Okume vs. Meranti Plywood for Boatbuilding

[MarkR]

In preparing for my build of Joel White's Shellback Dinghy I started to think about the plywood planking that I was going to use.  Okume is very lightweight, but it is typically used when covered in glass & epoxy.  Meranti is also available in BS1088 layup, but is slightly heavier.  (Hydrotek)  It is rated as medium to good in rot resistance while Okume is typically rated as low rot resistance.  Since I would be planking without fiberglass in the clinker ply method, I wanted to know more before I chose my planking material.

Charlie Ehm was kind enough to let me have some Meranti scraps for testing and I had Okume scraps on hand.  The linked PDF shows my first attempt at testing, which was a water soak up test:

https://www.dropbox.com/sh/uw9kp57wzsg28mg/AAB376wl_jU1hgrHQQz_FBN4a?dl=0

The caveat is that I am not trained at materials testing, so take this as you will.  Based upon this test alone, I will probably plank in Meranti as I want the boat to be as durable as possible.  

Mark

[Charlie43]

Mark,

I'd again argue that it is surface capillary action you're seeing on the Okume sample. Repeat the test as you ran it and then immediately cut each sample horizontally at 1/2" increments and inspect.  The photo of the edges suggests both are wicking water to the same degree, but then one of two things is happening. The water wicked by the 2nd and 4th plies is bleeding through to the first and fifth plies on the O, but not the M, or else, it's capillary action (due to differing surface grain structures) that enables/promotes/exacerbates the surface moisture seen.

Again, frankly, I think you're wasting time that would be better spent building the boat itself. No boat is truly 'durable'. Each boat can be only 'durable enough' given one's time/budget/intended use. No question about it, a "well-made" boat, "properly cared for" can offer 30-50 years of service. But something that's "good enough" can also be the means to do something better the next time, or simply different, when the time comes to replace it. As such, each boat is "a foot print in the sands", not a monument for all eternity. (I will never, ever "wear out" any of my boats for already being two more boats along the journey before my current build has even seen another season.)  If you need more Merenti, I'll be back in town in a week's time. But I'd say you need to stop testing and to start building.

Where some testing time would be worthwhile is how the paint you intend to use will adhere to epoxy and how to get an even, dust-free, bubble-free, run-free finish. That 0.002" of surface coating will be the most obvious feature an eye sees, and the quality of a paint job can make or break an otherwise superbly-built boat. In fact, painting boats is beyond my skill set. So I varnish, which is way easier, no matter its reputation otherwise, though a truly high-end varnisher would snicker at my work. But because of its transparency, varnish can borrow from the underlying, and because an all-bright wooden boat is such a rarity, even a less-than-perfect varnish job can be attractive. But a bad paint job is just a bad paint job that everyone can snicker at, because they all have experience with using paint or painted objects, but not so much with varnish. Hence, they are "in awe" of it. Oiled boats really amaze them, and that's by far the easiest of finishes.

Charlie

[MarkR]

Charlie,

I don't think I need to do further tests.    I did a second water soak test and it can be found here:

https://www.dropbox.com/s/gcyk87hvmlx09mh/Okume-Meranti%20Water%20Soak%20%232%20-%20PDF%20copy.pdf?dl=0

With these two tests, I really think I can stop testing and feel comfortable choosing Meranti.  This has been an interesting experience, just to get a feel for these materials.  For my use, I plan to hand select Meranti panels for weight.  That should result in a relatively lightweight and more durable planking material.

Something that I find interesting is that a hand selected for light weight Meranti hull panel is probably about the same weight (or maybe lighter) than a glass and epoxy coated panel of Okume.  Which is more durable?  Depends upon the use and the maintenance, I suppose.  I know that I will be VERY careful with any surface penetrations below the waterline on my Okume boat.

I've never thought that my boats will last forever.  But if someone cares about them, maintains them - they'll last a long time.  So I want them to be a certain way.  Have an honest durability and some craftsmanship.  It's not that different than what you do.  

Cheers!

Mark

[Charlie43]

Mark,

One thing your testing does establish is what a sponge Okume is compared to Merenti. That, I would never have suspected, for figuring that one hardwood marine plywood were interchangeable with another. But obviously, that isn't true. Species differences matter hugely, and that is a valuable result, because it argues that when Okume is used below the waterline, any injury to its surface coating cannot be ignored, and anywhere Okume is used and not adequately sealed is prone to rot.  

What's Okume cost for a 1/4" sheet? $65? compared to Merenti's $55 (at Crosscuts) and $40 at Shurway? Or two-thirds more cost for no weight savings but increased building problems.  No, thanks. I'll pass. I do have two sheets of metric Okume that tape out to 98 -1/2" long that I've been saving for a boat I would want to be a true 8' feet long without scarfed planks. But now I'd be reluctant to use them unless it were for a boat I didn't care about.

There's Buddhist saying that goes like this. The obstacle is the path. The fact that Okume is a sponge does mean that if epoxy saturation is intended, it'd be the preferred choice over Merenti. I can't think of an instance when I'd want to saturate a hull with epoxy. But maybe the need will arise as it might if a boat were built below the waterline with Okume just to get rid of the stuff, but planked above the waterline with something else.

In fact, as I write, I can think of a such a hull, a three-plank, lobster skiff where the cedar planks normally used for a cross-planked bottom would be replaced with Okume ply.  Slather the bottom --inside and out-- with epoxy, and oil the rest of the boat. I've had the cedar air-drying for two years now. Just haven't gotten around to building with them and didn't really want to cross-plank a bottom. So, problem solved, and two more boats can happen with materials already owned. Use a full sheet of Okume for the bottom and cut the strakes from 10' boards re-sawn to 3/8" thickness. Cedar's easy to work. So cutting lap bevels shouldn't be a problem. And since I'd be framing and doing all planking but the bottom with cedar, a building weight of 5 pounds per foot ought to be possible, or a 9' skiff under 50 pounds. Cedar and Okume do work well with each other in terms of their colors and could be a gorgeous boat on the roof racks.  

Thank you.