Results 1 to 8 of 8

Thread: Transom Height/ shaft length

  1. #1
    Join Date
    Nov 2001
    Location
    Mount Pleasant , SC, USA
    Posts
    333

    Post

    When repowering a boat What is the proper shaft length for a transom that is 20" tall. Will a 20 inch shaft do, or do I need a longer one?

    Thanks Rick

  2. #2
    Join Date
    Apr 2003
    Location
    Adelaide, South Australia
    Posts
    219

    Post

    Perhaps the question should be: what part of the outboard should extend below the bottom of the transom?

    No, I don't know the answer, it's a question I'll be needing an aswer to soon.

    Cheers
    Richard

  3. #3
    Join Date
    Mar 2000
    Location
    Acworth, GA
    Posts
    4,723

    Post

    What I've read is that the cavitation plate should be dead even with the bottom of the transom, and I built my boat accordingly. In retrospect, for my boat, I think I should have put it a bit below the bottom of the boat, maybe a 1/4" to 1/2" or so. I think it would have made adjusting the motor trim a little more effective as well as made the prop less likely to cavitate as the motor was trimmed up.

  4. #4
    Join Date
    Dec 1999
    Location
    Eagan, Minnesota, USA
    Posts
    11,073

    Post

    As a kid I was taught that the bottom of the cavitation plate should be about an inch below the bottom of the transom, or at least an inch in the water if the bottom of the transom is not in the water.

  5. #5
    Join Date
    Jul 2003
    Location
    Pullman, WA
    Posts
    8

    Post

    When I built my flat bottom skiff, I made the transom exactly 20" to correspond to to a 20" long shaft motor. I purchased a Mercury 15hp 4 stroke. It seemed to be the correct height based on measuring the motor before the boat was built. However when I installed the motor, positioning the cavitation plate within 1" of the bottom resulted in a transom height of 21 1/2". Luckily it was easier to add to the transom than remove material. Lesson: a 20" long shaft motor is an approximate guide for transom height. Only installation of the specific motor will provide you with the exact measurement.

  6. #6
    Join Date
    Mar 2000
    Location
    Acworth, GA
    Posts
    4,723

    Post

    Originally posted by Dan McLaughlin:
    When I built my flat bottom skiff, I made the transom exactly 20" to correspond to to a 20" long shaft motor. I purchased a Mercury 15hp 4 stroke. It seemed to be the correct height based on measuring the motor before the boat was built. However when I installed the motor, positioning the cavitation plate within 1" of the bottom resulted in a transom height of 21 1/2". Luckily it was easier to add to the transom than remove material. Lesson: a 20" long shaft motor is an approximate guide for transom height. Only installation of the specific motor will provide you with the exact measurement.
    The shaft length on outboards definitely isn't approximate. The shaft length on a motor is defined as the vertical distance from cavitation plate to the top of the transom when the motor is in a straight up and down position.

    How you measure transom height depends on what you're measuring. See below. The shorter vertical measurement is the transom height, not the longer measurement in the plane of the transom.

    I suspect in your case, you made the transom 20" tall, not accounting for any aft rake of the transom.

    In the drawing below, the rake is 15 degrees. Judging from your example yours must be raked aft at 21 degrees.



    [ 02-26-2004, 03:03 PM: Message edited by: John Bell ]

  7. #7
    Join Date
    Jul 2003
    Location
    Pullman, WA
    Posts
    8

    Post

    John, good point about the rake of the transom. However, I took this into account and my transom was 20" on the vertical. What turned out to be approximate was Mercury's definition of 20". When mounted, the motor need over 21" vertical on the transom to bring the cavitation plate into the correct alignment.

  8. #8
    Join Date
    Mar 2000
    Location
    Acworth, GA
    Posts
    4,723

    Post

    Hmmm. My brand new 2002 15 HP Mercury 4-stroke with the 15" shaft is exactly 15". Go figure...

    I'd appreciate if you'd measure your motor as I've shown here, which is my understanding of how shaft lenghts are determined. When I draw a boat for a particular motor shaft length this is what I assume. If it's really as arbitrary as you indicate, then that's a real bitter pill.



    [ 02-27-2004, 08:16 PM: Message edited by: John Bell ]

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •