Proper wood materials for marine use

[GreenLake]

It's mainly in the glue and the absence of any internal voids. Even when waterlogged, the MP should not delaminate. And there may be some focus on using rot-resistant species of wood for the plies.

I'd expect you should be able to find further details with a quick online search.

[talbot]

Glue, I believe. Although I think the quality of the laminates makes a difference. The last piece I bought, for making a hatch, is a bit puckered after a couple of seasons. I cared for our wooden boat when I was a kid, and I'm using the same brand of varnish I used in 1965. I expected more. I think next time around, I will shop online and not take what my local yard happens to have in stock.

[GreenLake]

An aside: you can't assume that the formulation for any finish (paint or varnish) has remained stable over time because there's been increasing pressure on manufacturers to reduce the amount of organic volatiles. This has nothing to do with one brand over another.

The problem is that brand X, reformulated, may now outperform brand Y, while earlier the reverse may have been true.

The only way to get a somewhat reasonable idea of the quality of different products is to ask for recent experience or to consult recent reviews. However, the limitation there is that stuff may no longer be around when the results come in.

[jdoorly]

Wikipedia says "Marine plywood is specially treated to resist rotting in a high-moisture environment. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. Each wood veneer will have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water Boiled Proof (WBP) glue similar to most exterior plywoods. Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood: the cost for a typical 4-foot by 8-foot 1/2-inch thick board is roughly $75 to $100 US or around $2.5 per square foot, which is about three times as expensive as standard plywood. Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd's of London stamp that certifies it to be BS 1088 compliant. Some plywood is also labeled based on the wood used to manufacture it. Examples of this are Okoume or Meranti." They failed to say marine plywood also has more layers than regular, which makes it even stronger, more stable, and better looking, like me!

If the load is in shear you don't need much of a backing plate, I use mostly fender washers. The one plate I put in was the gudgeon for my battle axe halyard leverager's 3/8" pintle pivot. It was much thicker than needed at 3/8x2x2".

About bending wood, It's not the water that makes wood bendable, it's heat! The easiest way to heat long pieces of wood is in a long PCV tube. It's usually not level and has a tea pot (over a heat source) at the low end, and a small 'chimney' hole at the high end. The water vapor/steam helps to conduct the heat into the wood. The thicker the wood the longer it takes. 10 minutes will due for a .5x.5, a 2x4 might take an hour, depending on the farenheights. If one edge of the piece is laying on the pipe interior that edge will not get hot, so the piece needs to have bits of transverse support to heat uniformly. I would think you could use a home oven for small pieces, just put a small pot of water in there with it cook it at 400F and test with a toothpic or until golden brown.

[GreenLake]

If you want to conform to the cuddy top, I'd simply use a double layer of 1/4" plywood. Each layer will bend separately (without heat or moisture) but together they'll be fine as backing plate (or to beef up the cuddy opening). If you are worried you can glue them together in place - use epoxy, so you need little clamping pressure.

Some cleats will need very little backing, because in many locations, the laminate on the DS is rather substantial while the loads, depending on the specific use, might not be very strong. For example, for the barber haulers I installed in the rim of the cuddy opening (on a DS1) I did not bother to do anything special.

If I were to replace the mooring horn cleat in the middle of the foredeck, I would glue a solid strip of plywood underneath, the full lenght of the deck, because not only can mooring loads be significant, but the deck needs reinforcement there anyway (and the existing lumber there is suspect after nearly 50 years...).

So it depends...

[GreenLake]

Someone I talked to suggested that "double A" exterior plywood might be sufficient for many uses. I tend to agree, but would always epoxy coat this anyway. The "A" grade designates limited voids, and exterior glues are waterproof. However, unlike many specialized maring plywood types you won't get any particularly durable wood species for the plys.

I think, with some care, and in a location like the underside of the deck, you'll probably be OK, because this wouldn't be a constant immersion type of application. The results may be more durable than some of the structural wood that O'Day put into these boats... esp. the DS1.

[jdoorly]

If you want to use non-marine ply you can inspect the wood by moving a bright light around on the far side to locate any existing core gaps. If gaps are found you can use a syringe to pump epoxy into it or just reject that area from use.