Autopilot Hydraulics Brain Teaser -- Overpressure ?

[Screwdiver]

i think i have a mechanical problem in my hydraulic steering. Mechanic fixed symptom but problem prevails. Any Ideas?

This is the story

I am cruising northern greece at the moment on my 38 feet ketch single handed. A few days back my autopilot suddenly stopped working. In a harbor entry channel under motor and autopilot suddenly the ship turned starboard. I managed to deactivate the AP, was releived that manual steering still worked and got clear of the harbor. At anchor i found some hydraulic oil next to the plunger and found that oil was leaking out of the starboard bleeder screws of the plunger. I tightened the screw, hand steering works. So next day i had hopes but again -- ship goes instantly starboard and lots of oil. Again hand steering works, so next days no autopilot but needed an oil top up.

In Kavala, the next harbor, the local mechanic replaced the bleeder screw with a cap screw , so the system is not leaking any more. On dock all tests show green lights, even if the AP Pump gives starboard rudder only slowly. Sea trial : Same symptoms. Now the mechanic got in the setup of the autopilot and increased electrical values for the pump. Next seatrial - the boat goes to starboard unly reluctantly and will not reach the desired course in short time.

the Mechanic wants to further increase the Values for the pump.

This gives me the feeling: We only fixed the symptom (like blocking the overpressure pipe on a steam engine). In the system somewhere a huge resistance is building up but possibly where ?

The assembly is quite typical and straigthforward for sailboats. Handsteering/Wheel/Handpump, twodirection Robertson AP-Pump, Vetus Plunger and in the middle this no-return-valve-block that gives the two pumps acces to the plunger.

The non-return-valve-block and all pipes are new this winter and the steering/AP worked flawlessly for hours on my trip from rhodes to the north. the motor is rather old

Any ideas ? (And perhaps a contact for a hydraulic wizard in thessaloniki, where i will go next.)

Sorry for my english...

[Screwdiver]

ahem an if some mod knows a better place for this thread, please feel free to move

[thesaltytar]

Hello Screwdriver.

I have not worked with your style of autopilot, so this may be of limited use but is based on experience with some other autopilot system failures... Please forgive the multiple assumptions I am making below.

It is unclear to me if hydraulic operation functions normally with the autopilot disengaged? It is my impression from your post is that it does.

If everything works until you engage autopilot I am suspicious of the electrical signal. Many systems use a 4-20 mA signal, and in the absence of this signal will default to either hard port or hard starboard (I have, on three occasions, been onboard for an autopilot failure and each time it placed the rudder hard to starboard - I really wish these would fail amidships!). I would want to rule out your electrical control system before proceeding.

I don't think any settings would change overnight, so changing those may only affect the symptoms, as you say. I would run tests with a multimeter and check for loose connections or chafed wires.

If all of this is correct, I suspect you already had a minor leak but this became noticeable when the pump was maintaining maximum stroke hard to starboard.