Hydraulic Propulsion

[Jmarkley]

I'm convinced that it's possible to use hydraulic fluid to turn a prop instead of using a direct mechanical connection. The advantages are less maintenance, less vibration, placement of the engine anywhere on the boat.

Does anyone know of a firm which markets this type of drive system?

[yipster]

read Trolling valves and more using the search button above

[JR-Shine]

Those drives range from $1,800 to $3,000.

Joel
Boatbuildercentral.com

[FAST FRED]

Most pumps are about 92% efficent , there are cooling,piping & valving losses , then the motor is only about 92% .

Multiplied together your loosing about 25% of your fuel in heating Hyd fluid.

Great if you wish to go from Full foward to Full reverse in an instant , but for cruising?

Most hyd parts have long but limited service life, so will need rebuild , timed about with the engine.

Best use would be for multiple apps , hyd windlass, hyd steering , hyd crane , and very low speed ops. Electric can be made quite stably from hyd , and it can power refrigeration system compressors too.

FAST FRED

[jehardiman]

Originally Posted by Jmarkley

I'm convinced that it's possible to use hydraulic fluid to turn a prop instead of using a direct mechanical connection. The advantages are less maintenance, less vibration, placement of the engine anywhere on the boat.

Possible yes, practical no.

In my experience, the only advantage of a hydraulic drive is in arrangements, and in this it has only a slight advantage over electric drives due to size of the power head.

They are about half as efficient as a gearbox or electric drive, and about twice as costly to maintain (ICE-gearbox or ICE-GEN-MOTOR vice ICE-HPU-HD). They are about as heavy either a gearbox or GEN-MOTOR, and only provide a reduction in vibration if the right components are selected. Additionally they are very noisy if the wrong components are selected.

All this is based upon design studies to replace existing electric drive systems of 5-250 Hp systems. ICE-GEN-MOTOR is far more suited for for low power application on a boat if arrangements warrant not fitting a gearbox.

The only other reason to fit a hydraulic drive is the very special case of a vessel that needs a huge hydraulic plant to perform its mission, but transits while not working. In this case, there is an argument to be made for saving space by not fitting a prime mover, but using "free" hydraulics.

[woodboat]

The trash boats here in Baltimore inner harbor use hydraulic. They move slowly around the harbor, go from forward to reverse alot and the pump also drives some scoops and a track belt to pull trash up and into the hopper. The drive units look like outboards.

[MikeJohns]

Gives some interesting design options. Can put the engine anywhere and any orientation. The same pump can drive thrusters, winches, Prop. Seems a good auxiliary option for a blue water sailing yacht. As for life and reliability they seem better than the modern cheaper mechanical boxes.
Horrible mess when bleeding the system uness you are careful.

Aside.......I've seen a 70' fishing boat pull itself off a sandbank with a hydraulic capstan winches. Would have burn't out the equiv electric one, first try it ripped the tree right out of the bank!

[John Perry]

Back in the '80s I worked on a big R + D project to develop hydraulic drive for warship propulsion with the hydraulic motor within the propeller hub and power output up to about 40MW. After building and testing expensive prototypes the conclusion was that the power losses would be prohibitive, not just because of the cost of the extra fuel but also the weight of that fuel or alternatively the loss of range. At that time the conventional wisdom was that the propeller hub must not be more than 30% of the propeller diameter and this ruled out an electric system with an electric motor in the propeller hub. Nowerdays such electric systems are the preferred option for many ships, espeicially cruise liners (eg Azipod drive) and I think they do have a larger propeller hub diameter than was considered acceptable at the time of our project.

Cruise ships and warships have a high electrical demand from the 'hotel load' and this usually requires a number of diesel generators to be installed. It is then logical to add a few more similar generators so that the power available is enough to cater for the main propulsion motors giving a flexible system where the number of generators running can vary according to the total load while keeping each generator operating close to its optimum power setting. The efficiency benefit this gives can more than compensate for the losses due to the need to transform mechanical power to electric power and back again.

For ships such as tankers which have small hotel load and which operate mostly at one power setting the most efficient system is still a simple mechanical drive from engine to propeller and with large direct drive reversible diesels the losses due to a reversing gearbox are avoided hence mechanical efficiency is almost 100%.

John

[dereksireci]

I've used a hydraulic power take off pump on the front of an engine to run a water pump on a fire boat. All that fluid can be heavy

[Ilan Voyager]

Hydraulic systems are used on harbour tugs or servitude boats where the inherent losses of hydraulics (and the fuel consumption) do not matter. These boats do not travel, and hydraulics is the solution for the numerous equipment requiring power on these boats.

