To scarf or not to scarf

A vacuum table is very useful and very easy to make. If I remember well there are the drawings of one in the Epoxyworks of West System. A little trick, use MDO as it's not porous at all with a very hard phenolic surface, and make at the perimeter with a router a 1/2 inch wide by a 1/4 deep line. Fill it with a liquid rubber like Thokiol or a very cheap polyurethane compound for filling the cracks in concrete like the Pennsylvania. It must remain slightly tacky, so the high quality polyus like the 3200 are just not good. Sand it to remain proud only of a 1/500 inch. It's too small to affect the precision of the scarf but that helps a lot the vacuum pump as you have a seal.

Car air conditioning compressors make good vacuum pumps for this kind of job (for more serious work a true vacuum pump is better, and with Ebay you'll find at any price) Venturi systems look cheap but have no good efficiency (about only 25-30%) and end to kill the air compressor.

In my shipyard as the fishing boats were made with 2 to 3 plies of marine plywood all scarfed, we used a modified Makita 8 inch with ball and roller bearings each side of the blade, the Makita was sliding on tracks and had a air cooling of the blade. Very precise and very fast, a 8 feet long scarf in a 9mm plywood was made in less that 3 minutes. With a good protection the system is very secure. In France you don't try to play with security in a shop, the fines are dissuasive...and you may go in jail.

For the other scarfs made "flying" we used another 7 inches Makita with a scarffer always modified to have ball bearings each side of the blade. If you're interested by this easy modification I'll make a rough drawing.

The saw must have a fast engine, and I've found that it's better to use cheap 40 carbure teeth blades and change them often. The glue lines in marine plywoods are killers. I think that the routers are dangerous and too slow.

Here I use the 7 1/4 Irving blade made in NZ sold at 125 pesos, about 12 bucks. It cuts also the fiberglass very well...

 

The profiles of the floor boards do not have to cope with high stresses nor bending issues. The lone good way is to scarf 6/1 is a mini with epoxy if you won't bend the ply. 8/1 is the good ratio with epoxy; if highly stressed the plywood will break rather than the scarf itself. That means that all the stresses are handled through the scarf until the ultimate strength of the wood.

Another advantage of scarfs is that do not develop cracks in the joint as the other systems, because there no concentration of stresses or hard points, or solutions of continuity. And do not hope that a 10 oz cloth will save the situation if your structure works at the maximum of its safe possibilities.

 

Does the vacuum have to be that good? I was planning to use my shop vac; 1 psi is 144 bl/sq ft which is a lot of force over a 4 ft wide ply sheet.

 

No the vacuum has not to be too high, just enough to hold the panel and to keep it FLAT, not so easy with some cheap plywoods...it's why I use the trick of the slightly tacky polyurethane compound; with a very moderate vacuum the panel can't slide even on a MDO.

For short vacuum uses you can use a shop vac but only if you add a bleeder( a controlled leak) that limit the vacuum and permits to some fresh air to cool the shop vac. These animals suck an enormous volume of air, begin to scream over-revving and die prematurely by overheating or explosion of the ball bearings. Also the noise is horrible.

For short uses you have venturis you can install on the air compressor for about 50 bucks, so the use of a good vac shop, with the risk to kill it, it's no worth the saving. It's finally expensive being too greedy....

For less than 150 bucks in Ebay you'll get a small vacuum pump that will last forever and silent. It's not infusion, but just holding plywood for cutting or gluing. A complete kit cost about 400 bucks and has many uses as gluing plywood, making sandwich or honeycomb panels, veneering etc...Epoxy glue needs very little pressure.

The use of good meranti BS 1088 plywood is finally cheaper than a crappy fir plywood. It's flat with no voids in inner plies, or knots, holes or splits on the faces. Well smooth sanded so finishing it is a pleasure, nothing to fill. It can be cut, routed, scarffed and machined without chipping. And bends smoothly...

More, the consumption of resin is lower than on fir, it doesn't check so fiberglass covering is not needed in most applications, and it's stronger so a 1/2 replaces a 5/8 AC Fir.

Make the calculations in materials and work...you'll be surprised to find that a AC fir ends to be more expensive that a BS 1088 Meranti.

 

I had planned to run a large vac line to the vac table with a smaller hose to a dust chute on the saw which would proved the bypass as you suggest. Even my cheap little wet-and-dry vac will pull at least 2 psi, I used it to drain an underground water pipe last month; sucked it dry in seconds.

Agreed, in spades! And a thing of beauty is a joy for a damn long time if not forever.

 

The best is to try as it's not dangerous nor expensive. I'm afraid that will be impossible to get vacuum and to suck the dust in same time with the same vac cleaner...It's curious but I've used very good fir plywood 30 years ago, what happened that all these plies have disappeared?

 

Any plywood from a respectable manufacturer is done to a specification of some kind. The questions is which. A way to find out is to ask, which I haven't done yet but intend to do before I use the plywood for any serious boat building.

