Okoume & Meranti long-term effects

Discussion in 'Materials' started by Skeezix, Mar 25, 2016.

  1. Skeezix
    Joined: Feb 2016
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    Skeezix Junior Member

    I apologize in advance if my searches have missed answers to this question ... which likely is the case. If so, links or suggestions for better search terms will be appreciated.

    I've read plenty here and elsewhere about okoume ply vs. meranti ply vs. sapele etc.

    Meranti is slightly cheaper ... but initial cost in any built object is eclipsed over time by use and upkeep costs and all other factors. I believe one should find a way to buy the best materials within reason.

    Okoume is lighter. For my build of a trailerable cruising tri of about 3000 lbs. displacement (Marples DC-3), a 20% weight saving in hulls and decks is a substantial difference.

    Okoume is more flexible. While my project is a Constant Camber design, so that the flexing is moderate across every span, that extra bit of spring might be nice during layup.

    Meranti is harder ... it is generally interpreted as being more impact resistant. Cosmetic dings don't worry me, but reistance to structural damage from rock impact does. Ultimately it seems the meranti will better resist a complete puncture. But with a 30 mm hull thickness, is it not going to be all kinds of trouble if any wood boat hits something that hard? I have seen it suggested that for lesser impacts okoume might flex and rebound better than meranti.

    Meranti is more rot resistant. While I will use best diligence to seal and maintain, it seems inevitable that water will find wood on a wooden boat.

    Colors, grains, sanding seem like minor concerns for me.

    So finally to my question: I chose a wood composite method because I believe over the long term that wood will better survive the thousands of flex cycles than will plastics, especially in a tri designed for sizeable seas. So what I am wondering is whether the additional flexibility of okoume gives it an advantage over meranti as those flex cycles accumulate, especially in a hull that already is tortured in design. I can see it probably is true for a design that tortures large sheets, but am not so sure for the CC biased laminations. And over the long term will the flex of the okoume help the life of the boat more or less than the impact and rot resistance of the meranti? These factors always seem to be discussed in the short term instead of in terms of decades.

    I also will ask John Marples this question before I embark on the build, but I am curious what the general thoughts of designers here might be.
     
  2. gonzo
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    gonzo Senior Member

    Flexibility and fatigue resistance are totally unrelated. Carbon steel has infinite life cycles when loaded below its elastic yield point.
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    A flexible material may also experience fatigue and collapse by fatigue. Maybe they are not "totally unrelated".
    It seems exaggerated to say "Carbon steel has infinite life cycles." You've probably tired of counting cycles and, in short, you have decided that were infinite.:D
    "Elastic yield point" is not a redundancy?
     
  4. Skeezix
    Joined: Feb 2016
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    Skeezix Junior Member

    Thank you for that clarification. I don't plan on building a boat from carbon steel. Am I correct in th8nking the flexibility of wood has some degree of relevance to fatigue cycles? It seems like the main reason trees have some flexibility is to survive a lot of cycles without fatigue failure.

    Now of course it gets more complicated when we take those mostly lateral fibers from the tree trunk that serve in mainly one direction, and we then bias them in layers of plywood. And then again bias those plywood strakes across each other in a layup.

    So can anyone offer a discussion of fatigue differences between okuome and meranti, and how that works in with other factors for long term performance?
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    This discussion needs to be refined a bit and taken from hypothetical to reality. Assuming we're taking plywood, not solid, the primary failure rates will be rolling shear, within the panels from repeated stresses. These stresses eventually will tear and break fibers, typically along the glue lines, reducing their grip on neighboring veneers.

    Small craft, with not very heavily loaded parts (most commonly plywood appendages) can live a long time, some never seeing much failure rate, but larger craft or those with more highly loaded bits, will eventually see a rolling shear failure, within the panel.

    Theoretically, this isn't supposed to occur, if the loads aren't very close to the max, but in reality and enough cycles, the less than perfect fibers, defects and other inconsistencies within the veneers, will rear up and show some level of breakdown.

    As to the choices and which might be better for an application, well most of these decisions are also driven by other considerations. So you have a good hard look at the design SOR and the light usually makes it obvious in this regard.

