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Old 30-07-2008, 18:50   #16
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Haven't found many, but there are hydraulic motors that work as pumps, in either direction. (I thought they were all the same when starting out the search...)
Actually a long time ago back in 1978 when I had my first job. My boss built a twin paddle wheeler up on the upper Mississippi. It was driven by twin hydraulic torque hubs powered by twin 62 HP diesels. This was a 62 ft steel boat 2 decks and could party 50 people easy drawing 2.5 ft with 4 rudders. It worked quite well and the fuel consumption was very very low. Maybe 1.5 gallons per hour. Top end was about 6 knots but it could spin on it's own axis. The pilot house was 22 ft above the water. He was not a favorite person but he is dead now. It's a lot easier when they are dead and 30 years passed. Heck of a boat though.

It did use keel coolers to cool the fluid. That was a critical part of the boat!

If you are going to drive a hydraulic torque hub go diesel. They don't do high rpms all that well but maybe they could.

Paul Blais
s/v Bright Eyes Gozzard 36
37 15.7 N 76 28.9 W
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Old 30-07-2008, 20:12   #17
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A hydraulic drive is convenient, variable speed, forward and reverse, you can drive the winch, thruster etc. but it will not be as cost effective fuelwise as the conventional transmission boats normally use now. Check out the following:
Hydraulic pumps convert mechanical energy into hydraulic energy. A high-performance piston pump can convert mechanical energy into hydraulic energy with an efficiency of 92 percent. If the pump drives a piston motor, the motor is able to convert this hydraulic energy back into mechanical energy with an efficiency of 92 percent. The overall efficiency of this hydraulic drive, without considering flow losses, is 85 percent (0.92 x 0.92 x 100 = 85).
Table 1 shows what the typical efficiency would be if a gearbox accomplished this same drive transfer.

Table 1. Typical Gearbox Efficiencies


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Old 30-07-2008, 23:14   #18
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Steven, you are correct about the efficiency of the pup and motor. Those two things have the least loss in a Hydraulic system. The big issue is the energy it takes to push the oil through the hoses. Then you have to have flow controlls, pressure reliefs and the fact that whether the hyd.motor is turning or not, the pump is turning and pumping oil of which is being pumped through the system back to tank. A system can only be hopeful of 50% efficiency in reality. And then add the energy being wasted while the pump is running. Add to that the noise. They make a very noisy system and most likely would drive most boaties nuts.
However, Hydraulics have their place. Kinda like a Plane vs a Helicopter. Both fly, one is far more expensive to run than the other, but each have their own purpose that neither can fit the others role of.
Here is an example of what can be done with hydraulics. These propulsion devices have big Cats powering them. The props are on hydraulic motors and the entire motor/prop can be spun around 360deg and the entire system can be raised and lowered.


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