Hydraulic prop. drive.

[jeanne]

A friend here in Spain has bought a Petter mini-twin 12hp motor with a Volvo-Penta hydraulic prop drive, to fit to a locally built 5.5M yacht. This is a hydraulic Pump on the engine, connected by pipes to a motor on the prop. shaft, not a hydraulically operated gearbox.
He is ready to buy the prop., and any number of firms are ready to sell him one, but the first question they ask when asked for the appropriate size and pitch is `what revs at the shaft?´. The Petter does 3000rpm, but what will the hydraulic motor do? Is there a reduction ratio, as in a gearbox, or is it dependant on input horsepower? If anyone has experience of this combination, or knows of a good website, we would be grateful for the information.

 

[gianenrico]

Pump and motor have a design dependent ratio for turn of the axis/ cubic inch of fluid moved and vice versa as well as maximum design speed (turns/minute).
This should be checked on the technical documentation of the different parts of the "system".
Good luck

 

[jeanne]

Thanks for your reply, Gianenrico, but I didn´t get a lot out of it. Perhaps I should have said that the equipment has been purchased second hand, and that we have no handbook or other information on it, and that the Volvo Penta web site has not helped.

 

[Mudplugger]

Jeanne, I confess to NOT knowing anything about Diabolic drives, but, am led to believe that the ratio achieved at the shaft depends upon the volume of Hyd. oil absorbed by the pump, as opposed to the capacity of the engine pump. e.g if the capacity of the E.P is 100litres per minute and the shaft drive the same, ratio would be 1-1, I am told that the only way to identify this info, is from the details shown on the pumps, and discussion with the manfg. HTH

 

[William_H]

As a very rough guess you would compare the physical size of the pump and motor. This might give you some idea of oil displacement per turn which would give you the ultimate gear ratio which is what you need. So if both units look about the same size that would indicate 1:1 ratio in which case you might look for a prop same as if the engine drove direct. A pump half the size (volume internal) to the prop motor would indicate a 2:1 reduction.( lower prop speed). Of course you would want more reliable info before forking out for a new prop. good luck will

 

[Sui Generis]

To determine the correct prop you need (at least) waterline length, engine hp and shaft revs. You have one of three so; you need to determine the reduction ration of revs (if any) from the engine to the drive motor; you also need to determine the power loss in the system, which may be up to 30%.
I personally have no problem with the drive system you have because you can place the engine wherever, as long as you take the power loss. However, I haven't solved your problem have I. . . Suggest you contact Volvo -Penta for details of their drive system to get the above info.

 

[supermalc]

Is it possible to rig the motor to run and then measure the rpm of the prop shaft with a hand held rev counter.

 

[boatmike]

Understand your problem. Without more info there is no way of knowing. Suggest you contact Volvo direct or any local agent quoting model no etc that ought to be marked on it. Their web site is not informative as you have found out but local agents usually are. Usually they are approx 2:1 but you can't rely on this. Failing all else you will have to install the beast and crank the engine over and count. The most reliable way would be to hook up a rev counter to the output shaft. If you have a hire shop near they may have one with a rubber contact pad on the end you can just hold on the end of the rotating shaft.

 

[WayneS]

Are you able to get the model numbers of the pump and motor? From those we can work it out. I went through the same process a year ago when we re-engined our boat.

There is a piece of the jigsaw that you have not mentioned which I hope is there. That is an oil reservoir. The size of this depends on the oil flow through the system. Our pump/motor ratio is 1:1 and we installed a reservoir holding 50l of oil.

I have various contact around the UK dealing in marine hydraulics and I have found them to be very helpful. PM me if you need more details

Cheers

Wayne