    In your situation and not knowing the SOR, the usual choice is okoume for it's weight, sized appropriately for it's service expectations. Meranti might permit thinner sheets to be employed, is more durable and better modulus, but plywood sizes are fixed and you might not get the reductions you need with available sizes. In the end, I'd still choose okoume, simply because this is a composite and any concerns, over flexural modulus or durability can be handled with the sheathings and proper maintenance..
     
  6. mike Banks
    Joined: Oct 2013
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    mike Banks Junior Member

    You quote a hull thickness of thirty millimetres? That seems rather a lot. My 42 foot trimaran has hull thickness of ten millimetres. That is quite strong when coated with glass and epoxy saturated. No boat should ever hit rocks--but if you intend doing so--steel is the best material. For a trailerable tri this is out of the question anyway--so I would opt for 9mm meranti, well saturated with epoxy and glassed over on the outside. Inside must also be saturated. Some may find that this is a bit heavy for the amas--but I have the same in amas as main hull.
     
  7. SukiSolo
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    SukiSolo Senior Member

    I thought this was a small(ish) trimaran..... 30mm is getting on for a minesweeper.....;)
     
  8. PAR
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    PAR Yacht Designer/Builder

    Agreed a hull shell that's over 1 1/8" thick seems quite excessive, especially for a tri.
     
  9. Skeezix
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    Skeezix Junior Member

    Thanks for the replies! Top of my head is not a good reference location. The plan is three layers of 1/8" (3mm) ply. And I agree that lightness probably trumps other concerns. John replied to me that any marine ply passing a one hour boil test, e.g. BS1088 standard is fine, and that many builders would choose okoume lightness.

    Thanks for the discussion on rolling shear along glue lines.

    The SOP for the DC-3 is "expedition cruising trimaran" ...
    -Here is the page on his website.
    -A short summary that repeats those model pictures and adds plan drawings is here. Also a good audio interview.
    -More info is in Wooden Boat #223 available for free download online.

    While I hope the boat eventually sees saltwater, it first will sail a variety of Upper Midwestern lakes and sailable stretches of the Mississippi. I might motor it in Miss channels (wingdams to avoid) and surely will use it on Lake Superior. So while rock impacts are to be avoided by anyone but whitewater drinkers, for me they are a legitimate concern. Also noteworthy are the rougher waters that can blow up on lakes like Superior. Beating up through that stuff should create some substantial stresses.

    It also just occurred to me that a lighter boat will hit an obstruction with less force and will rise over it with more net buoyancy. And won't be as deep in the water to start.

    Thanks all for your input.
     
  10. Steve W
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    Steve W Senior Member

    BS 1088 spec is more like a 72 hour boil test. Ive had some Chinese interior ply last more than an hour in tests but I wouldn't build a boat out of it. Okoume is usually selected on weight alone, its not durable or particularly pretty but it is light but epoxy makes it work.
     
  11. PAR
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    PAR Yacht Designer/Builder

    The type 1 WBP test is a little more complex than described, but the BS-1088 standard is more than the phenolic or melamine adhesive employed. This "panel standard" is the key, assuming a real type 1 glue is employed.

    I can't imagine a need for a 27' tri, having a 30 mm hull shell. This is way over the top. I also think you can get a good set of stock tri plans, for a fair bit less than the grand they're asking.
     

  12. Skeezix
    Joined: Feb 2016
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    Skeezix Junior Member

    Marples' plan only calls for a 10 mm hull. I was writing from memory, I think. I don't remember.

    To be fair to Mr. Marples, he described the standards accurately:
    "Use any ply that will survive boiling water for an hour. BS1088 (British marine ply spec.) is the best."

    I have had others comment that tri plans are available for less. If you were to look at the DC-3 plans for but a few minutes, you would see that the extra plan cost comes back manyfold in savings building the boat. Mr. Marples has made every effort to design a solid tri that can go anywhere between the 40s without worry, while keeping it economical and easy for a home build.

    Plan cost aside, it is one of very few designs I found with a center cockpit and horizontal folding possible while underway.
     
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