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TOPIC: 2859 transom thickness?

2859 transom thickness? 09 May 2017 13:46 #1

  • sketch96
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From what I can find it looks like 2 1/4 " is the Max thickness of the transom per merc standards. With 2 pieces of 3/4" plywood and the existing transom fibreglass im measuring 1 13/16". That leaves me with 11/16" to make up with fibreglass which is a lot to make up.

Of all the threads I've read most guys just use 2 pieces of 3/4". But im thinking of using a piece of 1" and 3/4" to bring me to approx 2 1/16". Which leaves about 3/16" fibreglass to make up the rest.

Is 3/16" enough glass to cover the new transom plywood? Or am I getting too fussy about hitting the 2 1/4" Mark to get the Max thickness for strength.?
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2859 transom thickness? 09 May 2017 14:25 #2

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I haven't hear of any transom failures due to lack of strength. All the ones I've seen on here have been due to rotten wood caused by water intrusion. If you're worried about that, look into what the really high end boat manufacturers do. They don't use wood in transom or stringers.
I think the stock setup is plenty strong. At least until water gets in there.

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2859 transom thickness? 09 May 2017 14:34 #3

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IMO, you are being a bit fussy. Two layers of 3/4" marine grade fir plywood is plenty strong IF installed well. Grinding to get a good mating surface to the transom and bottom of the hull is the first key. I go at least 6" beyond any new part if possible. I also soak any new wood in a 50/50 mix of catilized resin and acetone by pouring and spreading until it won't take any more. When I have the plywood dry fit, I fit four 4x4's, two in and two outside, and all thread to form a clamp after layup. The all thread goes through the outdrive keyhole as high as possible and as low as practical. I use a heavy mat against the transom extending onto the overlap, then a ply between the layers of plywood. Then I install the clamps and tighten the heck out of them for good squeeze out of excess resin and a maximum of bonded surface. After the resin has cured I remove the clamps, fill the gaps to the hull bottom with thickso and cover the plywood with a layer of heavy mat. After cutting out the keyhole I also do the resin soak to the exposed plywood and cover it with mat. I then drill the hole for the drain and slide a piece of PVC tubing covered in 5200 into the transom side of the hole. Then I paint on a thick layer of gel coat.
That's just how I do it and it seems to work. If I'm telling you more than you need to know, I apologize. Like I said, just one guys way of doing a transom repair. There are other methods and I'm sure others will chime in with their opinions and suggestions. That's part of the fun of this forum, the exchange of ideas as well as information.
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2859 transom thickness? 09 May 2017 14:40 #4

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2-2 1/4" mines a bit shy of 2" think it's plenty stronge ;)

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2859 transom thickness? 09 May 2017 14:53 #5

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So you should put a layer of mat on the transom before installing the new plywood and a layer between the 2 sheets of plywood also?

I was just going to use PL premium to laminate the 2 sheets together and use a trowel with thickened epoxy for the transom to plywood bond. I have 1708 mat and some 16oz mat. Would the 16oz be ok to use for the transom to plywood bond as well as the plywood sheet bond instead? Would add some needed thickness to my transom I guess.

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2859 transom thickness? 09 May 2017 14:57 #6

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IMO no mat is neccasary between the plywood and the inner transom when your using thickened epoxy. All your trying to accomplish is to fill any voids between the two. I'd use thickened epoxy between the two layers of plywood also.

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2859 transom thickness? 09 May 2017 15:50 #7

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My small bit of information for transom repair, thickened resin troweled on the existing transom FG. if the resin is runny add a layer of 1.5 OZ. mat to keep the resin from draining brush resin onto the wood , the side going on to the transom. repeat that for the 2nd layer but I like a 1.5 mat for the wood sandwich, hoping you have o good clamp system planed out, thinking back Dave had a great system with pictures to copy from. Good luck.
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2859 transom thickness? 10 May 2017 10:53 #8

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sketch96 wrote: From what I can find it looks like 2 1/4 " is the Max thickness of the transom per merc standards. With 2 pieces of 3/4" plywood and the existing transom fibreglass im measuring 1 13/16". That leaves me with 11/16" to make up with fibreglass which is a lot to make up.

