Less expensive products versus "marine grade".

[Stranded Mariner]

Gord, you are an amazing resource of knowledge and information. Thanks!

[sneuman]

Originally Posted by GordMay

Exterior Grade is NOT the “same as” Marine Grade Plywood.

Marine-grade plywood is made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better.
B-grade veneer may have knots but no knotholes.
A-grade veneer has no knots or knotholes.
Both A and B grade may contain wood or synthetic patches.
Panels are sanded on both faces or Medium Density Overlay (MDO) or High Density Overlay (HDO).
The maximum core-gap size permitted is 1/8 inch.
Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive.

Plywood panels rated as EXTERIOR but not Marine, such as A-B, A-A or C-C EXTERIOR, may contain any other permissible species and contain C-grade veneer.
Unless specially improved, C-grade veneer is permitted to contain knots and knotholes up to approximately 1-1/2 inches across, and the inner plies may have core gaps up to 1-inch wide.

Notwithstanding, there may well be many applications where Exterior Grade plywoods may suffice.

The detailed description of veneer grades and Marine-grade plywood is contained in Voluntary Product Standard PS 1-95 Construction And Industrial Plywood.
Goto:
http://ts.nist.gov/Standards/Conform...oad/ps1-95.pdf

What some others have to say on Plywood construction:

The Elements of Boat Strength: For Builders, Designers, and Owners ~ By Dave Gerr
Goto page 179 etc at:
The Elements of Boat Strength: For ... - Google Book Search

Understanding Boat Design ~ By Ted Brewer
Goto page 86 etc at:
Understanding Boat Design - Google Book Search

Is there a difference in the glues as well? The most important difference between ordinary plywood and exterior is the fact that the former is made with water soluble glue - guaranteed to eventually delaminate in a marine environment.

[GordMay]

Originally Posted by sneuman

Is there a difference in the glues as well? The most important difference between ordinary plywood and exterior is the fact that the former is made with water soluble glue - guaranteed to eventually delaminate in a marine environment.

I’m not certain, but I recall that both Exterior Fir and Marine Fir plywoods use the same waterproof “resorcinol” type glues (phenolic formaldehyde resin) - but check the previously-linked “Product Standard”.
With marine grade plywood , the lowest grade veneer is "B", which means that there will be no serious voids on the surface and interior plies. Because it’s the voids that cause poor adhesion, marine grade will have good adhesion throughout the lamination. Also, marine grade can be cut without getting a void in one laminate on a freshly cut edge. The edges will be totally solid.

[Southern Star]

A word of caution about non marine-grade 'stainless steel' hose clamps - while the band itself is stainless, the adjustment hardware is typically only galvanized steel.

Brad

[seafox]

Originally Posted by Alan Wheeler

I baffled a kid the other day with an explanation of how we stop the boat from sinking if we hit something and have a hole in the hull. I very simply said, you cut another hole the same size just below so the water can run back out. He eventually clicked on, but I must say it was a worry there for a few seconds.

Huh, what do you mean that's a worry. It is a perfect explanation I what you do isn't it?

[TexSail]

I priced a drop in hatch A/C unit (for use at the dock) at West Marine. If my memory serves me it was around $1100 for a 5000-6000 BTU unit. It looked to me like a cheap window unit in a plastic shell which fit through the forward hatch.

I built a 3/8" plywood housing that drops in the hatch and is custom fitted to my hatch. I put it together with stainless screws and marine epoxy. I gave it a sealer coat of epoxy and three coats of marine enamel. I got a GE window unit at WalMart for $90. Set the housing in the hatch, push in the window unit. I bought a large outboard motor cover at Wal Mart, split it on one side, and attached it to the housing with SS screws and grommets. The cinch cord tightens the cover around the open hatch and assures a rain tight seal.

The unit work like a charm. You will need to build baffles for cool air and return air. The controls are accessible from the inside of the cabin.

Total cost was about $175. The filters are cheap and readily available. If the unit needs service, it is about as common a unit as is found. If it bites the dust, a new GE window unit is available almost anywhere, including many Carribean islands.