distributed hydraulic systems on boats? why not?

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

  1. TeddyDiver
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    TeddyDiver Gollywobbler

    Thats the same thing as sailing with sails a bit out of shape, and it's also a matter of priorities. For a racer every % lost is too much. For a commercial actuator it's a difference in the bottom line. Private boat owners on the other hand have a such large scale of different priorities and circumstances that's it impossible to recommend one and only best solution for all. Instead it's considering pros&cons for each purpose.
    Like I said in some other message I considered also hydraulic transmissions but going mech VPP main propulsion & electrics for everything else now so in that regard I have allready agreed most of your opinions.
    However there are also tasks more suitable for hydraulics.
  2. Portager
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    Portager Senior Member


    Your post makes it sound like hydraulic motors are intrinsically less reliable or durable than electric motors, which is certainly not true. In reality hydraulic motors provide comparable reliability and durability to electric motors and in some applications they can provide superior reliability and/or durability. For example, hydraulic motors can operate in much hotter conditions than electric motors, and if you are cooling the hydraulic fluid such as with sea water, they can provide virtually an unlimited duty cycle because you are effectively pumping coolant through the motor.

    Operationally, most motors never reach their predicted life. With the average boater putting ~200 hours a year on their main engine(s) the hours on the bow thruster and anchor winch will never reach a significant number.

    The optimum choice of a motor must be made on a case by case basis depending on the operating conditions and the expected duty cycle. This tradeoff is highly dependent on if a hydraulic system is already onboard or not. Once the decision is made to install pump(s), reservoir and filters, the cost of adding a couple additional lines and a motor is more competitive with an electric motor.

    Mike Schooley

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  3. Guest625101138

    Guest625101138 Previous Member

    Just stating the obvious.

    As far as high operating temperatures electric motors win on all counts. Here is something similar to a Siemens motors I have operated in 80C ambient.

    In these conditions we could not find hydraulic ram seals that lasted more than 12 months. We did look at engineering an electric drive to replace the rams but the re-engineering involved too much cost.

    Rick W
  4. Poida
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    Poida Senior Member

    Hi ijason

    I do some design work in materials handling although not an engineer and not qualified to make any comment on this subject at all. But I'm going to.

    When I do my bit of designing I have to decide whether I will use Electric, Pneumatics or Hydraulics.

    All have their advantages and disadvantages.

    Hydraulics are used in boats. Winches are often powered by hydraulic motors because they are positive drive (mainly piston driven) and can be speed controlled and, work in the wet.

    As for driving props. As already pointed out there is a power loss. If you convert one type of power to another you have power losses. So a straight drive from an engine to a prop will provide more power than transmitting from engine to hydraulic then back to mechanical.

    No you can not use the same pump for everything else.

    To long to explain here but there are two hydraulic systems, an open circuit which is only under pressure when required and a closed circuit which is under constant pressure. Earth moving machinery uses the later but is a high power user. In industry we use the open circuit as the drive motor is only under load when the pressure is required. Boats would have to use an open circuit system or fuel costs would be huge. The problem is you can only use it for one application at a time. Think of when you are in the shower and somebody turns another hot water tap on and your shower goes cold because the pressure in the hot water pipe has dropped. The same thing would happen in your boat, turn something else on and your boat would slow down.

    The advantages of using hydraulics to drive a boat may be valid but hydraulics is used where slow high power is required. Maybe a slow moving motor driving a larger than normal prop could be OK a hydraulically driven paddle boat for example would probably work well.

    Personally, having worked with hydraulics over many years, I would only use it where absolutely necessary. trying to fit hydraulic hoses in confined spaces is no fun. Leaking is a pain in the butt, it is bad enough where you can throw some absorbent granules down and then mop it up, in the bilge of a boat? No thanks.

    But they told the Wright Brothers they couldn't fly, go for it ijason build a hydraulically driven boat. Don't forget to post the pics of your project.
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    This is true only if you have a fixed displacement pump and constant rpm. Most of the high end closed circuit hydraulics have variable displacement pumps, and if needed also variable displacement motors. Closed circuit offers much better total effiency than open..
  6. Guest625101138

    Guest625101138 Previous Member

    In 2009 we will see Formula 1 cars erode the exclusive hold hydraulics have on vehicle braking systems. They will not be replaced but the bulk of the braking will be done with regenerative systems.

    Interestingly the so-called KERS will not be the exclusive domain of electrics. The competing systems are fundamentally electric and mechanical energy storage:

    Like so many technology advances we see electrics displacing hydraulics in various ways.

