Single engine / dual shafts?

Discussion in 'Boat Design' started by dreamer, Apr 26, 2009.

  1. Guest625101138

    Guest625101138 Previous Member

    Yup - more than likely in certain circumstances. See post #14!!!

    Rick W
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    "...Taking an example of a 26 x 16 prop running at 1500rpm and pushing 10000N at 10kts the efficiency is 60%...."

    Is that 60% theoretical for the prop, or for whole PC for the design, since that is what it really is all about? The efficiency of the whole installation.

    If somone is getting 60% as a PC on a standard vessel at 10knots??...i would love to know. In real world it is next to impossible owing to the flow into the prop and the losses in the whole system.

    Perhaps some real world stats are in order, from real built vessels (well ones I've designed and worked on). A well designed Prop/hull in general have an efficiency (PC) ranging around 45~55% -in the boat measured on sea trials. The most I have ever had has been 72% and that was on a SWATH. Excellent flow into the prop, almost 'open water' characteristics. When i say 'boats', i mean more than 5m models running up and down rivers! Boats carrying 400 passengers, or other running at 35 knots chasing drug runners for days at sea with a crew of 14.

    The numbers spited out of a program may say 60% or 70% if 2 props used versus one may sound "great", but you wont get that when the props are in the boat attached to the shafts and you take the boat out on sea trails for the first time. You'll get a lot of disappointed faces!

    The flow of water into the prop is so varied and affects the efficiency significantly. Hence hull design aft is very important on prop boats. Not to mention almost all installations are angled shafts, often owing to lack of adequate flow into the prop and space in the engine room.

    With angles shafts it has an even more major effect of the propulsive efficiency. In general the majority of designs the real thrust is more likely to be in the order of 5%~10% less than shaft line thrust. Just this alone affects the efficiency and so the downward spiral goes.
     
  3. Boston

    Boston Previous Member

    I was thinkin as I read along that if the trany eats power and the shaft causes so much resistance then an electric in an azipod would both allow for the maneuverability and the electric allow for the variability while at the same time eliminating the need for the shaft and its supporting structures being replaced by the pivoting system for the Azipod.
    would also allow for a for and aft configuration rather than a side by side one, as long as they were far enough apart it should work.

    the biggest slowest blade you can fit being the most efficient

    kinda makes a good case for electrics

    thing I really like about the Azipod pivoting system is that the rotating mechanism can easily be designed to be above water line allowing for very little chance of water infiltration through the shaft, stuffing tube area
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Electric drives = heavy

    Only really suitable for large ships.
     
  5. Frosty

    Frosty Previous Member

    I was thinking about 2 engines and one shaft with hydraulic motors and drives.

    The result should be simplicity with reliability using either or both to a single shaft. Hydraulics could drive windlasses etc.

    Economy with one engine with the stand by being available for higher speeds.
     
  6. mydauphin
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    mydauphin Senior Member

    Everyone says hydraulics are cheap and easy. I understand how it works and how to make it work, but me tell where to buy the stuff to put it together. Large hydraulics pumps aren't cheap or easy to find. Any places anyone?
     
  7. Guest625101138

    Guest625101138 Previous Member

  8. mydauphin
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    mydauphin Senior Member

  9. Frosty

    Frosty Previous Member

    Heavy machinery, oil rigs, ships etc. Try a ship breakers yard or yellow pages.

    Bolt it up and pipe it, no bleeding just get a tank of oil.

    They don't need to be big , you would be surprised at how small they are.

    Dirty oil is the only killer. Well filtered oil is good. Once tolerences are lost the pump or motor looses power.
     
  10. rasorinc
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    rasorinc Senior Member

    Add to that all tractor companys even the ones that make small garden tractors. Don't bother to call CAT. Theirs would sinlk a small boat.
     
  11. fpjeepy05
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    fpjeepy05 Senior Member

  12. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    1 person likes this.
  13. Boston

    Boston Previous Member

    ya but that seemed like a lot of money for a couple of 90° gear boxes
     
  14. dreamer
    Joined: Nov 2004
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    dreamer Soñadora

    well, costs can be dealt with. The conclusions of the design are interesting.
     

  15. Lurvio
    Joined: Jul 2009
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    Lurvio Mad scientist

    For the thrust steering discussed above, there would be the setup used by John Deere to steer their rubbertracked tractors. It has an angle gear (after engine and gearbox) and on both sides planetary gears. Their ring gears are connected to a hydraulic motor, when the motor is stationary, the output rpm's on both sides are the same. When the motor starts turning the ring gears, it creates a difference in rpm, hence the vehicle turns. A steering wheel is used, a computer reads the position of the steering wheel, more the wheel is turned, faster the motor turns. The steering is accurate enough to work on the tractor up to speeds of 35 km/h on public roads(john deere 9030T).

    I'm not going to go on the feasibility of such system on marine use, this is just an example on the though of taking the rudder of the equation.
     
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