Non-marine ply build

Hello experienced people,

First, please don't jump down my throat and tell me how stupid I am - I am a qualified mechanical engineer & structural engineer and have a serious question .....

Why is marine ply the 'only' material for S&G construction ?

Seeing as a finished boat is encapsulated inside & out with at least 3 layers of epoxy resin, then numerous coats of paint &/or varnish ..... what is so wrong about using good quality non-marine plywood ?

Provided the epoxy layers are done to a high standard, and the paint/varnish too, the carcass is never going to get wet ..... hence the question

Thanks in advance for any replies


13 replies:

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RE: Non-marine ply build

>>the carcass is never going to get wet

Ask Planned Parenthood about barrier reliability :-)

>>good quality non-marine plywood

Next to impossible to find

If you can find it, go for it.

Laszlo

 

RE: Non-marine ply build

Tony,

I believe that you are correct in that fully encapsulated non-marine plywood is protected from water absorbtion and is probably a structurally sound option. I've seen beautiful boats built with it. That being said, I will continue to use high-quality marine plywood for stitch-and-glue construction, primarily because I've also seen the additional labor necessary to make non-marine plywood work well. Some of the issues are:

  1. Marine plywood is consistent in internal structure and bends in fair curves. Non-marine plywood is often less consistent (different woods and/or voids in inner plys) and is prone to less fair curves.
  2. Difficulty with checking and splintering on surface plys of non-msrine plywood.
  3. Patches in surface plys of non-marine plywood.
  4. Greater difficulty filling edges on non-marine plywood.
  5. Most stitch-and-glue designs assume the bending characterists of marine plywood.

Regards,

Dick

RE: Non-marine ply build

   Laslo / Dick

Thanks for your replies - really appreciated

The reason I am asking is because I would like to make more than one boat of the same design, and for multiple components I can program my laser cutter to cut the sections.

However, the recommendation is that the laser is only used to cut non-marine plywood.

I'm not sure of the reason for this - its either got to be the laser won't cut the resin used on marine ply (which I find difficult to believe, as it will cut 20mm acrylic and 25mm MDF) , or its just a general recommendation with regard to the fumes (though my machine has full extraction)

Hence the question in the first place

Thanks for your replies

RE: Non-marine ply build

According to christinedemerchant.com (which I think you will find useful to read):

/*

Both marine and exterior plywood are regulated so that the adhesives used in their manufacture can withstand severe conditions including boiling for several hours.

The main differrence is in the quality of the plies used. Number of knots, joins, repairs, voids are all strictly regulated in marine ply and less so in outside ply.

*/

I can't think of a reason why the machine would work less well on plywood with fewer knots, joins, repairs, and voids.

From the above reference it is clear that it couldn't be because of the resins used, as you had reasonably surmised, since the classification "exterior or marine" doesn't specify a resin. 

RE: Non-marine ply build

According to christinedemerchant.com (which I think you will find useful to read):

/*

Both marine and exterior plywood are regulated so that the adhesives used in their manufacture can withstand severe conditions including boiling for several hours.

The main differrence is in the quality of the plies used. Number of knots, joins, repairs, voids are all strictly regulated in marine ply and less so in outside ply.

*/

I can't think of a reason why the machine would work less well on plywood with fewer knots, joins, repairs, and voids.

From the above reference it is clear that it couldn't be because of the resins used, as you had reasonably surmised, since the classification "exterior or marine" doesn't specify a resin. 

RE: Non-marine ply build

   For what it is worth, before I would invest all the time and money into building a boat or boats with non-marine ply I would find out specifically why the instructions for your laser recommend not using MP.   Also, I don't care how thoroughly you coat and recoat the wood, you will undoubtedly damage it to some degree, somewhere on the boat, at some time and that little nick, cut, scrape or whatever is going to spread very fast.  All it takes is that one little area for rot to begin.  Why risk it if you can afford the MP and your laser will cut it?

RE: Non-marine ply build

Tony,

I guess that I'm surprised that one would cut a flammable material, wood, with a laser. Do you cut in an oxygen-free environment?

As an engineer, I'd probably choose my cutting technology to fit the material I want to use, rather than choose a material to fit my cutting technology. I suggest that you will expend more labor making non-marine plywood work for you than you will save using the laser cutter. If you have the data necessary to drive the laser cutter, could you not use a CNC service to have the plywood cut?

Just a thought,

Dick

RE: Non-marine ply build

   I have built sailboats with exterior grade plywood. They worked fine. But the ply was thicker and heavier than that used in S&G. The footballs were obivious but I painted it. They still telegraphed though the paint without a lot of sanding and filling.  The edges got filled. But no amount of sanding or filling got all of the voids. The void remained a weak penetration point for point loads. You know, rocks, docks, knocks and other bumps. 