I've found that construction fir plywood is crappy. "Furniture grade plywood" with pine at the surface and fir in the middle seems to behave very well on the other hand, and is available in several shops in the neighbourhood. All fir plywoods are not equal and BS 1088 possibly isn't the only spec for a usable plywood. "Furniture grade plywood" is much more expensive than construction plywood for the same dimension, but for the same strength I think the cost is almost the same. I haven't compared prices with BS 1088.

Erik

 

Construction fir plywood is crappy but fairly weatherproof, and it's about the only softwaood ply you can get from most lumber suppliers. We can get Baltic Birch ply here, lovely stuff but heavy; it's so good I planed the edge at a fine angle to expose the veneers as a decorative feature on one boat. It comes in 5 x 5 sheets insted of the usual 4 x 8 which is handy sometimes.

BS1088 isn't all it's cracked up to be at times I have sometimes found, but it's usually good. There's also BS6566, a bit cheaper with thinner outer veneers, and BS1088 Lloyds register approved, which is a bit over the top for most of us.

 

Kayaker's got a good point about not all plywood's, that may be rated the same, being up to snuff
you have to really watch what you buy
the mill's around here received huge fines for "mislabeling" there products as exterior when they had used interior glues and for not compressing the product during the manufacturing process sufficiently to meet industry standards
they accepted the fines and went on manufacturing substandard products

they were later found guilty of conspiring to sell fraudulently advertised products
http://papers.ssrn.com/sol3/Delivery.cfm/UVA-BP-0375.pdf?abstractid=907802&mirid=1

they were also found to be fixing prices
http://stage.homechannelnews.com/story.aspx?id=67661&menuid=305

and sued in a class action
http://www.bingham.com/PracticeMatter.aspx?PracticeID=223

I wont launch into a monologue on the use of ply in marine construction
Ild be frog marched outa here in a heartbeat
but I will second our Mr Kayaker with a song and dance you might remember

happy building
and clear sailing
B

 

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.

 

A comment about "furnitue plywood". The label put on the plywood at the store doesn't say anything about the glue used or the quality of the inner plys. I think the label focus on the cosmetic qualities. All "furniture grade" plywoods I've seen in my part of Stockholm have thin plys and are specified to have Water Boil Proof glue. This I have verifyed by boiling and splitting test samples of the plywood I used.

Thank you Ilan for tests you suggest. Plywood for boat building is a very small market and it is not available at many places. Testing the plywood that is locally available seems better than to order plywood according to a British Standard and hope that what I get meets the spec.

Erik

 

The Mosquito plane was a remarkable creation with an amazing performance for its time. It was a bit of a problem when used in the tropics, I have heard, due to local fungi and insects which found it delicious!

 

Erik, you're wellcome. You're in Sweden, a country of high standards, and making some of the finest plywoods of the world,it won't be very difficult to get marine equivalent plywood. I've worked in a shipyard of Malmo in the seventies, and the plywood for concrete molds was good enough for long lasting fishing boats... You're a lucky man.

The plywood I find in Mexico is not worth to start a fire...Everything has to be imported at great expense after a fight with the Customs, and its corruption. French also are lucky, as the norms CTB are high and there are very good plywood makers, among the best of the world ( Joubert and Toubois are well known).

The Mosquitos were glued with casein, a protein from milk. You can imagine the feast for fungii and insects with a hamburger Royal of untreated birch plywood, balsa and cheese tasting glue, topped with oil paint...miam, miam...the king food for wood destroyers. But it remains that this plane made in war time by furniture shops and piano makers, with Canadian plywood of veneers hand ironed by women, was able of 755 km/h and was one of the safest and strongest planes of WWII. I knew one english naval engineer, Mosquito pilot, who told me they came back once from Germany with 30 shots of 20mm cannon in the tail, and a few dozens of 1/2" machine gun holes in the fuselage and the wings... good fail safe structure demonstration.

Now with epoxy glues and polyurethane compounds, plus treatments: no match...you'll get better quality, and best; within the skills of a home boat builder.

 

Many of the Mozzies were stripped to cut weight and sent over on reconnaisance flights. They were too fast to catch, at least until the Reich had jets. Lots of tall tales about the good old Mozzie.

 

I was actually talking about exterior grades used in the housing industry
they are worth about kindling when you get down to it
and that is about the extent of my knowhow on ply
as I said we always used some variation of plank on frame
dam its been a long time
the Mitsubishi Zero was also originally plywood and its still a hell of a plane
it was later built with metal skins but I think it remained wood framed through out the war
I could be dead wrong about that though
haven't been to the Smithsonian in ages
anyway that blurb about testing ply was perfect and I saved it so thanks
oh
and the ply that I was specifically relating to wasnt a marine grade ply at all but some oriented strand board I swore off when I first saw the stuff
so I got a bang out of there being sued over it being so bad
thank the Gods I didn't build anything out of it
regards
B