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Old 16-02-2017, 07:57   #1
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Leveling surface for backing plates

Howdy Folks,

I'm in the middle of mounting a new Hydrovane on the transom. Unfortunately the inner surface of the transom in the area we need to mount it, isnt level. I've been planning to trim hardwood (oak) pads to create a level surface for the backing plates.

However, being that I'm not the worlds best wood worker and this will likely take me a long time...I've been considering using an epoxy putty (West Marine Rx or M-T). My thinking is, this would make it fairly easy to build up the interior surface to create a level surface by simply shaping the putty and then once cured I can drill it and finish the mounting. Seems like a solid structural option for spreading the load of a backing plate...any concerns with this route?

I guess I could even wrap the bolts in something like saran wrap and once the putty is in place use tighten down the backing plates a bit to ensure a good tight fit. The once cured I should still be able to remove the bolts?

Thanks,
EB
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Old 16-02-2017, 08:22   #2
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Re: Leveling surface for backing plates

Can you give the epoxy-wizards here some idea of the angles / thicknesses of the amount you are trying to build up? Are you dealing with two dimensions, like changing the transom to vertical and eliminating the curvature? Pictures would help. BTW: That two-part 'putty' is essentially epoxy, so it is permanent (like 3M 5200 permanent!). If you ever need to remove that filler, it will be with a grinder so be careful.
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Old 16-02-2017, 08:23   #3
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Re: Leveling surface for backing plates

Sounds good. You can thicken epoxy with filler like cabosil or microfibres and do this. (Probably not microballoons). I've made an in-place mold with kid's modelling clay, like a wall around the structure.

Mix it really thick like stiff peanut butter, so it doesn't run. Put some nylon release cloth between your plate and the epoxy if you ever want to get the plate off again. Any fine nylon woven fabric will do. I think I'd put grease or vaseline on the bolts rather than saran wrap.

Alternatively you could use 3M 5200 in a similar manner, I suppose. It might be a bit too runny, but I've seen it done.
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Old 16-02-2017, 08:27   #4
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Re: Leveling surface for backing plates

Quote:
Originally Posted by ErBrown View Post
Howdy Folks,

I'm in the middle of mounting a new Hydrovane on the transom. Unfortunately the inner surface of the transom in the area we need to mount it, isnt level. I've been planning to trim hardwood (oak) pads to create a level surface for the backing plates.

However, being that I'm not the worlds best wood worker and this will likely take me a long time...I've been considering using an epoxy putty (West Marine Rx or M-T). My thinking is, this would make it fairly easy to build up the interior surface to create a level surface by simply shaping the putty and then once cured I can drill it and finish the mounting. Seems like a solid structural option for spreading the load of a backing plate...any concerns with this route?

I guess I could even wrap the bolts in something like saran wrap and once the putty is in place use tighten down the backing plates a bit to ensure a good tight fit. The once cured I should still be able to remove the bolts?

Thanks,
EB
This is more or less my approach that I have garnered from this forum. I shaped the backing plate as best as I could using a grinder with a sanding disc, then bedded it with 4200, which is what I was using the bed the transducer. No through bolts in my set up, so nothing to say about that, except that you can always drill through the epoxy if it seeps into the bolt holes.
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Old 16-02-2017, 08:31   #5
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Re: Leveling surface for backing plates

5200 is your self leveling friend
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Old 16-02-2017, 08:39   #6
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Re: Leveling surface for backing plates

I have done this with marine Tex and it worked out well. Wrap the block in Saran Wrap and butter up both sides. Appply it and clamp or bolt if possible. Once it begins to kick, remove block and trim excess, clear holes. Once hardened reapply and bolt fitting with appropriate sealant.
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Old 16-02-2017, 08:46   #7
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Re: Leveling surface for backing plates

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Originally Posted by John_Trusty View Post
Can you give the epoxy-wizards here some idea of the angles / thicknesses of the amount you are trying to build up? Are you dealing with two dimensions, like changing the transom to vertical and eliminating the curvature? Pictures would help. BTW: That two-part 'putty' is essentially epoxy, so it is permanent (like 3M 5200 permanent!). If you ever need to remove that filler, it will be with a grinder so be careful.

Thanks for the quick response folks. The Hydrovane folks warn against using something like 5200 for the build up, stating due to the loads involved anything between the transom and backing plate needs to be structural like wood or epoxy. My gut is that this is because their recommendations have to be generic and fit for much lighter built boats than ours. Still, if I'm going to do it, I want it to be right.

