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Aluminum transom repair and reinforcement.

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  • Aluminum transom repair and reinforcement.

    I have a small 12 foot aluminum boat that has a small piece of wood mounted on the transom to clamp the outboard to. It's old and the clamps are starting to crush the wood. So, I'd like to remove it and replace it with a piece of marine-grade plywood...possibly.

    The questions is...would starboard be a better material to use or stick to two sheets of plywood?

    Other questions: Hypothetically, if the boat has a HP max of 12 hp and I'm running a 9.9hp on it now that's not quite powerful enough and I wanted to bump up to a 15hp (discussions of the illegality aside), what are the physical consequences (my boat has welded seams, would I start tearing the seams, warping/bending the transom, etc...) or would the bump up be not-advised but not necessarily something that'll tear my boat apart.

    If I'm replacing the small piece of wood on the transom, is there any way to reinforce the transom to better handle the added power of a 15hp. Should I have a local welder build a triangular brace to go from transom-to-floor? Should I replace the small piece of wood with a piece that covers the entire transom of the boat (thru-bolted with stainless bolts and sealed with 5200).
    Surely I'm not the first to attempt this.
    Any suggestions (besides sticking to the 9.9hp)?

  • #2
    Re: Aluminum transom repair and reinforcement.

    If your boat is sound in all other respects, replace the wooden transom pad with the material of your choice...plain old exterior ply painted with exterior latex works well...and then measure your transom. A 15 hp might be too long for your transom. Whatever you put on the transom pad should have it's cavitation plate even with, or within an inch of, the bottom of the boat. Too long and it will be slow and doggy in the water. That would be my main concern upping from 9.9 to 12 or 15 hp. The OMC's of the 50's and 60's are pretty bullet-proof in all hp camps. Can you post some pix of your boat? We love pix here.


    • #3
      Re: Aluminum transom repair and reinforcement.

      My Johnson 9.9 and my Evinrude 15hp are basically the same motor. OMC started with a 15hp and detuned it down to 9.9. So, it's same length, weight, height, props, etc...
      Right now cavitation plate is even with the bottom of the boat.

      My worry was damaging the boat with too much HP and what could I do to reinforce it, if necessary.

      If not, what would be a good material to replace the existing transom wood with. I was thinking starboard because as far as I know...it's not porous, won't expand, won't crush under the motor clamps, and won't rot.


      • #4
        Re: Aluminum transom repair and reinforcement.

        Wood lasts a good, long time and is easy to find/buy/replace when the time comes. Again, material for pad is your choice. I wouldn't be concerned about going up 5-6hp on a small boat if it is sound...especially if you won't be hot-rodding it and running it WOT in rough chop. It's wise to have a safety chain, cable on any outboard that just clamps to the transom...another food for thought item. Don't know anything about starboard, if you try it let us know how it works out over time.


        • #5
          Re: Aluminum transom repair and reinforcement.

          got your pictures.

          remove what you have, take a sheet of 3/4 inch exterior plywood, cut them out and fit. glue them together, you can use 1 1/4 deck screws to hold it together. now 3 coats of oil base rustoleum paint. fit it back into postion.

          you want a complete transom, where the black line is, the red/yellow are aluminum angle brackets, for each side. the red are 3/8 inch thru bolts with washers inside and out. you can use rubber washers to seal, or 3m4200 to seal the thru bolts. this leaves the transom drain alone. this should be able to slide directly under the cap rail across the transom.

          any holes you need to fill, you can use JB Weld. or just put a bolt back thru it and seal it.
          Attached Files
          TEBOW Country

          Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
          That is what these forums are for. Post your questions, in the appropriate Forum.


          • #6
            Re: Aluminum transom repair and reinforcement.

            there is my boat done. with a 1955 15 hp. i had to thin mine to get under the cap across the transom. it also had a piece of angle that ran all the way across the bottom of the transom, as a stiffener.

            FLORIDA GATORS
            TEBOW Country

            Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
            That is what these forums are for. Post your questions, in the appropriate Forum.


            • #7
              Re: Aluminum transom repair and reinforcement.

              I've got a similar 12' boat, I've also been considering a wood free alternative and a 15hp. I will most likely use 3/4" nylon(?) panel to make my transom, it's as sturdy as plywood in such a small width and will never rot. My boat has only the single panel as shown in the pic above too, I'll most likely do a full transom leaving the lower area open but I will go as close to the bottom of the boat and drain as I can. I'll also make a small motor pad on the outside as well to protect the aluminum. A 12' boat should fly with a 15hp on it. Mine does real well now with an older 9.5 hp on it, an increase by 50% in power would make it just plain fast. (I went with the 9.5 since it was lighter than the newer 9.9).

              I'm actually not sure what the white plastic material is that I have here, its used for commercial cutting boards, mock up machining and layout and wood substitute on convertible tops as well. It doesn't break or crack and won't split. It's a bit more flexible than wood but in a short 3/4" piece as in such a transom, there is no noticeable flex. A buddy gave me a sheet of it, they use it in the machine shop to do test runs on the CNC machines since it's cheaper and won't damage tooling if the programming is wrong. I was told it was nylon but it could be something else, maybe ABS? I know it's tough since I made a few mallets out of the thicker scraps he had left over. It don't chip or crack, even when used as a hammer.


              • #8
                Re: Aluminum transom repair and reinforcement.

                I'd love to see some other transom repairs, see what other owners have done and used.


                • #9
                  Re: Aluminum transom repair and reinforcement.

                  Here is the link to the Starcraft boat I did. Take a look at the transom photos and you will see a knee brace which is original and adds strength.... I also used Plastic on the outside but let me warn you..... it is very VERY slippery and I etched groves into it and I also put additional stainless steel bolts in to stop the outboard from shifting.

                  It is a 14' with a 18 ho evinrude..... have pulled people skiing with it.

                  This is a great link to boat specifications http://boatspecs.iboats.com/
                  Please, shop iboats first!!


                  • #10
                    Re: Aluminum transom repair and reinforcement.

                    I started to replace my transom today after having it apart to replace some corner cap screws that were missing. On mine, I can't put in a full width transom, the transom has a junk bin welded to the transom just below the top edge which I assume is a place to keep a spare plug and your spare oil bottle?
                    The boat is real clean so I want to leave it all original
                    I gave the slippery issue some thought and did decide against the plastic insert and outer piece. I instead am going to laminate two pieces of outdoor 3/4" plywood with a glass center and an aluminum cap where the motor bolts and maybe an aluminum outer plate to reinforce the outer skin and prevent wear on the aluminum. My concern is putting something on the outside that will trap salt and harbor corrosion if I venture into saltwater.
                    Wood may still be the best bet there especially if laminated with epoxy. I will also be coating the hull where it comes in contact with all hardware and wood with epoxy as well to seal out any water. I may even be better off not installing any outer wood or plate just for this reason. I've pulled off many rear panels only to find severe salt corrosion from trapped salt that got trapped in between the wood and the aluminum hull behind the motor.