marine grade vs exterior grade

[xsailor]

Hello Everyone, I am new to this forum and wooden boat building as well.
I am interested in building a 9.5' boat which I would be using for mostly lake fishing. In preparation for this endeavor, I would like to hear from you what is the difference between Okoume, Meranti and Luaun or exterior grade plywoods?
I have seen some sites mention small wooden boats can be built with exterior grade plywood and other sites opppose to this. If I will be using this boat for fishing maybe every other week, does it really matter which type of plywood I use?
I appreciate your time.

[mizzenman]

Marine plywood should have no voids. Allso have a look at the number of plys in the sheet. if they'r much thicker than 1mm thats a bad thing.

as for different wood types, Im certain wikipedia can tell you about the marine suitability regarding rot and strength.

For a small lake boat that will get wet once every second week and is your first build too, Id use exterior grade stuff. Especially if its a simple flat bottomed thing.

What does the designer recomend?

[Paul Pless]

from the glen l site
Marine-grade plywood is made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better. B-grade veneer may have knots but no knotholes. A-grade veneer has no knots or knotholes. Both A and B grade may contain wood or synthetic patches. Panels are sanded on both faces or Medium Density Overlay (MDO) or High Density Overlay (HDO). The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a "premium" panel grade for use in situations where these characteristics are required. It is available in 4x8-foot sheets of 1/4, 3/8, 1/2, 5/8 and 3/4-inch thickness. Sheets up to 5x12-feet are also available. Available grades are A-A, A-B, B-B (face-back), MDO and HDO.there are other grades of plywood that may be rated better than marine grade, such as BS1088. . .

[SMARTINSEN]

The luan that I have seen at the big box stores lately is so full of bondo that I would strongly recommend against use on any kind of boat. Maybe for making templates, but that is about it.

For a couple of sheets on a 9.5' boat, pony up, buy the good stuff, and never look back.

[Richard Jones]

Years ago, I built a couple of 8' tenders out of quarter inch exterior ply (AC grade). Used lots of oil exterior paint. They sat on the beach all summer and did fine.

That said, they were ugly things with all the grain showing through the paint. I built them super cheap because I didn't want to spend a lot on a boat that had a high risk of being stolen off the beach.

If this is something you want to look at with pride and will last forever, I'd build with a good marine ply. Good luck and send pictures!

[Cuyahoga Chuck]

I posted this on another thread but it may have some valid points for this discussion too.

I have to give you the quicky evaluation of US made exterior grade ply.
It's all made from softwoods like fir and pine. That means it's got bands of early wood and late wood that are of different hardnesses so any surface preparation like sanding will make the grain stand out more than it already is. It's a big drawback if looks matters to you. The exposed grain can be faired out but it takes time, involves a lot of sanding and will increase the weight of the boat markedly.
None of the plywood made in the US is made to a common standard. Various regions where ply is produced have their own associations which make and enforce quality standards. None of them press their members very hard because they can all sell everything the make. Most of the exterior they make is full of flaws like ragged veneers that create numerous voids when they are used as interior laminates. This can produce weaknesses and places that can trap moisture. Go to any bigbox lunber store and look at apile of exterior ply. You'll see gaps along the edges of every sheet. There are a lot more you can't see.
Our softwood plywood is all about 1/3rd heavier than a similar panel in BS1088 okoume. If you add that to your inexperince in S&G boat construction your 130 lb GIS will probably be closer to a 200 lb GIS.
I havent seen the GIS plans but if it calls for ply thinner than ¼" there is no suitable US ply.
There is more. If your ex-grade ply is fir it will eventually check. I.E. all the exposed surfaces will become covered with little splits in the grain and ruin the finish. The only remedy is to glass all the exposed surfaces. This wou,ld mean more weight yet.
Using the expensive imported marine grade ply will cut the amount of backlabor significantly so there is a payback for spending the extra money. This better ply has none of the drawbacks of exterior grade. It's structually better, and has more veneers in the thinner panels, and has exterior faces that are of furniture quality.

[Canoeyawl]

I think it only matters if you worry about drowning.
If you do that, build the best boat you can.

[Steve Paskey]

But if your goal is to build an inexpensive hull, perhaps as a means to gain some building skills, then other materials will serve. Many boats are successfully built with plywoods that are less than ideal for the purpose.

