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outboard mounting on transom? how high to mount the motor?

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  • outboard mounting on transom? how high to mount the motor?

    it seems that i have been hearing and reading alot about outboard installations. i have a 86 14 foot bayliner. the force blew a piston, so now im repowering with a 72 merc 402.
    the question, is do i mount the motor, so the big cavitation plate is 1" ABOVE the bottom of the boat? the original installation, was too low (i think) because the large cav plate was buried under the transom. at full throttle the plate was all the way in the water.
    so is it true that the cav plate creates unnecessary drag? and bringing the plate up 1" above the transom is better? also i have read that for every 10mph, u should raise the motor 1" for ever 10mph, so if i can get 30mph, i should raise the motor 3"?????
    help!
    dino
    46 johnson td 20
    54 evi 7.5
    56 johnny 5.5
    54 johnny 10hp
    72 johnny 20hp
    76 johnny 15hp
    68 evi 18hp elect start
    82 johnny 15 sailmaster
    83 evi 9.9 shorty
    84 evi 50 hp, remote, freebie
    85 johnny 30hp ls
    86 evi 4hp
    89 evi 15hp
    87 merc classic 50
    lots o parts
    81 12 foot valco wide body
    85 15' 9" bayrunner
    87 14 foot bayliner
    i dont have a omc problem, so i have a few motors..


  • #2
    Re: outboard mounting on transom? how high to mount the motor?

    Not sure what a merc 402 is but if you're talking about an outboard then start by mounting it with the cavitation plate even with the bottom of your hull. See how the boat performs with that then you can raise it up one notch at a time until you start having problems with the prop blowing out in turns.

    Generally the higher up the motor is mounted the more speed you will get. Usually the best setup is even with or slightly above the bottom of the hull.
    sigpic

    1969 Slickcraft SS170v

    In the garage:
    1996 Evinrude 115hp Rebuild in progress
    1995 Evinrude 70hp
    1997 Evinrude 70hp
    1994 Evinrude 4hp

    Comment



    • #3
      Re: outboard mounting on transom? how high to mount the motor?

      Originally posted by trendsetter240 View Post
      Not sure what a merc 402 is but if you're talking about an outboard then start by mounting it with the cavitation plate even with the bottom of your hull. See how the boat performs with that then you can raise it up one notch at a time until you start having problems with the prop blowing out in turns.

      Generally the higher up the motor is mounted the more speed you will get. Usually the best setup is even with or slightly above the bottom of the hull.
      sorry, i have a 1972 mercury 40 hp outboard (for some reason they call it a 402).
      46 johnson td 20
      54 evi 7.5
      56 johnny 5.5
      54 johnny 10hp
      72 johnny 20hp
      76 johnny 15hp
      68 evi 18hp elect start
      82 johnny 15 sailmaster
      83 evi 9.9 shorty
      84 evi 50 hp, remote, freebie
      85 johnny 30hp ls
      86 evi 4hp
      89 evi 15hp
      87 merc classic 50
      lots o parts
      81 12 foot valco wide body
      85 15' 9" bayrunner
      87 14 foot bayliner
      i dont have a omc problem, so i have a few motors..

      Comment



      • #4
        Re: outboard mounting on transom? how high to mount the motor?

        You are on the right track. The plate needs to be even or slightly (up to an inch) higher than the running keel.
        This is a great link to boat specifications http://boatspecs.iboats.com/
        Please, shop iboats first!!

        Comment



        • #5
          Re: outboard mounting on transom? how high to mount the motor?

          Originally posted by Bob_VT View Post
          You are on the right track. The plate needs to be even or slightly (up to an inch) higher than the running keel.
          the cav plate will be up another 3/4", bc im too stupid to figure out how the lu goes back on. had to pull it to change the impeller. now i cant put the thing back on.... and i cant install the shift/control till the lu is back on.
          hope i can get this thing squared away before next weekend.
          heres some picts of the work in progress.
          dino

          raised the motor by 1". got some plastic from tap plastic.

          new holes

          straight edge off the bottom of the boat. the lower unit will be up a little higher (lu not all the way in )

          another view

          46 johnson td 20
          54 evi 7.5
          56 johnny 5.5
          54 johnny 10hp
          72 johnny 20hp
          76 johnny 15hp
          68 evi 18hp elect start
          82 johnny 15 sailmaster
          83 evi 9.9 shorty
          84 evi 50 hp, remote, freebie
          85 johnny 30hp ls
          86 evi 4hp
          89 evi 15hp
          87 merc classic 50
          lots o parts
          81 12 foot valco wide body
          85 15' 9" bayrunner
          87 14 foot bayliner
          i dont have a omc problem, so i have a few motors..

          Comment



          • #6
            Re: outboard mounting on transom? how high to mount the motor?

            most of these have to be in revers to drop LU , is it still? try rotating the prop while pushing up to make sure splines slip togeather
            "Grounds 4 Divorce"

            Comment



            • #7
              Re: outboard mounting on transom? how high to mount the motor?

              Looks ok to me once you get the LU seated correctly. On my 73 Merc 500 50 hp, the manual said to pull and re-install the lower unit in N, but it was hard to deal with that way. I used to put it in D or R and it slipped back together easier/better...just make sure the lower unit is in the same gear as your control when you put the LU back on, and spin the prop or flywheel a little as you do it to get the splines aligned.

              Comment



              • #8
                Re: outboard mounting on transom? how high to mount the motor?

                Originally posted by tmcalavy View Post
                Looks ok to me once you get the LU seated correctly. On my 73 Merc 500 50 hp, the manual said to pull and re-install the lower unit in N, but it was hard to deal with that way. I used to put it in D or R and it slipped back together easier/better...just make sure the lower unit is in the same gear as your control when you put the LU back on, and spin the prop or flywheel a little as you do it to get the splines aligned.
                that thing was a bear to put back on.... somethings that johnny/evi engineered better....
                46 johnson td 20
                54 evi 7.5
                56 johnny 5.5
                54 johnny 10hp
                72 johnny 20hp
                76 johnny 15hp
                68 evi 18hp elect start
                82 johnny 15 sailmaster
                83 evi 9.9 shorty
                84 evi 50 hp, remote, freebie
                85 johnny 30hp ls
                86 evi 4hp
                89 evi 15hp
                87 merc classic 50
                lots o parts
                81 12 foot valco wide body
                85 15' 9" bayrunner
                87 14 foot bayliner
                i dont have a omc problem, so i have a few motors..

                Comment



                • #9
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                  Re: outboard mounting on transom? how high to mount the motor?

                  Agreed, OMC's from that period were easier to maintain. But now you know how the beastie works, it will get easier every time you have to service the LU and water pump. Like riding a bicycle...

                  Comment


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