Click for Delta Click for Perko Click for Westport Click for Walker Click for Nordhavn

Determining transmission gear ratio?

Discussion in 'Chris Craft Roamer Yacht' started by biodon, Jun 13, 2009.

  1. biodon, Jun 13, 2009
    #1
    biodon

    biodon New Member

    Joined:
    Feb 14, 2007
    Messages:
    34
    Location:
    Lake Ozark, MO
    The tag on my Model 71 Velvet Drives have been worn down so I can't read the reduction ratio. I tried counting engine revolutions vs. prop shaft revolutions. I don't think that method works when turning the motor over with the starter. I'm guessing the fluid coupling only locks up when the motor is running and the hydraulic pump is pumping full pressure.

    Any ideas short of tearing the transmission apart?

    It is a 1958 , 28' Express with 225 hp Nash/Gray V-8s and 12x14 props (original). Any clue as to what the reduction ratio might be?
  2. K1W1, Jun 14, 2009
    #2
    K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    7,396
    Location:
    My Office
    Hi,

    Get a hand held tacho.

    Put a bit of reflector tape on the shaft and when running at a known Engine RPM Measure the shaft speed.

    Example: Engine at 1000 rpm, shaft speed at 250 rpm, ratio is 4:1
  3. K1W1, Jun 14, 2009
    #3
    K1W1

    K1W1 Senior Member

    Joined:
    Sep 30, 2005
    Messages:
    7,396
    Location:
    My Office
    Hi,

    Get a hand held tacho.

    Put a bit of reflector tape on the shaft and when running underway at a known Engine RPM Measure the shaft speed.

    Example: Engine at 1000 rpm, shaft speed at 250 rpm, ratio is 4:1
  4. Capt J, Jun 14, 2009
    #4
    Capt J

    Capt J Senior Member

    Joined:
    Jul 11, 2005
    Messages:
    14,435
    Location:
    Fort Lauderdale
    Try wetting the tag on the transmission with saliva, sometimes that will allow you to read the letters or they'll raise enough to read it. Otherwise you have to do as K1w1 said or, look up old specs of your boat/engine combination on the internet for old magazine tests or a build sheet or something