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Old 27-05-2015, 18:10   #1
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Making fiberglass shims for outboard bracket

I'm installing an outboard bracket to replace my non-working diesel. Surprisingly, there's not a lot of good information out there and what there is is particular to the boat. Following the bracket manufacturer's instructions, I had two stainless steel backing plates made, one for the outside of the transom, one for the inside.
However, I did not consider that my transom has a shallow curve, and I'm worried that the rigidity of the SS will warp the transom when the plates are bolted and tightened. Reading mainesail's posts on backing plates, I know it's not okay to just use a bunch of 4200 as a bedding compound because 4200 won't distribute the load. Wood will rot. MaineSail instead recommends using kitty hair or thickened epoxy to bed rigid backing plates like this.
However, I'm not looking for a permanent installation as I plan to replace/rebuild the diesel. Would it be okay instead to just create thin but wide fiberglass shims along the vertical axis to push the steel plates away from the hull and allow a flush mounting? The area between the shins would be hollow. I would put marine sealant between the bracket and the shim, and the shim and the transom. Thanks!


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Old 27-05-2015, 18:33   #2
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Re: Making fiberglass shims for outboard bracket

I would make the shims out of Starboard...much easier to work with than 'glass. If that's too expensive for you, try using Trex composite decking.
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Old 27-05-2015, 18:52   #3
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Re: Making fiberglass shims for outboard bracket

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Originally Posted by brownoarsman View Post
MaineSail instead recommends using kitty hair or thickened epoxy to bed rigid backing plates like this.
However, I'm not looking for a permanent installation as I plan to replace/rebuild the diesel.


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While not a perfect solution, you could coat the stainless backing plate with mold release wax, bed it in a kitty hair/ epoxy mix making it fair to the hull, then bolt it up. Once cured, the stainless plate will pop off and the epoxy mold will be all that is left on the hull.

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Old 27-05-2015, 18:58   #4
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Re: Making fiberglass shims for outboard bracket

Do the lay up type shim but use a release agent on the hull so you can peel it off later. If you use polyester resin it will probably peel off with out any release (if you have gell coat.)
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Old 27-05-2015, 19:11   #5
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Re: Making fiberglass shims for outboard bracket

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Do the lay up type shim but use a release agent on the hull so you can peel it off later. If you use polyester resin it will probably peel off with out any release (if you have gell coat.)
Or simply tape a sheet of polyethylene to both hull and backing plate and then using some form of bog to fill the gaps as you bolt up the plate. This will ensure that the bog sticks to neither surface but conforms adequately to distribute clamping stress.

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Old 27-05-2015, 19:20   #6
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Re: Making fiberglass shims for outboard bracket

A sheet of polythene, or perhaps simpler, a sheet of Tyvek (the big FedEx and priority mail envelopes) taped to the hull and allowed to hang down. They'll confirm to any simple curve and when it is time to remove the backing plate, they'll just come off attached to it, leaving the hull clean. They're thin enough so any bumps, etc. will just telegraph through them and you'll still get a "perfect" fit.


Place some newspaper under the area and around it, so any extra epoxy that gobs out doesn't stick to the hull.
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Old 27-05-2015, 19:41   #7
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Re: Making fiberglass shims for outboard bracket

Don't over think it. This is temporary? Just use some soft wood blocks.

Western red cedar comes to mind as a likely candidate. It will crush and conform to whatever slight curve you need to accommodate.

Squish a piece with a C-clamp. You'll see what I mean.
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Old 27-05-2015, 19:50   #8
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Re: Making fiberglass shims for outboard bracket

Great advice everyone, thanks! Using the plastic sheets as a barrier layer is such a simple solution. I need to work on my out of the box thinking.
The reason I was going with thickened epoxy is that I already have it on the boat, and trying to find new materials (like a soft wood) and the tools to shape it usually takes me about a day of walking around stores here!


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Old 27-05-2015, 21:24   #9
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Re: Making fiberglass shims for outboard bracket

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I would make the shims out of Starboard...much easier to work with than 'glass. If that's too expensive for you, try using Trex composite decking.
Donīt use Trex decking on a boat. The stuff will rot away. Even the best composite decking does not have enough zinc borate to keep the recycled wood in it safe from from rotting away in a boat.
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Old 27-05-2015, 21:37   #10
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Re: Making fiberglass shims for outboard bracket

The big home improvement stores have composite shims for shimming doors that may work for you about 2 dollars for a pack
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Old 27-05-2015, 22:59   #11
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Re: Making fiberglass shims for outboard bracket

Or you could use a rubber mallet and a vice to introduce a bend into the plates that approximates the curve in your transom...
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Old 28-05-2015, 11:05   #12
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Re: Making fiberglass shims for outboard bracket

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Or you could use a rubber mallet and a vice to introduce a bend into the plates that approximates the curve in your transom...
Best idea.
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Old 28-05-2015, 14:41   #13
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Re: Making fiberglass shims for outboard bracket

Thanks for the continued tips. The kitty hair apparently has an open working time of only five or six minutes, so had to nix that. I could substitute the gflex I've been using with silica to fill holes, but my last attempt went poorly with a 4-day cure needed! Most likely I measured poorly.
Called the welder, and he believes the 1/8" stainless plate will curve to meet the transom before the transom warps, so I'll dry fit it and try it gently with frequent pauses for observation.
In retrospect, it would have been easier to epoxy a piece of wood or fiberglass backing plate inside the hull! Lessons learned.


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Old 11-06-2015, 20:10   #14
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Re: Making fiberglass shims for outboard bracket

Thought I'd update; kitty hair had too short a working time and the epoxy I was using took forever to cure, so rather than bed the plates I took the advice to bend them. A rubber mallet and a bench vice would have been too professional, so instead I used a log of buttonwood and some scrap ply.
The gently rolled plates fit the interior and exterior of the transom very well, and I bedded the holes and the plates with 4200.
When I mounted the outboard, however, I could see the transom flex a bit as I put a bit of force on the outboard to move it forward and back. Is this anything to be concerned about, or is a thin solid fiberglass transom similar to a pure fiberglass deck in that it is expected to flex a bit? Pull the installation and build the transom thicker with epoxy and fiberglass, or monitor the transom for signs of cracking? The swim ladder that was in place before bore more weight (me) than the bracket is bearing, with less backing, but I never noticed any flexing.
Thanks for the continued help.
Oh, and pictures!
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I had to use regular nuts first as I had to thread the nut without the benefit of anyone holding the bolt from the outside, so once the nut was past the tail a pair of channel locks held the bolt in place while the regular nut was tightened. Then I used nylock nuts to hold the regular nuts in place. I was tempted to use aircraft marker to see if any of the nuts would eventually vibrate loose, but couldn't find any. If anyone has a better way of getting nuts and bolts together from inside a hull solo, I'd love to hear it as this was a bit of a pain. I guess one suggestion would be to make more friends and ask them for help!


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Old 11-06-2015, 21:16   #15
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Re: Making fiberglass shims for outboard bracket

How thick is the transom?
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