To scarf or not to scarf

Discussion in 'Boat Design' started by lazerus, Nov 26, 2008.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    Speaking of the Mosquito and Mitsubishi Zero, wasn't there a mostly wood North Korean plane that caused the US pilots a bit of grief, partly because it was hard to spot on radar.
     
  2. Ilan Voyager
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    Ilan Voyager Senior Member

    Boston, you're welcome. I'm happy that the Testing Plywood Ole Smelly Recipe was useful for someone.

    One of the purpose of a forum is to share experience, so I think that I'll start a thread about plywood, wood and resins, giving the information on how to test them for marine use, and I hope that some will add their salt grains.

    There are a lot of myths about polyester and epoxy, and I see now companies marketing "cheap" epoxy resins claimed as of marine use (and there are some not truly naval) and others selling at too high price rather common resins, because of the brand written on the cans...

    I did not know about the North Korean plane, maybe its was a copy of the Yak 2, made in plywood by the Russians in WWII. The Mosquito, because of its qualities, partly due to the wood construction, has been used in a surprising variety of difficult missions and it's true that it's only at the end of WWII that the German jets ME 262 were able to intercept the Mosquitos. A search in Internet gives a great deal of infos.

    About the use of wood as a successful engineering material, there are a lot of examples as the German S boats, some Newick trimarans, a lot of small planes, a lot of yachts and also in building.

    A look at the APA association, the US Forest Industry Laboratory and other internet sites is very instructive. It's unhappy that wood is linked to antiquated designs, sometimes very poorly and dangerously engineered, with lots of bright varnish. Anybody can christen himself yacht designer and sell plans...

    For the home builder wood/epoxy/fibers composite is a material which needs common tooling, moderate skills, and very forgiving. It permits to work at its own pace, and best, you can get very good results, for a good price. I do not mean cheap, boats are always expensive, but lets say less expensive.
     
  3. PAR
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    PAR Yacht Designer/Builder

    This is a highly over rated aircraft. It was quick, particularly unloaded carrying cameras. It could muster 390+ MPH which beat the ME-109, pissing off the German pilots that chased it. Then the FW-190 showed up and all bets were off, because it could get over 400 MPH with the last version capable of almost 440 MPH.

    Against the ME-109 it could out run one in level flight, but if the ME-109 was one of the later variants like the E-7 and flown by a good pilot (most were dead by that point of the war) then the Mosquito driver was in for a surprise.

    These were the only two German aircraft capable of giving the Mosquito a run for it's money, until the ME-163 and 262 showed up. In fact the FW-154 was the German attempt to make a Mosquito. Their result was slightly faster, but they couldn't keep it in production because of the glue used to bond the laminates (the glue factory kept getting bombed) so only a handful ever were built.

    Compared to American aircraft of the era, it was pretty good, but we had several that could chew it up and spit it out their tail pipes. Fortunately, we were on the same side, as more then one of our aircraft had British engines (or copies) in them, of which they owed lots of they're stellar performance.

    It's the wise builder that runs occasional tests on product combinations. I've tested several glues on different species of wood, subjected them to different loading situations and done so repeatedly, to insure reasonable accuracy.

    The boil test is a good one, though I don't know many that will anchor their boat over active, underwater volcanoes. It does somewhat simulate long term submersion. I find wet/dry cycling a more informative test for trailer boats.

    Saturation tests for coatings, like paint, epoxy etc. can offer some insight to what works better in your climate.

    Ilan, though we usually agree on most things, I've yet to see a design that was compromised in any fashion by having excessive varnish work( :rolleyes: ). Of course I have seen some pretty poorly designed boats, from several angles of approach, engineering, hull form, rig selection, lack of building experience (a major issue, particularly with the new low cost and free software available now). Some designs can't be built as the plans show, simply because the designer didn't understand how much effort it would take, to bend in a piece of material to the shapes they'd drawn. Identifying these designs is very difficult for the novice, which I consider a very dangerous thing.
     
