Mystery: autopilot loud banging has 2 hour fix??

[niftyc]

Our boat has a custom hydraulic system, likely installed by a custom hydraulic consultant in Canada. The total documentation is the one-sheet diagram attached. This boat is new to us and it is something of a mystery we are solving step-by-step. From what I can tell the hydraulic system operates at two pressures: low for steering, but when you flip a helm switch labeled "hydr. pump" this energizes the high pressure portion of the system and powers the hydraulic bow thruster and hydraulic windlass.

I don't know anything about hydraulics and I've had a hard time finding a boat mechanic who will touch it. For peace of mind I just wanted someone to look it over. I finally found someone but he does not have marine experience -- it is a hydraulic company that does custom assembly line machinery work.

He could read the diagram, so that's a good first step. I had him look over the system when we bought it and he said it appeared to be operating correctly and he could see no problems. I also did not see any problems in performance, BUT when the autopilot is operating (low pressure), there is a very loud banging whenever the autopilot makes steering adjustments -- in other words, constantly -- that makes rest impossible in the aft cabin.

He said that we had what the hydraulic aficionados call a "bang-bang valve" in our engine room and there is no reason to use a valve that opens and closes instantaneously. If you add a valve with a short open-close delay of a fraction of a second, this will make the valve silent. The hydraulic lines transmit sound very well, and they run through the ceiling of the aft cabin. OK sounds convincing, let's replace the valve.

He replaced the valve and we tested the autopilot. It was silent!!! However, after he left we ran for about two hours and over that time the banging noises slowly returned. They may be slightly softer, but not much. A 2-hour fix?! With these strange symptoms this feels like a Sherlock Holmes mystery. I am thinking now that it must have been some other problem. When I read the forums about banging and autopilots it seems like it is air in the lines, and not the valve open/close speed.

Basically I'm just not sure what to do here and I wonder if there are people with similar experiences who might be able to point me in the right direction? Or any direction, I guess.

In case it matters we have a Robertson AP20 autopilot.

[coopec43]

Have you seen these links?


https://www.hydraulicdistributors.co...normal-noises/


https://www.google.com.au/search?q=6...client=gws-wiz

[niftyc]

Originally Posted by coopec43

Have you seen these links?


https://www.hydraulicdistributors.co...normal-noises/


https://www.google.com.au/search?q=6...client=gws-wiz

Thanks!

Well, I haven't found these particular links until now but I did quite a bit of reading and searching and that is what led me to the air-in-the-lines theory. But my hydraulic guy doesn't like that theory, and wanted to replace the valve. So I thought someone with experience in these systems might have a second opinion.

For example if you read the first link you sent it seems that air ingress from a failing pump could be a common problem leading to banging noises. And it's weird that my guy is not interested in that. So perhaps I should get a different hydraulic guy. I don't know!

[coopec43]

That's a worry. I think in the circumstances you should do you own research.

"Cavitation occurs when the volume of fluid demanded by any part of a hydraulic circuit exceeds the volume of fluid being supplied. This causes the absolute pressure in that part of the circuit to fall below the vapor pressure of the hydraulic fluid. This results in the formation of vapor cavities within the fluid, which implode when compressed, causing a characteristic knocking noise."

https://www.machinerylubrication.com...ic-root-causes

[deblen]

Originally Posted by niftyc

Our boat has a custom hydraulic system, likely installed by a custom hydraulic consultant in Canada. The total documentation is the one-sheet diagram attached. This boat is new to us and it is something of a mystery we are solving step-by-step. From what I can tell the hydraulic system operates at two pressures: low for steering, but when you flip a helm switch labeled "hydr. pump" this energizes the high pressure portion of the system and powers the hydraulic bow thruster and hydraulic windlass.

I don't know anything about hydraulics and I've had a hard time finding a boat mechanic who will touch it. For peace of mind I just wanted someone to look it over. I finally found someone but he does not have marine experience -- it is a hydraulic company that does custom assembly line machinery work.

He could read the diagram, so that's a good first step. I had him look over the system when we bought it and he said it appeared to be operating correctly and he could see no problems. I also did not see any problems in performance, BUT when the autopilot is operating (low pressure), there is a very loud banging whenever the autopilot makes steering adjustments -- in other words, constantly -- that makes rest impossible in the aft cabin.

He said that we had what the hydraulic aficionados call a "bang-bang valve" in our engine room and there is no reason to use a valve that opens and closes instantaneously. If you add a valve with a short open-close delay of a fraction of a second, this will make the valve silent. The hydraulic lines transmit sound very well, and they run through the ceiling of the aft cabin. OK sounds convincing, let's replace the valve.

He replaced the valve and we tested the autopilot. It was silent!!! However, after he left we ran for about two hours and over that time the banging noises slowly returned. They may be slightly softer, but not much. A 2-hour fix?! With these strange symptoms this feels like a Sherlock Holmes mystery. I am thinking now that it must have been some other problem. When I read the forums about banging and autopilots it seems like it is air in the lines, and not the valve open/close speed.

Basically I'm just not sure what to do here and I wonder if there are people with similar experiences who might be able to point me in the right direction? Or any direction, I guess.

In case it matters we have a Robertson AP20 autopilot.

I do not see a steering helm or jog valve nor an autopilot valve in this diagram. I do see a pump feeding a thruster motor& it's valve plus an anchor? winch motor & it's valve.
What am I missing.
Cheers/Len