I used to race beach cats), but not relevant since part of the windward rudder is always exposed. Thus, the windward rudder is functioning at a different aspect ratio, hence different angle of attack.

And a Prout is not a beach cat!

[mikereed100]

they should be parallel. If they were toed in then they require a bit of adjustment. Some cats employ Ackerman geometry with the rudders, but I believe this is more useful on cars than boats and would still be parallel when amidships.

[thomm225]

Originally Posted by a64pilot

I assume a lot of that racing is done with one rudder in the air?
Hobie's used to have asymmetrical hulls, I assume they still do?

I believe only the Hobie 14 and 16 had the asymmetrical hulls. The Hobie 18, F-18 and 20 and the rest of the newer racing boats did not because they used daggerboards

[thomm225]

But speaking of multihulls and Hobie 16's, I feel I just have to post this super well made video.

Great way to get into sailing and be physically active

https://www.youtube.com/watch?v=QO0LkUDALNk

[geoffr]

Thanks all - I will go with parallel.

Geoff
ps. Not surprising - Prouts do not have Ackerman geometry - it's a straight tiller off the rudder stock.

[MarathonJohn]

great, great video on the thrills and fun of 14-16' cats. A Venture 14 'banana hull gave many thrills of quick maneuver and fast acceleration. A bit frustrating when your mate wants no more than easy sail time and you want to ride like those in that video. From her end, we were in our sixties.

[44'cruisingcat]

Originally Posted by geoffr

We have just removed the rudders off our Prout Snowgoose as they are getting new interprotect and antifowling along with fixing the tiller arm to tie bar connection.

Antifowling on a Snowgoose. I like it.

[Seaslug Caravan]

Originally Posted by donradcliffe

Check out the Hobie tuning guides. After decades of racing, they all call for some toe in.

You are correct. The hobie 14/16 had toe in. The reasoning being that as the boats had a lot of aft rig rake and hence weather helm, the helm required a continual pressure to counteract this. By having toe in the windward blade which is pretty airated and doing very little is now aligned perfectly fore and aft and providing minimal drag. Some were skillful enough to sail to windward with the windward blade raised. All raised the windward blade off the wind.

As most have said your blades should be aligned.

[a64pilot]

I am the furthest thing from a racer, but if the rudders were toed in and you lifted one out of the water, then the other blade will seek the least resistance and will align itself and all the toe will be on the blade in the air.

Maybe it has to do with the asymmetrical hull and the water flow off of that, so that in fact with a little toe in, that it is aligned with the water flow and therefore having the least drag possible?

Just a guess

[Dave_S]

I am surprised that all cats don't have Ackerman built in, it must help a great deal when tacking.

Are there any reasons against. It is simple geometry and at very little or no cost at design stage.