Thread: Marine Ply vs Standard Ply test

Originally Posted by b.o.a.t.

It would be interesting to see those tests repeated with a square of ply
suspended only at the 4 corners, and the load applied to a small diameter circle
in the middle. I expect the panels would break in their 'weak' direction rather earlier... J

Hi b.o.a.t.,

i was searching the web and came across a test that is somewhat similar to what you decribe in your quote. Its called the Janka Hardness test and it is an ASTM standard. A small diameter (11.28 mm) steel ball is pushed into the wood half the ball's diameter to produce a 100mm2 circle.

Oddly enough Okoume is one of the weakest woods (1.7 KN). This was surprising to me, since it is one of the most desirable woods to use in boat building.

The meranti (Shorea spp. laun -meranti group) faired much better. Depending upon color, 2 KN to 5.1 KN. Meranti from the baulau group had a Janka rating of 7.9 KN.

You can find the results of these and other species at the following site:

What is the janka hardness rating for wood?

Concerning the videos i posted, i am not sure what specs we would need to know. The standard ply was of poor quality - it had many voids and the layers were not of the same thickness. The marine grade ply was of exceptionally good quality - no voids were noted and the layers were of uniform thickness. I suppose knowing the type of wood would be helpful, but would the manufacturer waste good quality wood and a make poor product or use poor quality wood and spend time and labor to make it flawless?

But i agree with Mik that drawing conclusions from a single data point is not wise.

What i found interesting is how the plys failed. As PAR has experienced, the standard ply failed explosively, all five layers failed. One or two layers (cant tell for sure from vids) of the marine ply held together. A difference between sinking and making it back to shore.

Originally Posted by PAR

The AA grade your local supplier has suggested, says absolutely nothing about the construction of the sheet, just that it should have two, perfect, sanded faces, free from defects (that's what the AA grade means).

Sorry forgot to mention it, he did say that the veneers where of equal thickness and since it is marine ply, we also know that it meets a certain standard of construction.

Originally Posted by PAR

This said, you surely can us a construction grade on a canoe or kayak with good success, mostly because these boats are lightly loaded. I know many that use "underlayment" panels for just these types of builds.

And this is my intention, to build a quick canoe, though i have already been convinced to use something better than standard ply.

Originally Posted by PAR

The price they've given you for that AA sheet is high, so maybe you need to shop around.

Of the 4 places i had checked, this was the only one who had marine grade ply. I spent some time today and found another lumber yard that has okoume, and two types of meranti, aquatech and hydrotech. They do not list prices on their website. I plan on calling them on Monday.

Originally Posted by PAR

Also, Douglas fir in an AA sheet is a waste of time, you can't finish it. As soon as you touch it with a sander, it turns into a washboard and you'll never smooth it out without gallons of filler.

I dont understand this. If as you say it has "two, perfect, sanded faces, free from defects..." why would i need to sand them?

Originally Posted by PAR

What's all this stuff mean? Well lets say you have a canoe design and it calls for Okoume panels and weighs 30 pounds. Well if you build it exactly the same way, but use Meranti, it'll be a 37.5 pound canoe. If it's Douglas fir slightly more and if you are toting this canoe around the woods, looking for a place with thick enough water to splash it in, you'll pretty soon wish you'd used Okoume.

Within 1.5 hrs, if i remember correctly, there are between 12 and 15 lakes and harbors all with very easy access. So weight is not a concern.

I picked the quick canoe for 2 reasons.

1. I figured this would be an easy way to introduce myself to boat construction.

2. if i didnt like it, i wouldnt worry about all the money i wasted (though i am sure i will like it).

But i dont want to go too cheap. I have a strong desire to see something i built with my own hands endure, especially the very first one.

Originally Posted by blue sailor

Hi b.o.a.t.,

Oddly enough Okoume is one of the weakest woods (1.7 KN). This was surprising to me, since it is one of the most desirable woods to use in boat building.

The meranti (Shorea spp. laun -meranti group) faired much better. Depending upon color, 2 KN to 5.1 KN. Meranti from the baulau group had a Janka rating of 7.9 KN.

Hardness will be different from strength and different to stiffness. Most tests are testing different things. There will be some relationship between this test and strength.

Generally the most difficult thing to achieve in boatbuilding is stiffness. This is why gaboon is nice.

Stiffnesses of all but exotic materials are almost identical per unit weight.

Aluminium is six times stiffer and six times the weight of average boat building timbers. Steel is 16 times heavier than average boat building timbers so is 16 times stiffer.

In most boats you have a weight target. So if you use 6mm ply then the aluminium will be only 1mm thick. Clearly not stiff enough unless you add framing or form the skin into ridges.

So lets take a really heavy wood ... double the material stiffness of gaboon.

But lets see what happens if you want to make it the same weight ... obviously half the thickness.

