Marine Plywood Testing

Several years ago Dave Nolan started building a DIY Great Alaskan Boat as designed by Brian Dixon, a long time Tolman Skiff Builder.

Dave got a great deal on some Marine Grade Fir Plywood. But as he started cutting wood he noticed that the plywood seemed dry and brittle and he worried about the breaking strength of this plywood and whether or not he got a good deal. Dave sent me some samples of his plywood, and I used my rather in-exact science to see how strong his plywood was compared to my own various plywood samples.

Initial Findings (pounds exerted until breakage):

Nolan Sample 1 42.6
Nolan Sample 2 41.6
Nolan Sample 3 77.6
Marine Grade Meranti 76.6
A/B Exterior Fir 111.6
Superply 46.6

Method:

Set up a saw horse with a piece of 3/8" plywood covered with a piece of 1 1/8" subfloor ply to create a 3/8" slot in which to accept the ends of the sample pieces. The middle of the sample boards was placed at the edge of the sawhorse. I stood on a bathroom scale and started out with the rather portly 196.6 pounds. Then I placed each board, one at a time in the slot. Hung a bucket of clamps from the samples to see how much they bent, then the break test. Pressed down with my right hand directly at the edge of the board, and had my left hand resting directly to left in order to balance myself. I pressed down with the right hand until the sample board failed, and made note of the new measurement on the bathroom scale at the point of breakage.

One thing I noticed is that two of Nolan's boards made a cracking sound at about 25 pounds and then failed at 41-42 pounds. The one Nolan board did not exhibit this behavior. The clear winner by and far was the A/B Exterior Fir that I've had kicking around for quite a while. It took almost 112 pounds of pressure before failing. The loser was one of Dave's boards which failed at 41 pounds.

Marine Grade Plywood Testing Images: