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Old 07-05-2018, 07:49 AM   #1
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Hydraulic help requested

I have a hydraulic Dickson stern thruster.
Basically its an engine belt driven hyd pump with a mag clutch, some directional valves, a rsevoir tank and a hyd motor mounted below the waterline on the transom.
Yesterday while doing other work in the engine room I found that the reservoir tank had overflowed out of the breather/fill neck. Normally there's a few inches of empty space. The fluid was frothy pink instead of Dextron cherry red.

So I am assuming it is sucking in seawater since there is no cooler of any sort.

The motor has 2 hoses going to it, one for each direction the prop turns. And the motor obviously has a shaft seal.
The hydraulic lines come out of the transom several inches above the waterline. and are about 12 inches long.

Any guesses as to where the failure is? I have not run the thruster and looked yet to see if I can see a leak or sheen but not sure with the prop running if I would.
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Old 07-05-2018, 08:46 AM   #2
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If it overflowed, you do have a problem.

The beauty of the dickson design means it should be fairly simple to remove the thruster and hoses to test everything. Chances are that you have either a blown line or the shaft seal has leaked, and the line is more likely the culprit.

It has two lines and the flow reverses to reverse the direction, so if you have a leak in the return line (depending on which direction you're going) it will suck oil and water / air into the reservoir.

I would start with taking both lines off where it goes through the hull and the thruster hydraulic motor and take it to PIRtek or some other hydraulics shop for leak testing. I would also empty reservoir of pink mayo and clean it out thoroughly and refill with oil. I would hand turn the pump to make sure it gets the mayo out and has good oil since it is most likely a gearpump and they can rust quickly.

I'm assuming you're in Conn, not Fla now (it's hot down here). Look for a hydraulics shop nearby, but Pirtek is in Avon, if needed.

https://www.pirtekusa.com/
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Old 07-05-2018, 09:20 AM   #3
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[QUOTEI would start with taking both lines off where it goes through the hull and the thruster hydraulic motor and take it to PIRtek or some other hydraulics shop for leak testing.][/QUOTE]

Thanks.
Yes that is where I am at. Our yard mgr is on vacation this week so I can't even talk about a short haul until he gets back.
Sooo I think I'll get in the water later today and see if I can remove the lines and motor.
There is a hydraulic shop right down the street from the marina and hopefully they can help me out. If not a road trip is in order, thanks for that lead.
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Old 07-05-2018, 09:48 AM   #4
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Does system have an oil cooler?
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Old 07-05-2018, 11:38 AM   #5
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No.

OP said "there is no cooler of any sort."

Only way to overflow it is suck in seawater on a return line. Since the flow alternates depending on which way the thruster is pushing, he needs to check both lines, and there is a possibility to suck water through the seal on the hydraulic motor turning the thruster blade.
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Old 07-05-2018, 12:09 PM   #6
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Yes, NO COOLER.
Just got out of the water pulling the motor off the bracket. A little water dripped out of the two hydraulic lines.
On my way to the local hydraulic shop to see if they can/will test the system so I can verify where the leak is.
Stay tuned.
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Old 07-05-2018, 12:19 PM   #7
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Is the system below the water level? If so, the outer seal on the shaft is likely bad, and the motor is admitting water through that seal when idle. This is the most likely scenario.



If not, it's possible that the valve closing rapidly (it's electric solenoid, I assume) is causing a slight cavitation due to the motor's inertia, particularly if the valve has a motor spool (small vent openings on both ports to tank when closed), which in a non-drained motor like a gear or gerotor motor (only two hoses) can pull a vacuum on the seal, possibly admitting a few drops on each stop.



Least likely is port reliefs that aren't cross-port connected and/or with anticavitation checks, so rapid stops that vent a little over the port relief pull a vacuum in the other line, which on a non-drained motor causes a vacuum internally. I say least likely because a system like that doesn't really need port reliefs - being open-loop (fluid from pump to valve to reservoir only moves in one direction), just a single supply relief is all that's needed.


