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  • Transom Questions

    Finally got the boat moved to my shop in Washington state and getting a chance to move forward on restoration.

    I started the restoration work on the transom. The transom measures about 3 feet high and about 7 feet wide on the inside (more like 4 feet high on the outside) and I have pulled out most of the plywood. Lower foot was pretty saturated with water and so are the stringers. I cut the floor and stringers back from the transom about 8 inches wide across the stern and cleaned out the hull of foam and wood to get to the bare hull. I am leaving the rest of the floor and stringers in place until I complete the transom replacement for added stability of the hull.

    So, now for two questions on the transom.

    First, the boat had an OMC outdrive and I am going to a Mercruiser Alpha 1. The specs call for the transom thickness to be 2 inches to 2 ¼ inches. The inside of the hull behind the transom wood is cleaned off and I measured the hull thickness at the OMC opening. It tapers from the bottom to the top of the OMC cutout. At the base it measures 0.550” thick on the port side to 0.604” on the starboard side at the edges of the OMC hole. At the top of the OMC hole it measures 0.460” thick on the port side to 0.439” on the starboard side. I need to find a tool to measure it farther out to see if the taper runs all the way across the transom. I am wondering the best way to make it flat for mounting the Alpha. Assuming the outside of the transom (gelcoat side) is flat, then the taper bottom to top is on the inside and underneath the plywood. Should I sand away the excess thickness at the bottom of the transom or add glass at the top before replacing the transom wood? It looks like the magic number for hull thickness will be 0.50 inches. Would it be better to lightly sand the bottom and fill at the top maybe to get it flat over the Alpha mounting surface?

    Second, I need to buy the new transom material so what layers of resin and glass should go between the new board and the hull? I understand that you should put a layer of fiberglass on the hull to bond the new board. I am new to fiberglass so how thick will this bonding lay-up be and then how thick will the top finish layer(s) of resin and fiberglass be? Need to know the stack up of thickness so I can order board to meet the 2 inch to 2 ¼ inch specs. Right now I am leaning towards Coosa board in 1 ½ inch thickness but that might be too thick once glassing is done. Next size down in Coosa board is 1 inch but I don’t want to do a lot of fiberglass build up either.

    Thanks.

  • #2
    a layer of CSM and PB between the hull and your new transom

    CSM and two layers of 1708 after tabbing in your new transom.

    that should put you at about 2-1/8 - 2-1/4" thick

    use regular exterior grade plywood, do a good job and it will outlast your grandchildren. no need to buy Coosa
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari

    Comment


    • #3
      Thanks. Is that with 1 1/2 inch core board or 1 inch core?

      So, in layers the target thickness would be:

      Hull - 1/2 inch
      Layer One, CSM and PB - ?? inch
      Layer Two, Transom Core Board - 1 inch, 1 1/4 inch, or 1 1/2 inch?
      Layer Three, CSM (1) and 1708 (2) - ?? Inch



      Comment


      • #4
        I’d shoot for an even 2” before you apply Layer 3. With a half inch hull, I’d want a wood layer a bit less than 1.5 inches. A good thick layer of thickened resin between the hull and the wood layer will allow you to clamp down to 2” total. Add the third layer of fiberglass and you’re golden!
        Location: West Central Illinois, USA 1997 Larson 186 SEi Bowrider I/O Mercruiser 350 #0F747565 Mag Alpha One Gen II #1A270120 Transom and Deck Restoration Project on my '97 Larson Bowrider

        Comment


        • #5
          Most 3/4 ply is actually 23/32. that's 1/16 shy. X2 that's 1/8 shy so your core will be 1 3/8 plus the resin mix...if you clamp it tight and true, you wont exceed 2" with the hull...with 2 layers of 1708, you will end up ranging from 2 1/8 to 2 1/4.

          make sure there is parallelism around the entire key hole.
          "We all come from the sea. But we are not all of the sea. Those of us who are, we children of the tides, must return to it again and again."

          Current Boats
          10' tin gamefisher/Johnson 7.5hp ob
          1991 Sea Ray 170br/AlphaMerc
          1996 Century 1800 CC/Yamaha c85
          1988 Imperial 280SF (under the knife)
          1972 Checkmate mx-15

          https://forums.iboats.com/boat-restor...cs-598307.html

          Comment


          • #6
            Originally posted by 76SeaRay View Post
            Thanks. Is that with 1 1/2 inch core board or 1 inch core?