My best advice to you as a new builder is to pick a plan by a respected designer and use the materials specified.

Forget what the designer says... any designer who recommends cheap ply does so only because it's cheap. The same boat built from bs1088 marine okueme will be lighter, better looking, longer lasting, and easier to maintain. And if you move on to another boat, you'll be able to pass the first boat along to someone else. Plus, it's better for the environment -- the last thing we need to do is chop down more trees to build ugly disposable boats.

[wtarzia]

Cheap plywood will require a layer of glass cloth epoxied over because of the checking: a pain in the ass but very strong result. If you glass the panels first flat on the bench, the glass job will come out fine and you will avoid some typical problems. I also went around the bare hull with a syringe full of epoxy and tried to fill all the voids I could find; overkill perhaps, but what the hell.

Later I reflected that I put in a lot of hours making up for the fact that I used cheap exterior plywood. I have a pile of marine grade Okuome for my next project, but the drive to give that a layer of glass too is very strong, making me wonder reverse-wise if maybe I spent too much money on marine grade! But the Okuome has other qualities, such as lightness.

So many variables and trade-offs in boat building, so little time to explore them all, and so much psychology involved in the whole thing.... good luck with your own time and variations. -- Wade

[Keith Wilson]

There's lots of heat in discussions of plywood grades. You need to understand a couple of basic concepts before any of it makes much sense. There are several characteristics of a sheet of plywood:

Wood species: US-made plywood is generally Douglas fir or southern yellow pine. The pine is heavier and less rot-resistant, both will check very badly if not covered by some kind of sheathing fabric (epoxy-fiberglass or =). Imported plywood is generally hardwood; various species are available:
Lauan ("Philippine mahogany") - highly variable, generally construction-grade plywood.
Khaya ("African mahogany") - heavy, strong, fairly rot-resistant, medium reddish brown.
Okoume - quite light, not very rot-resistant, light brown.
Meranti - Intermediate in weight and strength, reddish color.
There are others; Teak, Sapele etc, but they're more often used for interiors on bigger yachts where appearance is more critical, and are very expensive.
There's a lot of high-quality birch plywood available, but I wouldn't use it in a boat; most is interior grade, and birch rots easily.

Glue: Exterior and marine ply all use waterproof glue. One shouldn't ever use interior grade plywood in a boat, for obvious reasons.

Grades: This refers to the uniformity of the veneers, the number of plies (more is better, within reason) the number of interior voids. Voids are bad because they can make the sheet bend unevenly, and can serve as a place where rot can start. US plywood is rated by letters, referring to the quality of each face, AA being the best, AB having one really good face, one less good, CD being very cheap and nasty. US marine plywood is usually AA or AB. Imported marine plywood is usually BS1088 or BS 6566. 1088 is nicer and more expensive, but the major difference is only that it has thicker face veneers.

You basically have three choices:
- US-made constriction grade fir plywood: Cheap, has an occasional interior void, and checks badly if not sheathed. Strong enough.
- US-made fir marine plywood; better quality, moderately expensive, and still checks.
- Hardwood marine ply - moderately to very expensive, best quality and appearance. Use Okoume if light weight is very important;

It also makes a difference what kind of boat you’re making. Taped-seam and ply-on-frame boats can be built with cheaper plywood, but not ply lapstrake. If the boat isn’t going to have glass sheathing, fir ply will check. The better stuff is more durable – but not that much more durable; I made a 15’ boat with lumberyard ACX fir and polyester resin 20 years ago which is still going storing, despite very indifferent maintenance. If you were going to build a boat and sail to Borneo, I’d say buy the best. If you’re going to putter around in local lakes, you have more of a choice. The good stuff is more pleasant to work with, but plenty of perfectly functional boats have been built with lumberyard plywood.

[jonboy]

I agree with KW.... plenty of good long lasting boats around built with even scrap wood...I've built two identical Vaurien sailing dinghies, about twelve foot...one from expensive marine ply, that still had some faults and the other from construction supplier exterior grade...the main difference was in the end the ex grade came out more expensive in the long run due to more frequent repainting, and general stuff, but the marine grade was expensive, about twice the price in initial materials cost but as maintenance free as a ply boat can be....If you are on a tighter budget go for ex- grade.... it is a bit like buying a boat on the hire purchase....I never sheathed or glassed either just slathered the paint on....visually they were indistinguishable.

