I know everyone says that when using plywood aboard a boat that one should always use the best marine plywood you can get.
So here’s where that advice does not quite add up.
Some time back I built a hard dodger with marine ply, saturated it with epoxy and glassed then painted it. I was content that it was both bombproof and would last forever.
A couple of years later I built a Bolger long Tortoise dinghy and used cheap exterior ply because it was a very temporary boat and just a quick build to use until I built a different boat. I did use epoxy but because Raka had introduced a new UV resistant epoxy I did not bother to paint it using it as a test bed to see just how long the dinghy would last in the tropical sun.
Years have gone by…
A while back I noticed some issues with the dodger. A few soft spots developed in places they really should not have. I cut away the soft wood and replaced it with new marine ply using epoxy to glue it up coated with more epoxy and glassed it. Problem solved I thought to myself.
Meanwhile the temporary dinghy still got used every day the glass and epoxy on the boat was becoming a sad sight to behold but the boat still worked but there were bare spots exposing end grain and I pretty much accepted that the boat would be toast in a few months or so.
A couple of years later…
The dodger seems to have developed some sort of fatal infestation. The soft spots are back with a vengeance and the spread is now so widespread that it makes sense to just build another one to replace it. Actually not a bad thing at all is I’ve always thought the proportions were not quite right and I’ve been wanting to do another one for ages. Still, I had expected better from the marine ply in question.
On the other hand, the dinghy is still with us. All of the interior glass and epoxy coating has gone leaving bare plywood and exposed end grain. The dinghy was also sunk in hurricane Maria, abused on various dinghy docks, dragged up on sharp rocks, and just generally abused but is still in surprisingly good shape in spite of me purposely doing everything I could to get it to fail.
Over the expanse of time I’ve done a lot of ad hoc testing of ply and lumber offcuts to see just what would or would not rot and how they’d hold up to various tropical insects. Almost always the best results of such tests have leaned towards the opposite of what passes for common knowledge as to what works best on boats in the tropics.
So, now that “H” season is almost passed it’s time to get with the program and build a new hard dodger as well as a new dinghy for “So It Goes”. Doing either project in marine pretty much doubles the price of the dinghy or dodger so I’m inclined to just go with pressure treated exterior ply for both. Once finished, glassed, and painted no one is going to know what sort of ply it is and considering the experience I’ve had it will last as long or longer than what passes for so-called marine ply these days.
Interesting observations and it does beg the question of just why the less expensive product is performing better. Sure it is not a purely equal comparison as one is usually immersed in salt water and the other is exposed to a different mix of fresh and salt exposure. But setting that aside, the "toughness" of the conventional ply versus the marine ply seems to defy the conventional story from the marine industry.
It would be interesting to see if the wood properties of the ply materials that likely need to be tighter grained and more clear of knots and imperfections for marine ply actually inhibit how the epoxy is absorbed and migrates through the ply structure.
Another thought is that the glues involved in the different plywood types might resist the through ply flow or whetting of the center layers creating vulnerability.
I think I would be with you on selecting the less expensive ply for the next project, pressure treated for outside applications.
Pete Cinq Mars
Any thought to using MDO instead? Okoume is stupid expensive, not particularly durable, but light.
What do you think about using MDO instead of marine plywood for catamaran hulls? Okoume is stupidly expensive these days, not terribly durable, but it is light
What do you think about using MDO instead?
I like MDO and Formply quite a bit and they make a lot of sense where weight is less of a concern. Quite a few successful builds I'm aware of have used MDO and Formply to great advantage.
We built our yacht out of exterior grade hardwood plywood, fiberglassed exterior and epoxy coated interior. In 112 sheets I only found one void in the plywood laminations. Six years later and the plywood still looks like new.
Cheers