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I need to build a small shelf for an additional battery. My existing batter boxes are molded in fiberglass, in the lazarette, and stepped up the angled hull. I’ve made a template with curved bottom supports add a plan to screw in the sides of the existing battery “trays”. I need to make this in a couple pieces, then screw it into place, so that its secure, and strapped.

Marine plywood would be my default choice, however its hard to find where I am, and expensive to ship. This area of the boat is never wet. I’m considering just getting some A grade plywood, birch or something like that, and coating the pieces with bilge paint. If there are any voids in the laminate edges, I’ll fill them with epoxy or something.

Does this sound reasonable or should I really try and find some “marine” plywood? It needs to last.

Thoughts?
 

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If you don't or can't get marine plywood, I would wrap non-marine grade plywood in a layer of glass. Great practice.
 

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My preference would be marine plywood, but a good 13 ply AB Baltic birch properly sealed, with a layer of glass on the corners, epoxied and painted might work in a pinch...as to the longevity of not using marine ply...that would be your call in the end.
 

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The only difference between marine ply and grade A ply of the same wood is that marine ply has no voids and more ply layers. Neither of them are rot resistant or waterproof or anything "marine-ish". Your application doesn't require the bit of extra strength more ply layers provide, and any voids are inconsequential.

You should consider glueing the pieces together when you screw them in place. Thickened epoxy is good, but you could use a liberal amount of a premium construction adhesive too. Edge sealing is good also, although the paint will do some of this.

Glassing isn't necessary, but a layer on the outside carried over to the existing box would tie everything together nicely.

Mark
 

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If you think you can keep your plywood dry and want to use regular ply, it should work. The same reasoning goes for having a cored fiberglass hull. They never delaminate because of water infiltration. ;) Boats are wet. Wet plywood delaminates. Marine plywood delaminates more slowly. If the shelf is simply for an additional battery, is plywood even required? Others have suggested using Starboard. Red cedar might be cheaper, is rot resistant and won't delaminate.

On another point - why are you putting heavy batteries in the lazarette? Running cables all the way back there adds to the weight too. Would a more central location shorten the wiring and improve the boat's balance?
 

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I used either marine or CDX 3/4" for a "box" / compartment to house 2 8D AGMs. The compartment utilized 2 existing fore-aft OEM plywood 14mm vertical panels. The "floor" was supported on hardwood cleats (oak) screwed to the OEM panels. The athwartship vertical panels... we also screwed to hard wood cleats. The Box was painted with bilgekote.
 

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Marine plywood is intended to take a finish (varnish, paint), support structure, resist mold, and stay submerged. That means you pay extra for defect-free veneers of tropical hardwoods, no voids, more plies, and waterproof adhesive.

I'm guessing that there are fewer, less stringent requirements for holding batteries in a mostly dry, hidden corner of your boat.
 

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look for Baltic Birch plywood, made with water proof glue and several small plys of all birch. easy to work and finishes nice, many lumber yards will have it and some have smaller cut sheets.
 

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Historical note: back in the 60's, the cost and construction of plywood sort of rose based on cost of glue and integrity of the layers.
Basic ply had voids and non-waterproof glue. A step up got you plugged (patches) on the voids. Next level was less voids and waterproof glue. The best product was better veneers with few plugs and waterproof glue, and was called "marine plywood". Cost more, but both the labor in added inspection of the veneers and cost of glue went up, too.
Sometime in the last 20 years (?) most plywood was laid up with waterproof glue, with the interior layers and their integrity adding or lowering the cost.

Then there are products that use just fibers and pieces of fiberous wood... and cost less.
Now we live in a world of "engineered" wood-derived products and prices range over quite a long scale, depending on required strength.

FWIW, if the wood in any project is just there for "coring", you can use less expansive ply and lay some glass and resin on both sides and encapsulate it.

Example --- for a couple of decades I have used honeycomb panels for all interior projects, joined with epoxy and glass where needed. (I salted away a quantity of this "scrap" paneling, in a lot of odd sizes, before the Boeing Surplus Store closed up. They used to dispose of it for fifty cents a pound..... Oh My.
Sure do miss that place!
 
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No - for inside the boat top grade exterior fir ply works just fine.

Voidless marine ply is essential if you are building a hull or a dinghy etc but not for shelving, bulkheads etc. As noted, coat it with epoxy then paint.

I have rarely seen a production boat that used marine ply inside. In fact I've rarely seen it sealed with epoxy either, frequently left bare when it can't be seen.

Production boats are using variations of particle board now so anything is an improvement over that.
 
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Just to be clear, marine ply will delaminate, won't resist rot, and cannot be submerged unencapsulated. This is the same for Grade A exterior ply, which is also made with waterproof adhesives, is made to take a finish, and delaminates and rots at the same rate as marine ply if continually wet.

The important point between the two is that marine ply is generally made out of only one or two wood species (lots of finish veneer choices), while the construction choices of exterior ply is much larger.

As long as the wood is the same, the only difference between them are numbers of voids and plys. There is no magic around marine ply - it is just higher quality ply. Certainly useful in some applications, but not necessary in all applications.

Mark
 

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Discussion Starter #18
If you think you can keep your plywood dry and want to use regular ply, it should work. The same reasoning goes for having a cored fiberglass hull. They never delaminate because of water infiltration. ;) Boats are wet. Wet plywood delaminates. Marine plywood delaminates more slowly. If the shelf is simply for an additional battery, is plywood even required? Others have suggested using Starboard. Red cedar might be cheaper, is rot resistant and won't delaminate.

On another point - why are you putting heavy batteries in the lazarette? Running cables all the way back there adds to the weight too. Would a more central location shorten the wiring and improve the boat's balance?
It’s a small boat. The existing batteries, charger, and switch are located against the Cabin bulkhead, inside the lazerette/cockpit locker. Right next to the engine compartment. Short cables, just off to the port side of the engine. It’s a bit off center. The builder made fiberglass molded battery shelf/pockets for Group 24’s. I need to add a 3rd battery. It’s the only place to put it, right next to the others.
 

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It’s a small boat. The existing batteries, charger, and switch are located against the Cabin bulkhead, inside the lazerette/cockpit locker. Right next to the engine compartment. Short cables, just off to the port side of the engine. It’s a bit off center. The builder made fiberglass molded battery shelf/pockets for Group 24’s. I need to add a 3rd battery. It’s the only place to put it, right next to the others.
The forward end of the cockpit locker is a much better location than the lazarette. Lazarettes are usually aft of the cockpit, up against the transom. That far aft would not be a good place for batteries.
 
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