Norway Spruce for strip planking?

Discussion in 'Materials' started by Ranger1973, Oct 1, 2012.

  1. gonzo
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    gonzo Senior Member

    As PAR says a sharp stick works well to clean epoxy. I usually make several wooden "chisels" to clean the ooze. Hard wood works best.
     
  2. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Ranger, is there a chance you can take some of your fir to a mill and have it cut thin to make 3mm laminates? Is there any way you can procure some lightweight cedar? Here's why I ask. Let's say you need a total thickness of 22mm. If you do it the way we've been discussing:

    18mm fir + 4mm GRP
    [(18mm*35 lbs/ft^3)+(4mm*115 lbs/ft^3)] * 0.003281 ft/mm
    (630+460) * 0.003281 ft/mm
    3.58 lbs/ft^2

    Now let's say you strip plank with 14mm white or western red cedar, but with every 7th strip plank being fir to create a network of internal stringers. Then you face glue two 3mm layers of diagonal fir laminate on the outside. Since you have diagonal strength, you need less fiberglass sheathing. Here's what you get as your weight per square foot:

    14mm thickness 6/7 cedar & 1/7 fir (equivalent to 12mm cedar + 2mm fir) + 6mm fir + 2mm GRP
    [(12mm*24 lbs/ft^3)+(8mm*35 lbs/ft^3)+(2mm*115 lbs/ft^3)] * 0.003281 ft/mm
    (288+280+230) * 0.003281 ft/mm
    2.62 lbs/ft^2

    You've just reduced hull skin weight by nearly 27% without sacrificing strength (much, by any meaningful measure).

    Now I ask you Ilan, PAR, Gonzo, Teddy and other experienced builders: in the latter scenario, might it make sense to strip directly to bulkheads and frames without temporary station molds? Could one just sheath between the bulkheads on the inside? Would this save time? How much of the time spent on the double-diagonal outer wood layers could be made up by doing this?
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    Smile...Old fashion...but that works. Unhappily mine broke several years ago and it's no way to fix it. These things are rather expensive small jewels.
     
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    And the results are very basic. Gerr's book is for monohuls and is totally useless for multis. SCT is not better, being more than basic. Multis have enormous surfaces and such scantlings would be far to heavy. There is no book for these scantlings.
     
  5. redreuben
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    redreuben redreuben

  6. Ilan Voyager
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    Ilan Voyager Senior Member

  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    SCT is "more than basic" because ISO 12215-7 (draft version) is? Do you think there's an improved version of 12215 - 7 coming? Or do you think the issue is the SCT software? So I take it the way to proceed is to make educated guesses at slamming loads, design pressure head, and sailing loads, and calculate panel deflections (in addition to comparing to proven designs)?

    If scaling from a known boat of a different size, can one at least use the tables in Gerr to establish the slope or exponent of the scale factor?
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    Stephen:

    A first principle; a composite wood strip plank is a monocoque, ie a structure where the skins bear ALL the loads. For such a purpose the skins must have the better ratio strength/rigidity/density. Any bulkhead or other reinforcement is a latter addition to the skins. in reality most times reinforcements are made adding or more fiber or a stronger fiber as you get a better efficiency on the structural beam represented by the hull. The boat must be seen as a whole and all the stresses, local and general must be defined as precisely as possible and the skins designed to bear them. On a a small cat it's easily done par an experienced NA.
    The best illustration of the monocoque is the WWII Mosquito, you'll find plenty of information in Internet.

    Consequences of the principle:
    -NO solution of continuity in the skin as it's the total bearer of all loads; that means that the fibers must run fore and aft (the wood) and side to side (the glass fibers). There are some "exceptions" as the openings, and the joints.
    -The bulkheads must not interfere with the working skins except just maintaining the general shape, and not to create hard points where the skin can be fractured by an excess of ridgidity.

    In practice:
    NEVER GLUE a bulkhead to the bare wood, the bulkhead must "float" on an elastic bed glued to the glassfiber so it does not make hard points. Furthermore its not practical to glue the bulkheads to the strips even on a monohull; the sanding and glassfiber jobs become a pain with all the bulkheads on the way...the result would be awful after an exhausting work.
    Multihulls are pretty slim and cramped. The lone practical way is to strip in half-hull female molds, sand and fiber inside, finish inside as much as possible, place and glue the bulkheads in one half hull, join the 2 half hulls, glue the bulkheads to the second half hull, sand and fiber outside. Finishing.