So what stiffness do we lose. The formula for the stiffness of the piece/section (nothing to do with the material) is the cube of the thickness. Classical beam theory - the width times the cube of the thickness. Width is the same for both pieces of timber because they cover the same boat!

So 6mm gaboon has a section stiffness of 6 cubed = 216

The 3mm thick heavier timber has a section stiffness of 3 cubed = 27
but we need to correct for the double material stiffness.
Material stiffness x sectional stiffness = 27 x 2 = 56.

So the lighter, thicker gaboon is 216/56 or 3.8 times more stiff than the thinner heavier timber.

So there you go.

Generally within constraints of surface strength (the ball test) it is best to build the hull of the boat out of the lightest material possible as thick as possible. You can also increase surface strength when building bigger boats out of light materials by fibreglassing the surface. But for the loads that a smaller boat might suffer from .. maybe painted wood or painted epoxied wood will resist dents well enough.

You can't do the same with a mast ... as it would make the mast a much larger diameter. But for the structure of a hull it makes sense as the skin will always be thin compared to the dimensions of the hull so alternatives can be quite thick for the primary skin compared to some of the others.

The advantages of a stiffer skin like this is the principle of modern boatbuilding. instead of having all the ribs, stringers, floors, bulkheads, frames and deckbeams of a traditional boat, the much stiffer skin needs much less support. So often a few seats or a galley or settees are enough to stiffen up the structure.

Do you don't have to build the frame then build the skin then build the furniture. So the building process is much simplified.

The other thang ... is the above makes it clear why Aluminium, fibreglass or steel don't work so well for smaller boats but start to make sense with bigger ones. A fibreglass or aluminium canoe will be well above the 40 - 45lbs of a canoe built in gaboon.

Hope this helps explain why this new data is not so relevant.

Again ... a perfect question and observation.

Michael

Originally Posted by blue sailor

. . . he did say that the veneers where of equal thickness and since it is marine ply, we also know that it meets a certain standard of construction . . .

If it's an APA sheet there is no guarantee of construction standards. The APA standards are voluntary and often not strictly adhered to. Most of us look for the Lloyd's stamp or prefer getting known products, such as those previously mentioned.

Originally Posted by blue sailor

. . . I dont understand this. If as you say it has "two, perfect, sanded faces, free from defects..." why would i need to sand them? . . .

On Douglass fir, the winter and summer growth rings differ dramatically in density. Meaning if you sand (you will), it will create a washboard effect. An AA sheet on any other species does mean nice, neat, perfectly sanded sheets, but not Douglas fir. It's the nature of the species. You'll sand to insure "tooth" for coatings, shaping, trimming, finishes, etc. In short, Douglas fir is reserved for none visible applications, where it might be covered or other wise out of site.

Originally Posted by blue sailor

. . . Within 1.5 hrs, if i remember correctly, there are between 12 and 15 lakes and harbors all with very easy access. So weight is not a concern . . .

Weight is always a concern in small craft, particularly if human propelled.

Originally Posted by blue sailor

. . . I have a strong desire to see something i built with my own hands endure, especially the very first one . . .

A refreshing desire from a first timer. Welcome aboard BTW, have fun and post some progress photos as you can. The choice of good materials is usually the best one. It can help insure durability, certainly convey quality and improves resale value too. A side benefit of good materials is, they are often easier to work and make finishing chores more rewarding. In other words, it's easier to get good looking results from good materials then less. Douglas fit plywood is a classic example of this. Yep, it's cheap, but you'll spend hours with filler and a DA just to smooth it out, whereas a different species of plywood don't have this issue, so you look good without having done much. After you have some experience with good quality plywood, run down to the local Lowe's/Depot and see if you can find an exterior grade sheet that comes even remotely close.

Further to PAR's comment about weight being an issue with human powered boats...
Think of it like this.
A boat displaces water.
As it moves through the water, it moves an amount of water equal to its
total weight down & to the sides, then back again as the boat passes on.

So if the all-up weight of the boat and load is 250#, by dint of muscle power,
for every boat length travelled, 250# of water is moved maybe 6" to the side,
and back again.

If heavier materials are used & it weighs an extra 10#, that's an extra 10#
that has to be moved aside & back again, for every boat length travelled,
for ever and ever, amen. There are 350 x 15' boat lengths in a mile. So with
that extra 10#, for every mile travelled, an extra 3500# of water is moved
aside, then back.

At this point, feel free to call me lazy !!
All the best with the build. Look forward to the photos.
Alan J

The same is true of all low power applications. The biggest factor in sailboat speeds increasing so dramatically in recent times, is weight or the lack of it compared to previous, similarly shaped craft.

With all this discussion on strength and hardness, of plywood and durability etc, I built a JARCAT 5 , back in 1984, the boat is still going and was, until recently used frequently, until the owner passed away earlier this year.

This boat was built with MARINE PLY LLOYDS 1088 Specification, my next build used 10mm exterior ply with a layer of 200grm glass, built in 1994, and is used extensively in open water on the Sunshine Coast north of Brisbane, the new owner has not to my knowledge had any issues with the PLY.

