Catamaran setup? doesn't go straight after a turn!

[ED66677]

hi all!
I've got a 42" catamaran (ABS hull) powered with 2xSpeed 700 9,6V, K39 props, running correctly under 14,4V, that thing runs fine, fine enought according to the cheap setup it represent!!!
Here's the problem : when turning (right or left) at the end of a turn the thing continues slowly turning the same direction, I think (tell me if I'm wrong) when turning, the cat tilt a very little bit on the outside of the turn (while a mono would tilt on the inside) so the inside prop comes out of water while the outside one goes deaper, then the outside prop generates more thrust while the inside one generates less thrust, that cause the cat to continue turning!
what do I have to do to fix this?
Is my COG too much in the back?
Are the props not enough deap in water?
Shall I use trim tabs?
Prop are 3" away from transom, is this too much?

[Doug Smock]

I'm just guessing here Ed.
Could it be that there is a little play in the rudder linkage, or maybe the servo is having a hard time finding it's centerpoint?? Nothing is binding?

Doug

[ED66677]

Servo's working fine even it's a cheap one, but you are right, when we got back in the workshop, after a complete checkup I remember now that we found a little play, not in the linkage but in the rudder shaft, it's a Hopf rudder so the shaft is a screw, I need to fix that for sure!...
Good point D.Smoke!
I'll remove the rudder for next run? :D

[Steven Vaccaro]

Doug may be on to something, a 42" boat is going to need some beef to turn the rudder.

[SJFE]

Hey Ed I may look you up to order me some stuff fro Hopf some day....lol

[SJFE]

Oh and in reference to your question:D I agree in regards to a high torque steering servo. They are inexpensive enough these days. Try one out. You may also want to try a push pull steering linkage system. You would need a double horn rudder and servo arms with twin linkages.

[ED66677]

well it's a 5Kg servo under 5 cells so it is fast and accurate, sure I could use a 8Kg and I will I have some in stock, but the cat actually turns short at any speed.
I checked at the shaft play I was talking and it doesn't seem to make any rotation of the rudder when servo's at centerpoint!
I'm really thinking of a kind of power-turn due to one prop remaining deaper in water at the end of a turn.
Any play in a configuration like this, where It's not a push-pull linkage, would certainly generate a turning effect only in one direction, I've done many RC plane (giant scale ones) so I'm very carefull of these things!

[ED66677]

Ok guys I checked for any play, change the COG to have the cata running flater
but still continues turning alone!
I'm deseperate!

[Doug Smock]

Ok guys I checked for any play, change the COG to have the cata running flater
but still continues turning alone!
I'm deseperate!

Send a PM to Jay Turner "Fluid". He has alot of cat experience and will be able to help you. I hope you get it nailed down.

Doug

[ED66677]

OK D.Smock done!

[NorthernBoater]

You may also want to try a push pull steering linkage system. You .

How do you setup a push pull steering system.

[saleens7]

..............

[Fluid]

Backwards Saleen. Actually, both methods will push and pull the rudder, but using a dual horn on the rudder with two pushrods reduces binding since the total servo force is split between both sides of the rudder. This is the preferred method for a heavy outboard motor because it reduces pushrod bending/flexing. A single pushrod setup could cause binding - although most R/C race boats get along just fine with the single pushrod setup.


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[saleens7]

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[ED66677]

no guys!
I really think it doesn't have to do with any play or any bind, the cat can start a nice straight run of 50 yards and suddenly turn right or left
but I will remove the rudder tomorrow if the cat goes straight then this will mean a tiny play or bind has a huge impact if not, I'll add 4 or 5 lb of battery in the hull to keep those damned prop in the water ! :)