Hydraulic Propulsion

[Jmarkley]

I'm convinced that it's possible to use hydraulic fluid pumped from a normal marine engine to turn a prop. If the connection between the engine and the prop is simply hoses, then one could place the engine anywhere on the boat. There would also be less problems with alignment, universal joints, vibration, maintenance, etc.

Does anyone know of a firm that builds this type of drive system?

[Tall Timber]

How bout a stern paddle wheel. Seen them built here in the 60 to 300 foot range with hydraulic power. Pretty noisy, I would think a prop would be inefficient.

[Portager]

Hydraulic propulsion is certainly feasible, the problem is efficiency. The hydraulic pump efficiency is typically 90% to 94% and the hydraulic motor efficiency is typically 98% to 99%. This equates to an overall efficiency of 88% to 93% compared to conventional gearbox efficiency of 98% to 99% or a net loss of about 10%.

Effectively you will need a 10% larger engine to achieve the same speed and >10% more fuel to achieve the same range.

You will also need an hydraulic fluid cooler to remove the additional heat because all the lost power ends up as waste heat in the hydraulic fluid.

One of the main advantage of a hydraulic transmission is it allows multiple engines to drive a single propeller.

Let me know if your still interested and I'll provide some links when I get home tonight.

Regards;
Mike Schooley

[Sean Herron]

Hello...

I have installed and worked with Engineers (over the phone ) on a few of this companies systems in the past...

See http://www.quantumhydraulic.com/...

How about a couple of modified bow thrusters - Korted. - http://www.marinepropulsion.net/hrp/hrp.htm ..

Yikes...

SH.

[PAR]

Hydraulic drive units are in production and used quite a bit over seas, yet to catch on in the states. The engine can be run at the best RPM for the conditions, like an idling engine in a smooth sea with the prop spinning fast or in heavy going, slowing down the prop and spinning up the engine. You get the best economy for this arrangement. Tied to a variable pitch prop the unit can be rather efficient to use.

The engine can be mounted where the weight can do the most good or be out of the way. Heat can be used as well, both the engine and fluid lines can have heat extracted for other uses, like hot water or cabin warmth. According to most hydraulic drive manufactures, the current units are near as efficient as a good marine transmission.

[Portager]

Hydrostatic transmissions approach the efficiency of gear transmissions however they achieve this efficiency by closely coupling the hydraulic pump and motor and optimizing the fluid passages between them. This defeats the benefit of being able to separate the engine and drive shaft.

To allow the propeller rate to vary relative to the engine with high efficiency, then a variable displacement pump must be used (it is possible to decouple the engine and propeller speeds with a bypass valve, but this just wastes the excess energy so in general it is undesirable for long term operation). Variable displacement pumps are considerable more complex and more expensive that continuous flow pumps.

If you using a fixed pitch propeller and it is optimized to operate near the engines maximum continuous output point, then there is no benefit at the design speed, a conventional transmission can provide the optimum gear ratio more efficiently than a hydraulic drive. The benefit of the hydraulic drive is at lower boat speed, where the propeller power demand curve is lower than the engines optimum power output curve. In this case the hydraulic drive allows the engine to operate at its optimum speed independent of the required propeller speed. This not only improves engine efficiency, which may compensate for the hydraulic drive efficiency, but it also extends engine life by eliminating operation in an under-loaded condition and by reducing engine revolutions (some people believe engine life is more closely to revolutions than hours of operation). If you have a system that will operate for significant periods at much different power outputs, like a commercial tug, then this may be a good alternative.

It should be noted that during the low power operation mode the propeller will be operating far from its design point so propeller efficiency will suffer. A better solution might be to use a controllable pitch propeller, which would allow the pitch of the propeller to be optimized for the desired boat speed and allow the engine to operate at its optimum speed. The controllable pitch propeller provides the same benefits to the engine as the hydraulic drive, but it also provides higher propeller efficiency, so overall efficiency is higher.

Regards;
Mike Schooley

[david carey]

I have seen a richard woods catamaran with hydraulic drives . It has a single engine mounted to port of the hull centerline in the port hull ,close to midships . Driving two hydraulic drives situated aft of midships inboard of the hulls .