The low pressure line, which is just the outflow from the motor and depends on which way it's being driven, should never see a vacuum in normal operation, as the outflow from the motor is pushed back through the valve to tank by the inflow to the motor, which is doing the work, aka 'backpressure'. It may be small, but it's not nothing.


And do make sure the vented breather is breathing - otherwise it can pull a vacuum when it cools.
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Old 07-05-2018, 01:42 PM   #8
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Quote:
Is the system below the water level? If so, the outer seal on the shaft is likely bad, and the motor is admitting water through that seal when idle. This is the most likely scenario.

The motor (and prop) and 6 inches of the two hydraulic lines are under water.


The shop will pressure test in the morning. Regardless of the outcome both hydraulic lines will be replaced.
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Old 07-05-2018, 09:47 PM   #9
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As Tomandjeri stated " if the tank vent is plugged the system will draw a vacuum on the return line when the pump starts" this will cause the system to suck in anything where there is a damaged seal or hose.
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Old 07-06-2018, 07:13 AM   #10
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The vent is not plugged
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Old 07-06-2018, 10:17 AM   #11
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Is the reservoir above the thruster location? The thru-hull fittings are above the waterline, correct? The tank and pump are above the waterline?
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Old 07-06-2018, 10:55 AM   #12
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Is the reservoir above the thruster location?
YES

The thru-hull fittings are above the waterline, correct?
YES

The tank and pump are above the waterline?
Tank is on the engine room floor, pump is maybe 2 inches above the floor. The floor is just below the waterline.
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Old 07-07-2018, 08:02 AM   #13
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Test showed a leaking seal on the motor, and a leak in one of the lines, but that was well above the waterline.
I can't find anyone local that can replace the seal, (including the Pirtek location near me) so I guess it's off to UPS to ship it back to Dickson for rebuild.
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Old 07-07-2018, 09:32 AM   #14
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So the motor output shaft has oil on one side and sea water on the other? Is the shaft SS?

Sealing in that scenario is tricky. Can't use the same seal arrangement as when oil is on one side and air on the other.
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Old 07-07-2018, 09:49 AM   #15
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So the motor output shaft has oil on one side and sea water on the other? Is the shaft SS?

Sealing in that scenario is tricky. Can't use the same seal arrangement as when oil is on one side and air on the other.
Yes SS shaft, and yes oil vs salt water. There is also a bushing inside that is worn out as well, so I'm convinced sending it back to Dickson is the right thing to do even though it's going to cost an arm and half a leg.
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Old 07-15-2018, 11:34 AM   #16
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It may be worth telling a local hydraulic shop that the motor is a standard Char-Lynn Geroler motor. They may be afraid of an unknown (to them) hydraulic device. I had mine rebuilt at a local shop with no problem at all.
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Old 07-15-2018, 01:08 PM   #17
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Wish I knew that last week. Motor should arrive in Ca tomorrow I should get it back a week from Wednesday.
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Old 07-15-2018, 02:02 PM   #18
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Just one more piece of advice regarding the repair. Don't run into the same mistake that I did. My motor repair was fine, and I decided to replace the hoses since they were 30+ years old. Got new ones made up by a hydraulic shop, and they did properly use stainless steel fittings, BUT the way they work is that the hose fits onto the fitting (kind of like a hose barb) and then there is a sort of ferrule that is fitted on the outside of the hose and crimped to make the connection secure. The problem is that they did not use stainless for that ferrule and it began to rust off rather quickly. GRRRR.
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Old 07-15-2018, 02:27 PM   #19
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wrap with Denso tape.
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Old 07-16-2018, 06:21 AM   #20
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"I had mine rebuilt at a local shop with no problem at all."

This is the norm, hyd stuff lasts so long that in many areas 20- 50 year old items are no challenge for the hyd repair folks.
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