SailNet Community banner
  • SailNet is a forum community dedicated to Sailing enthusiasts. Come join the discussion about sailing, modifications, classifieds, troubleshooting, repairs, reviews, maintenance, and more!

Is Marine plywood a must?

7K views 41 replies 26 participants last post by  SanderO 
#1 ·
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?
 
#8 ·
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
 
#9 · (Edited)
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?
 
#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.
 
#10 ·
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.
 
#11 ·
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.
 
#12 ·
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.
 
  • Like
Reactions: Zemurray
#15 · (Edited)
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!
 
  • Like
Reactions: Zemurray
#16 ·
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.
 
  • Like
Reactions: Zemurray
#17 ·
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
 
#22 ·
The most important consideration with plywood is weather the glue is waterproof. Cut off a sample and boil it in water for 10 minutes. The difference will be apparent.
 
#23 ·
Think my labor is more expensive than materials. I just want to go sailing. If I do a job want it to be one and done. For this project still like the idea of starboard. Have done things in G10 and starboard by asking yard if they have scraps they’re going to toss or get rid of for short money. Would ask before blowing money.
 
#28 ·
Think my labor is more expensive than materials. I just want to go sailing. If I do a job want it to be one and done. For this project still like the idea of starboard. Have done things in G10 and starboard by asking yard if they have scraps they're going to toss or get rid of for short money. Would ask before blowing money.
I thought about starboard, However the nuance of this installation, I have to screw the thing together like a box, after two vertical sides have been screwed into the adjacent battery box. Short version is that I need to screw the starboard together at a 90 degree angle, on two sides, and it will have to support the full weight of the battery (strapped down to the shelf) in a knock down. My understanding is that starboard works great when screwed through, but doesn't have as strong of a hold when using wood screws to hold two pieces together.

After all the feedback, I think the easiest and most logical is for me to get a small high quality piece of birch plywood with no voids for $30, and epoxy paint it. That's pretty much where i was leaning. Again, this part of the boat is a bit higher than the low spot and never really sees water. It should outlive my ownership of the boat.
 
#24 ·
As others have said marine ply is just exterior ply with fewer voids. That feature is so it will bend event around a hull without developing hard spots where the voids are. Species of wood is more important than the marine rating. The ratings only address adhesives and construction. Rot resistance is not considered and some of the "marine" plys are terrible in this regard. They don't delaminate, they dissolve into mush. Use a good exterior fir or pine ply, coat it with epoxy, and it will be fine.
 
#25 ·
I live 1000 miles from the ocean in a small community and I can get marine plywood delivered. Ask your local lumber yard. My delivery took about a week to deliver and is similarly priced to the online prices I see without the delivery fees. There are many variations, I think mine was marinetech, which costs about $140, I know a sheet of 3/4" marine grade fir can be had for around $90. It is nice to know that 10-15 years down the line the owner then won't be asking, "Who the hell did this and what was he thinking?"
 
#26 ·
Side story, related.

My garden trailer base is plywood. It rotted. I'm old, and perhaps rotted a bit myself.

I go to the lumberyard. I want to fix it once. I run into the guy who owns the yard, a friend. He says, "what are you looking for?" I say "marine plywood." "What for?" I tell him.

He starts to laugh, and says "how long are you going to live?"

Needless to say, I walked out with a sheet of plywood, not marine.
 
#27 ·
Ive been using Advantech plywood as a subfloor on the houses I’ve built over the past 20 years. Unbelievable product that seems to be unaffected by moisture. Ive often thought it would be a good and inexpensive product for certain marine applications as there would be no need to epoxy coat. About the same cost as a CD plywood.


Sent from my iPad using Tapatalk
 
#29 ·
We cant see the batt space.
Just wondering....
If the factory batt boxes are deep enough to allow for stacking above and not needing to build out.
The support is already there.
Then youre building and supporting off of the original boxes
A thought....

I5s easy ro create cut outs in a floor for maintenance
 
#30 ·
The battery “boxes” arent really boxes. Inside the lazarette, in the port side hull, it’s just a wide open space. The builder molded in a pocket for batteries, two little pockets about 1” deep, and leveled off the curve hull, staggered as they climb up the curve of the hull, so each battery is like a step. Inside those pockets are just cheap plastic battery boxes, all that just straps down to the fiberglass pockets.

I’m essentially making a 3rd step, adjacent to the others. All of this is in a wide open hull space. Stacking unfortunately wouldn’t be stable, and its relatively easy to just make a new level spot next to the others.

I’d upload a pic, but it’s not an easy task on an iPad without hosting somewhere.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top