[DG] [DG11] CR hydraulic drive versus Type 3 linear hydraulic

[shparrot]

I have a 57', 20,000 kg, sailboat with a semi balanced rudder weighing 54 kg. After stiffening the rudder post, redoing the ram mounts, connecting the rudder transducers directly to the tiller arms and updating software, both my type 3 linear drives, (dual ACU 400s with EVO), occasionally go into rapid oscillation. The pedestal / wheel movement becomes quite violent and I need to grab onto the wheel to settle the system. I can usually set damping to 7 or 8 in calm conditions and must raise it to 9 downwind or when seas are over 2.5 meters. This was an upgrade to an older 400 system which worked extremely well so I'm not sure what is going on. There was a second system, CR pump with balanced ram. (Guess I should have kept it.)
Would converting one of the ACU 400s with a type 3 drive to a ACU 300 with a CR2 pump solve the hunting issue. I wonder if relays respond more quickly than a pump stopping, starting and changing direction. Somehow, I need to reduce the system inertia.
Thanks
steve

[Derek - Raymarine - Moderator]

Hi Steve,
Thank you for your enquiry.

I assume from your description that you are using T3 hydraulic linear drives to operate the rudder quadrant on you boat?

One aspect of a larger boat is that there is more inertia in the steering system, there is also some flexibility in the system too - the steering cables will all stretch very slightly, the quadrant will flex very slightly and there are microscopic air bubbles in the hydraulic fluid of the rams which will also allow a little bit of compressibility.

All the above, coupled with the inertia of the system, the large rudder and the flywheel effect of the wheel will trigger some positional instability of the rudder and this is picked up by the rudder reference unit and causes the ACU to constantly reverse drive direction leading to the wheel cycling to and fro - sometimes quite harshly.

the effect is less upwind when the natural dynamics of water flowing past the rudder will damp this motion out, but with slower water flow past the rudder, either alongside or downsea, the motion will be exaggerated.

Check the software version of the Pilots onboard, there was a software update in November 2019 in the ACU to 3.04 which changed the damping structure of the ACU and you will find that whereas previously you might have tried a level of 8 or 9 and it almost controlled this oscillation, you can now select a level of 5 or 6 and it will have a better effect.

We would recommend that you update the software across all pilot components, the EV1, the p70 and the ACU and you will notice a big change in performance. Note that if you have early software, you will need to recalibrate the pilots before putting to sea.

Regards

Derek

[shparrot]

Thanks Derek
I don't have the Nov update. I will install it when the boat goes back in the water. From your message, I'm assuming converting one of the T3 linear drives to a CR pump and balanced ram won't make much of a difference.
Regards
Steve

[Derek - Raymarine - Moderator]

Hi Steve,

You are right, converting one of the T3 Hydraulic Linears to a CR type system will not resolve the issue, you are still fundamentally left with the issue of system inertia, the software update will equally apply to both types, so I would go that route .

Kind regards
Derek

[shparrot]

Thanks.
I mistakenly sent a second reply. Please ignore.

[Derek - Raymarine - Moderator]

No worries
Regards
Derek