On boats needing a good range of cruising these losses and very bad total efficiency are unacceptable; any % of efficiency gained has important economics consequencies. We made in 1983 a study for a mines sweeper and this technical option has been abandonned. It was far better to put a propeller with an electric motor on the rudder and start a 250HP gas turbine generator while hunting mines at low speed, the main diesel engine being stopped.

The current engineering trend, as it has been written by Johnn Perry in a former post, is electric; good global efficiency, no problems of reliability (the hydraulic hoses and connections may be problematic), no problems of cooling a fluid running in hoses inside the boat. Or the absolute simplicity; big slow diesel (85 to 120 RPM), no gear box, simple shaft, big slow propeller. The global efficiency is excellent: almost 50% for the engine (to compare a marine gas engine about 25%), no gear box losses, propeller about 85 to 90 %. Very reliable also, it's the choice for tankers, container carriers etc... It handles very well very big powers: the biggest diesel engine gives 110000 HP at 102 RPM.

On a yacht, a semi fast diesel generator and an electric engine would be a good solution. The mixte buses (diesel and electricity) equipment should be a starting point as they have similar requirements and are in the range of power as a small yacht.

I have seen hydraulic transmissions a race sail boats in the 70's, and they were always unreliable and often noisy.

[Lda]

In electric transmission you have two kinds of solutions:
1) Without electronic control (DC generator and engine directly connected...) simple and reliable but heavier than mechanical. Expensive if you use marine grade equipment. Nearly as efficient as mechanical. Ships don't use this solution any more.
2) With electronic control (AC generator and DC or AC-self-piloted motor connected through intelligent "drive" electronics). Light and very efficient. Control electronics is expensive if you want it reliable. For low power systems, electronics might be prohibitely expensive because part of it does not depend from the size of the engines. Might be dificult to maintain in some area. Efficiency is typically better than mechanical because transmission has continuously variable ratios you can optimise depending on actual use.

In hydraulic transmissions you also have two kinds of solutions:
1) closed circuit, piston type pump and motor (classic "earthmover type" technics). Light and #80% efficient. Efficiency can improve if you use variable-displacement or automatically-adjustable pumps, because transmission will have continuously variable ratios (transmission losses remain at #20% but chances are you will more than recover them through better continuous adjustment of propeller rpms). Reliability is good for fixed displacement pumps, lower for variable displacement. Generally rather expensive.
2) Open circuit, gear type pump and motor ("agricultural" or "log-splitter" type technics): cheaper but limited in size due to availability of bigger components (<=#150 hp). Also requires rather cumbersome hydraulic reservoir (about 4 liters oil per transmission hp). Efficiency #80%.

I understand that US is more conservative than Europe for hydraulic solutions although it looks like they are still quite interresting for yachts vs. electrical, depending the needs.

[PADDLEGUY]

An All Hydraulic Drive System Is What I Have Designed For My Twin Sternwheel 28` Week Ender. A 25hp Diesel With Pump, Running Twin Low Speed, High Tourque Hydraulic Motors Bolted Directly To The 6` X 2` Wheels With 8 Buckets Each.. My Hydraulic Engineer Tells Me This Will All Be Fine,, With Just One Problem Remaining.. Keeping Saltwater Out Of The Motors.. Any Ideas, Comments, Advice Will Be Well Taken,, Thanks

[gtsmarine]

for information on drive systems contact Duckworth Steel boats
Tarpon Springs Fl

[gtsmarine]

contact Duckworth Steel boats in Tarpon Springs Fl

[John Perry]

I think that 80% is a bit optimistic for the efficiency of a hydrostatic propulsion drive. A good axial piston pump or motor may manage 90%+ but only under an optimum combination of rpm and torque. You have to add the losses in pump, motor, pipework, plus mechanical losses in shaft seal and bearings. Hard to quote a general figure, it will depend on the operating conditions anyway and also on wear in the pump and motor but I think 60-70% is nearer the mark than 80%. That is a rather large power loss, you also need to dump the heat somehow. A small system based on gear pump/motor can be expected to be rather less efficient than a larger one based on axial piston machines. This is the main reason that hydrostatic transmission has never been much used for ship/boat propulsion.

Many modern ships have electric drive for controllability, fault tolerance, ability to match number of active prime movers to varying power requirment and the ability to supply both propulsion and 'hotel' loads from the same set of generators. But for ships where these aspects are less important, that is ships which run mainly at a steady cruising speed and which dont have large and/or variable hotel loads the preferred and most efficient drive is still a mechanical one, preferably without a gearbox. Large tankers and containerships use a direct drive from a reversible diesel engine to the prop and no alternative transmission is going to beat that for efficiency.