problems with length?????

Discussion in 'Boat Design' started by toocanshan81, May 2, 2009.

  1. toocanshan81
    Joined: Mar 2009
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    toocanshan81 Junior Member

    hey guys i have a well what i was told 1970's MFG and i bought this boat just for the motor that was hanging on the back. little did i know that the 150 hp x-long shaft evinrude was too long for the boat. anyway when i ran this boat it looked like the motor was on backwards. all this water was slamming up against the transom and it was not pretty. anyway to sum it up i did a lot of investigation and it turns out that my chances on getting a 20" shaft is next to none. even if i could find the whole lower end i would need a pro. to do the work.

    what are the theory's on extending my transom about 4-5" so the cavitation plate sits where it is expected to be????

    any advice and\or help is very appreciated thanks toocan

  2. Gilbert
    Joined: Aug 2004
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    Gilbert Senior Member

    Yes, that is definitely the thing to do.
    You should first determine if the present transom is in top shape. Tap on it all over with a mallet or something and listen to the sounds. If it sounds hollow to any serious extent you should probable replace the transom anyway and then just make it taller and well reinforced. If it seems to be very sound you could probably get away with putting a doubler over most of the present transom that extends high enough to make it the right height.
  3. toocanshan81
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    toocanshan81 Junior Member

    can someone be a little more specific on how exactly how to build on to this transom, assuming it is still solid. thanks toocan
  4. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi toocanshan,

    Making sure the existing transom is solid is of course the first step. If it has any rot or delamination, you may as well do as Gilbert suggests and replace the transom.

    If it is sound, and you want to make it higher, there are a few possibilities.

    One would be to weld up a steel or aluminum filler, 5" high, to add the needed height. For a 150hp, it would have to have beefy plates welded on both the interior and exterior sides that extend down far enough to be securely bolted to the existing transom. (I did this with my runabout a few years ago when switching from a 15" to a 20" shaft. Done properly and well bedded, it's a strong and durable option.)

    Another option, and one that would better preserve the look of the boat, would be to cut a 5" high wood filler piece and fibreglass it in. You'd have to sand off the gelcoat, of course, and would need enough layers of epoxy/fibreglass to make the addition stronger than the original transom.

    In both cases, it may be necessary to add additional knees or bracing to the surrounding hull structure in order to properly distribute the stresses. Looking at your second photo, I'd also consider making the front of the motor well a bit higher while you're at it.
  5. tom28571
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    tom28571 Senior Member

    Matt's last paragraph is the most serious thing to remember. Raising the transom 5" will also raise the thrust pivot point 5" and no existing transom is likely to take that stress without some additional bracing up high. The added braces must transfer stress from the higher transom to hull stringers or other strong structure well forward of the transom.
  6. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    When the motor is down the cavitation plate (just above the prop) should be about in line with the bottom of the hull or it will not run correctly.

    Is that a slash well just in front of the motor ? Looks like it.

    If you're extending the transom it better be really really strong. A 150 have some kick in it, if not done well it's going to rid you of itself very quickly, besides be vey dangerous.

    I suggest you go to someone that worlks on boats and show them the setup, least get some suggestions what you can do there.
  7. Rangerspeedboat
    Joined: Apr 2009
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    Rangerspeedboat Senior Member

    Sell the motor and buy another one?

  8. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Frankly, I would do what Ranger suggests. But, an other option is a jack plate. Rather than screwing up the transom, a jack plate bolts on and can raise and lower the transom by the push of a button.
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