To scarf or not to scarf

Guest62110524 · Dec 17, 2008

Ilan Voyager said: ↑

Thanks Boston for the very interesting infos you have given to us. A bit frightening....

About furniture plywood, no it can't be used in boat simply because of the glue; formol urea which is clear, and do not resist permanent humidity.

The BS 6566 has its place inside the hull, and in small boats where the stresses are low compared to the thickness. But in "big" boat it's better to use the BS 1088 or the Finlandese or Russian equivalent because the BS 6566 admits small voids in the inner plies, and the outer plies lack of thickness.

The BS (British Standard) is the best known in the USA, but there are a lot of national norms, civilian and military. The Russian norms are largely equivalent to the BS1088, and I'm happy of their commercial success with the baltic ply because they have worked very hard to get a very good product.

I bought many times pine and birch boat building ply to the Russians and I've never had the smallest problem even of the middle of the turmoil of the Gorbatcheff era. The quality is astounding. The ply imported in the States in in 5*5 because this size is handful in furniture and plywoods easier to fabricate so the price is kept fair. But you can get 12*6 (3.10m*1.53m) and 8*4 sheets.

Birch is heavy but very, very strong. If you calculate the structure in function of the characteristics of the birch plywood and better make a monocoque structure of 2 or 3 layers of plywood, vacuum epoxy glued, and all scarffed plus fiberglass you get a resilient structure that only can be beaten by high cost composites... but at 1/2 to 1/3 of the price.

To give you an idea of the strength the bottom of a fast pro fishing 40 feet boat, 25 knots @ heavy displacement (9 metric tons), Det Norske Veritas standards with a big safety margin is 1 1/2 inch (18mm) made of 3 plies of 6mm plus 2*6oz glass outside and 1*6oz inside. The weight@ sq foot is about one half of the same strength structure in monolithic fiberglass and far more rigid!
The oldest boat is 21 years old now, fishing every day, and is as pristine as the day of the launching. The boat is good to work 20 years more.

The method is very interesting because it doesn't need high tech tooling nor expensive materials at 100 US$ a pound.

Make a search in the Web about the Mosquito bomber and fighter plane, made by simple furniture shops during the WWII in Balsa and Birch plywood and you'll understand the possibilities of wood structures. With epoxies these structures are at the hand of the common boatbuilder.

Ok, carbone and kevlar, Nomex, infusion, post cure oven permit to get lighter boats but the technological level and the price are not the same...

To conclude you can make a crude test of a plywood to estimate its quality.
1-Buy a sheet and count the number of inner plies 7 for an 1/2" and quality and thickness 1/20" mini of the outside plies (knots, discoloration, splits etc...)
1-line glues must be dark brown (phenol)
2-cut the plywood in squares of 4*4 inches and look for voids and other defects.
3-with a chisel try to separate the plies: the glue lines must be stronger than the wood itself.
4-take ten squares and boil them 1 hour. Delamination while boiling is forbidden.
5-Destroy one boiled square with a chisel trying to separate the plies: the glue lines must be stronger than the wood itself at least at 80%.
6-take 2 squares and make it dry totally in a oven at 120 celsius degrees. Delamination while drying is forbidden.
7-Destroy one dried square with a chisel trying to separate the plies: the glue lines must be stronger than the wood itself at least at 70%.
8-meantime reboil one hour more the remaining 7 squares.
9-Destroy one re-boiled square with a chisel trying to separate the plies: the glue lines must be stronger than the wood itself at least at 70%.
10-take 2 re-boiled squares and make it dry totally in a oven at 120 celsius degrees. Delamination while drying is forbidden.
11-Destroy one re-boiled and dried square with a chisel trying to separate the plies: the glue lines must be stronger than the wood itself at least at 60 to 70%.
12-Clean the kitchen and buy flowers for you wife. Don't forget to open the windows because of the smell.

You bought this 1/2 inch at Home Depot for 29.99 bucks? Buy all the stock, you've got marine plywood for the price of crap.That happens, a friend of mine bought 120 sheets found in a Castorama (Home Depot similar) in Brittany France...mistake of a provider? The miraculous of miracles is that happens sometimes...

Seems stupid but I have done that on every new brand of plywood, even from a renowned provider. Better to destroy one sheet than to have a failure on a full size boat. There are also simple tests of bending and resistance.

Next time I can give you simple tests for evaluating the compatibility of the plywood with the epoxy, and the quality of the epoxy itself.
Click to expand...

Brilliant thanks

Guest62110524 · Jun 21, 2010

thought this may be a good way for long joints, 2.4 m long. the saw is pulled along layed over a bit and dragged

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To scarf or not to scarf

Guest62110524 Previous Member

Guest62110524 Previous Member

¼” x ¾” strip plank scarf joint spacing?

Scarfing Techniques