OK, an automotive A/T handles a prop just fine;

Discussion in 'Propulsion' started by Squidly-Diddly, Sep 24, 2011.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    if you don't worry about the added fuel for driving that 3spd C-4 or C-6.

    How about an automotive manual trans?.....as far as shifting?

    In a car, the wheels keep turning at the same speed as you shift, as they are glued to the road with 2500lb of car regulating their speed.

    What about when a prop is turning at 2-6(?) X "water speed" and then suddenly looses power for that 1/2 second while you change gears?

    And what is the typical ratio of a prop's turning under power VS what it would do 'free spinning'? Obviously, it would vary greatly from starting from stop to cruising.


    WOOPS, this should go in propulsion section.
     
  2. Poida
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    Poida Senior Member

    Not sure on the question here squids?

    Inertia of car doesn't equal that of gear box during gear changes, hence need for syncro, double de clutching, AKA shuffle clutching or racing change, where you speed up or slow down gearbox to match revs of tailshaft.

    Note high range of speeds of car vs low range of speeds for boat.

    Difference between positive drive for car vs non positive drive for boat.

    A jet aircraft takes off from 0 KPH and reaches over 1000 KPH does it change gears?

    What does this mean to the man on the street. Stuffed if I know.

    Time for bed. Goodnight.
     
  3. jim lee
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    jim lee Senior Member

    2 speed automatics are common in V-drive Drag boats. I think I recall them being beefed up old GM powerglides. I would not be surprised if a manual would work fine as well.

    -jim lee
     
  4. DCockey
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    DCockey Senior Member

    Cars and trucks don't necessarially stay at constant speed during a gear shift. Going up a hill a car or truck will slow down during a shift. So there shouldn't be any problem shifting a manual transmission in a boat if a clutch is used between the engine and the transmission. The transmission can be shifted the same as if it was in a car or truck using the clutch and closing the throttle during the shift..

    If the clutch isn't used it becomes more interesting. If the transmission has synchronizers then a synchonizer will try to slow down or speed up the gears in the transmission, the prop shaft and the propeller as needed. That will be usually be hard on the synchronizer and not good for its life.
     
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Poida, unlike cars wheels, a prop that WAS spinning at several times

    'freewheeling' speed(governed by boat speed) would suddenly reduce RPMs to freewheeling speed when power is cut when clutch engaged.

    Think of a prop like a car's tires Burning Rubber. Now think about shifting from 1st to 2nd during a prolonged Peel Out(or just spinning on slippery road).

    I think that would subject the gearbox to unplanned for stresses.

    I'm just wondering if those would be stresses that while unplanned for, don't bother it a bit, or would tend to kill the unit, and if certain models of gearboxes can handle this, and others can't.

    Probably a question for master mechanics at some 'deep' "GearboxRebuilder" forum.


    DCockey, that is what I was thinking, 'hard on the synchos', but doable....

    but I don't see too many instances where a boat would need to shift quickly like a typical car in traffic, or shift even 10% as often, or downshift to use engine braking, so it should be fine.

    How useful would having a 4sp manual be spinning a big (3') prop on a displacement 'trawler' or motorsailer, especially fuel economy wise? I don't see any refs to multiple gear ratios, and certainly this has been tried.

    Is it all about the variable pitch props, and RPMs pretty constant?

    PS-"transmission" normally means A/T and "gearbox" refers to the gearbox of a manual(and it is understood a clutch exists) at least here on West Coast.
     
  6. DCockey
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    DCockey Senior Member

    Manual transmission shifting using the clutch: Disengage the clutch and stresses inside the transmission are very low compared to the stresses when power is passing through the transmission. When the shift lever is moved the synchronizers only need to change the speed of the input shaft so that is at the speed needed for to engage the new gear. Engage the clutch and the speed of the engine and driveline are matched.

    Shifting without using the clutch, which someone doing a "peel out" might try, is a different story. In that case the synchroizer has to match the speed of the engine to the driveline. That's much harder on the transmission, though not as bad in a boat as in a car or truck.


    Automatic transmission shifting is a different story.
     

  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    thx, DCockey, I was thinking of an auto derived 4sp on a cat similar

    to Wharram's 'no frills' concept where it looks like the engine, shaft and prop are hung from a crossbeam and tilted in and out of the water.

    I figure gears might be helpful on a cat that might be sailing fairly well but still need some power to make schedule, OR might need a lot of torque to push upwind.

    I'm pretty sure a good auto/truck motor,clutch,gearbox,shaft can be had used for 1/3(or less) of the cost of a variable pitch prop....and cheaper to install Wharram-style than a boatyard fitting a VP prop.
     
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