What makes for a ‘racing’ lower unit?

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  • 1aquaholic


    Replies: 22
    Topics: 4
    #18938

    Show us the "Tractor Drive" lower unit! That is amazing to me!
    1Aquaholic


    Mumbles


    Replies: 5764
    Topics: 298
    #18940
    quote retiredoz:

    And the ‘Green Hornet’ is nowhere in sight ?

    For you, Oz. 😉


    Mumbles


    Replies: 5764
    Topics: 298
    #18941
    quote 1Aquaholic:

    Show us the “Tractor Drive” lower unit! That is amazing to me!
    1Aquaholic


    Here’s a ’36 Caille with dual carbs and tractor unit.

    And a Speeditwin with tractor.


    benchrascal


    Replies: 24
    Topics: 6
    #18953

    Some bathtub racers in Nanaimo BC experimented with racing units on their 6hp motors .Not sure now but at the time that was the top hp allowance .I could not believe how small a profile an lu could be and what high thrust you could get out of just six horsepower .Some of those bathtubs really fly .The one thing I saw that was common (irregardless of make ) was that they were all direct drive to reduce size and weight.


    legendre


    Replies: 389
    Topics: 8
    #19063

    It seems that tractor LUs fell out of favor long ago, particularly in general service engines. So what was the theoretical advantage of the tractor design?

    The obvious difference is that in a tractor, the prop is first through the medium, and the tractor LU structure tails the prop – whereas with a standard pusher, the LU structure leads the prop.

    So was the leading-prop arrangement believed to be superior in some ways? Was it ever so, and if yes, why isn’t it still used?


    Mumbles


    Replies: 5764
    Topics: 298
    #19068

    With the propeller in front of the gearcase, the blades are biting into undisturbed water but when the prop is at the rear, the blades have to grab water which is swirling around having just come over the gearcase and this makes for less tractive force. When being raced, the tractor units would out perform a conventional rear mounted unit in the straight stretches but would lose any advantage gained while going thru the turns. Apparently the tractor units became a handfull to operate in the turns and would have to slow down and then be passed by the rest of the fleet.


    legendre


    Replies: 389
    Topics: 8
    #19070

    Classic.. leads in the straightaways but lags hard in the corners. Thanks Mumbles.. interesting info.


    bh


    Replies: 22
    Topics: 1
    #19095
    quote benchrascal:

    Some bathtub racers in Nanaimo BC experimented with racing units on their 6hp motors .Not sure now but at the time that was the top hp allowance .I could not believe how small a profile an lu could be and what high thrust you could get out of just six horsepower .Some of those bathtubs really fly .The one thing I saw that was common (irregardless of make ) was that they were all direct drive to reduce size and weight.

    The motor of choice during those times, ie ’50s >’70s, (+?) was the 7.2", Mercury, KF5, Mark 5 & Mark 6.
    This was also the motor used in APBA JU & later JSR racing, ie; ‘Junior Stock Runabout’.
    A recent post on Hydroracer plots the rise in speed of these rigs, from 29mph in ’59, and 30mph in ’63,
    to 33mph in ’73, and 36mph in ’75. At some point in the ’80s the carb restricted OMC motor became the J engine,
    and speeds went up. The ‘bathtubs’ that I saw running in the ’70s were nothing more than JUs with plastic
    tubs mounted on them. and their straight away speeds were comparable to JU/JSRs.
    One design change was made in APBA racing, in that period, the introduction of the ‘J60’ by Mercury,
    a stock Mark 6 with a very small special Quicksilver racing l/u. It would appear that,
    all other things being equal,[??] the switch from the stock fishing unit to a ‘racing l/u’
    at most resulted in an increase of ~20% in straight away speeds.
    Not as great as one might expect, as the fishing l/u’s were direct drive, and a 16:21 ratio in some cases,
    whereas the Quicky would have been a 1:1 ratio, making for a small case, but not necessarily higher speeds .
    Another good comparison is in the large l/u of the Johnson racing PR40, and the small streamlined unit
    of their later PR50. Speeds in that period doubled, from the 20s to the 40 mph range,
    but for many reasons.
    And one could compare the fishing 30hp Johnson RD vs the M30H with quicky,
    36"Run vs CU, ~40mph vs 60mph, ie; roughly 50% +
    Food for thought………. ❓


    chris-p


    Replies: 2456
    Topics: 153
    #19444

    How about the racing Soriano!?

    It had twin props, one tractor style, one traditional. Pretty neat. Enjoy the pics here

    http://www.soriano-outboard.com/12395.html


    Mumbles


    Replies: 5764
    Topics: 298
    #19448

    Thanks for posting that link Chris!

    These people were definitely thinking outside the box with their opposed four and six cylinder engines and unique propulsion system!

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