marine plywood

[bulldogdaddy]

been looking at a few things on this. now a supplier here say's he only carries fir. i've seen a few websites that has this listed as the 2nd strongest, with kerring as the strongest. at $92 a sheet. has anyone used this for their transom?

[WATERWINGS]

I was told by Lowes, that fur? fir? has the same water proof glue in it as marine, an is cheaper.

He sold me a sheet, but it was not for a boat.

[bulldogdaddy]

gonna call lowes and see what they say. thanks

[Reese]

boats for a living I'd use marine ply.

Why? Two main reasons...marine ply is usually kiln dried and is very dimensionally consistant from sheet to sheet.

But I don't build boats for a living...I'm only building one boat...and I use douglas fir ACX.

Look at woods (plys) bending strength (Modulus of elasticity) and you'll notice it's about the same...if it makes you feel more comfortable using marine ply that's fine...but it not mandatory.

[bulldogdaddy]

found this on a websitehttp://www.resourcesintl-wa.com/properties.html

[bulldogdaddy]

why can't i get it to redirect?

[pyro]

need a space between "website" and "http"

[Reese]

Modulus of elasticity is 2350 vs. 1950 per square inch...not even worth the math...because a very small increase in size makes them both perform the same.

Also notice that okume is very weak (relatively speaking) but is one of the most widely used woods for boats. One more thing to keep in mind...wood is not like aluminum or steel, each finished sheet is different because different trees were cut to make them. You can make general statements about a particular species of wood but each final piece will have different properties.

BTW...the biggest single factor of strength is how little humidity in in the wood...regardless of whether it's marine or not if it gets wet is looses a lot of strength.


Plywood strength... (http://www.resourcesintl-wa.com/properties.html)

[Gordie Miller]

All wood will turn to **** if it stays wet for any extended period. You have to seal it completely no matter what wood you use. The stuff from Harbor gives you the best possible chance of success.

[Techno]

My short answer is not to use crap plywood for the transom. This is a highly loaded part and weight sensitive in my opinion. The more unneeded weight the larger the performace loss.

A few things to consider when comparing plywood.
Plywood is made up of layers of slice or sawn laminations- right?Wrong!
The cheaper the plywood the more bark filler is used and sometime there are actual gaps in them. The plies aren't continuous. Look at any construction type plywood and the surface will have unfilled holes where the knots where cut out. Reduce the strength accordingly.
Plywoods grade system is based on these knots. I've pretty much forgotten but A is a nice face, B is nice face but with filled patches C is unfilled patches. Forgot D. CDX is the 2 faces like any graded plywood is telling. C might be unmatched patches and D unfilled- forgot.

Marine plywood, furniture grade plywood and the ultimate if you can find it, baltic birch plywood( too small for transoms). have no voids & no bark filler . Baltic birch has many more laminations than the other types and is much stronger. I think it's 15 compared to the normal 5.

The other plywoods have a lot of bark filler and gaps, look at the edges and you can see this. The strength is decreased by every void and by any bark thats added, bark has no strength. Also by the unfilled patch holes. Reduction in thickness.

As a comparision of the difference in strength. If you used a 1/2" thick furniture grade plywood( similar to marine) not only do you get the pricey wood facing but this wood can span a greater distance like as used in a bookshelf. To span the same with construction plywood you need closer supports or thicker wood. Thicker wood is heavier. More expensive. 3/4" instead of 1/2" or 3/8"s

The plies are cut in 2 ways- A log is steamed to soften and is peeled like a potato. Rotary peeled. You can see this on most plywood in the grain patterns wierd expanded look. The other is sawn or sliced. It's more expensive to do it but is visibly more appealing and stronger. This is why it's placed on the faces of furniture grade plywood. this looks like bookmatched veneer. Like boards placed side by side.

The strengths of woods listed isn't always the best guide to go by in things like boats. Sitka spruce is used in boat constuction because of price/weight/ strength. Even if price isn't considered the weight to strength is more favourable than most others.
Most furniture grade woods are weather resistant. Never use maple for weather resistance, it isn't.