.

You would also want to use at least an A/B ply. Elsewise you sand a lot of inside surfaces and do a lot of filling of inside "C" knots, checks, etc.  Yet I'm told sanding is fun...........sanding is fun..........sanding is.......

 

RE: Non-marine ply build

You will spend way more on epoxy and glass than what you will save on the plywood. I understand that is not the motivation for your question. I built a "Cheap Canoe" from Bateau.com. I needed a boat for a trip. I used underlayment plywood. Supposed to go under tile in kitchens and bathrooms, no voids and waterproof glue. That boat currently has several patches from the plywood delaminating. It has one spot that has no wood left, translucent area. You could use the laser cutter to make patterns in cheap plywood, the use a router to cut the real parts. Good luck. JRC.   

RE: Non-marine ply build

My first build was a prototype Eastport Pram build.  I was building from plans, so doubted my lofting abilities vs. CNC cut kit.  I also could not afford okoume, so sticking with marine grade plywood would mean that I would not build a boat at all.  With these factors in mind, I used 1/4" oak veneer plywood (for wainscotting) from Home Depot to build the entire boat.  This plywood looks great finished bright, matches my oak gunwales, and the parts were planned so that the bad sides were all painted.  The boat turned out great.  It has no footballs, bent perfectly, was about a third the price of marine grade.  I do have some dark spots from water intrusion, but for the price, the experience, etc. I would not have the boat that I take my son sailing on almost every weekend during the summer if I had waited until I could afford marine grade at almost $100/sheet for 6mm (I think I paid $38 per sheet).  Please don't let the purists dissuade you as long as you know what you're getting.  Yes, marine grade is better, will last longer, etc. but I'm already about to embark on building the next boat.  We'll see if I go marine grade or not.  After all, wood is wood and it gets wet, swells, wick, etc. and the adhesives are similar.  Good luck!  

RE: Non-marine ply build

Also, keep in mind that digitally reproducing multiple boats is getting into a very gray area legality-wise.  Purchased plans are for a single hull and some sort of royalty would be due the designer in the case of multiple boats.  But whether you cut out a boat by japanese dozuki like I did or with a CNC machine, it shouldn't matter the level of tech used.

With that being said, I also plan on using a CNC machine to digitize templates and using a 1/4" downcut spiral bit cut at least one boat out that way.  This has all been with John's blessing via email.

The CNC machine in question is a 1000mm x 1000mm X-Carve, so I will need to perfect a tiling process in order to cut out 12' long parts.  This is my winter project.

RE: Non-marine ply build

Thank you all so much for all your replies !!

To answer most (I hope) of the points raised :

1.

The laser is VERY hot. It doesn't really burn the wood, it more 'instantly vapourises' it. It cuts in normal ambient atmosphere but the speed the cutter head moves means the laser isn't stationary long enough to ignite the timber. It just leaves a very fine cut edge, a little bit charred, but only superficially.

2.

The legality issue is well raised and I will look into it (when I get round to doing a second boat ........ I still haven't got to the 'fun' sanding part yet of my first build)

This said, the design I'm using is from a book which, as far as I can see, doesn't limit one to building only one boat. The design itself is no longer commercially available

3.

The idea of the laser was to incorporate both cutting the sections and a puzzle-joint simultaneously. At present I am cutting PJ's independantly after cutting the section - with an applicable allowance for the additional lengths.

My PJ template is (yes, you guessed it) laser cut in 12mm acrylic. The cutter is a top bearing-guided 9.5mm cutter, cutting 12mm wide apertures

4.

I'm going to give the laser a try on some marine ply next week .....

5.

I'm guessing Home Depot is a shed ( a large chain of building supply products) in the US ? We don't have them here (UK) but I do have a very good supplier of sheet materials. 1088 Marine ply costs me £19 a sheet, non marine ply of the same thickness costs me £4 a sheet  (4mm)

 

Thanks again for all your replies

RE: Non-marine ply build

Just a hunch about why the recommendation about not cutting marine ply...

It might not actually be about the high quality boat building marine ply (okoume, meranti) that we are used to.

It might be a rec. that was developed for other plywood formulations. I bought "marine plywood" at a local lumberyard 20 years ago because I had to hang some instruments on a piling for 12 months. This wood was impregnated with anti-whatever chemicals and was very heavy. The rec. could be about CCA plywood too. The density of these "marine plywoods" is very high. And you would be burning up some nasty stuff too.

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