How come we don't see hydraulic outboards?

Discussion in 'DIY Marinizing' started by parkland, Feb 11, 2013.

  1. parkland
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    parkland Senior Member

    I can think of a huge benefit - 360 degree thrust, and impossible to leak water into boat from prop shaft.

    Even if not really energy effecient, neither is a gas outboard compared to a diesel.
     
  2. parkland
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    parkland Senior Member

    I don't think underloading will harm an engine at all.
    Idling is another story.
     
  3. alan white
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    alan white Senior Member

    The chief advantage of hydraulic drives is that they can be installed without regard for mechanical constraints like alignment, space, and orientation. You could put the driving pump anywhere and in any orientation, as if dealing with a battery bank and wires. Also, hydraulic systems lend themselves to infinite gradations of ratios. This can make for engine efficiency beyond what is possible with a single-ratio drive. However, because drive system losses through friction are pretty high, bigger system are more efficient. The reason is that bigger tubes and pipes have far less internal skin surface than small tubes. They become more efficient the bigger they get. For instance, quadruple the size of a pipe and you don't quadruple the internal surface, you only double it. Strange but true.
    Of course, I'm just guessing.
     
  4. El_Guero

    El_Guero Previous Member

  5. parkland
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    parkland Senior Member

    You could potentially change out the entire powertain without ever pulling the boat out of the water too.

    The possibilities are endless too;
    You could have 1 engine, 5 props; 5 engines, 1 prop....

    It would be so easy to have a giant high HP gas motor for emergency situations, and a small fuel effecient diesel for cruising economy.

    Does anyone know what effeciency is possible?
    Even a marine transmission isn't 100%, so what are we really looking at here as a difference?
     
  6. powerabout
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    powerabout Senior Member

    look at the size of the engine in a drag line versus a hydraulic excavator and you can see hydraulics have huge losses
     
  7. parkland
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    parkland Senior Member


    You could run an entire escavator off a 1 hp hydraulic pump if you were patient enough :).
     
  8. broke_not
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    broke_not Junior Member

    It most definitely will. We rent generators, mainly to contractors. Since they're not typically engineers or electricians, they often rent more power than they need. We just got a 50KW unit back because it was wet-stacking and had started using oil. The end-user thought it was okay to underload it, and had had it running pretty much 24/7 on a jobsite powering up 35-40 amps worth of 120 volt single phase loads. It was shut down routinely for fueling and oil/coolant level checks, so it wasn't being neglected at all....it was simply being underworked.

    The paragraphs on wet stacking here explain what's going on when diesels aren't adequately loaded:


    http://www.plantengineering.com/ind...tacking/e0b399904af236d9f863742ca8f03440.html

    As for hydraulic transmissions and when they're suitable, others have already covered the pros and cons....just thought I'd add this. A few years ago when gas prices were skyrocketing, I came across a website that detailed some hydrostatic transmission vehicles that were in the works. They had built a motorcycle already, and had a car about halfway done. I joined their message board and asked what it was they were trying to accomplish. They replied by saying that hydrostatic transmissions were infinitely variable, and therefore could be coupled to an engine that could in turn be operated at a speed closer to optimum for power and economy. I replied that I understood the benefits of a CVT, but didn't think hydrostatics were the way to go considering the inefficiencies of the pumps/motors/plumbing losses themselves. That, and the simple fact that hydrostats themselves were nothing new, and that if it were a valid concept someone would already be doing it.

    A while back, a family member bought me a box of 50's and 60's Popular Mechanics magazines, and in the August 1963 issue there's an article on a prototype hydrostatic car built by the British National Engineering Laboratory. Given that the NEL wasn't strapped for cash or other resources and yet weren't able to make the project practical kind of says it all. The overall mechanical efficiency they ended up with is in the ballpark with what off-the-shelf piston pumps and motors in common use today offer.

    Hydraulic propulsion works very well for some applications, but the smaller something is or the wider variety of speeds it has to be able to produce....the less desirable it becomes. A lot of the equipment we have is hydraulically-propelled for convenience and ease of use, but the majority of it is designed for high-torque/low-speed operation. Going "fast" while being pushed along by oil is another story altogether. Speed in a hydraulic transmission builds heat, and that heat needs to be dissipated.
     
  9. hoytedow
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    hoytedow Wood Butcher

    You took the words from my mouth. Sorry about the saliva. :eek:
     
  10. mike Banks
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    mike Banks Junior Member

    I have a pal who used diesel electric and diesel hydraulic systems on a 38 foot sailing cat. The diesel drives electric motors and generators. It also drives a hydraulic pump which drives a hydraulic motor atop an old 60 HP outboard shaft. The whole slides on rails and is hoisted out of the water when under sail. The system turns with the helm or can be disconnected and locked in position. The forward and reverse gear is original. As far as I know it works fine--it is a Kubota diesel and the pipes, except for the rubber coated flexible hydraulic ones to the outboard itself, are ordinary high pressure steam tube. It seems to work OK and is not a great deal heavier than a gearbox plus shaft. Of course there are some losses because the hydraulic oil needs to be cooled--and that represents a loss of energy. It works OK though and berthing is easier--and sailing without propeller drag has to be a plus.
     
    Last edited: Oct 18, 2013
  11. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Maybe because people don't want to fool with the extra weight and complexity of a diesel electric or a diesel-hydraulic arrangement. If you spring a hydraulic leak and any oil washes overboard, you could be looking at a big pollution fine as well.
     
  12. michael pierzga
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    michael pierzga Senior Member

    Ahh...just when you need one a naval architect swings by.

    Oil pollution. Its my understanding that mineral oil is no longer used below waterline for pollution reasons. New biodegradable oils are used. For instance Shell Naturelle

    http://s07.static-shell.com/content...ll-for-businesses/shell-naturelle-leaflet.pdf
    Any experience with this oil ?

    Is it a direct substute for mineral oil shell tellus ?

    I ask because i recently tangled with a fishing net , damaged the shaft seals and blew 400 litres of mineral oil into the sea.

    Not good

    What do you specify for new builds ?
     
  13. jonr
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    jonr Senior Member

    > wet stacking here explain what's going on when diesels aren't adequately loaded

    Note that generators don't have the option of running at higher load and lower rpm (same HP) to increase cylinder heat to acceptable levels. Some other applications do.
     
  14. FAST FRED
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    FAST FRED Senior Member

    Note that generators don't have the option of running at higher load and lower rpm (same HP) to increase cylinder heat to acceptable levels. Some other applications do.


    ITS in the works , some folks in the RV world are using 300A - 24V DC alternators spun by a small (Kubota) diesel that is properly sized to create full output.

    At lower loads the RPM is reduced to only create the DC amperage required.

    A 4000W (or a pair for 8000W ) of inverters create very nice sine wave 120V or 240V AC to run required stuff.

    This is what Honda does on its better gas generators , but their production volume is probably 100x the boating market.
     

  15. jonr
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    jonr Senior Member

    Agreed, I should have said "the diesel generators in the article". Variable RPM generators provide advantages for engine life and for part load fuel efficiency.
     
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