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Subject: Fitting a Hydraulic Drive

We have a 60 foot long 13 foot beam wooden 1937 passenger vessel. She has a 2.5 BMC (50 hp I guess) situated in the bow, which in turn means a 40 odd foot prop shaft. With a PRM - rather large (cant remember the numbers) -gearbox which changes direction with quite a bang. The shaft rotates on 6 double roller bearings with from front to back, a nylon half coupling, a universal joint, an aluminium cum rubber cushion drive thingy and just before the stern gland, a "Python" drive (2 cv joints and a forward/reverse thrust bearing).

As you can imagine that lot fitted to a wooden drum (hull) creates quite a lot of noise for the passengers (despite a rather large sound system). Now for the question, I think that a hydraulic propulsion system is needed, I am led to believe that up to 25% of engine power is lost by using hydraulics, is this true?

Maybe a Ford Transit 70+hp could be the answer (easily marinised). Do you have any recommendations of a pump/ motor/ control package (rather expensive) or can I adapt something from say a fork lift truck or buy parts of the shelf from an industrial hydraulic supplier.

Many thanks




Dear Adrian

Funny you should send your email on that topic.

Many years ago on our hire fleet we had a very similar 12 berth vessel to what you describe, except it was about 45 feet long and we did convert it (and may smaller boats) to hydraulic drive. Those used BMC 2.2s.

I am sure ARS in East Anglia could give you chapter and verse and sell you the complete package.

I would not dispute the efficiency figure you quote and might put it at 30% loss myself, rising to 50% as wear sets in.

ARS will use piston pumps and motors because they offer maximum efficiency, however because of problems with failures associated with seizures cause by wear and dirt particles we used gear pumps and vane motors (as used on refuse vehicles and pre-mix lorries at the time). We accepted the loss of efficiency as being acceptable with the increased reliability.

There is nothing particularly difficult about specifying the system. The reduction ration is obtained by the ratio of pump to motor capacity (large motor smaller pump). You need a good oil cooler, control sliding spool valve (three position with central "dump back to tank", safety valve and reservoir. If you can get a filter to stand up to the pressure pulses it should be used. You also require a b****y great magnet placed inside the oil reservoir, where you can get at it, to catch metal particles. Regular inspection also gives some idea bout wear rates and incipient failure.

I would fit the motor directly to the front of the Pythondrive because it saves you aligning it and also removes the need for a motor that will accept axial thrust in both directions (cheaper motor).

I have no idea about what pumps and motors are now available, so I think talking to a few local hydraulic specialists would now be a good idea, but a word of caution. As this is a passenger vessel make sure anything you do is acceptable to the MCA.

Lastly I would never use any engine that uses a timing belt in a boat - let alone a passenger vessel that has to earn money, so that puts the small Ford range out. If the belt snaps you might as well fit a new engine - not a very good idea for a commercial operation!

I hope this helps

Tony Brooks


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