Transom Fabrication





Introduction: Transom Fabrication

About: Software QA Test Engineer/Technical Writer

Typical small boats have transoms made of wood, as the years pass the wood will rot and become useless. This instructable will guide you in removing, fabricating and replacing the transom of that old boat you just bough againt your wife's wishes.


16 - 2" stainless steel carriage bolts
32 - Stainless Steel washers
16 - lock-washers
16 - Nylon threaded stainless steel nuts
2 - pieces of extruded aluminum "L" (optional)
1 - 6oz tube of 3M 5200 Marine Sealant
1 - 8' x 4' -½" Pressure Treated Plywood
1 - Can of fiberglass resin (Jelly)
1 - Bondo applicator
1 - 6oz tube of Gorilla Glue

Tools needed

Circular Saw
Palm Sander
Wood Clamps (you can use any type of weight if you don't have clamps)
Tape Measure
Straight Edge
Drill with stepped drill bit
Welding punch

Total cost: around $125

Step 1: Removing the Old Rivets

The first thing to do is identify all rivets that run through the transom. In my case I had 16. After identifying the rivets to remove, mark the outside of the rivet head with a black dot with a sharpie. No use drilling out a good rivet.

After marking the rivets to be drilled out get your drill and get to work. Besides replacing the rivets with carriage bolts, this is probably the most difficult task of the process. Use a spring loaded punch to start a divot on the rivet head so your drill bit doesn't go all wild. Drill the rivet head as least as possible, once you see the head is about to be drilled through, stop immediately, no use making the hole in the hull any bigger than it needs to be. Repeat this task for all of the marked rivets. When you are done, the transom should be loose and with a little coercing, be ready to be removed.

Step 2: A Little Side Note:

Just a little side not here: If you are going to be doing any additional hull work on this boat, now (after the transom is removed) is a great time to do it. With this boat, I sanded and painted the exterior and sanded and used a Rhino type bed liner on the interior. I would highly recommend doing this type of work before reinstalling the newly fabricated transom.

Step 3: Learning From My Mistakes

Now heres chance for you to learn from my mistake:

Once I got the transom removed, I noticed it was 1" thick, so being the numb-skull I am, I made the decision to add another layer to it. Big mistake. The holes in the transom reinforcement, didnt line up to well because of the added layer in thickness. The thought that I might want to add a larger motor to the boat in the future led me down the path of knuckle-headed-ness. So if you want my advice. Stick to the original design, if the transom is 1" thick, rebuild it 1" thick.

Step 4: Fabricating the Transom

On to fabricating the replacement transom: Most, if not all wooden transoms in smaller boats are made of plywood. From the looks of the transom in this boat, I would estimate a typical transom will last about 20 years or so before they need to be replaced. This boat is a 1972 and the pictures were taken in late 2007 and as you can see, the transom was in bad shape. It should have been replaced years ago.

The first thing to do is to take the old rotten transom and use it as a template as best as you can. Some of it could bee missing due to rot and whatnot. Do your best to draw that area in when tracing the template. Since we're using ½" pressure treated plywood, you're going to be doing this twice. Go ahead and cut out your first piece after tracing it out, you're going to use this newly cut piece as the template for the 2nd piece.

After cutting the two pieces of transom, you going to need to sandwich them together to form a 1" piece. To do this, I used Gorilla Glue; it works like a dream in this type of application. Be sure to clean all of the sawdust and loose wood from both or the surfaces, then apply a light mist of water to the sides of the wood to be glued. The water helps the Gorilla Glue cure. Lay down horizontal lines of glue from edge to edge then cross those lines with vertical lines of glue making a checkerboard pattern. Line up the edges and press the two pieces of wood firmly together. Now you can either use some wood clamps or you can lay it flat and put some bricks on it, either way you want the two pieces pressed as firmly together as possible. I would leave this in a place where it can sit, undisturbed for at least 48 hours. Almost forgot to mention, put something under it like a drop cloth or newspaper, since glue will seep out and onto the floor.

Step 5: Applying the Fiberglass Jelly

Fast forward 48 hours.

Your transom is ready for some light sanding and then a copious amount of fiberglass resin (Jelly) Go along the edges with a palm sander (you manly men can hand sand if you like) try and level up all your edges and remove excess glue that seeped out from the edges. Clean up the wood and remove dust and lay it flat on some newspapers for the first application the fiberglass resin. I used a piece of cardboard for mixing; put down the recommended amount of resin and apply the pea sized amount of cream hardener. Mix it as briskly as possible with the applicator and start applying it to the transom making sure to get the sides and cover all exposed wood. Don't worry about getting it perfect, we're going to be sanding this bad boy once it cures. After the first side is complete, let it cure overnight and flip it over and do the other side.