Of all the threads I've read most guys just use 2 pieces of 3/4". But im thinking of using a piece of 1" and 3/4" to bring me to approx 2 1/16". Which leaves about 3/16" fibreglass to make up the rest.

Is 3/16" enough glass to cover the new transom plywood? Or am I getting too fussy about hitting the 2 1/4" Mark to get the Max thickness for strength.?


If I may:

The core material need not be Marine grade plywood. Good dry, fully plugged, Fir skinned and Fir cored plywood will do just fine.
Your new core will be of much better quality than what B/L did with their cheap CDX type plywood and the chopper gun technique.

Using a 3/4" and a 1" layer is OK, but you may want to double check your anticipated final thickness! I.E., original hull/matting/1st core/matting/2nd core/matting and roving! It may be that two 3/4" layers (plus the matting/roving) work out just find..... or it may be that a 3/4" and 1" layer (plus the matting/roving) gives you the correct thickness.

Take the old core material out using a router with a straight flute bit set to depth, and by cutting it into small workable squares.
Grind/sand the original hull material nice and clean and flat, but leave a rough surface.

Dry-Fit your first core layer. This may require many attempts in and out of the boat, in order to get a good fit.
If need be, the first core layer can be sectioned!
Temporily secure the first core layer using the stern drive transom unit mounting holes, and any other holes that are available..... swim platform mounting holes, etc
Be sure to also secure the perimeter areas.
DO NOT pre-cut for the transom unit. Let this new core material run right on through the factory cut-out! You will make the cut-out much later!

Now Dry-Fit the second core layer. Again, this may require many attempts to ge a good snug fit.
This core layer can also be sectioned if need be. Stagger the cuts so that they do not align!
This layer will eventually be screwed to the first layer.
Set up for the all-thread and 2x4 strong-backs as suggested.

When ready to install the first core layer, it will be bed into resin wet matting.
Now re-install your all-thread and 2x4 strong-backs. (the strong-backs will ensure that the actual finished transom will cure in a flat and straight plane)

After the first core layer's resin has cured and after the strong-backs have been removed, now you will install the second core layer into resin wet matting.
Screw this layer into the first layer, and don't be shy with the number of screws!
The screws will remain.

The final structural component will again be resin wet matting, topped off with resin wet roving.

The sum of the original hull skin, the matting, the first core layer, the again matting, the second core layer, the again matting and final roving, will create a "Box Beam"!
We are after the "Box Beam" strength!


I drew this up several years ago as to help one of our members who was also doing a transom core replacement.
His was a Volvo Penta stern drive. The basic idea is very much the same for Merc.


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2859 transom thickness? 19 May 2017 19:47 #9

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What is the best tool to use to cutout the keyhole when it comes time? Also you describe a layer of resin wet heavy matting between layers of plywood. I have 1708 and some 16oz biaxial cloth. Would the 16oz work or are you referring to csm?

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2859 transom thickness? 20 May 2017 00:03 #10

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I used a saber saw to cut the keyhole. I cut it out of each piece of ply before attaching it to the transom. My worry about doing the cut out last, was if I screwed up the cut with everything in place there would be no easy "do-overs". I fit the inner (aft) ply sheet in place, outlined the keyhole, then did the cutout out of the boat. Once that was done I used that to outline the keyhole on the inner ply. After that was done a couple of screws held them together for one last test fit. The two sheets were epoxied together outside of the boat and both sides coated too. When it was time to attach it to the transom, the keyhole was double checked for line up. Another plus for cutting the keyhole first was being able to pass lumber through the hole. I used 2x4's as clamps to hold the ply against the transom. If your interested in pictures, I've got the rebuild in my signature. -Tom
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