    Cold molded wood is very nice but a pain to make. Cold moulding over strip planks is for monohulls (read the Gougeon Brothers! free 400 pages 10 megas PDF) Keep it simple; fiberglass or carbone on the wood strips.

    As PAR, I won't give any scantling on a forum, but those you proposed are far too heavy on 34-35 foot cat. Scantlings are the job and responsibility of the NA making and/or selling the plans.

    To give you an idea, I give you scantlings of proven racing multis; ADRENALIN, trimaran 40 feet, main hull WRC 3/8. 200 gr of carbon fiber each side, 3 bulkheads. A "F40" catamaran I have worked on in 1988; North Pine 480 kg/m3 10mm strips, 2 * -+45° unidirectional glass 180gr outside and inside, 1*165gr satin cloth outside, some additional glass uni 180 gr, plus carbon uni 120gr at strategic points. Only 3 bulkheads plywood 9mm and fibers. Rigging plates in carbon-kevlar glued in the hulls, no screws. This cat has never had structural problems.

    It's not a strip plank but a sandwich race F40 cat in 1987. 12.10 m * 7.75 m, mast 18 m high, 90 m2 sail plus 100 m2 gennaker. Weight ready to sail; 1800 kg. Nota this cat is always navigating in excellent shape in 2012, never had structural problems nor reparations. Skins : kevlar 0.9 mm thick each side, core 20 mm divinicell 75 and 100 kg/m3, plus some carbon (only 10 kg used in the 2 hulls). It's a race boat able of 27-28 knots max, 25 knots on a mile, and 21-22 knots during 24 hours. Able to navigate at 16-18 knots in a state 6-7 sea without problem. As you see, it's needed just enough material, no more, if you're able to calculate the stresses and figure the way of dissipating them.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Strip planking can be considered a monocoque structure, but the skins bearing all the loads is only true of the cored composite types of strip building. Strip planking runs the full range of narrow carvel like structures over frames to the truly composite sandwich build types. In each of the sheathed methods, a progressive amount of load bearing is absorbed by the skins, of course dependent on the sheathing schedules and the build method ideology. Since we don't know what the strip method being employed is . . .
     
  10. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Now THAT was useful, gentlemen. Thank you for sharing your knowledge & experience.
     
  11. Ilan Voyager
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    Ilan Voyager Senior Member

    Yes, PAR the word strip plank recovers very different methods having in common only the fact that the wood of the hull is cut in strips, it's why I wrote composite strip planking. It's practically the lone suited for modern multihulls, the other methods (classic and molded) being too heavy for this use but very suitable for others.

    I'll add that if the job is relatively easy, it's rather tedious when the surfaces involved are large (but the width of the strips can be modulated in function of the compounds et the severity of the curves) and it's best suited for cruising trimarans which can have complicated shapes. The main advantage is that the inside is not cluttered by the stringers and ribs (look at the pics inside Rogue Wave and you'll understand...). The other advantages are the ratio strength/weight (to get a better ratio you have to go to advanced composites) and the durability as the thick "coats"of epoxy-fiber are truly waterproof. The main disadvantage is that in case of big accident it's rather difficult to repair.

    There is a medium term for a home builder if he wants to build a hull with rather simple shapes like most cats; the bottom of the hulls in composite strip plank (so you get an exact hydrodynamic shape and the wanted displacement plus the needed strength), topsides in slightly compounded plywood with a minimal frame, decks made for example in honeycomb/plywood/glass on a light mold. So they can be finished inside before installing. That saves a lot of hard work overhead inside a cramped hull...

    I do feel that compounded plywood and variants like the cylinder mold are beyond the mental grasp of a lot of people, it's a lot of eyeball building and do ask for some previous experience to understand the system, which is the opposite of the monocoque in terms of structural engineering. Another problem is to find nowadays the suitable thin plywoods.
     
  12. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Just to remind, the boat under discussion is to be 28-30 feet per Ranger, below. It's to be a strong cruiser, not a racer, and it's to be finished bright. In my own defense, all the possible thicknesses I've ventured are between the 1" thickness Ranger suggested and what you, Ilan, seem to be recommending. I agree with you that Ranger would do best to use plans and scantlings by a NA experienced in multihull design (and would again point out that Richard Woods is experienced, value oriented, in the U.K., offers an excellent design in the size under discussion that has been built in wood strip before, and is currently working on an updated version of that design).

     
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    You're welcome.
     