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    Hi, Par
    The Mosquito has its legend as the Spitfire. Remains that it was a remarkable achievement for a plane mainly made with simple non strategic materials by furniture and piano makers on concrete molds with steel straps to get the bonding pressure...Remains also its capacity of survival. That shows that wood is a good material with no high tech technics.

    The Testing Plywood Ole Smelly Recipe is similar to the Italian RINA tests, it represents crudely the stresses induced in the glue joints by the dilatation and contraction of the wood, and also an accelerated aging of the glue itself. It permits also to control the quality of the inner plies. At least it permits to eliminate the badly made plywoods or/and with not waterproof glues. The test is simple and costs almost nothing.

    I would be interested if the USA made MDOs are able to withstand this test as they look interesting for some parts in boatbuilding. I've tried with a Chilean MDO sold here and the auraco or radiata pine swelled so much that no glue in the world would withstand the stresses. Also these pines have not any durability, nor strength. Good for furniture, not for boats.

    The tests I want to purpose for epoxies are principally informative. Saturation depth is not the best test, as any epox with solvent will soak wood. More interesting are adhesion, peeling, brittleness and water absorption. All tests easy to make.

    I have seen ads for epoxies (specially a 1/1 mix resin-hardener- already suspicious as generally the 1/1 epoxies are not the best ones in boatbuilding-, which has poor elongation, and soaks as much water as a polyester) rather bothering because claimed as of marine use and I'm sure that a lot of people will try to save 5 bucks a gallon buying it... On the other hand some companies are selling at 80 $/gallon resins which are not better that the 45$/gallon resin sold by some honest guys.

    About plans many are properly terrifying for a trained naval engineer. A lot are totally outdated, as if you wanted to remake a Ford T or 1908 Biplan, many have no idea of decent scantlings, worst you won't find any detail of scantlings, or use over-complicated or unsure methods, a few are totally crap/junk. In old good times the so called designers would be exposed to the infuriated customers provided with a good supply of rotten eggs and tomatoes :p

    Yes it's too easy to produce a drawing with some freeware and to sell it, with a lot of bla-bla. I have just seen in internet the drawings of a multichine 7.5 meters trimaran with 4mm plywood hulls!!! Mamma mia!!!:mad:
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    The de Havilland Mosquito was designed originally as a fast, unarmed light bomber that could outrun fighters. It had an remarkable range and was also a highly effective night fighter with its load-carrying and loitering capability. However, there were quite a few legendary planes during WW2, British, German, Russian, US and so forth. It's not necessary to knock a legend in order to recognize another.

    3 mm ply on a (fairly) big trimaran! Sounds risky. but some boats are built for speed, some for durability, then there's comfort, endurance, single handling ... hope it never hits a log.

    That said, ply strength can be amazing; I built an ama for a sailing canoe using 3 mm door skin, that's really cheap luan ply, $10 for a 4 x 8 sheet. Lousy performance, but when it came to discard it, I wanted to see what punishment it could take. Think of a box, 9 inches (23 cm) square, 7 ft (2.1 m) long, pointy ends, two very light ply inner frames, no epoxy (Gorilla Glue), no glass (3/8" or 1 cm sq Poplar chine logs), a brick at each end to support it. I stood in the middle, no effect. Jumped up and down, nothing. Got my buddy to stand on it with me, about 440 lb/200 kg, didn't even creak! We even stamped on it (not enough room to jump) and this was crap ply! Swung a heafty axe at it; punched straight through the skin of course but bounced off the corners. To destroy the damn thing I had to cut it up with a saw.

    Back to scarfing;

    I made a series of test joints a couple of years back. varied the scarfing angle from 90 deg (butt joint) to 6:1. Then broke 'em. Only the test pieces with butt joints broke at the joints. Every joint with a scarf ratio 2:1 or better broke across the wood. I compared the strength of the butt joints (epoxied) to the wood, all fell in the range 55% to 70%. I did some more butt jointed pieces but added an epoxy fillet (no glass or fiber) to one side, about 30% of the ply's thickness, then broke them. This time they all broke across the wood except for one that turned out to be a dry joint.

