diagonal planking

Discussion in 'Boat Design' started by Boston, May 8, 2009.

  1. Boston

    Boston Previous Member

    of all the many types of planking I could go with for my wood build, diagonal seems to be the lightest and strongest of my options. Its glue intensive, which in my book is a strike against it, but, the stiffness it so obviously imparts is a strong consideration in its favor. The main issue I have with it is Ive not any experience in a cold molded technique and so far nothing about this build is outside of my experience in building. Im not adverse to learning something, its just learning it the hard way, I prefer to avoid.

    most of you know the boat Im working out the preliminary design for, but since the size and hull configuration is so critical to sheathing considerations Ill picture her again


    57' of easy radii
    no problem to get a bend around with one small area of compound curve
    the hardest turn in the hull being about a 5'8" diameter radius at station #7

    so far I'm going with a laminated keel out of Black Locust under a white oak skeleton
    ( turned out to be a pain in the *** to acquire BL in timber sizes )
    with a BL keelson and bellow wl stringers the rest being WO

    frames are steam bent solid and Im upping the scantlings from 1 5/8 x 1 5/8 each 9" to 1 7/8 x 3 tapering to 1 7/8 x 1 7/8 each 9"
    midway through the turn Im about 1 7/8 x 2 3/8 on edge and I think thats about what I can expect to safely bend out of WO without risking eventual damage to the frame over time

    Ill make a few and see how they survive
    I may have to laminate the frames to get the bend and the size I want
    maybe as I think Im in the ball park on that bend limit
    this vessel was not originally designed to be a open water voyager so Im thinking it needs some stiffening ( storm shutters and stiffening in the superstructure as well but thats no big deal )

    reason I start this thread is that if I went with diagonal planking I might gain sufficient strength to pair down the frames some considering that true cold molded hulls generally dont even have frames in em just bulk heads and stringers.

    I want a frame ( for me its half the fun of building ) but the sheathing, laminated keel, and stringers on a diagonal planked hull do most of the work

    question is
    how much strength does a monolithic skin impart to the structure as a whole
    for each four layers ( two diagonal two linear ) of what wood do I gain what over the same thickness of the same wood

    Im guessing its significant but Im guessing and thats not good enough
    someone of you builders have got a chart somewhere
    so if you could post it
    Ild sure appreciate it

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  2. nero
    Joined: Aug 2003
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    nero Senior Member

    Strength is only part of the compromise. Weight is another.

    If you take the surface areas and add up the hull thickness as weight-per area then is becomes amazing how fast things get too heavy.

    You would be much stronger, faster, fuel efficent to make the plan as a constant radius and use pannels construction. Or simply redue the plan in something like TouchCad and let it spit out the unrolled shapes.

  3. Boston

    Boston Previous Member

    so whats its strength to weight ratio as compared to other planking styles in a given wood

    I can calculate weight and get this boat to its original displacement and be stronger in the long run

    once I get this thing further along Ill post the strength to weight ratios of each member and then on the whole structure to derive the strength gain over original
    assuming someone has the numbers on diagonal planking

    cad is not necessary friend
    I prefer to do things by hand

  4. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Actualy, I doubt that you will find a chart comparing like thickness. Double diagonal cold molding is so superior in strength to conventional caravel construction that you usualy end up with a much thinner skin.

    For conventional construction, the plank thickness is selected to support buckling and shear load between frames with each plank acting as an individual. The frames (actualy the plank fastenings) attempt to hold all the planks in relation to one another, but are relatively ineffective in this (i.e. racking and working) and at distributing load between planks.

    In cold molding, the monolithic structure of the shell allows for load to be distributed throughout many planks with the diagonal layers locking the inner and outer layers together athawartships. These diagonal layers are far more effective than frames (with modern glues) in maintaining the geometry of the shell and the relationship between planks.

    FWIW, conventional hull design have been trying to solve the individual plank flexture problem for hundreds of years. The Diagonal riders in the Humphries frigates, the iron or bronze strapping in the clippers and 1870's yachts, the iron framing and diagonal strapping of the composite tea clippers, right up to the Ashcroft system, have all be trying to make a monolithic wooden hull.
    Last edited: May 8, 2009
  5. Tad
    Joined: Mar 2002
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    Tad Boat Designer


    I see you've morphed a traditionally built pilot schooner into a cold-molded power boat? There's a leap....

