distributed hydraulic systems on boats? why not?

Discussion in 'Boat Design' started by ijason, Jan 25, 2009.

  1. ijason
    Joined: Jan 2009
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    ijason Junior Member


    as a heavy-equipment operator for some years now, i've gained a huge appreciation for hydraulics. and as an utter novice in all things nautical, i've got some questions about how the two subjects might get along.

    almost all of my boat knowledge is from the internet, so please do correct any misunderstandings i have :) it seems to be there are several considerable advantages to using a distributed hydraulic system to do much of the work on a boat, with relatively few disadvantages. i'll make a list for easier refutation!

    1) wear on the engine. when you tie an engine to a hydraulic pump, that engine will almost always run at a steady rate. there is no revving up or down as all throttling is handled by the hydraulic pump. this clearly means less wear on the engine in terms of needing it to run at any speed other than it's optimal rpm. also, having your engine and hydraulic pump connected to other devices via lines instead of mechanical linkage isolates the engine from vibration imparted from the propeller or waves.

    2) more options to locate your engine. if you're free from needing a linear connection between your engine and your drive propeller you are free to mount the engine in a way that makes maintenance easier. big plus there! it also gives more options for sound insulation and water-proofing your engine compartment.

    3) no direct linkage between engine and prop. by using a hydraulic-drive motor to run the prop you eliminate the chance of damaging your engine/transmission by fouling your prop onto rocks or chain.

    4) you can use your engine for more than one thing. you could power bow-thrusters, winches, deck-cranes, or any number of systems from the same hydraulic pump that runs your propulsion. keeping things simple means less spare parts to take, and less things to go wrong.

    5) heck, you could even replace your generator! tie a generator head to another hydraulic-motor and you can say goodbye to needing an entirely separate generator. simply run it off your main engine.

    6) hydraulic lines don't mind getting wet or short out. this seems like a huge advantage to me. just for the safety aspect of things, if you lose a hydraulic line you end up with a mess, but no chance of electrocution.

    7) you can get better health readings. if you put an inline pressure-gauge on all your systems you can very easily establish a normal operating pressure and any deviation from that pressure would immediately indicate problems with the system, the pump, or a system leak. something much more difficult to determine with many mechanical systems.

    all of these seem to be pretty substantial advantages. of course, if it was a perfect solution everyone would be doing it! so here are the disadvantages that are obvious even to me :

    1) weight. this would add weight to your boat, and potentially in substantial ways. while i think the advantage of having one drive motor may help off-set the issue, there's no doubt that a hydraulic-drive motor is a lot heavier than an electric.

    2) many people are unfamiliar with hydraulics. at least, compared to electronics. if you've got a pair of dikes and some electrical tape you can work on electronics, the skill-set to trouble-shoot and repair hydraulics is a bit broader.

    3) lack of redundancy. if you've only got one main motor and one main hydraulic pump attached to it, you're out of luck if either fail.

    i'm sure i've missed many obvious things, but it seems to me that the advantages out-weigh the costs... and yet, i've seen almost no mention of hydraulic systems on boats. there must be a reason!
  2. Guest625101138

    Guest625101138 Previous Member

    You need to be very selective with the materials used in a marine environment. If standard cad plated connections are used they could be a mass of rust in a short period of time. The cost of corrosion resistant components need to be determined. A larger boat, where things are accessible and can be painted with room to work in, will not be as much of an issue.

    If something fails catastrophically the debris can find its way through the entire system meaning the whole lot needs to be replaced. High pressure filters to overcome this issues add expense and another source of inefficiency. Also it could be the high pressure filter that fails when it gets a dose of crap.

    You cannot make an hydraulic light so you need some electric power anyhow. Why not use the power system for all the other distributed uses.

    Energy storage in a hydraulic system is heavy and bulky. It does not match the energy density of a battery. So each time you want to operate something with the distributed hydraulics you need to run the main engine. What happens when you are anchored and the bilge pump needs to kick in. How does your hydraulic system sense when to start the main engine and kick in the bilge pump.

    You could set up to start the main engine off hydraulics but what happens after the boat has sat on its mooring for three months and the accumulator has bled down. Hard to find a solar cell for pumping up hydraulics.

    I do not know of any hydraulic GPSs, autopilots, depth sounder and so on.

    On review I doubt that you can avoid an electrical system. And likely a couple of voltages on larger boats. So why complicate the boat with something that reduces efficiency and increases the fire risk with high pressure hydraulics being plumbed around the boat.

