Backing Plate Material and Adhesive

[HangOutdoors]

I have been doing some reading regarding backing plates for boats and different applications, light load vs heavy load and structural, etc.

Looking for some thoughts on the better/best material and thickness to use for a bow gunwale trolling motor mount backing plate. Also the type of adhesive to use. The following are what I was coming up with.

• HDPE / Starboard - It appears that this is not structural and not recommended, plus I read that it doesn't bond well with Adhesives as others do.
• 1/4" aluminum plate - Use 5200 or similar to help bond it.
• Marine Plywood - Easily shaped, but need to epoxy it fully in holes, edges etc.
• G10 - Seems to be the best, and can use 5200, or preferably glass it in with fast cure Epoxy.

I have been reading a lot last night on the web, so I wanted to validate some of the info that I found.

Of course the plate will be as large as I can fit to spread the load. It appears I have some local suppliers who have the different types of material available so will be going out to purchase today.

 

[HangOutdoors]

Going to take a road trip about an hour or so a way. Found a supplier of plastics. Purchasing 2 sheets of 1/4" x 1' x 2' of G10 (FR-4 is all they had). I plan on layering two pieces as blocking and staggering the edges a bit so I don't have a hard end.

This seems like the best material to use. Two sheets are just under $100 in total.

I am thinking about using West System 105 Epoxy Resin with Fast Cure Harder as well. Not sure if that is the way to go to bond the backing plate.

 

[ididntdoit]

You can probably use six10 - depending on how much you need, it could get pricey though - just a lot easier to apply/use

 

[HangOutdoors]

Going to need a good bit since the hull there is slightly curved. I do like that the Six10 is thickened though. Unfortunately you are correct with those size tubes it is going to get quite expensive. Great Suggestion!

 

[Beachbummer]

I'm a fan of aluminium myself. Strong and durable. and If you can get nuts behind it, It makes for a very beefy installation.

 

[Babin Farms]

Have you thought about something like this?? It's textured so adhesives will bond to it & lightweight but yet strong.

https://www.amazon.com/Commercial-Plastic-Cutting-Kitchens-Approved/dp/B01LZZRBOJ/ref=asc_df_B01LZZRBOJ/?tag=hyprod-20&linkCode=df0&hvadid=198060092138&hvpos=&hvnetw=g&hvrand=17877400121586039050&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9021880&hvtargid=pla-349517985000&psc=1

 

[Beachbummer]

Sorry, a follow up thought on Aluminium... You can use angled or flat long pieces too, one on each side, nuts behind it...very strong and the flat pieces will bend to contour. Wide enough to give you the backing you need, and thin and separate to make for easier install. Just another thought. Good Luck however you proceed.

 

[HangOutdoors]

@Babin Farms Yes I looked at something similar. It is basically some form of HDPE. I guess, and I am no expert by any means, just doing a lot of reading, that HDPE, Starboard, etc, has the propensity to deform in a structural use such as being used as a backing plate and is not recommended from all the stuff I am reading. It would be fine for a lot of things that don't have a lot of holding power required. I have both HDPE and Starboard in the garage and was going in that direction at first.

Since the trolling motor will have different direction force when in use and vertical up and down forces while in the locked position, it made me second guess my original decision. The catch is, is that I don't know which way to go and I have no common frame of reference or experience.

 

[HangOutdoors]

Sorry, a follow up thought on Aluminium... You can use angled or flat long pieces too, one on each side, nuts behind it...very strong and the flat pieces will bend to contour. Wide enough to give you the backing you need, and thin and separate to make for easier install. Just another thought. Good Luck however you proceed.

I have been thinking about that all night. I could source aluminum angle from the big box store. and I have some aluminum plate left over from the transducer (and the finger accident). I felt like a good move, till I started reading around the internet and you know how that goes. I would have no problem using the aluminum if that is a good solution.

What would you use for the bond to the glass? Just 5200 so there is full contact across all the minor voids, between the aluminum and glass?

 

[Beachbummer]

Do you have enough access to reach the back to use bolts? or will you rely entirely on screws? If #1 I think the answer is less important as the set up works in your favor. If you have 5200 Handy, that is certainly very strong (Maybe too strong? ever plan on removing?) Silicone and you can in the future remove/replace if you change motors, etc, but 5200 is very strong and permanent. a very FINAL solution

 

[Babin Farms]

If you look at the backing plates of the towers there is really not much there, at least on my boat. A small piece of plywood glassed into the hull or if you look at the cleat and tiedown mounts there's not much there either. Granted the wood in the glass helps with spreading the load but your only looking at a trolling motor mount so I would think that anything that you listed in post 1 would work. Your not going to be using it for climbing in & out of the boat, getting towed from or anything that is going to put tons of stress on it are you?

 

[HangOutdoors]

@Beachbummer Yes I have a case 5200 in caulk tubes, don't ask it was on sale Also 4200 and 4000 Been using on different things. I have full access under gunwale. You can see this thread to see where I am at and what is going on.

https://jetboaters.net/threads/2020-ar210-trolling-motor.32843/post-533001

If Aluminum is the way to go, I will definitely save the cash on the G10.

Also I have 2" and 3" I think #10 SS Machine screws with 2" SS Washers and Nylock nuts. Was goin to use 4 or 5.

 

[Beachcampin]

for what its worth, I remembered reading this on the Monster Tower webpage regarding reinforcement for their towers. if it works for the tower, I would think it would work in this application as well.