A simple strength test is place a sheet or strip on 2 supports that are the distance that will be used. Stand on it and check how much it deflects. Is it acceptable? This is a greater deflection than when actually used since the ends aren't fixed.

There is also 4'x8' plywood and 8'x4' plywood. The major strength direction is changed to the 4' direction. So if you used 8'x4' plywood in a boat floor it would be stronger than the other since you have continuous support in the 8' direction from the boats stringers. You could also cut the normal plywood and turn it 90* to mimic the width strength rather than the length strength.
I think thats kind of confusing but regular plywood is strongest along it's length which you don't need in a boat as much as the width strength.
Think floor joists and how the plywood is used for a floor. For transom it's good though.

The comparision between Keruing and fir is worth the math.
Keruing is 12% stronger
Fir is 27% lighter. Increase the fir by 12% and it's still lighter. 1" to 1,1/4" is a 25% increase. But I thinks would be the same weight as the Keruing but even stronger due to increased thickness and plies.
Okume is 52% strong but is 56% the wieght. So the ultimate strength numbers can change with wieght being constant.

[bulldogdaddy]

gordie ,you have a # handy for harbor sales? and techno,love the technical advice,but please put it in a laymans term for me,lol. you think the fir would be ok with 2 layers of 1/2in across the transom,with 2 more layers in between the kneebraces,or 3/4in?

[Techno]

That was laymans terms.

I would just replace with the thickness that was there, assuming marine. If anyone was going to use something else then definately go thicker. I have no idea how much since who knows where the bark is or where the gaps are.
The thickness depends on the boat I guess.
I doubt your going to gain a thing besides poundage by increasing thickness between the knees. If you want to go thicker do it with out the gaps created by the knees. In other words the knees get shorter, and butt up against the new thickness of transom. The whole transom gets thicker without the knees cut into it.

[lyle]

sandy luon or veroila pywood are very good for what there made for cabinets. marine ply has a diffrent glue and drying prosses use marine ply will last longer

[typhoon]

As Techno said, Marine ply uses selected veneers and the whole process is carefuly quality controlled.
I have had a 3'by 2' piece of 12mm 5 ply marine ply on my roof here under a downpipe(used to spread the flow from the downpipe over a wider area) for over six years now. It gets full sun 6+ hours a day, and lots of water. It is still strong. he face plies have split a bit, wouldn't look great painted, but solid.
You are paying extra for a far superior adhesive, and a process that guarantees you get plywood that is solid and bonded consistently over the entire sheet.
The amount of ply you buy to repair a boat is not much, and marine ply is a tiny amount extra to pay!

Andrew.

[bulldogdaddy]

thanks for the #,they have the best prices by far,even if it's a litlle bit of a drive. beat the local guy by 40 bucs a sheet

[Gordie Miller]

just be careful of the fire hazard, bein you is haulin wood. Seriously, how far is it?

[bulldogdaddy]

it's over the bay bridge,towards chestertown. got a bro in law over there anyway. he'd be more than happy to help load a few beers and drink a few sheets,lol. take me about 1 hr to get there.

[Gordie Miller]

around here, we alll pool our orders and have it sent by truck. Usually keeps freight to around $50.00 each. It would be nice to be able to go get one sheet if I needed it. Glad I could help. BTW, nice avitar.

[bulldogdaddy]

is that where you get your wood from? and if so do you use the fit,okumfe,or mahogany? and btw a tech question for ya seeing your a marine mech. how much more hp is gained with a 225 bored out 30 over?

[Gordie Miller]

mahogany to repair my race boats. As far as the overbore, if your thinking of doing it just to gain HP, DON'T, if you go .030 over now then have a problem, you may run out of sleeve and have to junk yhe motor. The HP gain from .030 overbore is negligable compared to the costs/risks involved. If you are racing and need that last 1/10 in the 1/4 or to the corner maybe, but for lake running, no way. That's my opinion. Others may disagree.

[bulldogdaddy]

nah,bob's motor was bored out due to no water temp or pressure gauge,toasted a piston. not looking to gain a 1/10 in a pig like my ride:D