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Consort
Consort

Posted: Tue Oct 17, 2017 9:49am

Post Subject: VOAC Hydraulic Transmission

My 70 ft narrow boat is fitted with a VOAC Parker hydraulic transmission (by original owner) to enable having the engine to one side in the engine room. The system works okay but I've been told that my prop speed is too high as, unlike mechanical transmissions, there is no 2:1 or 3:1 gearbox reduction. It means that slow maneuvering is difficult. Does anyone have experience with this brand as I'm wondering if it's possible to have a valve to limit pressure to the hydraulic motor to reduce prop speed. The existing valve only allows Forward, Reverse and Idle with no obvious regulation in between. Stephen

Tony-B
Tony-B

Posted: Tue Oct 17, 2017 3:24pm

Post Subject: VOAC Hydraulic Transmission

I have nor experience of that particular make but considerable experi9ence of hydraulic small boat system on a Thames hire fleet in the 70s. First of all it is not true to say there is no 2:1 reduction because the reduction is set by the relative displacement of the hydraulic pump to hydraulic motor. The reduction can be almost anything you want providing suitably sized motors and pump are available. The systems I worked on used vane, gear, or sine pumps and motors but modern equipment seem to use swash plate piston pumps and motors. Hydraulic transmissions are not particularly efficient at any time and trying to stricture the flow to the motor or dumping excess flow back into the reservoir will make it even less efficient. Not a very good idea I would suggest. Restricting the flow could also cause the oil to overheat. Often hydraulic transmissions sound as if the boat is going faster than it really is but as you say you have problems with slow speed manoeuvring I can assume there really is a speed issue. The way to cure it is to fit the correct sized prop or get your prop re-pitched to suit whatever reduction is built into your system. First look for plates on the motor and pump that should indicate their displacement. That will tell you the built in reduction and from that you can go online to any one of many prop calculators to see what size prop you need. If the pitch is reduced the boat will move forward less per revolution so move more slowly at idle but it will reduce the stopping effect of reverse to some degree.

Tony-B
Tony-B

Posted: Wed Oct 18, 2017 6:46am

Post Subject: VOAC Hydraulic Transmission

Having done a bit of digging it seems that VOAC are the makers of the pump and motors but not necessarily the valves in the system. Interestingly they list www.arsangliandiesels.co.uk/ as a dealer who are also arguably the premier supplier of marine hydraulic transmissions so I suspect that you probably have an ARS transmission system.

Consort
Consort

Posted: Wed Oct 18, 2017 8:24am

Post Subject: VOAC Hydraulic Transmission

Hi Tony, Thanks for that. Yes, both the pump and motor are of the angled piston pump variety. There is no makers plate on the valve so you are likely correct that the latter is made by someone else. I have considered a shallower pitch prop if only to enable the engine to clear its lungs from time to time. She's barely above idle at canal speed. She does stop well with a bit of throttle though. Moving slowly and winding requires delicate on/off control at idle engine speed which is not ideal. Thanks for the link, I'll contact Anglian and see what they suggest. Many Thanks Stephen

Tony-B
Tony-B

Posted: Wed Oct 18, 2017 9:22am

Post Subject: VOAC Hydraulic Transmission

As you suggest it may be a self fit-out boat I suspect the prop may well be the one that was "near enough" but at the right price. I feel getting it re-pitched will be the easiest way to solve the issue.

Consort
Consort

Posted: Wed Oct 18, 2017 12:45pm

Post Subject: VOAC Hydraulic Transmission

Hi Tony, I've done some investigation via Parker's website and their info literature. My pump outputs at the rate of 30cc/rev whereas the motor is rated at 80cc/rev so you are right, there is a 2.5:1 reduction in the drive. It looks like I need to focus on the prop as suggested. Thanks again for the insight. Stephen

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