Most competition ski and wakeboard boats, as well as more expensive boat brands like Sea Ray and Cobalt, have decks which are at least 3/8" thick and do not need to be reinforced. If your deck is thinner than 3/8" you should reinforce it with additional fiberglass to build it up to 3/8". Fiberglass kits are available at many online stores including www.WestMarine.com, www.iboats.com in addition to local boat supply companies. Some of our customers have added a 1/4" or 3/8" piece of plywood under the deck attached with 3M Model 5200 Marine adhesive and then covered this with fiberglass. This is the best possible solution as it adds significant strength for the mounting location. This would be recommended for Bayliner, and similar brands where the fiberglass thickness may be closer to 1/4" or 3/16" thick.

 

[2kwik4u]

I'm not sure what G10 is. Can't comment there.

From a structural standpoint, when you laminate materials, you want them to be as near the same stiffness and strength as you can get. This helps distribute the stress/strain equally.

For instance consider an 1/4in aluminum plate attached to a 1/8in thick fiberglass plate. Lets say the aluminum is 8in square and the fiberglass is 12in square. Then you put a bolt through the center, and pull on the laminated assembly through that bolt while holding the edges of the fiberglass. The fiberglass will yield before the aluminum, so you've essentially just moved most of the stress to the edge interface between the aluminum and the fiberglass. There is both bending and shear in this arrangement, and VERY LITTLE shear is transferred because of the difference in relative material stiffness, even with a good adhesive on the edges, you're most likely to puncture the fiberglass at the corners (heavily rounding them will help some, but that will still be the high stress area.

A really good example of this is if you take a thick paperback book and bend it. Watch the edges of the pages. Notice how the top of the book has to bend further than the bottom. This difference in length between the ID and OD of the bend is what creates the shear forces that will rip the adhesive apart. Now do the same thing with an aluminum (or steel) plate behind it, and you can see the mismatch in stiffness and how it creates problems.

HOWEVER, if you are just loading it in pure tension, and need to make sure you don't "pull through", then a giant washer/plate can be a good thing from a tearout perspective. So there are multiple load cases to consider, and each joint will be slightly different. In general though, you want to attempt to match the stiffness for bending and shear, and add stiffness for tension/tear out.

In your case, I would probably go with a large sheet of plywood, then back that with some large structural washers on the connection. Once it's all in place I would consider skim coating (within reason and reach) the whole setup with some fiberglass to seal it in, and adhere them togethor.

 

[HangOutdoors]

@2kwik4u excellent informaion, very interesting I was reading about the ends and the different strengths of different materials and exactly what your talking about I believe I saw the recommendation that you would either taper the ends or if you can't do that build-up the backing material I in a staggered way so that way the end's are stepped coming in so you dont provideba hard edge.

 

[2kwik4u]

@2kwik4u excellent informaion, very interesting I was reading about the ends and the different strengths of different materials and exactly what your talking about I believe I saw the recommendation that you would either taper the ends or if you can't do that build-up the backing material I in a staggered way so that way the end's are stepped coming in so you dont provideba hard edge.

Tapering the ends helps. Definitely round the corners.

What you're trying to avoid is a large and abrupt change in thickness. This creates a stress concentration area with factors in the 1.2 to 5.0 range (depends on thickness, load, etc). The smoother you make that transition in thickness the better off you will be. Conversely, you can purposely oversize your backing plates for lamination in terms of length and width, and place that thickness transition outside the nominal load path, and avoid the problem. This usually leads to excess material, and unwieldy pieces though.

For your trolling motor mount I would probably throw a 1/2in or 5/8in thick piece of marine grade plywood in there. Make it generously oversized as best you can. Then through bolt with large washers where possible. Adhere the plywood to the FRP hull with 5200 adhesive and bolt in place to cure. Once cured I would consider removing the bolts, and covering the whole area with some fiberglass matting and resin to both seal, and reduce the abrupt thickness changes. You can redrill the holes and do final assembly once the fiberglass has cured.

*edit* what the heck is G10?

*edit #2* is this it? Jamestown Distributors

*edit #3* This thread has a comment about it. Alternative backing plate materials - Cruisers & Sailing Forums Assuming it doesn't get brittle like bakelite does (the "see also on wikipedia about G10 point to bakelite), then it could be a very attractive alternative to marine plywood. Sounds like it's harder to cut and shape than plywood, but adheres to epoxy better, and doesn't have to be coated to remain water intrustion resistant.......damn it's pricey though.

 

[mrcleanr6]

g10 is an excellent material. maybe i missed it but where are you mounting this? i wouldnt use anything less than 5200. despite what alot of people say, 5200 is certainly removable down the road. if you want a more permanent bond then the west system will work but you will also need a thickener/filler for it since is far too runny to be used like a straight epoxy adhesive for most things. west system has a few different fillers. fumed silica is a good one. mill fibers also. you can add a little of this to the resin to thicken it a little or alot all the way to making a putty/paste out of it. as a boater, west system is always a good thing to have on hand in the garage to repair all kinds of things, boat related or not

 

[mrcleanr6]

g10 is a fiberglass material. its made in sheets or extruded under super high pressure. you have probably seen it and did even know. its usually a translucent light green. you really cant see the glass in it like a regular hand laid glass. it looks like a solid sheet of plastic. very heavy though and extremely strong

 

[HangOutdoors]

They already cut the G10 so i am on the road to get it

 

[HangOutdoors]

Here is what I picked up. 2 sheets of 1/4" 1' x 2' G10. The guy was really cool, was telling him what I am doing and he gave me some rods, pieces that where in a box to play around with. 1 is nylone, the other two I don't remember what he said they were but he said extremely strong. Could drill them out and pass bolts through as spaces and shims, etc. Or cut them thin, drill them out and make my own washers.

It doesnt heat shrink, doesnt need to be glassed over. Stonger than hdpe and plywood, fully waterproof. And they also make one that is heavily fire resistant. To cut it requires a diamond blade, like for tile, it will drill with carbide

Interesting side note, they make a lot of computer mother boards out of it as well as knife handles is what I was told. Thanks everyone for the input.