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Old 21-05-2013, 11:21   #1
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Diesel/hydraulic drive

Do any sailboats use hydraulic motors for power?

I have a decent amount of experience with hydraulic power on my diesel farm tractor, plus I own a tracked vehicle that has hydraulic drive powered by a gas engine.

It just seems to me to be a perfect solution for driving a multihull sailboat. - A single diesel engine centered in the boat, powering a hydraulic pump, which then leads to a pair of hydraulic motors.
- The drive motors could be placed anywhere you wanted near the back of the boat, and are relatively light. (alot lighter than having a pair of huge diesel engines stuck way back there!)
- You can go from stopped, to full speed forward, to full speed reverse, without problems
- infinitely variable speed control
- No gears to shift
- the whole system will be lighter and cheaper since there is only a single engine instead of two (though you could have two for redundancy and safety if you wished... for that matter, you could have two smaller diesels, and only use one for moderate power, and fire up the second for full power)
- steering under power would be simpler and not even use the rudders, as power can easily be shifted from one prop to the other. Turning the wheel more than 90 degrees could even put one motor into reverse so you turn on the spot. (this is how my tracked vehicle steers via a steering wheel and I can spin on the spot if desired) So you wouldn't even need two power levers. Just one to set your speed (with the diesel engine having a regulator it would automatically adjust throttle as needed), and the steering wheel to vary power between the two motors.

Do any boats use this already? I have very little experience with motors on sailboats, so is there some obvious reason why this isn't done?
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Old 21-05-2013, 11:34   #2
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Along the same line I often wonder why diesel electric hybrid drive is not more common on boats. You have the battery bank and motor already. With a good solar setup I would think it would be the way to go.
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Old 21-05-2013, 12:02   #3
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Re: Diesel/hydraulic drive

I know that the SMG 50+ (Home) uses a diesel electric drive system. It also has all the benefits that I mentioned for the diesel hydraulic set-up.
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Old 21-05-2013, 13:41   #4
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Re: Diesel/hydraulic drive

You certainly could, and in a multi-hull boat like you describe the tradeoffs might be worth it. The down side of a hydraulic drive system, which is the same problem that a diesel-electric system poses, is the loss of power within the system itself.

An engine driving a prop through a transmission has very little loss in the drive system. In a hydraulic system, the motor drives a pump which pushes oil through lines which then drives a motor. There are losses in each of those steps. The cooling system required in a hydraulic system is a good indicator of the energy loss which comes out as heat.

The same is true for a diesel-electric system. Engine drives a generator which drives a motor. Both items suck energy out of the system and dissipate as heat.

The extra losses can start to be overlooked when there are other compelling advantages that come from the hydraulic or electric drive. Your example of gaining dual props on a multi-hull from a single engine is a good example. Trains use electric drives (the most prevalent use of diesel-electric) because it provides very effective power delivery from zero to cruise speed RPM which is great fro getting a heavy train rolling from a stand still. Giant cruise ships use electric drive for control and because they need huge generators anyway for all the house loads.

The big miss-conception is that diesel-electric drive in a boat is a way to gain efficiency over a gear drive, probably inspired by the efficiencies gained in cars. The laws of thermodynamics pretty clearly show that's not the case. The benefits to cars mostly come from power load leveling on the engine (something which happens naturally in a boat), and recovery of momentum energy when baking (another thing that doesn't exist in boats). The energy recover opportunity just isn't there in a boat like in a car.
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Old 21-05-2013, 16:16   #5
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The main reason is cost, a dual hydraulic with a single larger diesel is more expensive then two smaller directly coupled engines, easier to service and easier to get parts for

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Old 21-05-2013, 16:19   #6
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Re: Diesel/hydraulic drive

Some years back we looked at a 52' cat, Rudy Choy design. It had twin 27HP diesels. Each engine was hooked up to a hydraulic motor. The engines were over the motors and had to be moved to service the motors. It had built in rails so you could relatively easily pick up the engine and pull it out of the way.

Either engine would drive either prop.

Very neat boat, but with only 1/4" ply and some fiber glass we thought it too light for our purposes. Ketch rigged to boot.
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Old 21-05-2013, 17:17   #7
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Re: Diesel/hydraulic drive

I skippered a dinner boat with hydraulic drive. It was a dual paddle wheel each independently controlled. There were two BIG gensets for the house load (all electric galley); each genset had a hydraulic pump mounted off the front of the crank for propulsion. Either pump could run both paddle wheels, but we typically ran both before and during dinner, mostly for electrical demand, then ran back to the berth on one. The snipes did their maintenance on the idle genny.

That boat was in service over 30 years and was hardly ever down due to a propulsion issue. Were I to build a hydraulic drive, I would do a similar set-up, except I would have one genny much larger than the other. The big one would be for general steaming and high house electric demands such as air conditioning and cooking. The smaller one would be just enough to keep up lighting and refrigeration..... and limp home hydraulic power.
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Old 21-05-2013, 17:38   #8
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Re: Diesel/hydraulic drive

I've thought so too....It's been discussed here before... but consensus is it doesnt seem to work out well.... why I have no idea, fishing boats use hydraulics for varous things all the time. Your average 4WD Kubota tractor is hydraulic drive and they just keep chugging along without issue.... with hydraulics the engine could go anywhere you want it.....
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Old 23-05-2013, 15:10   #9
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Re: Diesel/hydraulic drive

I've heard some of the earlier catamarans had these but I belive the cost savings weren't there.

Also, what about leaks? Eveny hydralic piece of equipment I've been around has hydro fluid coating it. Now what I would want in my boat. As someone mentioned, likely not very efficent.

The diesel-electric seems much more appealing as it may not be any more efficent but offers a variety of benefits if set up correctly:
- Battery backup if the engine goes down.
- Same no transmission, instant full torque, full forward to full reverse with no issue operation.
- Operate high load electrical appliances when not motoring.
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Old 23-05-2013, 18:55   #10
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Re: Diesel/hydraulic drive

Yeah, leaks may be one reason it's not done on yachts much. I had a maintenence supervisor who worked for me whose motto was ; "if it aint leakin' , it aint hydraulic". I used to get pissed off about all the drip pans under hyraulic units... my motto was: "just fix the damn thing so it doesnt leak" Evidently that wasnt a "real world" expectation.... :>)
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