    As a result of that experiment, I concluded that the only reason for long scarf joints in plywood is, they are less inclined to slide apart when clamped. I now use 4:1 in ply, but I increase that to 6:1 for solid wood because the end grain can suck some of the glue out of the joint. I think the usually recommended ratios of 12:1 or even 20:1 is a hangover from the days before decent adhesives, when fasteners were used and the only thing inside the joint was sealer.
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    The 7.5 meters trimaran looks like a daysailing one with possibilities of coastal cruising. The hulls are 4 mm MULTICHINES. That's a lot of pieces to cut accurately, joints to make, smooth and seal, and surely an dense frame of bulkheads and stringers to build. And nope, the skins are too thin.

    I think that a 6mm, 2 plies of 3 mm, cylinder mold or even ployed or stress-formed will be stronger, with better hydrodynamics, and easier to make for a trimaran of same program. Kurt Hughes has some in his catalog, the Gougeon stressform tris were interesting, and the old original drawings of the Tornado in ployed plywood are avalaible at the Tornado Association for a few bucks. More: in Gougeon Bros book on boat construction (a bible) has the drawings in chapter 24 on tortured plywood the drawings of the panels for a 28 feet class C trimaran. A good base to start.

    Plywood can be indecently strong. When I discarded the hulls of my first 18m2 sport catamaran like you I ended to use a chain saw. The last one made almost 20 years ago, in 3mm okoume plywood with 4oz glass cloth, sails always, but the owner has reduced the sail.

    A 18m2 sail on a 30 feet wing mast on a 200 pounds 18 feet cat is a bit hard to dominate alone where you're softened by age...It was a wild beast. A 200 pounds skipper, able to run 5 miles singing and to lift 200 pounds smiling was required in a breeze over 10 knots. That gives an idea of the stresses held by this cat.

    The Class C Yellow Pages (34 knots max, 12 knots in a 4 knots wind with rigid wing) used 3 mm okoume ply and stiffeners about 1/2 inch...

    Some french aerobatic planes were all wood and cloth, and designed to stand 20 G stresses...So on a yacht no problem.

    You're wright about the scarfs, the 12 to 20/1 are when glues were no better that boiled floor. The 8/1 has been fixed to give safety margin and to work on both hard and soft woods. Also for a smooth bending with no hard spots. On a deck a 4/1 or 6/1 is enough it depends on the wood used in the plywood.
     
  7. PAR
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    PAR Yacht Designer/Builder

    I think most of the longer scarf ratios were to insure a fair bend, rather then a hard spot at a short glue line. On free standing masts, I'll use longer scarf ratios also on pieces that I want quick, but fair bend, other wise I'll use 6 or 8 to 1. The old school lapstrake builders used 8:1 on mechanically fastened planking and after many years of repairing these types, the method works very well and bend fair.

    I didn't mean to suggest the Mosquito wasn't a cool ship, it was, but not as much as the legend suggests. The same can be said of the Spitfire, who's wing (tip) is highly touted as being the best. In fact, testing has concluded it wasn't. Every other aircraft of the era and since has ignored the benefit of less tip vortice generation in favor of slightly more drag, but much more effective area and lift at the tip, provided by a square plan form. The same is true of yacht appendages. Yes, the elliptical tip does generate less tip turbulence, but this benefit is offset, by the advantages of more lift (square tip) at the end of the lever arm. The bulb verses a fin keel debate have similar arguments and benefit trade offs.

    When you look at the physical properties of well made plywood, it's difficult to rival it's attributes, especially in small craft. You have to go fairly high tech to beat it and the associated costs may be grossly out of scale in comparison. Repeated designs (including aircraft) have born this out.
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    I agree with you, Par on a mast to get a fair bending better to go to 12 to 1. Scarfs on battens around the 1 to 1 1/2 inch are a breeze with a good circular saw and a jig (many designs). Mine are made with 1/4 thick 2 inches wide aluminum angles and incorporated press. Very simple and cheap with excellent results. Looks like a miter box.

