Why don't boats have gears?

Discussion in 'DIY Marinizing' started by jakefrith, Oct 28, 2008.

  1. MattZ
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    MattZ Junior Member

    The one litre every two minutes you mention for a 100HP two-stroke is very good. I would double check your calculations because the four-stroke would probably use almost the exact same amount of fuel. Your calculation for the motorcycle engine's fuel consumption is a far too optimistic.

    Some boats do have two speed transmissions. Have a look at the product offerings from ZF Marine: http://www.zf-marine.com/ This is mainly done to allow higher engine speeds for generators, compressors, and hydraulic pumps while cruising at low speeds.

    For low speed cruising the prop torque needed goes down very rapidly, so even the high speed cruising gear has more than enough torque. For optimum fuel economy you actually need a higher gear at low speeds. However, the increase in fuel economy is very low so the cost of an extra gear is not justified.
     
  2. FAST FRED
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    FAST FRED Senior Member

    The good folks at ZF would be delighted to sell you a 2 speed gear box. They're on the shelf , in a variety of ratios..

    We have been looking for 2 years to find a good used one for the Box boat project.

    The concept is to increase efficiency the boat would run at High Cruse at one ratio , and at max range cruise (1/3 slower) with less gear reduction. This would place a higher load on the engine at low speeds , increasing engine efficency.

    "However, the increase in fuel economy is very low so the cost of an extra gear is not justified."

    We have found sometimes a 300% difference between hard working engines and loafing engines per HP per gal of fuel consumed.

    Since most folks usually choose to cruise at lower speeds a 3X increase in efficiency IS of interest , at least to us.

    Look at the gen set articles by Nigel Calder in PBB , for some concept of how fuelish diesels are when unloaded.


    It takes a certain amount of power to push any boat at a desired speed , how much you pay (in fuel or mechanical complexity ) is your choice.

    FF
     
  3. tom28571
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    tom28571 Senior Member

    I think we are missing a main ingredient here. Look at the propeller slip curves for a boat. High percentage of slip at low speed which reduces as speed increases to near zero at very high speed. This a clue that the prop in water is acting like a fluid coupling as some cars have had. The water is acting like an infinitely variable transmission of sorts. The drive wheels of a car do not normally allow for any slip so, in order to get the engine into its high torque range, it needs variable gearing. Some small advantage could probably be had with multiple gear ratios but not enough to offset the cost and complexity.

    This is a separate argument from a selected gear ratio for cruising speed like Fred mentions. Most setups on outboards and inboards have this.
     
  4. CDK
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    CDK retired engineer

    A couple of years ago I read on this forum that some people installed an automatic transmission in their houseboats to achieve better (=less worse) mileage and acceleration.
    The ability to change gear does have advantages it seems and using an automatic gearbox straight from a car avoids the clutch problem. The propeller slip does make the power train much "softer" than in a road vehicle, but changing gear with a manual box without a clutch needs a very experienced helmsman.
     
  5. MattZ
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    MattZ Junior Member

    It would not be too hard to rig up a way to operate a hydraulically operated clutch from the helm. Then you could avoid the loss of operating the hydraulic pump in an automatic transmission. (Not insignificant at high engine RPM`s).
     
  6. Marco1
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    Marco1 Senior Member

    I was thinking that to compare shifting gears in a car to a boat it should compare to having a controlled pitch propeller...but then I thought that a CPP is like having a wheel that grows or decreases its diameter...a Controlled diameter wheel.

    It is difficult to compare a car to a boat..,.however a 2 speed powerglide auto gearbox would probably do a good job for such test
     
  7. DianneB
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    DianneB Junior Member

    In aircraft, which also operate in "a fluid environment" and where the best combination of performance and efficiency are key, a fixed reduction ratio and variable pitch propeller seems to have been proven to be the best combination. I would imagine the same would also apply to watercraft.
     
  8. powerabout
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    powerabout Senior Member

    Recreational ski boats/river cruisers in California have had 2 an 3 speeds trannys in them for over 30 years.
    Some of their boats are 25' plus.
    Torque converters are removed.

    Mercury also made a 2 'speed' outboard but never went into production. It had a duo prop setup and used the props independently apparently?

    and of course drag boats have used 2 speed Lenco's just like drag cars do.

    The other use for them would be in any diesel powered boat that does 30+knots as with such a small rpm windows it means they do almost 10kts at idle.
    Most large sport fishers have trolling valves, as in they let the trans slip so you can go slow.
     
  9. Zed
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    Zed Senior Member

    I think that is the other way they are slightly better than a manual in most cars I have looked at. The opposite to a traditional auto... FWIW.
     
  10. nukisen
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    nukisen Senior Member

    Automatic!

    :eek:
    It seems like many moore people then me does have this question in mind.

    As I am planing about maybe build a bigger boat somewhere in the future I have also planned about engine and transmission alternatives.

    My thought was to build it with a car diesel engine with an automatic gearing. But then I do think the output sped to the prop would be to much reduced. Then I was thinking about solve this problem by use an ordinary gearbox after the automatic but mount it backwards to gear out the rotation speed. Then it would be able to choose the output speed at the gearbox. Also it would be able to both forward and reversed.

    Maybe this is an option for us redneckbuilders who have to build low budget.
    :p
     
  11. CDK
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    CDK retired engineer

    The thought about the diesel is the good part.
    Automotive gearboxes are unsuitable, two gearboxes after each other do not change that.

    What hp range are we talking about? Small marine gearboxes are not very expensive and save you a lot of fruitless experiments.
     
  12. nukisen
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    nukisen Senior Member

    :idea:
    If you take an ordinary manual gearbox with a ratio at first gear 1:4 and second gear 1:2,25.

    Turn it around and then you do have gear ratio 4:1 vice 2,25:1.

    Then you have geared out again. ;)

    Everything is possible, unless drunken sailor and small babies.

    Cheers!
     
  13. FAST FRED
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    FAST FRED Senior Member

    In aircraft, which also operate in "a fluid environment" and where the best combination of performance and efficiency are key, a fixed reduction ratio and variable pitch propeller seems to have been proven to be the best combination.

    This is because it has been many decades since aircraft used diesels , although this may change.

    The usual gas aircraft engine MUST be at low power (compared to TO) for any cruise .The prop allows the load to be matched to the low (Say 2900 at TO and 1700 LRC) rpm needed to give some longevity.

    FF
     
  14. dand0_4
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    dand0_4 Junior Member

    I am currently building a 16ft aluminum boat powered by a 350 chevy with a turbo 400 auto transmission. I have an 18 pitch four blade which should push me in 2nd gear, 1.48:1 ratio, 4000 rpm at 40mph with excellent accelleration. I can then shift into 3rd gear 1:1 ratio and cruise at about 2500 rpm at about 38 mph. with much better gas mileage than an outboard. Cooling is a combination of radiators and keel coolers.
     

  15. gonzo
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    gonzo Senior Member

    On modern engines, the same power will require about the same amount of fuel. The efficiency is very close.
     
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