    Even if hydraulics were a clear winner they are in the Beta v VHS or Mac v Microsoft acceptance category. There is a lot behind existing electrical systems and I cannot see hydraulics gaining a foothold for widespread use in pleasure boat applications. Some specific components such as tensioners maybe but not distributed hydraulic system.

    I did not mention noise. This issue was previously discussed and I think noise was a big issue for many. Could you imagine sitting in a quiet anchorage and every hour or so that the bilge pump kicks in the main engine needs to fire up and you get the clunk of hydraulic valves, the piercing whine of pressure relief valve and the whirring of the gear motor on the bilge pump. Same thing for the hydraulically powered refrigerator, freezer and air conditioner. You could never sleep. Even if you could, your fellow boaties would soon get p'ed off to the point of asking you to find another place.

    I have not tried real hard here so I expect there would be many more disadvantages if I really thought about it.

    Rick W
  3. ijason
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    ijason Junior Member


    i think your reply is going a bit into the absurd realm. i was obviously not talking about replacing *every* system on the boat with hydraulics, so your reference to a hydraulically powered gps is just being silly.

    however, my point is that there are a significant number of power-intensive systems on ships of any size, and it seems they might be served better by a distributed hydraulic system rather than multiple, small electric motors. winches, anchor hoists, sail hoists, bow thrusters, main drive thrusters, and similar would be superior candidates for jobs done by hydraulics.

    lastly, to suggest that hydraulic fittings and lines are more susceptible to corrosion than electrical lines shows a real unfamiliarity with the profound durability of well-made hydraulics.
  4. Guest625101138

    Guest625101138 Previous Member

    I have worked in the mining industry for 40 years and see a good mixture of hydraulic and electric components.

    You are confusing power and force. Many people do this. Hydraulics are good for force applications. Unless size is an absolute premium you would go for electrics every time in a power application.

    Take a look at what is used on the largest mining equipment. No one would contemplate hydraulic drive for a 250t truck. Most trucks of this size are diesel/electric powered these days. They use hydraulics on lift rams and the like but real power uses electrics because it is so much more efficient. Have a look at locomotives. Usually straight electric or diesel electric. Again no one would consider hydraulic transmission on a locomotive unless it was a micky mouse application.

    Distributed hydraulic systems are typically messy. OK for commercial applications where leaks, noise and smell are accepted as part of the job.

    I have engineered both hydraulic and electric systems and there is no reason why electrics are any more prone to water than hydraulics.

    You have familiarity with hydraulics and I guess not as much experience with electrics. You have weighted the advantages over the disadvantages. I am just adding some balance to the case for not having hydraulics.

    All systems work better with development and certain applications suit different means of power transmission. For a small to medium sized pleasure craft electrics cannot be avoided but hydraulics can.

    Rick W
  5. Guest62110524

    Guest62110524 Previous Member

    da, look at qm2 diesel electric
    Le Tourneau, I think was the man who really bought dc to the wheels of earthmoving
    another thing is HYD , is very very expensive, compared with a stator and rotor
  6. Guest625101138

    Guest625101138 Previous Member

    Actually that reminds me of another issue with hydraulics. Efficiency in power applications is strongly related to oil bypass, which is a function of wear. You rarely see hydraulic systems with adequate filtering to avoid wear and gradual deterioration in performance. This does not happen with electrics. Electrons are always the same. They do not carry around fine debris that degrades the system.

    As we see more electric cars coming into operation I expect we will see cars with much longer life. The electric motors will not deteriorate at the same rate of IC engines. Might use a few batteries over the life of the vehicle but these will gradually get better.

    Rick W
  7. Guest62110524

    Guest62110524 Previous Member

    there are two threads same poster Rick, on the 2 mussel harvesters I built, I remember, the man who did the miles of hosing did not wash those hoses after cutting with cut off saw, despite my questioning him
    Filtration was good, but I prefer ss lines, flared
    why? do manufacturers of pleasure boat thrusters with dc , always limit the period they can run, ? they always say, "oh they will get hot"?
  8. ijason
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    ijason Junior Member

    @rick : you are doubly right; first in my experience with electronics being limited - i have years of experience with low-voltage, but nothing over 12vDC applications, and second with my hydraulic experience being in the construction field where noise and leaks are just taken as expected.