I definitely want to bolts to be removable and will likely go with the vaseline recommendation. I don't necessarily care if the backing plates (G10 high density fiber board) are removable but I guess they probably should be too, just in case.

Here's a picture of the surface I'm dealing with. The bolt on the left side is mounted on a 45'sh degree slope, then there is a slight transom curve with another slight ridge on the right side. I'm sure I could make this work with a sander and the wood pads, but epoxy seems like it will result in a better fit.
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Old 16-02-2017, 08:52   #8
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Re: Leveling surface for backing plates

Bondo glass, the same stuff your boat's made of. Though I'd recommend using MEKP accelerator, that red stuff makes it go off too fast. (or maybe I'm too slow)
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Old 16-02-2017, 08:53   #9
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Re: Leveling surface for backing plates

Lets try the attachment again...
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Old 16-02-2017, 08:58   #10
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Re: Leveling surface for backing plates

Why build a wood block? Wood will deteriorate and back there out of site you will not notice. Make it out substantial (as thick as the external base but wider if possible) aluminum, then as suggested with thickened resin or 5200. Pre drill and dry fit everything (allow oversized holes on backing plate), isolate the install surface like a surgeon does, then install.

Just saw the pic, go with aluminum or stainless for sure.
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Old 16-02-2017, 10:06   #11
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Re: Leveling surface for backing plates

It really depends on how exact you need it. But if you need a verticle base to mount the hardware to a 45 degree sloped transom, then get a wood block with a 45 degree face, and use thickened epoxy to act as the gap filler.

If you are just mounting the backing plate, then take some G10, rough sand one side, slather that side with thickened epoxy, and press into place until epoxy oozes out the sides. Clean up the epoxy weepout, add some duct tape to hold it in place, go to lunch (ok give it a day). Then drill out the mounting holes, and bolt the hardware on.
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Old 16-02-2017, 10:21   #12
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Re: Leveling surface for backing plates

We used a stuff called Chockfast (a 2 part epoxy) for aligning engines, guns, radars and so on on warships, so it is quite tough stuff. Other epoxies would probably do provided they are tough and not brittle when cured. However, it is probably not the stuff to use if the substrate is at all flexible. In that case you need something structural such as glass layup, ply etc. for that.
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Old 16-02-2017, 10:33   #13
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Re: Leveling surface for backing plates

  • Not wood. I've seen too many crack or rot. What's the point? Use precast FRP. It does NOT need to be G10, no matter what the internet mantra says; structural grade will be stronger than the rest of the boat.
  • Fender washers are totally useless. Take one and tighten a nut up over a soft surface, and see if it doesn't bend into the a cone just when it is needed. I've done testing and shown that they are no better than a common washer. Just a false sense of security. Extra thick fender washers are available, and they are the ONLY ones you should use. Funny, even new boats come through with bend washers under winches.
  • Bonding with thickened epoxy is best. Polyurethane is good.
Or, simply lay-up about 4-6 layers of 17 ounce triax on the surface and it will fit perfectly. This serves exactly the same purpose as the backing plate, but can be much thinner because of the fit and bonding. Then use a common bolting washer (not fender washer--too thin to serve a purpose). Now the boat is actually stronger, for less weight an less risk of deterioration.
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Old 16-02-2017, 10:36   #14
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Re: Leveling surface for backing plates

I helped a friend with a similar style backing plate for a rudder support, using an aluminum plate. We had odd angles and uneven surfaces to contend with. We pre-fitted and staged everything. Using West system and microfibers, we mixed up the batch to the consistency of thick peanut butter, slathered it on and bolted down until the goo started pushing out the sides. It worked really well and wasn't that hard. Depending out your accuracy of hardener mix, pot life can be short, so an extra set of hands is helpful. Grease bolts if you want the option of removing them.
My view on wood backing is that under heavy load it will crush somewhat, possibly requiring more tightening later, thus crushing it even more, unless you use another alum or steel backing plate on it. Skip the wood and use SS or alum, IMHO.
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Old 16-02-2017, 20:51   #15
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Re: Leveling surface for backing plates

I've experimented with backing blocks made from a dense compressed plastic lumber called "Trex." So far it shows promise. At 1 inch thick, it doesn't compress. No rot, no cracking. There must be a downside, but haven't found it yet.
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