Catamaran offset rudder placement???

[JIM MARCUM]

I've seen OS rudders placed 1 1/2" ahead of, parallel to, and up to 4" behind the prop on single motor cats. Not being a Hydrodynamic Engineer, I would really like to know the advantages, disadvantages, and recommendations on these various OS rudder applications, OS placement effect on prop thrust, and efficent rudder trim.

As to what effect OS rudder placement has on the prop, I'm clueless. A boat that pulls left or right will require the rudder to be trimed to go straight. The further the rudder is adjusted off axis the greater the rudders drag. My goal is to maximize prop thrust efficency (rudder placement), and minimize parasitic rudder drag (off axis alignment).

Anyone out there know what I'm talking about? JIM

[lenny]

You are looking at this for saw application or all around running ?

[Old Sloppy]

Single prop or twins ?

If twins, counter rotatiog ?

I owned a single motor cat (Drifter S) I mounted the rudder on the left sponson, this mounting exactly ballanced the prop torque.

This mounting ment minimum drag due to the rudder angle.

As for ahead of the prop, I believe this would disturbe the water flow to the prop.

As for behind the prop, the further back the rudder, the more powerfull the rudder becomes.

It is a leverage thing...too far back would make steering "touchy". possible prone to "spinouts".


Harry

[JonD]

I am intending to move my in line rudder on my Genesis, and offset it. I was going to mount it on the right sponson only because that would be simpler to implement bearing in mind my existing servo set up /position etc.

But based on what Old Sloppy says, the left sponson is preferable. I have to admit I don't quite understand this ......"I mounted the rudder on the left sponson, this mounting exactly ballanced the prop torque" I am trying to think of the "forces at play" here that would make left side mounting preferable. Can some explain this to me please?

Imagine for the moment a cat with no rudder (single motor - not twin). Would the boat want to naturally veer to the right under power? Why? If so, is this just "in theory", or is it quite noticeable? If this is the case, then I can see how a rudder on the left may cause some drag on the left and tend to offset any veering to the right. Am I close?

Jon

[JIM MARCUM]

I was refering to single screw catamaran hulls. Harry, when you say "mounted the rudder on the left sponson", it would be more helpful if you used the terms "Port" or "Starboard". Every off-set cat rudder I've seen has been mounted on the Starboard side of the hull transon or Starboard sponson.

Whether for Offshore, oval or SAW setups, I would think the effect of various rudder placement points would have consistent results.

Jon, about the effect of prop torque on a counter rotating single screw boat (cat or mono), I'm not sure (must have slept through that part of my boating 101 course). That or I've made too many deep dives & my brain has the bends. Maybe, whether the prop is fully submerged or is a surface piercing type results in different effects? SOMEBODY has to know the right answers! JIM

[Old Sloppy]

Imagine for the moment a cat with no rudder,Would the boat want to naturally veer to the right under power? Why? If so, is this just "in theory", or is it quite noticeable? If this is the case, then I can see how a rudder on the left may cause some drag on the left and tend to offset any veering to the right. Am I close?

Jon

You are exactly correct.
The common prop is a left hand rotation, as it rotates the botton half pushes against the water and the top half pushes against air.
the water grips much better than air, so the prop "walks" toward the left side (all of this is viewed from the transom with the bow furthest away from you)
this "prop walk" results in an arc toward the right. Unless there is a rudder (creating drag) to counter this force.
in "lap racing" the prop walk actually aids you in turning around the bouys, so a great many boaters (racers) mount the rudder on the right to make super sharp right turns possible.
In "offshore racing" we turn both right and left, so a rudder mounted on the left adds some drag that "ballances" the steering.
Some boat builders offset the prop shaft to the right about a 1/4", this is the "ultimate" method but the distance varies with each setup.
it allso varies with the MPH of the boat, the faster the boat the greater the offset.
I haven't personally tested a lot with prop shaft offset, but I do know for a fact it works "like a charm".

Harry