Which plywood to use for muffler mounting pad

Thanks John.

My local Home Depot doesn’t seem to have cdx. However, last week I found that my local Lowes has “3/4” Marine plywood”. While I can’t find a 2’ x 4’ piece anywhere, I will most likely buy the marine 4’ x 8’. Still debating on glasssing it but either way it has to be painted and I will buy bilge paint for that.

I am waiting to remove the plate before pulling the trigger on the full plywood sheet.

I looked at the suggestions above and decided to stick with the 3/4” because when I enter my bilge, I need to stand on these plates as I arrange my feet. I have to crawl in on my belly and slide my feet between the mufflers and stringers. These plates support my body weight.
 
Lowes might have the Canadian cdx. You just have to look at it.

From our research, there was only two factories that produced it in Canada. Both are the same processes and same company, just different sizes. And both had the same color sealant on the edging.

If I had to replace some, I'd put the bottom side up. Then you will have a smooth finish when done. If you have a small fiberglass shop in your area, ask them for the same color of tooling gel coat as what you have. The shop I worked at used the red, black and a white that is tintable. A pint of that might be lower cost and blend well for the repair. Just a thought.
 
Thanks John.

My local Home Depot doesn’t seem to have cdx. However, last week I found that my local Lowes has “3/4” Marine plywood”. While I can’t find a 2’ x 4’ piece anywhere, I will most likely buy the marine 4’ x 8’. Still debating on glasssing it but either way it has to be painted and I will buy bilge paint for that.

I am waiting to remove the plate before pulling the trigger on the full plywood sheet.

I looked at the suggestions above and decided to stick with the 3/4” because when I enter my bilge, I need to stand on these plates as I arrange my feet. I have to crawl in on my belly and slide my feet between the mufflers and stringers. These plates support my body weight.

Marine Ply is much better then the CDX I recommended, just harder to come by, or at least I thought it was. While CDX is a good ply for what you are doing, marine is perfect. IMO, you don't need to glass or epoxy. Especially marine. But if your going to paint use a primer first then coat with something like Total Boat Bilge paint or the like. It will last for years.
 
So much incorrect information on this post. If you go to the APA ( American Plywood Association) you will find detailed explanations of all the different grades of plywood. What makes Marine grade plywood different from CDX, BCX and ACx is the quality of the veneers. If you are looking for something close to Marine grade plywood try to find some BBOES plywood commonly called playform (it is used in concrete formwork). It has very good high-quality veneers. All exterior-grade plywood uses the same exterior resin (glue). ACX will be your next-best choice. There is no difference between plywood made in Canada and the US. There is a difference between FIR plywood and Pine plywood but they both meet the same grading rules.
 
So much incorrect information on this post. If you go to the APA ( American Plywood Association) you will find detailed explanations of all the different grades of plywood. What makes Marine grade plywood different from CDX, BCX and ACx is the quality of the veneers. If you are looking for something close to Marine grade plywood try to find some BBOES plywood commonly called playform (it is used in concrete formwork). It has very good high-quality veneers. All exterior-grade plywood uses the same exterior resin (glue). ACX will be your next-best choice. There is no difference between plywood made in Canada and the US. There is a difference between FIR plywood and Pine plywood but they both meet the same grading rules.
One of the major differences between the exterior grades and marine are the allowable voids in the plys and number of plys. In all of my years in construction I've never seen an A or B grade exterior glue plywood; it may be in the specs but a unicorn for sure. In fact you can't find AB (front and back surface grade) birch plywood anymore - everything is BC. For my kitchen remodel I had to special order the AB Hickory plywood which ended up being a special mill run; still it had some surface issues.
As far as marine grade plywood there are significant differences to the construction "X" (exterior glue) plywoods - There are no voids on the surface and internal voids are minimal to none, it is constructed of 100% hardwood (Douglas Fir), there are more plys and cross-laminated. The caution is the term "marine plywood" is loosely used at the big box stores; I've seen trash PT plywood called "marine" at HD. From a shear and bending strength aspect the marine grade plywood is far superior.
 
Having worked for Georgia Pacific for 26 years in plywood manufacturing, I know a little bit about it. I have been out of it for 12 years, but not much has changed. CDX, more commonly known as Rated Sheathing, uses the same exterior glue that other other grades advertise. As Tom states above, the grading rules deal with the defects, ie, a 3" knothole vs a 2", etc. BBOES would not be good to use because it has oil applied on both sides and is edge sealed with a water based sealer that is mainly for looks. The oil is anything from motor oil to used hyd oil. CC treated plywood, goes thorough the same treatment process as treated lumber. For building a house, CDX or Rated Sheathing is good. It will take paint, epoxy, etc. You might look for a 4'x2' or 4'x4' pc of BC and go the epoxy coating route and would last forever. I would look for a hardwood or fir plywood in 3/4" thickness with 6 or more plies and very few visible voids, paint it and move on. Often times, there is a lot of sub-par material hidden in the inner plies. Delamination from water or other liquid is the biggest problem with plywood.

Just my $0.02....

Bennett
 
So this is what my local lowes has.
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So after all your input, I decided on the marine plywood shown above. I will paint it with Total boat bilge paint. Since I had to buy a full sheet, I am making all new strainer mounts and the muffler mount that is not rotted. I’ll post some before and after pictures after I clean the bilge.
Thanks all.
 
Your 270 seemed to be pristine, Just sayin.
Ok - you shamed me into re-thinking. I spent the day glassing the marine plywood I cut. I know it’s probably way overkill, but it’s done. This week will be bilge paint on them.
I started on cleaning the bilge yesterday too. With the motors in, it’s so hard to reach everything but at least with the muffler plates out, I can reach under the motors somewhat.
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Scratch this idea. The starboard is deforming with the weight of the large center hatch on the ram. I am quite surprised with it being 3/4" thick. I guess the targeted pressure to a small area like the base of the ram is too much for plastic.
A concern of mine early on was would it warp? I never gave it a thought about strength. Thanks for updating us. Now I am really glad I spent the time getting marine plywood and glassing it.
 
Scratch this idea. The starboard is deforming with the weight of the large center hatch on the ram. I am quite surprised with it being 3/4" thick. I guess the targeted pressure to a small area like the base of the ram is too much for plastic.
If you don't want to go through all of the work with a wood product then consider Garolite (which is a form of resin impregnated fiberglass). The only limitation is things attached to it needs to be through bolted. It's incredibly stiff so a 1/8 inch thick sheet would have probably worked just fine.
 

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