    first a question: both the applications you listed where hydraulics would serve poorly as a main drive seem much different than for a boat's main drive. you will never see hydraulic final drives on a moving vehicle because the rpm range is much too limited; it would do no good to have 10,000 foot pounds of torque if your train or truck could only go to 15mph. so then you're looking at fitting a transmission to the hydraulic motor before it hits the drive shaft for the vehicle, and now you're loosing the simplicity, losing more energy to mechanical inefficiency, and ending up with a system complicated enough you may as well do direct mechanical drive or diesel/electric.

    now, i am a complete novice when it comes to boats, so my question may seem obvious, but it seems to me that a propeller would be well driven by a hydraulic motor. it just needs to rotate against the resistance of the water, and do it for a long time and steadily, correct? is it simply an issue of efficiency that gives the advantage of this task to electrics?

    and then a clarification : i believe the biggest advantage of using a hydraulic system comes into play specifically where electric motors are their weakest; small dc-motors are extremely vulnerable to corrosion in their bearings and windings, and - as woosh pointed out - tend to have limited duty cycles. but you could fit very small hydraulic motors to any number of tasks otherwise done with electric motors: hoists, deck-cranes, bow-thrusters, etc. and i believe the hydraulic motors would be much more robust and be able to be ran as long as needed. for a good example, look at the difference in performance between a hydraulic winch and an electric one.
  9. Guest625101138

    Guest625101138 Previous Member

    The relatively low efficiency with hydraulic drive for motive power works against its wide acceptance. Even with diesel electric you need to go to quite a large vessel before it becomes worthwhile.

    The sort of low power, high torque applications you mention are on quite large vessels and hydraulics are well suited and often used particularly on commercial craft. All large outboards to my knowledge use hydraulic tilt adjustment. This is a highly developed self-contained application similar to the starter motor on the same outboard. It would be possible to use a screw type jack for the tilt application but it would be heavier and more difficult to overcome corrosion than the neat little electric/hydraulic unit.

    As far as electric motors go you get what you pay for. You can get electric motors that will quite happily work fully submerged for decades.

    Look at the little Minn Kota electric outboards. Basically cost peanuts and yet they are serviceable for small boats. Try to engineer something in this category in hydraulics.

    Hydraulics offer some advantages over electric for certain applications but not for the main motive power on boats. Not to say they could not be used but any loss of efficiency means you have to increase size of the driver, you have increase power accordingly for the extra weight, you have to carry more fuel to have the same range, you have to increase the power again to compensate for the extra fuel and so it goes on. Efficiency is a key consideration in most applications and becoming even more important as users try to reduce fuel costs. Losses in an electric transmission are about half the losses of an hydraulic system engineered to the same standard when new and the hydraulic system will deteriorate over time as components wear.

    Rick W
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Not true. You should check what can be done with modern hydraulics. There are good diagrams (torque/rpm/power) both on Bosch-Rexroth and Eaton web sites..
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    Sorry Rick but electric components don't wear? :D And the losses put that way sound huge when the difference in effiency is only 12%.
  12. Guest625101138

    Guest625101138 Previous Member

    The performance of electric motors does not deteriorate as they wear. Losses in hydraulic components increase as the wear increases. The only wear related failure mode in modern electric motors is the bearings. Typically they perform without loss of performance until the rotor poles. Any component will fail but electric motors maintain efficiency up to the point of failure. Hydraulic motors begin to wear from day one and efficiency is always downward as the tolerances are lost. It can be sobering to do efficiency test on a 5 year old hydraulic system intended for motive power that seems in good working order.

    Yes - hydraulics are about half as good as electrics when considering the losses. This is significant when the compounding factors are taken into account as I noted in the earlier post.

    Rick W
  13. Portager
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    Portager Senior Member


    As a Mechanical Engineer by degree and a practicing Systems Engineer, I feel I am qualified to comment on this concept in general, although I have only designed one simple hydraulic system in my career, so I do not feel that I am qualified to design a complex system.

    I think the main advantage of the hydraulics system is that if you use a variable displacement pump, then the engine speed would be independent of propeller speed. A fixed pitch propeller is designed to never overload the engine, so it is only able to utilize the full engine power at maximum engine speed. Therefore, at all speeds lower than maximum speed the engine will be capable of producing more power than the propeller can use. At low speed the engine is operating in an under loaded condition. The hydraulic system would allow the engine speed to be adjusted independent of the propeller speed, so the engine could always operate at the optimum speed for the propeller power demand. This would be a similar benefit to a controllable pitch propeller.

