gearing for fuel economy

Discussion in 'All Things Boats & Boating' started by Waddie, Jan 14, 2011.

  1. Waddie
    Joined: Nov 2010
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    Waddie Waddie

    this question probably belongs on an automotive thread, but a lot of you boat people are engineers and/or mechanical types so you might have some good input regarding this question;

    Why can't we achieve way better gas mileage in our vehicles, including power boats, by improving gear ratios, adding more gears in a wider ratio, or utilizing devices like a Marlin Crawler ( which gives you a low/high gearbox behind your regular tranny, thereby doubling your gearing) ?

    18 wheelers have always used gearing to produce the power to the wheels that they require. Without all those gears they couldn't pull a tenth of what they do. How about a 2 cyl engine in a Honda Civic with a 15 speed gearbox? What kind of mileage could we get?

    just wondering,

    Waddie
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    It's been tried and you have a diminishing set of returns as you add gear ratio combinations. You're right in that if you discount frictional and other energy loses, a very small engine can "spool up" a substantial amount of power. I can use a cordless drill to operate my 6,000 pound, twin sling gantry. It's a 12 VDC cordless drill!

    Ultimately it boils down to a few key issues, Weight, frictional loses, complication and of course additional cost. In an 18 wheeler, you have to have a huge gear box, because it can't do it's job without it, so the only cost effective method is with lots of gears. In a car, you can get by with just two, so it's hard to justify the additional complication, weight and expense of additional gearing. Now this said and people being the finicky dolts they are, we have 6 speed transmissions, when a 3 speed will do fine. On a boat, the gear box is much less useful, as it is on an airplane, though you can employ them in special applications (read costly high preformance, likely to break applications).

    In short, sure, you can use the idea, but in reality, it'll cost more then necessary or that practicality will suggest is reasonable.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Actually we have something far superior to any number of gears, the continuously variable prop. It enables you to get optimum performance at any load condition by keeping the engine at the point of best economy.

    But it does cost a bit more than a fixed prop, making it too expensive for boats that are only occasionally used.
     
  4. Frosty

    Frosty Previous Member

    What you are talking about is keeping the torque on the button.

    Auto trannies do this with in the confines of the ratios available.

    The modern motorcycle uses a variomatic transmission like the Dutch Daff car in the 70's.

    Its all been done, yet Hummers remain popular,--go figure.

    Mind you there are two very differnt ecomony expectations that of European and of American iron push rod V8s of the last era that still survive as dinasours as far as Europeans are concerned.

    Economy sells, I think we have just about got it cornered, but in the US it seems of little concern as long as you can afford it --burn it.
     
  5. yipster
    Joined: Oct 2002
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    yipster designer

    was reading this recently..

    http://www.boattest.com/boats/boat_video.aspx?ID=1956
    http://www.boattest.com/boats/boat_video.aspx?ID=2416
     

    Attached Files:

  6. Waddie
    Joined: Nov 2010
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    Waddie Waddie

    gearing

    It would seem that friction and excessive weight are the enemies of developing more complex transmissions whether in boats or autos, though there is quite a lot of experimentation going on in this field.

    here is a link to one effort;

    www.orbitaltraction.com/marine.html

    As engines become more efficient perhaps transmissions will, too.

    I looked up variable speed props and saw that they use engine rpm to change the prop pitch, but several sites recommend them only for boats that are somewhat underpowered, claiming they aren't as efficient as fixed blade designs properly matched to the boat's power and weight.

    Does anyone make a folding prop for outboards ? Especially in the 6 to 10 hp range ? Or is this not practical for some reason ?

    thanks,

    Waddie
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A folding frop for an outboard might rival the cost of the outboard. This is also the case for huge gear reduction systems, they cost so darn much you can learn to live with less.
     
  8. Waddie
    Joined: Nov 2010
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    Waddie Waddie

    gearing

    Hello Paul,

    I would like to get away from having to raise the 6hp outboard when under sail.

    On my 19' Mariner it doesn't seem to produce all that much drag if I leave it in the water, but I have no way of accurately measuring speed so I can't say for sure. I don't race, just cruising around. If a folding prop existed, and wasn't priced too high, I would try one.

    I wonder if there is a market for such an item in the sailing community if the costs could be controlled? What would be a marketable price point for an outboard folding prop ?

    regards,

    Waddie
     
  9. CDK
    Joined: Aug 2007
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    CDK retired engineer

    There is more to it than just that. Several Scandinavian companies offer systems allowing the pitch to be changed by the pilot over a wide range including vane position (neutral) and reverse, eliminating gear box control.

    Here is an overview:
    http://www.nauticexpo.com/boat-manufacturer/variable-pitch-propeller-1966.html
     
  10. mark775

    mark775 Guest

    "I have no way of accurately measuring speed" - Can you possibly borrow a watch and a piece of string for this?
     
  11. Waddie
    Joined: Nov 2010
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    Waddie Waddie

    gearing

    OK, I'll bite.

    I actually own a watch and a piece of string.

    How does it work for measuring speed on the water.

    How precise is this method?

    Waddie
     
  12. wardd
    Joined: Apr 2009
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    wardd Senior Member

    you throw someone overboard to hold the string and time how long they can tread water
     
    1 person likes this.
  13. Frosty

    Frosty Previous Member

    What they did in the olde days was throw or place a beer can in the sea at the front of the boat then they would get it out again after a certain time and mark the position is was recovered. This distance from the front to the back was measures with a polypropelene rope with Knots in it.

    The measurement would be in knots. They would then take this information to the capatain who would check it against the GPS.
     
  14. Waddie
    Joined: Nov 2010
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    Waddie Waddie

    snipe hunting

    the watch/string is to sailing as snipe hunting is to, well, hunting.

    I get it now.....you're actually measuring how gullible a person can be..:)

    thanks,

    Waddie
     

  15. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Rofl. :D
     
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