    This is another application where electrics will displace hydraulics:

    Yet another application:
    Arguably still hydraulic with integrated electrics but these are pure electric:

    There is not much you cannot do better with electrics than with hydraulics. It is a matter of technological development.

    Think of the mess in your car if those 8 or so little electric motors in the seat were replaced with hydraulic adjusters. Or the electric windows were replaced with hydraulic adjusters. The boot release even.

    You may get the impression I am not an avid fan of hydraulics. You are right. Any time I have lost an argument on using hydraulics in an application I have regretted it. It is not to say I have not used hydraulics but if I can find an electric system to replace it I go for the electrics. Once you get 90% there you try very hard to go all the way and forget the hydraulics and all its mess. I prefer a busted electrical wire to a busted hydraulic hose.

    I have seen many examples of vehicle fires like the one described here:

    Rick W
  7. TeddyDiver
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    TeddyDiver Gollywobbler

    If that were so simple as you think these would be electric:D

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  8. Jim_Hbar
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    Jim_Hbar Junior Member


    One advantage of hydraulics is that when it leaks, you know where the oil is - When a high powered electrical system leaks, you never know where the electrons are..:D

    The only real advantage of hydraulics are the power densities available at the point of application - like you state, a force application, or a compact motor application where efficiencies aren't important.

    But many boats do have distributed hydraulic systems - winches (girdies), thrusters, cranes (davits), steering cylinders, stabilizer actuators etc..

    Really, it is "horses for courses".

    But like you, in our industry, we now only use hydraulics where we can't make electrics work effectively - basically high-speed/high-response linear actuators and hoist applications are the only ones left.

    For a hydraulic drive system, a closed-loop hydrostatic transmission would offer the best control and efficiency possible from hydraulics, but would be hard pressed to exceed 80% mechanical efficiency. And that circuit design would not be compatible with a distributed hydraulic system - for that application you would likely want a pressure compensated open loop type circuit.

  9. Guest625101138

    Guest625101138 Previous Member

    The biggest and best loaders and trucks use electrics for motive power.

    Where reliability and efficiency determine the bottom line you do not use hydraulics in power transmission applications.

    There is no doubt hydraulics are still used for power transmission where unreliability and inefficiency are accepted as normal state of affairs.

    Rick W

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  10. Poida
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    Poida Senior Member

    Hello Teddy

    Wish I had more time to debate, but I don't.
    Efficiency is not a prime consideration in design. Economy almost always is on top of the list followed by suitability.

  11. ijason
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    ijason Junior Member

    @rick. i think your using construction equipment is a poor example. of course, you're not the only one bringing them up... but it's not really a valid argument for my original question. you point at a big earth-mover and say, "see... it's wheels are run by a diesel electric system, electric motors are superior!". someone else posts a picture of a small bobcat and says, "see... it's 100% hydraulic, hydraulics are superior!".

    it's somewhat of a moot point, though. yes, we already talked about the fact that big earth moving equipment often uses diesel-electrics to turn its wheels... and i pointed out the fact that the limited rpm range of a hydraulic system makes them entirely out of the running for a final drive on anything that needs a broad range of speed. however, my front-loader gets along just fine with a hydraulic motive system. of course, it only needs to get up to 8mph :)

    "There is no doubt hydraulics are still used for power transmission where unreliability and inefficiency are accepted as normal state of affairs." wow, really rick? it seems that your bias is showing a bit now. in both examples of equipment, hydraulics are exclusively used to move the boom and bucket, or to lift the dump-bed. you're not likely to see a magnetic solenoid that can match the lift force generated by a small hydraulic piston. on the same note, you'll never see a hydraulic powered ceiling fan, or a hydraulic powered gate controller on someone's driveway... although, i would be tempted to argue this is more of a cost-point issue than anything else.

    the original aim of my question was not a debate over which is more efficient; driving with an electric motor or driving with a hydraulic motor. it was weather the advantages of having a single hydraulic pump; to drive the ship, run a generator, provide aux hydraulic use in place of small point-of-use electrics which are prone to weather exposure, would be a superior option due to the reduced complexity and reduced duplication needed. compared to a mechanical engine/drive, a generator with its own engine, a battery array and alternator to run an electric bow-thruster or winches which are at risk to corrosion and have limited duty cycle. etc etc. i don't doubt that electrics are more efficient for turning a prop, this is why all big ships use diesel/electric systems. my question was about simplicity, reliability, and more economy of use... not necessarily utmost efficiency of driving the boat forward.