    The biggest disadvantage of the hydraulic drive would be efficiency. The efficiency of a well designed pump/motor combination will typically be in the range of 85% to 90%, the efficiency of a mechanical gear transmission will be 98% to 99% and an electric drive is typically 95%. Assuming constant efficiency on the engine (not quite true, but go with it for the sake of this discussion) and using 85% power transmission efficiency the hydraulic drive boat would need to carry ~15% more fuel to achieve the same range as the boat with a mechanical transmission, whereas the electric drive would need ~3% more fuel.

    I would not recommend a hydraulic maid drive for a typical boat application, however I may consider a hydraulic "get-home" drive as a back-up system see http://www.keypower.net/take.htm .

    This sounds like a solution in search of a problem. Since you generally want the engine as low as possible for CG considerations, existing drive train options accommodate all the desirable engine locations I can think of.

    This issue has already been solved by shear pins and vibration isolators.

    If you use a transmission with a power take-off (PTO) and attach a hydraulic pump to the PTO you can drive the propeller directly and still use the main engine to power hydraulics.

    Yes, this is called a hydraulic generator. In the recent past, onboard electronics such as computers and some electronics, have become increasingly sensitive to the quality of electrical power, specifically frequency drift, and hydraulic generators were replaced by inverters. However, modern hydraulic generators have a built-in feedback control loop which provides acceptable frequency control and hydraulic generators are making a come back. See http://findarticles.com/p/articles/mi_m0FZX/is_5_70/ai_n6062479 also when I talked to Keypower Equipment Inc. http://www.keypower.net/index.htm they told me that they also do Hydraulic Generators and Hydraulic DC Alternators.

    True, but if you route electrical runs above the waterline you won't have a problem until just before you submerge.

    You can accomplish the same thing by mounting a microphone on the transmission housing and monitoring the noise level.

    However using a hydraulic generator and eliminating one diesel engine would be a weight advantage. On the other hand I do not like the idea of running the main engine at anchor just to have AC power. I would add an auxiliary engine to run the generator when the main is off. Another advantage of this approach is the auxiliary could run the "get-home" drive if the main is unavailable.


    As I mentioned above I would not rely on a single engine, however my concern is what if I have a major leak in the hydraulic system and I loose all the hydraulic fluid the hydraulics would be useless. If the hydraulics are critical system, then I'll need to add a complete hydraulics system fill to the on-board spares. Depending on the system this could require tens of gallons of hydraulic fluid.

    There are many hydraulics systems on boats these days and the number of systems is increasing every day. I believe you need to decide which system to power hydraulically on an individual basis. I do not want to have to run hydraulics while I'm at the dock on shore power, so I focus on systems that are only needed away from the doc. Therefore, no hydraulic powered bilge pumps, refrigerator, freezer or air conditioner. I also wouldn't use hydraulics for the davit for the dingy. Most boats add hydraulics if they want active roll stabilization. Then once you have hydraulics it makes sense to use hydraulic anchor winch and bow/stern thrusters. I would also add a hydraulic emergency bailer. Other options include the hydraulic generator discussed above and hydraulic DC alternators such as they are using on the 86 and 62 foot Nordhavn's http://www.nordhavn.com/86/overview.php4 , "An extensive list of standard machinery details includes a central hydraulics with 20-square-foot stabilizers, 50 hp bow and stern thrusters, hydraulic bilge pumping and anchor wash downs, high output hydraulic alternators ..."

    Mike Schooley
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    Well.. I definately agree more or less what you said about hydraulics, but done so much work with electric repairs that I have to disagree..
    Although the problems are somewhat different they are still severe, and at times from #$*&*§£# to repair...

  15. Guest625101138

    Guest625101138 Previous Member

    I am not implying electric motors do not fail.

    Electric motors achieve design efficiency most of their life. By comparison hydraulic pumps and motors start wearing the first time they are operated and this is immediately reducing efficiency. There may be a brief period where seal and bearing run-in actually result in improved efficiency but fundamentally they are on a downhill slide from day 1.

    Very few operators actually bother to measure the efficiency or replace components until the system overheats. By that stage they have tolerated the inefficiency for a long time.

    The loss of efficiency is not a bother in force applications but where you are using the hydraulics for motive power transfer then it can be a significant waste.

    Rick W
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