    Wood is "anisotropic", it's strength is unequal in all directions. Strength is dependent on grain direction and orientation to the loads imposed. Trying to mix traditional and modern construction techniques, or even compare them, is a complex exercise needing precise input.

    What's the load? Powerboats are subjected to various major loads, the big one is speed dependent, but there's also bending in waves. And what "strength" do you want to compare? Compression, Tension, Shear, or Static Bending? Parallel to the grain or perpendicular to the grain? Or someplace in between as with most boatbuilding problems? Understanding Wood, by Bruce Hoadley, is a good primer on wood engineering.

    You cannot address one part of the structure and ignore the rest, it must be designed as a system, otherwise there will be problems. A cold molded hull will take many more man hours, will need more clamps, you'll need to learn about vacuum bagging and get rigged up with plastic nails and gun.

    A properly designed and built cold-molded hull will be far superior to a traditional one, in stiffness, puncture resistance, insulation value, and more valuable in the marketplace, also long term maintenance will be far lower. Initial cost is far higher, and you have to deal with nasty goop.

    The boat in my avatar is 80', cold-molded of all diagonal skins on big plywood and laminated stringers, There was no benefit in running any of the planking longitudinally.
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  6. peter radclyffe
    Joined: Mar 2009
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    peter radclyffe Senior Member

    lloyds scantling rules allow, a reduction of 10% of planking thickness for oiled calico, double diag, if you dont want to use glue, & as you know, 1'000s of wonderful american boats have been built this way, & a reduction of up to 25% for cold moulded, but check your frame spacing, i repeat , up to 25%, so for safety 15-20%, as you wish, i am not a cold moulded specialist, i am sure there are many with far more exp than I.
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  7. Boston

    Boston Previous Member

    just seemed a lot more livable
    and then I started considering the upkeep on all that rigging and the sails and bla bla bla. The old Elco ended up being a way bigger bang for the buck with a far more habitable design, so, I jumped ship.

    glad someone noticed
    I was beginning to think no body cared :-(
    precise input being exactly what Im after mate ( thanks Tad for the numbers )

    you forgot stability across the grain transition, moisture content, fiber stress at proportional limit, both rupture and elasticity, and the all important impact bending.

    I will undoubtedly be considering the entire structure in the one part of this build that will be computer generated, a flex analysis.

    am working my way through that system now which is why I was after the numbers on diagonal planking vs other forms of planking

    Pete my frame spacing is per original specs and my sheathing will probably be tulip wood set in epoxy with either the inner and outer layers longitudinal or just the outer
    need feed back on that and how it relates to the overall stiffness of the structure

    c mon people
    I know someones got a comparison somewhere
    although that 25% from Lloyd's is pretty telling, I need to also consider wood species, thickness, number of lamination's and all that jive.

    basically I could alter species of the original specifications from ceder to tulip wood, go with the same thickness for a slight weight gain, go with diagonal planking for significantly greater strength "over all".
    also the take up would be nearly eliminated so in the end the weight gain in the species switch would balance itself out
    and Ild be dryer inside

    any flaws in that logic Ild love to hear it
  8. thudpucker
    Joined: Jul 2007
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    thudpucker Senior Member

    I keep thinking about the WWII Sub Chaser I had some time on.
    It was double planked. 4" for (I think) a total of 8".

    It was probably necessary for the WWII sea Duty, but I doubt if you need it now. It had two 16 Cylinder, Radial GM 71 Inch cylinders. Awsome power.

    I also had a Chris Craft of the 50's, single planked and it worked fine. Just that maint problem if you ever had to pull it out of the water.
    It had two little Izuzu four cylinder diesels.

    I'd stick with single planking and smaller engines.
  9. Boston

    Boston Previous Member

    that was closer to the mark than you may know
    I found lines and some details for the old elco ML's or Movies that were the sub chasers in WW1 for the English Navy. I detrended them and scaled that down to fit the dimensions of the 57. It worked perfectly and looks exactly like the 29 Elco flat top.

    The original elco 57 had 1 1/4 inch ceder carvel planking on the hull
    and 1 1/4 white pine on the deck all over white oak framing

    I think it was Par that mentioned having a 40 wooden fishing boat that lost a full ton in take up once out of the water for a few weeks.
    if that ratio of length to weight is scaled up to the 57' boat Im thinking of then thats about 1.5 tons of weight that I can play with in strengthening
    through strategic design.

