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

    Like for bigger boats, how come there isn't outboard rigs with hydraulic motor, and a motor in the boat with a hydraulic pump?

    Those azipods are electric, why couldn't hydraulic also be used?
  2. Petros
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    Petros Senior Member

    hydraulics are heavy and there are a lot of power losses in a small system. It take power to push hydraulic fluid through the lines and valves, in addition to drive the hydraulic motor. Larger systems have the benefit of much larger lines, and can also have space saving benefits since a drive shaft takes up a lot of hull volume as compared to hydraulic lines. Not really a benefit in an outboard. either way it is always a trade off, and for what ever reason (tradition perhaps?) outboard manufacturers have not found a benefit to a small hydraulic drive system mounted in an outboard.
  3. michael pierzga
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    michael pierzga Senior Member

  4. Submarine Tom

    Submarine Tom Previous Member

    Mine has a hydraulic tilt/trim on it!
  5. Milehog
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    Milehog Clever Quip

    Another clue. Ever notice how loud those trim-tilt units are?
  6. Red Dwarf
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    Red Dwarf Senior Member

  7. tomas
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    tomas Senior Member

    I'm wondering how much loss, percentage-wise, as I like the idea of operating a diesel-powered hydraulic unit with the diesel held around an optimal and constant RPM. This seems reliable as well. Perhaps the losses would still be acceptable for a 50 ~ 80 HP propulsion drive.
  8. Submarine Tom

    Submarine Tom Previous Member

    If you did find the losses acceptable I think you'd have a hard time with the added weight and then there is the cost of not only acquisition and installation but maintenance...
  9. Petros
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    Petros Senior Member

    this is really an engineering question, and a detailed comparison of system losses, weight, and cost would have to be made for identical systems in the size you are considering.

    Unless you have some other unique problem that is easier to solve with a hydraulic drive system, I would say the market has determined that the best small systems are mechanical rather than hydraulic. I have not done such an engineering analysis of two such systems, but I would say there is usually a reason there are no such system available. It seems to me it would be a tough sell comparing a system that costs and weighs more against a mechanical system. modern gear and drive shafts in small installations are very efficient, so the looses in a hydraulic system would have to have some really large advantages to be competitive.

    Every time I see someone's idea for a new product to fill some market nitch that is not being served, my first thought is that there might be a reason other larger companies are not filling that nitch; there is no such product there because there is no demand for it. And you will likely lose money finding that out.
  10. redreuben
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    redreuben redreuben

    I could see possible advantages in a largish cruising cat where one motor could service two drives and perhaps do double duty as a gen set ?
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    FAST FRED Senior Member

    Might be hard to create a house load that comes close to what is needed for 2 power units.

    Underloading will kill an engine rapidly.
  12. whitepointer23

    whitepointer23 Previous Member

    There used to be a hydraulic package available for cats with twin drive motors. I don't remember the brand though.
  13. BMcF
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    BMcF Senior Member

    One of Rexroth's divisions in Italy spent a great deal of time and money to develop high-power hydraulic drive systems that were targeted at mainly the hydrofoil propulsion market. When there actually was such a market. This activity would have been around the early 90s.

    If I recall correctly, they were shooting for an overall transmission efficiency to the prop shaft of around 85% but did not reach that goal.

    About 15 or so years ago we developed and built a smaller hydraulic drive system for a skipjack. The prime mover (60 HP diesel) and pump were in the skipjack and the motor and propellor were installed in the push boat. Not very efficient but it kept the vessel legally eligible to dredge.

    A 10HP system that was all hydraulic would be very much heavier and certainly less efficient than the 10HP 4-stroke OB it might replace.
  14. philSweet
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    philSweet Senior Member

    The usual place to find hydraulic drive is where constant maximum power is desired, and conditions vary frequently such that the torque must constantly be matched to the conditions. For example, a front end loader with hydraulic drive and a three speed gearbox can accelerate and transport a load at peak power without shifting constantly. Backing off the throttle in response to lugging increases the torque to the drive train and decreases speed, relieving the lugging condition. (In a gas or diesel engine, throttling back does not raise torque very much, if at all) In a boat, you don't need to play the throttle to prevent lugging like you would in a piece of construction equipment. You don't need a torque/speed relationship that is constant power. In fact, it is undesirerable. Lacking this requirement, all you get is extra weight, extra cost, and extra headaches, and a lot of extra noise. The link points to a product specifically marketed to the construction business. No surprise there- those folks know hydraulics. They can use their existing maintenance people and the units probably have parts commonality with their other equipment.

  15. jonr
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    jonr Senior Member

    In some cases, a mechanical link is difficult to route. Other than this or the ease of variable power, it's unlikely that complexity and power loss is worth it.

    > Underloading will kill an engine rapidly.

    Mostly you need to manage heat and cylinder pressures. So it's typically not a low HP output issue.
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