Hydraulic Drive for max efficiency

Hydraulic drive seems to have a bad name when it comes to efficiency.... This is mainly due to the way they have been built, rather than inherent inefficiency. Potentially far more efficient that hybrid electric, not to mention more reliable. Typical hydraulic drives use either variable displacement pumps or control valves. Neither are necessary. Engine RPM can control output just as with as mechanical drive. The most efficient pumps and motors are vane type, and the optimum system should have large lines (oversize), and as short as possible. Preferably the reversing should take place at the prop, through gears or variable pitch, or a simple flow reversing valve that provides no convoluted and restrictive flow path. This means a single pressure line, and a large low pressure return. Hydrostatic systems recycle fluid in a more or less closed loop, sending a portion to a cooler and replacing it from a charge pump. This added complexity has it's benefits, but is not worth the complexity. For a marine application simple is best. At 85% efficiency or more, vane pumps and motors exceed the efficiency that can be achieved by a motor/generator system, and eliminate the problems inherent in electrical systems in a salt environment.
I am admittedly biased toward diesel hydraulic, based on many years working with hydraulics and designing systems.... But I also have worked extensively with electrical systems in that time, and I find hydraulics inherently more robust in a harsh environment so long as the working fluid is kept clean and free of contamination.
I feel that electric drive has a long way to go. Efficiency has greatly improved with the better quality brushless dc motors, but the main benefit of electric is the ability to use stored energy from batteries, and as yet, battery technology does not seem "up to the mark".

H.W.

 

Energy losses ("efficiency") come from transitions from one form to another. The more you have the less efficient it is. Be it from converting the liner motion of pistons into rotary motion of a shaft thru a gearbox to converting the stored electrons in a chemical mixture into magnetic fields to spin a shaft. Hydraulic is just a means of transmission, but one that has many more transitions (power source > pump > lines > motor > output).

Each system has its advantages and disadvantages and places where they are more applicable than others. If hydraulic drive were more reliable or efficient in practice, it would be in wide spread use today.

 

Of course it's just a means of transmission of power..... Hydraulics as a drive system unfortunately have been poorly implemented in many if not most cases of marine application. In the application I'm looking at, a small sailing catamaran, it could offer many benefits that would outweigh it's disadvantages. Most significantly it could offer two "outboards" that would be driven by a single reliable and efficient diesel, and could be completely lifted clear of the water when not needed, without the need of a nacelle, etc, and could be submerged to an efficient depth. They could operate on a "common rail", merely restricting flow to one or the other pressure supply lines after they split off from the common rail for steering. The actual drives could be fairly light and compact compared to outboards, and the diesel could serve other functions such as operating the watermaker, charging batteries, etc, with the pump declutched. I'm talking of course about a sailboat, so this isn't a "primary drive". In this case 20 or so total horsepower as per outboard ratings is sufficient. Of course most sailors are really motorsailors.......... What makes sense for one person, does not necessarily make sense for someone else's needs.

I seen "similar threads" listed that didn't come up when I did a search..... I'll be checking them out.

H.W.

 

Do you have examples to justify that

?

 

Unfortunately that opinion is based on accounts I have read over time, including descriptions of what sort of components were used ..... against my own experience working with hydraulics. It's something that interests me, so I read everything I can find, and that is the conclusion I've reached.............. The goals vary, and efficiency while it is a high priority .... an overriding one... to me, may not be in other cases. Distance and line size are hugely important to hydraulic efficiency. The greater the distance, the greater the line size need be. The efficiency of different pump and motor types is quite different. Variable displacement piston pumps as used in hydrostatic systems are elegant, but complex and not extremely efficient. Valving can create efficiency problems by forcing the fluid to flow through a convoluted path, restricting flow, or creating pressure drops. Anybody can hang a pump and motor together and make it work with minimal engineering expertise, but making work efficiently and well is another story. I see "engineered" hydraulic systems that are sloppy or even deeply flawed. "Poorly implemented" is a term I would apply to many factory built hydraulic systems used in industry. I've been working with hydraulics for almost 40 years........... But the short answer is NO ....... I can't cite specific examples to satisfy your demand.

H.W.

 

"Poorly Implemented" usually means some design factors that aren't readily apparent to the end-user/operator. Either design compromises for "good enough" performance across a range of power through-put and amounts of neglect and abuse, or good old unit cost reduction.

I do like you idea though, even though I am not really a fan of hydraulic systems. It will be an interesting experiment.

 

Bumping this old thread. My Catalac has worn out drive legs and I am keen to go hydraulic. Does anyone have any pointers about where to go to source suitable underwater hydraulic motors?

 

Maybe re purpose bow thruster leg and prop it for an open prop?

 

Oh, yes thank you that is certainly worth looking into.

 

Hydraulic marine systems; marine propulsion manufacture https://www.hydraulicmarinesystems.com/page/page/4692398.htm

This is one of a myriad of marine hydraulic systems available. I had been told by a friend who managed a hydraulic firm in Alaska that many commercial boats use hydraulic propulsion
but perhaps Comfisherman will provide some input.

This was maybe 40 odd years ago, maybe things have changed

 

We use lots of hydraulics, none for primary propulsion. Can only think of one use for primary and it lasted only a year or so.

We use hydraulic thrusters quite a bit, would think some of the single prop bronze bowthruster legs would work. Think lewmar does some decent sized single prop thrusters. I know sleipner sells just a stripped leg for replacement, would make it so you could tweak with it a bit. Not positive they have proper bearings for continuous use... something worth looking into.

 

A vane pump may be more efficient than others, but it has considerable fluid slippage within its case and the system has fluid friction when transmitting power along the hydraulic lines to the drive shaft and propeller. A direct mechanical linkage from the drive motor to the drive shaft has zero slippage and very small gearing and bearing friction losses, so it should be considerably more efficient compared to hydraulic? JMHO, as I have no numbers or references to prove it.

 

Actually, piston machines of "swash-plate" design are more efficient, both as pumps and motors. If your design can hit their respective best efficiency point (which is not that easy....), each is ~90%, making the combination 81%. Add losses in valves, piping, cooler etc, and you seldom get more than about 75% efficiency at best. That is roughly 20% more losses than a mechanical reverse tranny!

 

Known in North America as hydrostatic drives I believe.
Their performance characteristics are impressive.
Expensive.

 

"Hydrostatic" refers to displacement pumps and motors in general; could be gear pumps, vane or piston. In the first note, the thread opener is stating an efficiency of 85 %, unclear if that is in total or the efficiency for each machine (ie pump or motor) and that the vane pump is superior, which is not correct.

If a transmission efficiency of 70 to 75 % is impressive must be related to whatever alternative is available. And, yes, cost per kW is high, but often there is no other choice. But for propulsion of a leisure boat......? Naah.