  10. rasorinc
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    rasorinc Senior Member

    My 2 bits. Bottom 2 layers 1st 1/2" 2nd 1/2" marine ply max 9" wide which should give you max usage out of a 4 x 8 Final layer 3/16" to 1/4" black locust.
    Sides 2 layers 1st 3/8" 2nd 3/8" marine ply. Final layer 3/16" black locust. I sent you that link for Black Locust a guy in Washington state has whole dry logs. Increase diameter depending on frame spacing. The Black Locust provides shock and dent resistance. The double ply laid diagonally gives fantastic strength and it is never seen. I'm assuming frames at 24" OC to 30" OC. Best, Stan PS this is a Minimum, not a maximum Use these composite nails, brads, staples to attach the plywood layers. Use bronze screws for the final layer and you can pre drill through the composites for the bronze. Raptor are used thoughout in boat building but they will not go through black locust
  11. Boston

    Boston Previous Member


    that was the magic last bit I needed Ras

    ply strips are week perpendicular to the surface grain so I was going to avoid using it in the sheathing of the hull, (at least 5 ply is, 9 is the bomb but I wont be finding it in 1/2"). Black Locust is great for structural members of the frame exactly as you previously stated but heavy, might add to much weight but would sure be worth using. Please send that link again as I must have missed it or misplaced it. Ply is also heavy and typically that stuff uses Resourcenol which out gasses formaldehyde and has been known to cause serious respiratory complications.
    Ive given serious consideration to increasing the spacing of the framing to a typical 16oc spec just to keep my brain on straight
    thats another reason Im after the stats on cold molding
    which I guess I got at +25% ( wood for wood ? )
    so if I consider same wood and same thickness I could go 25% greater spacing on the frames

    poplar is 25% heavier than red ceder and about 30% stronger depends on how you want to measure it with a modulus of elasticity of 1,120 lb/sq (ceder) vs 1,500 (yellow poplar) at 12% moisture content being about 34% stronger and an impact bending limit of 8,600 lb/sq (ceder) vs 13,500 lb/sq (poplar) or 57% stronger at the same 12% moisture content

    means poplar would allow me at the same thickness to gain ~30% spacing for a ttl of ~25% + ~30% or ~55% of 9" spacing is ~14" spacing
    and then I increased the size of the frames on average from 1 5/8 x 1 5/8 to 1 7/8 x 2 1/2 or ~ nearly 78%
    which gives me the 16" spacing for the frames with a significant gain in strength throughout the structure

    and I have yet to calculate the weight gain but it only needs to be bellow the typical take up of red ceder and Im good to go

    57 x 12 = 684 / 16 = 43 ribs
    as apposed to 684 / 9 = 76 ribs
    or 77% fewer ribs at 78% greater mass pr rib
    well that worked out

    but the greater spacing kinda makes me want to thicken the hull from 1 1/4 to maybe 1 3/4 or about 40% using the same four layer system and thus the same amount of epoxy say an inner layer of 1/2 two layers of 3/8 diagonal and one more of 1/2 exactly as Ras suggested
    although Im starting to warm up to the idea of a final layer of black locust, at least bellow the line, and since its so much stronger I could go thinner on that last layer, say whatever the weight difference dictates, maybe.

    so the sheathing material (poplar) weighs in at 28% heavier pr cubic foot and I added 40% which works out for each square foot of planking to be 2.2 lb/sq ft for ceder at 1 1/4 and 3.9 lb/sq ft for poplar at 1.75 or 77% heavier

    the boat is 57' long and an avarage rib length of 22' ( measuring strong just for the sake of argument ) and I get 1,254 sq ft of surface area
    cedar would weigh in at 2,758.8 lbs
    poplar would weigh in at 4,890.6 lbs

    and that's within the 1 1/2 tons I roughly calculated for take up in an old school ceder hull
    means the boat floats about were it should and yet is far stronger than it was

    I think
    if I screwed up anything obvious feel free
    thanks for the help guys
  12. mark775

    mark775 Guest

    Ferrocement! (Second choice; resin rich chopper gun).