            So, in layers the target thickness would be:

            Hull - 1/2 inch
            Layer One, CSM and PB - ?? inch
            Layer Two, Transom Core Board - 1 inch, 1 1/4 inch, or 1 1/2 inch?
            Layer Three, CSM (1) and 1708 (2) - ?? Inch


            hull - 7/16" - 1/2"
            CSM and PB - 1/8"
            two layers of 3/4" plywood glued together with tite-bond III - 1-3/8" (see Zool's comment above)
            CSM and two layers of 1708 - 3/16 - 1/4"

            your transom is now 2-1/8" -2-1/4"

            finish boat

            go boating.
            Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

            1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

            Past Boats
            1970 Wooster Hellion - Merc 9.8
            2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
            1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

            What’s behind you doesn’t matter.Enzo Ferrari

            Comment


            • #7
              Great technical data. Thanks a million. I am concerned with the parallelism at the keyhole since it has a taper top to bottom around the OMC hole. So, is grinding it down the best way to go and/or filling the best way to go? If using 3/4 plywood instead of Coosa Board, is it best to install one sheet first, then the second or bond the two layers of 3/4 on a bench before install?

              Comment


              • #8
                Originally posted by 76SeaRay View Post
                Great technical data. Thanks a million. I am concerned with the parallelism at the keyhole since it has a taper top to bottom around the OMC hole. So, is grinding it down the best way to go and/or filling the best way to go? If using 3/4 plywood instead of Coosa Board, is it best to install one sheet first, then the second or bond the two layers of 3/4 on a bench before install?
                You mean the thickness of the cutout is different at the top and bottom? or the transom angle?.

                I like to build the whole transom on the bench after test fitting each piece. You can glue and screw it and remove the screws after it set up, or leave them in. The trick is to make sure the outer hull skin secures evenly throughout the stern, most importantly around the keyhole. There are plenty of examples here of how we clamp the transom to the outer skin.

                The parallelism can be adjusted with the inner glassing and fairing. You just need the correlation between the inner and outer shield.

                Not sure about OMC, but the older volvo AQ setups were cantilevered with only front motor mounts and no inner shield. At some point, they standardized the cutouts around the Merc, so the parallelism wasnt an issue back then.

                "We all come from the sea. But we are not all of the sea. Those of us who are, we children of the tides, must return to it again and again."

                Current Boats
                10' tin gamefisher/Johnson 7.5hp ob
                1991 Sea Ray 170br/AlphaMerc
                1996 Century 1800 CC/Yamaha c85
                1988 Imperial 280SF (under the knife)
                1972 Checkmate mx-15

                https://forums.iboats.com/boat-restor...cs-598307.html

                Comment


                • #9
                  Yes, the thickness of the cutout is different at the top than it is at the bottom. It is thicker at the bottom than at the top.

                  Comment


                  • #10
                    I have all the old plywood off the transom shell. Getting ready to start prepping the transom for new plywood but still need to deal with the hull thickness around the OMC cutout. I got some long reach calipers and measured in from the edge of the hole about 4 inches. Found the same thing, that is, the hull is thicker at the bottom than the top. Near the top starting about half way up the OMC opening it measures 1/2 inch but from the bottom up to about 1/3 to 1/2 way up it measures 5/8 thick. So, should I grind along the bottom to reduce the hull thickness or put in a layer of fiberglass starting about 1/3 up from the bottom and go all the way to the top (about 3 feet).

                    Having trouble uploading pics.. Get an error message.

                    Comment


                    • #11
                      Trying again with a picture using Chrome instead of Internet Explorer... Click image for larger version

Name:	Transom_Inprocess_a.jpg
Views:	1
Size:	191.0 KB
ID:	10690613

                      Comment


                      • #12
                        You realize its 100x harder to work around stringers, etc. You have to replace the stringers and bulkheads and deck anyway
                        Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

                        1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

                        Past Boats
                        1970 Wooster Hellion - Merc 9.8
                        2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
                        1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

                        What’s behind you doesn’t matter.Enzo Ferrari

                        Comment


                        • #13
                          You have an omc stringer. The thickness isn't as critical as most other io setups. With those they mount to the transom by sandwiching it with 2 "plates" those have to be true and the right thickness or driveline issues will happen. I have the same setup as you. With a stringer the transom holds the boot, helps transfer thrust from stringers throughout the hull, and keeps alot of water out. As long as it's sound and strong a little wonkyNess is ok. Mine was 3/4 throughout with a second layer of 3/4 in the rear corners to beef up the ubolt tied owns. I will rebuild with 2 sheets of 3/4 except for by the dog door sized hole. So when I do swap out driveline I don't have a lot of tearout. Smarter boat people know more than me but if you think about the setup you have it should make sense

                          Comment


                          • #14
                            You do realize that the OMC keyhole and the Alpha 1 drive Keyhole are different patterns. You'll need to cut a New keyhole to accommodate the Merc outdrive. This may entail glassing in the old keyhole and then re-cutting the new one. I'm NOT an I/O expert so hopefully others will chime in.
                            This might be informative...
                            https://www.sterndrives.com/omc_to_mercruiser.html
                            1961 Lonestar Flamingo - SPLASHED...Kinda!!
                            Fabricating Decks, Stringers, and Transoms
                            Paint Your Boat with Tractor Paint...Say What!!!

                            Comment


                            • #15
                              Sign up today
                              I just reread post and realized you are not going back with a stringer....

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