V drive rear mount conversion dissapointment

Discussion in 'Inboards' started by 202_ski, Oct 19, 2008.

  1. 202_ski
    Joined: Mar 2008
    Posts: 12
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    Location: Australia

    202_ski Junior Member

    I converted my mid mount 6 cylinder, 14 foot ski boat (which was originally outboard), to a rear mount V drive using a suzuki jimny transfer case. The boat would only rev to 2400 as a mid mount, with a 12*11 prop (a inboard specialist sold me the prop, said it was ''the one''). The V drive box(transfer case) has a 1.32:1 ratio, iv kept the same motor, shaft angle and prop. The boat now revs to about 3200 - which works out to be about 2400 at the prop. No change! I thought the transfer case gears would multiply torque to spin higher revs and also the change in weight distribution. It does plane better but thats about it. Does anyone else think this is strange?
     
  2. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    You should specify at least this:
    - boat model
    - engine model, max power, max rpm
    - boat's operative weight
    - lenght overall, length waterline, beam waterline, draft
    - transom deadrise angle
    - max. speed you have with this configuration.
    Also, what speed (max, cruise) would you like to obtain?
     
  3. 202_ski
    Joined: Mar 2008
    Posts: 12
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    Location: Australia

    202_ski Junior Member

    -Pride starfire runabout, low sides. Ski boat style.
    -3.3L Holden straight six (about 100HP).
    -not sure on weight. Fiberglass, wood frame, full cast iron motor. 2 seats.
    -15ft LOA.
    -17 degree deadrise.
    -52 km/h WOT. Up from 48kp/h as a mid mount.
    -52km/h feels like a good criuse speed. My idea of max is about 70 km/h.
     
  4. Manie B
    Joined: Sep 2006
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    Location: Cape Town South Africa

    Manie B Senior Member

    motor revs still too low
    ratio's are wrong
    should rev to 4600 WOT
    now its just on the torque line
    change gear ratios or prop
     
  5. 202_ski
    Joined: Mar 2008
    Posts: 12
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    Location: Australia

    202_ski Junior Member

    As a midmount it was direct drive 1:1, as a Vdrive 1.32:1 and at WOT the prop will only turns to 2400 for both set ups. To reach 4600 from 3200, in theory i will need a 12*6 prop?- every inch decrease in pitch lets the motor rev out-roughly- a further 250 rpm. Which would comromise speed, defeating the purpose.
     

  6. Jango
    Joined: Aug 2005
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    Location: Mid Atlantic

    Jango Senior Enthusiast

    202-ski,
    The results with the vee-drive is what I would expect. Gear change does Not result in More power. It merely changes the shaft speed in relation to engine speed. As previously stated, you need to change gear ratio and/or Prop pitch in order to increase engine RPM,s to where Max Engine HP is developed - Probably 4400 - 4600 rpms. Since HP - Speed relationship is expodential, don't expect a large increase in speed.

    Assuming a total displacement of 1600 lb (with driver only) and 100 Hp with 1.32 gear, Correct prop should be a 12x16, 3 blade. At 4600 rpm (motor speed), max Boat speed is 80 km. If you are going slower than 80 Km, you have either less than 100 HP and/or more displacement and prop pitch will need to be less. Knowing actual HP and weight are essential for proper Prop sizing.
     
    Last edited: Nov 8, 2008
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