    rick, to reference your above comment about hydraulics only being used where "unreliability and inefficiency are accepted" i have a question: how many alternators have you replaced in the cars you've owned, verses how many power-steering pumps? personally i've replaced the alternator at least once in every car i've owned, and never yet needed to replace a power-steering pump. which of those two systems is electric, and which hydraulic? :) when you're going to a professional mechanic shop, do you see corded drills, grinders, cut-off saws? or do you see air-powered tools? do you think that almost universal preference for an air-driven tools is due to collective stupidity? maybe it's the fact that the tools break down less frequently and are more reliable, have longer duty cycles, and are even cheaper when air-powered instead of each one having it's own small electric motor. air and hydraulics are quite similar, i would expect they offer similar rewards.
  12. ijason
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    ijason Junior Member

    dang... i swear i saw a post on here where somebody was pointing out how absurd it would be to have hydraulic seat-adjustment motors, window motors, truck release in a car... and i was all ready to be snarky about my 1984 mercedes that uses a vacuum system to control all of those very same things. and does so without any problems. but now i can't find the post! :(
  13. Guest625101138

    Guest625101138 Previous Member

    Never replaced an alternator in a car I owned. Did rebuild a regulator that was overcharging and causing battery problems. Have had leaky power steering system in one car but, apart from the mess in the garage, lived with it until car was sold.

    I have set up workshops and avoided air powered tools for safety reasons. In Australia we have had mandated earth leakage systems on electrical outlets for about 30 years and this makes electric the safest choice. Too many hazards with compressed air. Hoses are heavy and deadly if connection opens. I once did the calculation that if you are running a compressor off electric to power your hand tools you get 16% of the original energy in useful work if the power tool is in good condition - most are not. Throw in a few leaks and it is like flushing money down the toilet. Most construction sites now use battery powered hand tools. The common exceptions where I have not seen electric equivalent are spray painting and nail guns.

    I have a compressor in my garage and its only use is to put air in tyres. I have done some spray painting with it but I am not set up to do this properly so usually avoid it.

    The point I am making with the big mining equipment is that where reliability and efficiency are important electrics are favoured. Also as things become more technologically advanced we see a natural progression from hydraulic systems to increased use of electrics.

    Your original post suggested using hydraulics for motive power as you referred to the flexibility of locating the main engine. You provided a lot of positives and very few negatives. I am just adding to the negatives. I have hundreds of photos of typical hydraulic power packs that I see. Not one of them is leak free. Many are a fire just waiting to happen. I simply would not encourage use of hydraulic systems in any application unless there is not a good electrical equivalent.

    There certainly are some applications where I like to see hydraulics. I prefer rams on luffing functions to using electric winch and steel rope. The common tipper is a good example. Early versions used reeved wire winches. These are slow moving high force applications where hydraulics have an advantage. A busted hose will result in slow decent at worst - a good system will have a piloted pressure balance valve that will lock the ram up.

    Rick W
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    There are two possibilities.. Either you have everything upside down in oz :p and the hydraulic fluid runs the wrong way... or you and hydraulics are just incompatible..
    Some people are incompatble with electronics and thats more common..

    By the way.. You know what keeps electric gear running? It's Holy smoke! When it escapes you see brown smoke running in the air from the gadget and it never works again:)

  15. Guest625101138

    Guest625101138 Previous Member

    Just as an example for comparison purposes, wander into your own garage/workshop and do a count of the electric powered items, air powered and hydraulic powered. All the things that make your own workshop function. Lets see how different it is to mine in Australia.

    Mill drill is my most frequently used item
    Bench drill rarely used since buying the mill drill
    Three 240V hand drills
    Two battery powered drills
    One battery powered hedge trimmer
    One batter powered leaf sweeper
    Two welders, one TIG, one portable AC, TIG has inert gas bottle
    One circular saw
    One centrifugal spray painter (not used any more)
    Two angle grinders
    One bench grinder
    One belt sander
    Three orbital sanders
    One hot air gun
    Three battery powered screw drivers
    Garage door opener
    One router
    Two jig saws (oldest no longer used)
    Two soldering irons
    High pressure car wash pump

    Electric powered compressor (mostly used to pump tyres)
    Spary gun (used infrequently)
    One nail gun packaged with compressor but never used

    1 tonne car jack

    Rick W
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