the two engines, one prop setup was 'slagged' here

Discussion in 'Boat Design' started by Squidly-Diddly, Mar 3, 2011.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    mostly because the prop was the most likely to stop working due to damage or bearings or whatever, and the modern diesel in near 100% reliable.

    Then I saw the "Steve Irwin" is a two engine, one variable prop setup, and it was built by a real ship yard for Govt service, so I figure it ain't a 'loony' design.

    I've also heard one thing that kills a diesel is running at too low power.

    Seems like a variable prop with two engines might be the way to go.

    You run one at correct power for slow cruising, and both for normal cruising. Makes more sense than running one engine for one prop, and dragging a second dead prop through the water.

    Plus, it might be lot easier to man handle two smaller engines than one huge one. You could do things like run just one if fuel quality is suspect, etc.
     
  2. jehardiman
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    jehardiman Senior Member

    I don't think it was the concept that was "slagged" but rather the reason. There are many propulsion concepts that only make economic and engineering sense when needed to solve very specific operational requirements. There is no need for the average vessel to labor under the added complexity, cost, and weight of a split gearbox/CP prop when twin engine/twin shafts is far more effective (do the FEMA). On the other hand, it make all the sense in the world to build a CODAG fisheries/SAR patrol boat. But even most of them are quad engine/twin shaft.
     
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Jerhardiman, what is "FEMA" in this context?

    some "total cost of ownership"?

    here is what FEMA normally stands for(hee,hee) http://www.freedomfiles.org/war/fema.htm

    Would a fishing enforcement vessel be doing a lot of low power slow crusing not done by other boats? Why would this make sense for them but not others?

    Yes, I know the average cargo ship is all about "set it and forget it" for that 4,000 mile, 11 knot cruise from China to USA. I hear standard cargo ships will ballast down if unloaded, just because their prop/engine is very efficient, but only at a very narrow set of specs (rpm, distance of prop under water, speed of water flowing over prop, etc).

    Once slightly out of any of those specs it falls from 90% to 20% efficient, so it is worth it for them to just haul thousands of tons of seawater just so their prop isn't flailing in the air.


    I'm thinking a twin engine, single variable prop setup would be good for Sail Assisted vessels, or those doing a lot of slower cruising in inner passages or down wide rivers, or just sight seeing.
     
  4. jehardiman
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    jehardiman Senior Member

    My bad, dyslexic engineerer gets no cookie...Should be Failure Modes and Effects Analysis FMEA http://en.wikipedia.org/wiki/Failure_mode_and_effects_analysis.

    You never want a "must work" system where you string 3 single point failures together. If you do then you have de-facto accepted the failure path, so there is no reason to have ganged engines over a single one. Adding complexity to a linear system doesn't raise its efficency or reliability, and in the end is only wasteful. Redundency only works when you actually have redundent systems.
     
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    the idea is to be able to run the boat at low power

    and efficient low speeds, AND run an engine at its most efficient(85% full power?) but still having the option to go to "max cruise".

    Also to prevent engines from damaging themselves from prolonged running at low power.

    Just how much of a bugaboo and gremlin is the extra gearbox to cut the engines in and out of the prop?

    You could see it as adding a "one" to the "two" of the prop/engine.

    I'm thinking of it as adding a "one" to the several dozen of:

    variable prop gears,

    prop main bearings,

    shaft through hull seal,

    forward/reverse gearbox and clutch,

    fuel tank and pump,

    hi-pressure injector pump,

    injector rail,

    valve train,

    engine cooling system,

    main bearings,

    engine lube oil pump,

    intake and exhaust manifolds,

    starting motor and batteries(or compressed air).


    So you take all those, with "reliability factors" of, say, 0.99999317 or 0.9999925, etc and multiply them together, right? and that gives your "system reliability".

    Or just take the prop, engine(s) and extra gear box for two-into-one prop and their reliablity factors of say, 0.9999123 and 0.99943 and 0.999678....

    my point being you are only doing one more very minor reduction in reliability, but also removing a main cause of diesel engine problems...running at too low power for extended periods....while solving another problem not solved effectively by any engine design...producing power efficiently at under 1/2 power. I want my kilowatt-hours/gallon high to make running slow worth it, fuel economy wise.

    Certain problems seem unsolvable and are rooting in natural law, like a diesel only being "happy" at fairly narrow RPM/power. But with CAD, etc. all sorts of mechanical gizmos are getting better and cheaper fast, and today you order up custom machine parts for $10,000 for delivery in two weeks that only NASA and $10,000,000 and one year wait could produce 30 years ago.

    Having two(or more?) engines on one prop also allows for use of engines with completely different 'personalities'. They don't need to be of same size, or one could be an small, expensive modern "clean diesel" for use in sensitive areas like canals and inside passages and one could be a salvaged old 'smoker' for open ocean.

    In this context it wouldn't matter much if the extra gearbox took 15 minutes to make a switch, as long as that doesn't mean 15 minutes of greasy, dangerous manual labor on the human end. I'm not talking about fancy quick shifting thing with 'digital only' glass screen user interface.


    Anyone got any firm-ish numbers of cost and reliability-factor of a off-shelf gearbox able to cut in-n-out on a single prop two different engines, say a diesel from a 2005 VW Golf TDI 1.8 liter and a 1970s Detriot Diesel 8-71.
     
  6. u4ea32
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    u4ea32 Senior Member

    Perhaps consider a single shaft (one transmission, one prop), and two engines that are connected to the shaft via belts. Belts on Harley Davidsons handle about 80 HP with ease. Google Gates for infor on such belts. They are used widely in industrial applications too.

    A belt system could easily allow the two engines to be truly redundant, thereby providing actual improvement in reliability. This approach avoids the problematic (expensive, heavy, noisy, custom, single point failure) ganged transmission, as used on Helicopters and usually included in discussions of two engines driving a single shaft.
     

  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I like the belts idea because I don't see how two cog wheels

    separated by an inch or more and connected by a drive belt could ever jam, even if the belt snaps, twists or whatever.

    Plus, it is pretty easy to slap on a new belt, and I remember when Harley-Davidson first came out with belt drive the bike had a "Emergency Repair" spare belt that could be slapped on in a jiffy, connected(no need to remove any wheels or gears) and was good for at least several hundred miles.

    Using belts would really help the concept of being able to use any engines or combination...anything you can bolt to the engine room floor and run fuel and exhaust.

    Those belts are quite, clean and do a great job absorbing misalignment and shock.

    And they aren't "your daddy's drive belts" that were just rubber and cotton.


    Motorcycle drive chains type chain is another idea, and that stuff comes in size/speed ratings from kiddies bike to bulldozer drive wheel chain.
     
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