Can I have two DD15 drive units on one NAC-3?

[Maika]

My old one-off 40ft catamaran has one Ultraflex push/pull steering cable to each rudder, connected together in a steering gearbox.
I currently have a Raymarine steering wheel autopilot I mounted some years ago, but this is both noisy and does not feel very sturdy.
I do not believe having just one drive DD15 drive unit on one rudder would work very well, pushing the cabel up to the steering gearbox, which then would push the other cabel down to the other rudder, so I guess I would need one at each rudder. Will this work, can I parallel connect the drive units into the NAC-3 computer?

[SailingHarmonie]

Short answer is “No”.

The computer needs the rudder position data to control the drive. It can not take two rudder position sensors and control the two drives separately.

[Maika]

Thanks, but I do not think I would want the rudders to be controlled separately?
I believe I would want them to be totally synchronized. i.e. having both drive units performe the exact same operations. Since the rudders are connected with cabel they do not go out of sync, and I believe it is necessary to connect just one of the rudder position sensors?

[SailingHarmonie]

Sorry, I misunderstood what you were suggesting.

But I am afraid the answer is still no. The drive motors are not stepper motors that can keep in exact coordination. Your drive unit should be connected before the gearbox. I have no idea of the mechanical configuration that might be needed, but a chain drive unit is likely going to be the way to go. Raymarine calls them "rotary drives".

[Sos]

Can you connect the rudders with a rigid tie bar?

[DougM]

no problem having two actuators in a servo system, if they are driving rudders that are tied together (one mechanical unit to move), and there is one position feedback device.

Caveat - drive current from NAC-3 must be high enough to drive both dd15's in parallel. quick looks says there is 30A continuous / 50 peak available from NAC-3. DD15 says 1-4 amp. So if those online specs are correct, then you are good.

Another caveat - make sure drive polarities are connected correctly, so they are pushing the rudder system together, and not fighting each other.

[JustMurph]

Originally Posted by Maika

My old one-off 40ft catamaran has one Ultraflex push/pull steering cable to each rudder, connected together in a steering gearbox.
I currently have a Raymarine steering wheel autopilot I mounted some years ago, but this is both noisy and does not feel very sturdy.
I do not believe having just one drive DD15 drive unit on one rudder would work very well, pushing the cabel up to the steering gearbox, which then would push the other cabel down to the other rudder, so I guess I would need one at each rudder. Will this work, can I parallel connect the drive units into the NAC-3 computer?

In this type of setup you would probably be better of going for a Raymarine type 1 rotary drive at the wheel or steering gear box. The two individual actuators (with rudder pos on one side) probably would work (they will balance out with the tie through the cables), but isn't ideal.

[FlyingScot]

I suspect you will have a lot of trouble as the two identical DD15s may not move an identical amount when driven. Maybe wire length matters? Maybe build tolerances, but I can't imagine they will stay in sync.
Our 40' Leopard Catamaran came with a Raymarine Rotary Type 2 drive unit for our "pull - pull" unit and worked reliably for years. It was located behind the wheel with sprocket and chain. We only swapped to the DD15 to drive the tiller directly in case of steering cable failure.