Transom repair, have a few questions

Discussion in 'Fiberglass and Composite Boat Building' started by GP1998, Sep 10, 2018.

  1. GP1998
    Joined: Sep 2018
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    Location: The Netherlands

    GP1998 Junior Member

    Hey all,

    First of all, this is my first post here. English is not my native langauge so it might be a bit hard to read but I hope you get the idea :D.

    I'm looking the repair the transom of my boat. Its a pretty old fiberglass boat and there are a lot of cracks where the engine is mounted. After some research I found that it was caused by a rotten transom. After removing the engine and a metal strip I could clearly see this was the case for my boat too.

    I've done quite a bit of research on how to do the repair but there are a few things I'm not sure about yet.

    First of all, its about where to cut the transom. I could cut the entire backplate backplate off as shown in the picture below (yellow line). If I were to do this, I would have to connect the new fiberglass to the sides of the boat which means I'd damage the color of that part too.

    In a few videos I found on youtube, ( and ) they chose to cut about 5-7cm (2-3 inches) from the sides so they could connect the new fiberglass there. I would then cut it like in the picture below (blue line). Only noticed now I made a little mistake, the upper, middle part of the blue line should be at the top of the orange but you guys probably get the idea.

    Ofcourse cutting the blue line would make it a lot easier to attach the new fiberglass, but here's where my question comes up.

    How do I get a full transom in? I could cut the transom in 3 parts as shown in the picture below, but wouldn't that harm the strength tremendously?

    I could also make the new transom a bit smaller but if thats the best option, what do you put in the remaining gaps? I thought maybe a small piece of wood or something but that won't really help with strength..

    Thanks for the help already :D

    GP1998


    boot.JPG

    afsnijden op rand.jpg
    afsnijdeninhouden.jpg

    back plate.png
     
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Welcome to the group.

    Many of our members use translating software. Language usually is not a problem.

    I like to repair/replace transom from the inside. Distirb the outside glass and gelcoat as little as possible. The interior furnishings hide the repair scars.

    Since your boat is an outboard, I see the desire to repair from outside.

    Cutting at edge makes easier to replace plywood core but increases difficulty of fiberglass and gelcoat work.
    Leaving a flange makes it harder to change plywood but easier to finish.

    What are you better at: wood working or gelcoat color matching?

    My approach:
    + remove as little fiberglass as possible (more glass can be removed later if needed)
    + cut 8:1 scarf into old plywood core.
    + epoxy in new plywood
    + grind 12:1 scarf on fiberglass
    + glass and gelcoat

    - if scarfing plywood to difficult then proceed with edge cut method.

    Best of luck
     
    Last edited: Sep 10, 2018
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The usual repair is done by cutting from the inside. It will avoid having to refinish the exterior of the hull which is much harder. Also, the interior laminate on the transom is much thinner, so you will use less materials.
     
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  4. GP1998
    Joined: Sep 2018
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    Location: The Netherlands

    GP1998 Junior Member

    Thanks for your replies, I didn’t know what scarfing was but did some research. Only now I noticed thats also what they do in the first video.

    So they are basically using the 3 part transom I was talking about in my original post. I’ve never scarfed plywood before but I’ll figure it out.

    What do you mean by 8:1 by the way? I’m assuming if the plate is 1 inch the scarf area needs to be 8 inches?

    One final question, how do you connect the two pieces? Epoxy glue and a few screws?

    Thanks again for the help!
     
  5. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Sometimes technical terms don't translate. Glad you figured it out.

    Absolutely. Your likely metric 3 cm thick so 24 cm wide.

    Glue with thickened epoxy. Remember to glue to inner fiberglass skin as well. I use notched trowel to guarantee plywood is fully bonded to skin.
    I have the equipment so I vaccum bag clamp. Screws, clamps you might have to get creative.

    You're more than welcome
     
  6. gonzo
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    gonzo Senior Member

    You don't need to scarf plywood used as a core in the transom. Simply stagger the joints.
     
  7. GP1998
    Joined: Sep 2018
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    Location: The Netherlands

    GP1998 Junior Member

  8. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Both the 1st and 2nd examples would work.
    A router could make the leap joint easily.
    The more steps and wider the laps the stronger it is.
     
  9. gonzo
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    gonzo Senior Member

    It is a core. While scarfing won't cause any problems, it is a waste of time and effort.
     
  10. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    I'm taking what gonzo meant was a plain lap, not a half lap. In the US transoms are usually 1-1/2" thick so you use two layers of 3/4" ply. Say the transom was 4' wide. The first layer would be two pieces say maybe 3' on the left side and 1' on the right with a vertical butt joint. The second layer would reverse that, being 3' on the right and 1' on the left, again with a vertical butt joint. Each butt joint would be lapped by the 3' piece. There would also be some mat sandwiched between the two layers of ply and it all clamped together with screws that would be removed before final glassing.

    The first thing to be done is to see if the stringers are all rotten too. Odds are they are not good. In that case it might be better to go ahead and take the cap off and repair the transom from the inside.

    A person might also consider a Seacast transom repair, or one of the other brand name pour able repairs.
     
  11. GP1998
    Joined: Sep 2018
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    Location: The Netherlands

    GP1998 Junior Member

    Even though the boat is pretty old, it wasn’t used that much. Most of its time it was used maybe 10 times a year max. It also never stayed in the water while it wasn’t being used but passed the days on a trailer in a shed.

    Therefore, I think the stringers are in decent shape, but thats just my guess. I’m not skilled enough to repair those unfortunately... Taking off the entire upper part is too much of a gamble for me.
     
  12. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    Unless the boat was stored indoors and was totally dry when not in use, then odds are there is far more rotten wood below the deck.

    Doing it from the inside is much easier and faster.
     
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  13. SamSam
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    Location: Coastal Georgia

    SamSam Senior Member

    Yes, it gets a little involved taking the whole thing apart, but it's not like rocket science, it's just more work.
    How big of a motor is on it?
    I see you have a pulley and cable steering system. Some people say they are not safe in that if a pulley gets pulled out or any such thing the motor will get yanked to one side or another and if you are at speed, excitement ensues.
     
  14. GP1998
    Joined: Sep 2018
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    Location: The Netherlands

    GP1998 Junior Member

    Its a 55hp yamaha, the boat was designed for 80hp and was designed as a fishing boat in greece for the middle sea. So I think it should be strong enough alltogether, but I’m definitely no expert on this.

    And as I mentioned its only used in sweet water without a lot of intensive use. These were pretty small lakes aswell so big waves were nowhere to be seen.

    All in all I’m just hoping for the reasons mentioned the stringers are okay. But the transom also rotted so its just a guess. I don’t know if these are correlated? The transom isn’t connected to the stringers at the bottom, right?
     

  15. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    Boats rot faster in fresh water than in saltwater. Boats rot faster in storage than in use.

    80% of boats brought to me for stringer rot also had transom rot. 95% of boats brought to me for transom rot also had rotten stringers of recently replaced stringers

    The transom and stringers should be well connected.
    • Prop pushes outboard
    • Outboard pushes transom
    • Transom pushes stringers
    • Stringers push hull
    Several methods used to make stringers. Some very easily rotted, others impervious to rot. Research how your boat was constructed. You might luck out and have solid fiberglass stringers.
     
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