I have just built my own design , a MANGROVEJACK 3.65MT drawn up by BOWDIDGE MARINE , using PACIFIC MAPLE PLYWOOD, I did have some issues with this PLY <
1. splintering when cut.
2 very stiff when bending panels.
3 grain variations.

the boat finished well and is solid , and durable, and behaving .

I am now finishing off, a power Canoe and I am using Aero-lite ply, which is made of Balsa , although 5 ply in construction, the outer plys' are thin, the ply cut reasonably well with minor splintering, and it took the bending well the design is stitch & glue, and I had no tear-outs with the wire.

PLYWOOD differs not only from manufacturers but also in their batches, so it will always come down to buyer be ware, and check each sheet as you laod it into the trailer.

my 2 cents worth.

Jeff

Originally Posted by PAR

The price they've given you for that AA sheet is high, so maybe you need to shop around. Also, Douglas fir in an AA sheet is a waste of time, you can't finish it. As soon as you touch it with a sander, it turns into a washboard and you'll never smooth it out without gallons of filler. A good quality 1/4" meranti BS-6566 sheet (typically called Aquatech) will run about $40. It's faces will be perfect or nearly so and it can take a finish without going insane. A better grade would be what's commonly called Hydrotech and it's also a meranti sheet, but to the BS-1088 standard, which is the best. This sheet should be about $50 and it's the usual recommendation for hull panels, unless light weight is desired (isn't it always). If you need a light, strong, quality panel, then you want Okoume sheets. These are often from Joubert Marine and are very good (what I usually buy). They'll runabout $80 a sheet for 1/4". There's also some Lloyd's stamped (the BS-1088 stuff) stock I've been getting from Italy that's also very good and about the same price.

I got a hold of the lumber yard and they sent me their price list. the following prices are for 6 mm ply with 5 veneers:

Okoume - 77.95
Hydrotech - 44.95
Aquatech - 32.95

I will go with the hydrotech, since the aquatech, according to the lumber yard, allows the use of putty "in the face and back to maintain a sound panel". Plus it is still $9 cheaper then what i was willing to pay for the douglas fir. I'll think about using the okoume when i build the eureka.

Thanks PAR for encouraging me to shop around and giving me an idea on how much i should expect to pay for the plywood.

.

Originally Posted by b.o.a.t.

If heavier materials are used & it weighs an extra 10#, that's an extra 10#
that has to be moved aside & back again, for every boat length travelled,
for ever and ever, amen. There are 350 x 15' boat lengths in a mile. So with
that extra 10#, for every mile travelled, an extra 3500# of water is moved
aside, then back.

Wow, good thing i'm beast (not).

edit: by the way, i was only thinking about moving the boat from the truck to the water. It didnt cross my mind the impact that aditional weight would have on paddling (not to smart, i know).

Originally Posted by b.o.a.t.

At this point, feel free to call me lazy !!

Since we are the same in this regard, i shall call you efficient.

I am not a boat builder, but nevertheless this thread is fascinating. Thank you.

Regards
Paul

Originally Posted by Bushmiller

I am not a boat builder, but nevertheless this thread is fascinating. Thank you.

Regards
Paul

fortunately I am a man of great humility, thus only allowing myself to take 99% credit.

The filler used on the Aquateck isn't an issue and usually is just in some checks on the faces. A fairly small percentage of the sheets will actually have this and if encapsulated, and particularly if sheathed, a non-issue. It's a 17% difference in price per sheet too.

Always shop around and let the folks you're talking with about supplies, that you are. This forces them to offer their best price.

Originally Posted by blue sailor

fortunately I am a man of great humility, thus only allowing myself to take 99% credit.

Humility duly noted. Percentages, unfortunately, have gone the way of inflation and while 100 used to be the maximum there is an increasing incidence of significantly higher figures which diminish the once revered 100%.

Nevertheless, thank you for raising the subject together with videos. Also, welcome to the forums with an excellent first contribution. A sense of humour too. You may find you need it.

In Australia we have quite a range of plys available and I am grateful to the contributors to the thread for explaining some of the differences and their advantages. I have to say I'm impressed by the quality of the posts too.

Regards
Paul

Originally Posted by PAR

The filler used on the Aquateck isn't an issue and usually is just in some checks on the faces. A fairly small percentage of the sheets will actually have this and if encapsulated, and particularly if sheathed, a non-issue. It's a 17% difference in price per sheet too.

Always shop around and let the folks you're talking with about supplies, that you are. This forces them to offer their best price.

I am not sure what you mean by "encapsulated, and particularly if sheathed". Are you referring to epoxy and fiberglass or something that is done by the manufacturer? I was concerned that the epoxy would not bond to the putty and since it is not to BS 1088, there would be core voids that would impact the integrity of the canoe.

But if this is acceptable, i would love to save the additional 17%.