  14. Ilan Voyager
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    Ilan Voyager Senior Member

    Cruisers do not need to be heavy to be strong, it's a common misconception that cruisers need to be built as war tanks and bullet proof.
    Weight is a silly spiral, where the rig has to be bigger and heavier with more sail, thus asking for more structure. Seaworthiness and performances will badly suffer. The cost skyrockets as you need more material, more work, more gear, more hardware, more sails, and bigger engines.

    A very old example of lightness with strength=seaworthiness : the 31 feet tri Third Turtle, a minimal coastal cruiser . The 1976 OSTAR, race where you have to go against the wind, was plagued by very hard weather; various depressions and three strong gales. Mike Birch simply slept while the tempest was too strong... and arrived rather fresh compared to the exhaustion of Tabarly, who had the 32 tons monohull Pen Duick IV.

    No matter the thickness of the hull, on grounding on a reef the hull will be largely punctured and broken. And more the boat will be heavier and more the shock will be strong.

    Besides an heavy cata is a pain to sail; slow with a characteristic short period rolling, unable to go upwind, burying the bows downwind, needing constant sail adjustments, and the engines in light wind. I'll add: dangerous in strong seas as it's unable to run and to stay maneuverable, plus the waves fully impacting on the hulls and superstructures. Better to have a monhull in this case.

    A cruising boat has just to be strong enough for the job and that does not mean overweight and over-engineered. You'll be surprised by the scantlings of a good modern 31 feet cruising catamaran in strip plank or/and plywood, buy study plans to Woods or Hughes. You'll get the answer.
     

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    Stephen Ditmore SCT is "more than basic" because ISO 12215-7 (draft version) is? YES PARTLY
    Do you think there's an improved version of 12215 - 7 coming? I DO NOT KNOW.
    Or do you think the issue is the SCT software? PROBABLY, NOT MADE FOR MULTIS.
    So I take it the way to proceed is to make educated guesses at slamming loads, design pressure head, and sailing loads, and calculate panel deflections (in addition to comparing to proven designs)?
    NO EDUCATED GUESSES ON MULTIHULL ENGINEERING. OR MUST BE VERY BUT VERY EDUCATED. SO EDUCATED IT'S NO MORE A GUESSING...
    GERR AND AND SCT ARE USELESS TOOLS ON MULTIS. GERR EXPLAINS VERY WELL AT THE BEGINNING THE LIMITS AND APPLICABLE FIELD OF HIS BOOK AND FOR PROBLEMS OF LIABILITY HE GOES TO THE SAFE SIDE IE RATHER HEAVY.
    If scaling from a known boat of a different size, can one at least use the tables in Gerr to establish the slope or exponent of the scale fact. NO, TOO MUCH POSSIBILITIES OF MISTAKE. THE EFFORTS INDUCED BY THE BEAMS NEED A DIFFERENT METHOD.

    Structural engineering is a profession like physician. Lot of knowledge and experience. There are not amateur physicians (my grandma used to say; not too young as they have not killed enough people and learnt the mistakes, not to old as they are out of the race) surgeons or dentists and there are not amateur structural engineers. And unhappily there are a lot of amateur NA, trying to pass as professionals in Internet.

    Empiricism may work on monohulls, and it can be a guide on multis. But, even empiricism needs good solid and reliable data ie the analysis of the scantlings and design plans of a lot of boats; the bad ones as you know where are the mistakes, and the good ones to know what were the factors of success. To make this analysis and thus eventually extrapolate, you need a solid education in several fields; hydrodynamics, aerodynamics, some maths, strength of material, boat building methods etc...There is not foolproof method in a book or software for "dummies". Even for cooking, a book of recipes is just a guide, you need some experience like burning several cakes, oversalt the roast-beef or undercook it, glue the rice in the pan and after these miseries and having learnt the how to do, you'll be a good "home chef".

    Plus when I see the ridiculous low prices of the plans of good boats by good reputable NA, compared to the total cost of the boat, I would not bother to try to design or calculate one. I'm making a boat trailer for a client: I bought the plans to an engineer who knows very well this field. Far cheaper and faster than to try to design a trailer, and I'm engineer... Probably because I'm engineer, I do imagine the eventual problems like stability, and I do appreciate the quality of the design work proposed by the trailer engineer. So I prefer to pay the rather cheap fees to save time, money and problems.
    The possible apparent savings are small, but the possibilities of big mistake are great. On a thing as costly and long to build as a boat I would not take the risk to try to design without the knowledge and the experience..
     
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