One thing I noticed is that the ridges are rather hard to sand, this stuff hardens up nicely. I didn't spend too much time trying to make it smooth since it wasn't going to be exposed in my boat. Once you have it sanded to your liking you're going to try and get it back to where it belongs. It took some time for me since I added the extra ½" layer to it. If you stuck with the original thickness, you should be okay. Now that it's in place break out the wood clamps again and clamp it into place, use as many as possible since your drill force is going to be pushing against it.

Step 6: Replacing the Old Rivets With Carriage Bolts

Choose a drill bit that matches the diameter of you carriage bolts and start drilling holes through the transom in the places where the old rivets were. As you finish each hole make sure it's big enough for the bolt to pass through, remove the bolt and fill the hole with the 3M 5200 sealant. (Quick tip: wear latex gloves. This stuff is a bear to clean up.) First put a washer on the bolt and push it through the freshly drilled and 5200 sealed hole. On the other end put another washer, lock washer and then bolt. Before pushing the bolts all the way through, apply a copious amount of 5200 to the areas when the washer will meet with the boat. Repeat this step with all holes left from drilling out the old rivets. Tighten the bots down as tight as possible using a normal ratchet or wrench. Give the 5200 sealant 48-72 hours to cure.

Note: The great thing about the 3M 5200 sealant is that it cures up fairly hard, but still has enough elasticity to flex when the boat itself flexes during normal operation.

Step 7: Transom Cap?

If there was an aluminum cap along the top edge of the transom and it couldn't be salvaged (like in my case) replace it with 2 pieces of extruded aluminum "L" pieces. It works just fine and give the transom cap a nice finished look.

Bolt on your motor and hit the water.

Next time: Building a fishing deck and installing pedestal seats in your boat.



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    26 Discussions

    Hi just saw your article after I replace my transom. Great job.

    I was wondering if you of a place to find corner caps?

    Its a 1963 StarCraft 12' V bottom?

    Also any ideas on adding outrigger floatation's to make this alum boat not so tipsy?

    Do you not glue the transom to the aluminum on the back of the boat as well?

    1 reply

    No, the bolts should be enough to keep the transom secured to the boat. The glue is to mainly waterproof the hole you drill for the bolts.

    Well done Sir!!

    Here's one I just done.

    unless indications to the contrary the transom board should stop 2" to 4" from the bottom of boat re: water flooding, capillary attraction to cracks in wood sealer and space against transom aluminum, . must count on surface sealer failures however small they may be. good luck,

    I read on the internet that AC Plywood is a great choice if marine ply is not available. ACX grade is not the same as AC grade. Make sure it is exterior grade.

    Like all PVA glues, Gorilla glue foams as it cures; often pushing the pieces apart. By all means clamp the pieces together so that it doesn't slide out of alignment.

    great 'ible! old boats should never be discarded because they are just, well, cool!

    i used to work for a yacht building company and that is where i learned about the 5200. always were gloves because once it dries on you it is darn near impossible to remove. have to pluck it off in little bits. i know this from experience. you can use denatured alcohol (so-so), toluene or xylene to clean it off of surfaces (not your skin 'cuz it is toxic)

    you will not ever find a better sealant/adhesive than that 5200. there used to be a 4800 that was considered semi-perm. the 5200 is rated for below the water also comes in black, brown, and gray. it also will work on wood.

    i mentioned this because years ago i helped a friend redo the transom on his grand-pa's old boat. I had a bunch of 1/4 thick alum which we made the transome out of. we cut it out with a circular saw (noisey and throws lots of hot chips -wear appropriate protection!)

    we laminated the pieces together with the 5200 after drilling multiple holes in the metal. we then stacked them and inserted stainles steel 1/4-20 tpi bolts with washers on each side and tightened them down. we repeated the tightening several times over the next few days. we may have gone overboard on the amount of 5200 as it kept oozing out. is there really such a thing as over kill?

    we then used the same process to install the new transom to the boat. it was about 1 1/4 inches thick when done.

    there was also a leak in the seam at the keel. we used 5200 there as well. no more leaks.

    the key to using 5200 effectively is make sure the surface is clean.

    Great and helpful article. What size carriage bolts did you use? I was thinking I'd do like 3/8 inch and realize 1/4 might be just fine. I plan on putting a painted piece of red oak in as the transom but as I understand wood or paint against aluminum shouldn't be done. So I plan to clean rear inside and then use an etching aluminum primer like Rustoleum's and then spray on two top coats.

    Now to find that fishing deck article mentioned..ready for that to happen next also!