    At the risk to look like an old guy saying always the same thing, for plywood under 3/8 a Scarffer from Gougeon Bros make life simple. On thicker plies, you'll get a good feather edge, the most difficult part of scarfing.

    No everybody is a king of the planer, able to shave the hair of a frog without damaging the delicate skin of this animal... A guide for a planer is not a hard project, but as hard glue lines have tendency to dull the planer blades fast and I'm very lazy so I prefer the circular saw, with "disposable" disc blade. The lone con is the vibrations of the blade and it's solved by adding a sealed ball or roller bearing on the other side, so the blade spins on bearings each side. That saves the main ball bearing of premature death.

    I keep my precious planer blades (imagine the price imported from the states with a Fedex shipping of about 70 bucks) for the nice job on clean wood. Basic matter of taste, not worth of a discussion, as several methods are as valuable.

    I agree totally with the conclusion, I'll add that the ratio price/efficiency is unbeatable with epoxies techniques and the last but not the least it has no hard smell and better: it is prefinished!

    Work with polyester and breathe the delicate styrene fumes, that disinfect the lungs...listen with the impassibility of a zen monk to your neighbors complaining about the horrible smell that stinks up all the street...Smile to the city officials and policemen...Count the number of hard work hours and of unbearable itching while trying desperately to finish a polyester sandwich on male mold that looks like a battlefield after heavy bombing...the sanding of the inside of the polyester hull, ahhhh, what a sweet pleasure.:D I suspect that polyester amateurs do love also big women in black leather with harsh voices and whip at the hand...
     
  9. PAR
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    PAR Yacht Designer/Builder

    The scarfer jig I show in an earlier post is now primarily used for those fast, thin plywood scarfs. Slap down the stock, toss a couple of drywall screws in it, maybe some vacuum or other clamping feature, fire up the circular saw, one quick pass and done. No attaching anything to anything (scarfer attachment) or fiddling around with setting up a router, just plow and go. With epoxy the feather edge isn't a concern, nor is neatness.

    Ilan, what's wrong with loving big women in black leather with harsh voices and whip at the hand. Some of my best friends own whips and have enjoyed using it on me. Unfortunately, many have been clients and I couldn't show my pleasure as much as I'd have liked, without them expecting some sort of discount for services rendered . . .
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    Thanks for explaining why longer scarfs might sometimes be called for. I haven't needed joints near sharp bends yet, in fact my boats to date have scarcely stressed the ply or lumber during construction. I'll bear it in mind.

    I'm always willing to learn. Speaking of learning, perhaps you can explain about the women in black leather as I've led a sheltered life.
     
  11. PAR
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    PAR Yacht Designer/Builder

    I have a video I can send you . . .
     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    :eek: :) :p :D I can't see the screen because of the tears of laughing... Nothing wrong, all tastes are in nature...I have a tendency to whip the customers...some seem very happy.
     
  13. PAR
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    PAR Yacht Designer/Builder

    But it's a reallllly good video . . .
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Now I'm not a professional and I hesitate to offer advise but Par, it sounds like Ilan has a very good class of client. Maybe a move to Mexico ...
     

  15. Boston

    Boston Previous Member

    The old school lapstrake builders used 8:1
    on a mast to get a fair bending better to go to 12 to 1

    old Robby would be smiling, but I can hear it now
    depends on the wood
    for a fair bend in a dam hard wood you need to go with a higher ratio scarf
    for a fair bend in a soft wood bla bla bla

    Ilan feel free to start that thread and keep me posted
    I wont be posting much but Ill be reading
    I dont know **** about ply
    nor do I know Jack about glue
    wood and bronze I got down but I am not to slick with epoxies
    thanks B
     
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