[Frank R]

Try using Ultraply XL available at Lowe's. Marine glue, no voids. Google it. $20 a sheet.

[xsailor]

I thank you all for taking the time to respond to my questions. I have some decisions to make. I will post pictures of the finished boat and list the materials I choose.

[Scot McPherson]

I have learned in making Canoes that the glass/epoxy coat costs half the cost of the canoe. Though in canoes we glass both the inside and outside of a wood strip canoe for strength and preservation.

[Cuyahoga Chuck]

Try using Ultraply XL available at Lowe's. Marine glue, no voids. Google it. $20 a sheet.

I just googled "Ultraply". The claim is made that it can be scored with a utility knife and snapped. I don't find that reassuring. I have built two small boat with a competitor's product, "Multiply" and can assure you it won't yield to anything but a saw blade.

[Frank R]

Many people have built with it successfully.

[coelacanth2]

You might also look for "sign painter's plywood" or "sign painters board". It is the plywood used for signs. My understanding is that it is usually faced with mahogany and the glue is rated for extended wet conditions, including submersion. I have a few sheets which will be the cabin top on my Seabird project (eventually). MDO is supposed to be pretty durable, too, and the paper covering would take lines easily

[Paul Mc G]

Hi there new to the forum hope everyone is well.I will soon be embarking on my first boat build I plan to build a 25 foot motor boat,9 foot beam.I am thinking the construction method might be a double skin of 12mm marine ply on pressure treated red deal softwood.Does this sound plausable and whats the best way to seal hull inside and out under waterline.Thanks

[Cuyahoga Chuck]

Hi there new to the forum hope everyone is well.I will soon be embarking on my first boat build I plan to build a 25 foot motor boat,9 foot beam.I am thinking the construction method might be a double skin of 12mm marine ply on pressure treated red deal softwood.Does this sound plausable and whats the best way to seal hull inside and out under waterline.Thanks

You will do better by starting your own thread with a suitable title to tickle the sensabilities of the crew here.

[Venchka]

I am of the opinion that virutally all manufactured goods have deteriorated over time to the point that most everything made today is junk. That said, I reckon real marine plywood is the best junk available today. Personal opinion only. YMMV.

In a constructive mood now..........

You could build your wee fishing boat from the finest materials possible. Leave it right side up and uncovered and let fresh water sit in it and it would turn to mush in a year or less. Best to store your wee fishing boat upside down on horses AND under a roof so the wood gets lots of fresh air ventilation. Or on a small boat dolly under cover with lots of ventilation. Dry boats are happy boats.
You didn't mention the design you are contemplating. John Welsford's Sherpa would make a nice multi-purpose boat in the length you mentioned.

[Wayne Jeffers]

The first boat I built, almost 20 years ago, was a Ken Swan “Little Gem.” I built her out of AC exterior fir plywood, mostly because that was what was easily available locally. My purposes in building her were to get a small boat for fishing that was at least a little more handsome than the typical aluminum skiff, and as an experiment to see if boatbuilding was agreeable to me. I had no expectation of the boat lasting more than a few years.

I’ve kept her under cover (silver polytarp, mostly) when not in use and she is still fine. If I had known that she would last so long, I might have used better materials.

BTW, if you soak fir plywood with raw linseed oil before painting, it will not check. (Note: Epoxy will not adhere to wood treated with linseed oil. Glue everything up first.) Yellow pine is not nearly so bad about checking as fir.

Wayne

[Venchka]

If only we could find stacks of 20 year old A/C exterior fir plywood. sigh.
Thanks for the tip on soaking fir plywood in linseed oil. Good to know.

[Wayne Jeffers]

Even better would be to find new 40-year-old stock of AC fir. ;)

But that's another story.

Wayne

[Venchka]

You didn't? You did, didn't you? Sigh.

[Wayne Jeffers]

You didn't? You did, didn't you? Sigh.

No, I didn't. That was just my comment on the quality of lumberyard plywood even 20 years ago. The last of the really good stuff, from old-growth trees, was probably sold in the 1960's or earlier.

But 20 years ago you could at least still get 5-ply half-inch fir plywood. I haven't seen any of that in almost 20 years.

Wayne