    Help! I have an 8 foot inflatable West Marine dinghey that has a completely rotted transom and am trying to work on it in my shop. The old transom has been removed along with the plate for the outboard motor. I removed the transom and plate, and scraped the rotten wood pieces out of the grooves on each end of the area where the dinghey "locks" into place on each side, so it is clean of old wood and debris. My question is: My old transom is so rotted, if there anywhere I can purchase a new transsom for this OR any kind of "cutout" pattern that I can get so I can make the proper cuts into the new piece of marine grade wood that is to be the replacement. I would rather repair it using the wood I bought and use fiberglass to reseal, Does any one sell used or new transoms for dinghey boats and/or anyone have a cutout pattern that I can trace and make my cuts? I am pulling my hair out tryiing to fix this with the proper fitting on each end... Any help is appreciated.

    1 reply

    Here is a good video of you boat and transom. You might be able to get a good idea from this video and a few measurements in your boat.

    This is awesome just what I needed. I have a 16 aluminum starcraft with a 75 hp motor, the transom has rotted and it's time for a replacement. I have a few questions. I plan on doing this project right away, so I will not have time to let the plywood dry. Should i go with marine grade plywood instead or do you think exterior grade A/B plywood with the fiberglass resin would suffice?
    Also, i was thinking about using 3m 4200 instead of 5200 because I read 5200 gives a permanent bond and would be very difficult to remove the bolts at a later date. What are your thoughts? Thx great job!

    2 replies

    yeah you can use regular plywood, just be sure to seal it up good, be sure to coat your drilled blot holes especially.

    The marine grade plywood in my area is about $85 for a 4x8 1/2" sheet. It's ultra expensive compared to PT or regular plywood. If you seal it up good, the regular should last ages.

    I've never used the 4200 before, but everyone I talk to swears by the 5200.

    Just wanted to send you some Kudos. I followed your advice and it came out perfect. The hardest part, was removing the old 30 year old rotted wood. I've used the boat now for 2 summers and have had no problems. I would highly recommend this for anyone trying to rebuild their own transom. Thx for your help.

    NEVER NEVER put pressure treated plywoom against aluminum!!!! It will eal away the metal where they meet!! This is a well proven FACT! Research it a little and you will see the horror

    4 replies

    Thanks for the comment Rob. If you read step 5, you'll see all the plywood is coated with fiberglass resin. So no worries.

    I read that, And it just confused me more. Why use pressure treated wood if it was just going to be covered with glass anyhow? There no strength difference between treated and non treated wood. All I see the treated wood does is add weight and adds a risk if failure of bolts do to corosion with the treated material. I know it seems I am harping about  nothing but I am a boat builder by trade and have seen the bad effects of aluminum and pt wood contact, Also PT wood to desinged to air dry over time and leach out the extra weather treatment. Hence thay say to let PT wood stand for 30+ days before sealing it like decking and such. If you cover the PT wood with glass before the wood drys completely it will never be able to and it will eventually mold and become weak. Seems like a small thing but can lead to big problems. There's such things as marine grade wood for just such  reasons. 

    Hey Rob -

    Once again thanks for the comment. Constructive criticism is always welcome. The reason I used PT is because I had 3 or 4 sheets laying around from a project earlier in the year, no use buying more if you already had some laying around. They had been against a wall in my garage for 3 or 4 months, so they were plenty dry. All holes drilled thru we coated with 3M 5200 sealant and I used stainless steel carriage bolts.

    I am currently redecking a 19' boat and I bought untreated plywood for this since it's cheaper. I priced marine grade plywood and it's close to $100 for a 4x8 sheet. I have some old time boat guys in my family (uncle and grandfather) and both agree that a regular sheet coated with an epoxy resin would be equally as good if not better than the marine grade.

    I appreciate the tips. While I am not a boat builder by trade, I tend to over research things before starting a project, I probably spent 10 times the time it took to build transom reading and researching the best way to do it.

    When I'm done with my larger boat this summer, I'll post some pictures of the progress.


    I've been building boats for years. And there's a product PL premium Construction adhesive. It's in the same class as 3m 5200 (which is great sealent) But it comes in a calk tube and is under 5 bucks! Lowes-Home depot. It's even good enough to make fillets with.I have used it for years on hundreds of things. I agree that regular untreated pylwood coated with epoxy is just as good! But epoxy and hardner is $146 a gallon. So really it's a cost/labour thing. And if it's something like decking the first time you drive a nail or screw thru it you just made a breach in the cured epoxy so it needs to be done again after the piece is installed. But I have done it this way. I put a new floor in my 46ft. Bertrum last may and using marine grade would have put me in the poor house! I used 23/32 exterior grade ply from Lowes and it was toung and groove! It made instalation easy and so far is holding up well! I used MAS epoxy and completely coated it with a roller both sides before I installed it and then re rolled this top side with a slightly thickened Q cell mix before painting and carpeting it.

     I see in step 6 you mentioned it's hard to clean 5200 off your hands, Try WD-40 it works pretty good to get it off.