    If anything could make us see eye-to-eye, it is the beauty of that vessel. I know you're from Colorado, and all, but the above was a joke.
  13. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Why are you bothering with steam bent framing? You have plans for a homogeneous hull shell, why not take this to full advantage. You'll save the trouble and costs of these elements, plus save considerable interior volume as well. Ditto the massive deadwood assembly. With modern engineering techniques, material use, application and build methods, you don't need these huge, ridiculously heavy and costly timbers. Put the savings in fuel and stowage capacity.
  14. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    I would go simpler: strip plank, epoxy and biaxial. Little or no framing, just bulkheads.
    Strong as a rock, easy to make, lasts at least 50 years, using a common affordable wood.
    I would stop to split my head about calculations and guessings, I would take the Gerr's book "The elements of boat strength" and recalculate the scantlings in less than one day of work. The results are in the strong side. And I'm a naval engineer with more than 30 years of experience and now 161 boats behind me...go simple and Gerr's book is enough good..
    With the weight and money savings, I would spend more time and money to nice amenities and good engines. Hull is just a very small part of the job.

  15. Boston

    Boston Previous Member

    the interior is going to be raised panel cherry and the decks in curly maple ( depends on the availability at the time might just end up being maple )
    the framing will have all non bearing surfaces beveled with a saddle left on all bearing points, just like all my other stuff
    and a lot of the structural will be exposed
    when I start in on the interior design Ill send you some of the drawings if your interested
    glad you liked the boat

    I have lines for the old elco but no prints
    and Im piecing together plans from bits and pieces of info and smatterings of scantlings then modifying them to my own idea of what I want the boat to do. What I come up with is still developing.

    I didnt know I was going with steam till someone suggested it
    nor did I know I would go with pellets for a fuel source till the steam engine was mentioned
    and that came from a consideration of electric
    the hole thing is a process that seems to have a life of its own actually
    its like following a trail of bread crumbs

    where I end up, who knows
    but I want to blend function and form wherever I end up and that means its not always going to make sense to the purely linear thinkers. That artistic considerations function must however transcend the purely aesthetic.

    make your best case Par, I am listening intently and you may find I am leaning in your direction
    I picked the boat and placed the stipulation that it be environmentally friendly using low embodied energy materials and fuels. I also placed the structural consideration out there that it would be transoceanic. beyond that the various alterations must have an intrinsic relationship to the era in which the design comes, if those alterations are going to be visible to the casual observer.

    in the begining of this project it was suggested I dont have to use glue at all
    but it seems the 21st century is calling and given maintenance considerations it might be wise to answer. also the desire for a transoceanic capable boat seems better met with a cold molded hull.

    the take up consideration presented earlier was the final nail in the traditional carvel planking's coffin ( that was almost a pun )

    we started talking about glues and I found I was stuck with epoxy
    ( had to sorry )
    even though I swore I would never use the stuff

    now its a mater of how much epoxy as its hands down the least environmentally friendly thing in the build

    you would suggest I slather the whole thing in epoxy
    I would suggest using as little as possible to meet the design criteria that it be as environmentally friendly as possible and yet open water capable

    I have 1,254 sq ft of surface area and four layers of sheathing in the hull
    thats 3,762 sq ft of glue
    I think I came a long way from my no glue days

    as for the keel
    I was going to begin another thread for that design
    but since the answer to the hull planking came so easily I guess I can move on

    there are three ways Im thinking of going

    A) a simple frame over keel with a keelson and rabbeted gar boards to accept the diagonal planking

    B) the Cutts method but with four layers instead of two and with no fabric in between

    C) or the standard cold molded, false keel sandwiching the sheathing between itself and a keelson

    Im inclined to blend method A with method C
    as Ill be laminating the keel anyway and I like the idea of clamping the sheathing and the frames between the keel and keelson

    also it eliminates the rabbet line, the garboards and reduces the thickness of the keel moving the main structural member to the inside where its better protected anyway

    Im not willing to give up the steam bent framing as it adds so much to the look of the interior, Much of which is going to be exposed. Honestly the inside of a cold molded hull just looks ugly. There is little room for the expression of craftsmanship nor the opportunity to have large open spaces as bulk heads are such an integral part of the cold molded method. I suppose if you made the hull thick enough, the stringers huge and used enough glue you could just leave the hole thing open, but that void of barren walls is just not what Im after


    Ill definitely pick up the book
    although figuring things out on my own is half the fun
    I will be not only checking my self against the references suggested but I also plan on having the drawings checked by someone who designs for a living before I start the build. I have a friend of mine who does computer modeling it will be interesting to give him the final design and see what it takes to break it.
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