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I have a 10' aluminum jon boat and when I installed a small outboard I noticed the transom flexing a lot. Is this normal? I was thinking of screwing a 3/8" x 10" x 15" plate of aluminum to the transom to strengthen it up. The aluminum seems a little flimsy and there is a piece of marine plywood on the upper half of the transom that seems to be in good shape. Any suggestions, or should this do the trick?

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When I was a kid, we used to go out in the aluminum jon boats all the time in salt water....we had to chaulk the rivets to keep them from leaking. Anyway, the transom was always flexing on them and I thought it was just normal....ran it like that for years. But, your strengthening suggestion above makes sense to me......it certainly couldn't hurt.

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Why couldn't you just get a piece of 1 x 8 treated board running the width of the transom and clamp the motor to that?

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What brand of boat is it? Does it have gussets for support? where are they? I have a 1961 Lone Star 12' lumie. It is very sturdy. On the transom there is a piece of about 1" thick plywood from the top of the transom to almost the bottom where a cross support is. Sounds like you might need to replace the plywood you have with a larger piece. It would look more original and not so much a patch job.
Mike

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It is a Landau. The plywood runs from port to starboard and 4-5 inches down from the top of the transom. I think a new piece of plywood down from the top of the transom maybe 12", might also do the trick.

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Yes, if you use a larger piece and with the adhesives available today (something like Liquid Nails) you should be able to shore it up well without using any fasteners below the water line.

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How much does a piece of 3/4 marine plywood go for? I would guess any lumber supply should have it?

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Would you really want to use PLY wood?

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As far as I'm concerned plywood is the way to go. You can often find scraps at lumber yards for free. I would go all the way to the bottom with it.
Mike

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And if you coated it first to protect it, you could also use exterior ply as the glue is waterproof given it's a light duty fairly dry application you're using it for.

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Hi, folks,
Yes, you do want to use ply wood. However, that is not unprotected plywood. You might want to follow these steps:
1. cut plywood to right size
2. encapsulate the plywood in a resin. Particularly the edges. GLUVIT is a 2 part epoxy that works great for wood, and will work wonders on any rivets you might have that are leaking, as well. GLUVIT is called an aluminum boat's best friend, and it is really good stuff. It is available at West Marine and likely other boats stores.
3. After the epooxy has set, paint the wood to the color of your choice. The expoxy will deteriorate in the sun....UV light will break it down.
4. Install on the aluminum boat.
5. Install the motor.

this set up will last for years and years...... IMHO

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thanks guys! I will try these great ideas.

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I think txmnt hit it dead right. Plywood is much stronger than a single board because of the multiple plys and glue and the resin will make it almost industructable! :D

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What is the best way to adhere the plywood to the transom? I had planned on cutting a piece the size of the entire transom, and glue it to the inside, then use 4-6 SS bolts to hold it all together. I figure between the glue (liquid nails) and the Stainless bolts it should be solid. Sound good?

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If you use plywood get marine grade void free which around here is $65 for a 4x8x3/4 Sheet.

txmntman has the right idea.

This is how a friend fixed his.

First he used a large piece of card board to trace a template of the transom, cut the card board and test fit it, once it fit well then he transfered the outline to the wood, cut and made other adjustments for it to fit.

Once it was like he wanted it, holes were drilled and it was attached with the thru-bolted fasteners (carriage bolts and washers). With everything in place and looking good the wood was removed again for coating.

Now using west systems epoxy he coated the entire thing with the west systems and chopped matte. Once coated the matte was rolled out and let it cure. Be sure and coat the fastener holes as well. Once cured he could have "bilge coat" painted the part or just installed it as is. He ended up painting his. Then using a liquid nails or gorilla glue type adhesive he bonded the part to the transom and finished by attaching the fasteners.

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Between Jeff's and TXmntman's ideas I think the transom will work great. Thanks guys!:)

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I think you are right. One bit I would add...Marine Ply probably isn't necessary for this small application if you will be encapsulating with glass. PT or EXT grade should be fine, but with PT you need to make sure it is dry or it wont take the glass well.

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Regardless of which type of plywood you use, make absolutely sure that the edges of the wood are sealed up tight and not even a small amount of water can invade it. If the edges get wet, you risk a chance of the plys delaminating and coming apart, even though they have an exterior glue on them.

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Yes, Tom is right...around all the edges, and the places you drill through for bolts.

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Why not put two pcs of 1inx4in alum channel welded inplace across the top and bottom of tansom. I just did this on a pontoon trans to mount a larger motor. Less than $125.00 with no fuss and no rust and no rot and no flex.

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Problem is Pete, the transom flexes in the middle right below where the engine is mounted. When power is applied to the engine, the aluminum flexes in. The top and bottom don't move.

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Might move bottom channel to flex point. On a toon, the transom tubs are VERY flimsy. I placed mine at the bottom as that is where the ob mount hit. Just a thought. Good luck

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I was looking into buying a piece of alum plate 1/2"x 13"x 15" and have it tig welded to the transom. That should give it plenty of strength. I'm not sure whether I should have it welded or just bolt it to the transom. Any welders out there?