Best wood veneer for a foam panel beachcat?

Discussion in 'Materials' started by TJ Cameron, Mar 25, 2012.

  1. TJ Cameron
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    TJ Cameron Junior Member

    I am designing an ultra-light sailing beach cat with what I hope will be increased durability. Many ultra-light beach cats constructed with epoxy, foam or nomex core and carbon or Kevlar skins are very susceptible to impacts and end up with surface denting after several years of use.

    My plan is to substitute a 0.040 inch thick 2-ply wood veneer for the outer layer of carbon cloth. Starboard sailboards has successfully used a similar 0.6 mm (0.024 inch) pine veneer to achieve light weight and superior ding resistance in their top boards.

    I plan to use either ¼” or 3/8” Divinycell or Core-Cell with 5.7 ounce carbon cloth at the hull interior. Both 2-ply with parallel veneers and 2-ply with perpendicular veneers as well as 0.020 inch thick paper backed veneer will be used at the exterior depending on the bend radius needed.

    The 2-ply parallel and paper backed veneers will be reinforced with 5.7 ounce carbon fiber cloth to compensate for the lack of strength in the perpendicular direction. The perpendicular 2-ply veneer will not be reinforced, but only epoxy coated and painted and so must adequately resist structural loads, checking and other deterioration.

    2-ply veneers are available with more than 40 types of wood. The backing wood veneer is available as parallel or perpendicular to the face veneer. The veneer used on the back side is often an imported hardwood of lesser value. I suspect it is typically Gaboon, which I plan to confirm. My top veneer candidates are:

    Clear Pine – Already proven by Starboard, relatively poor impact resistance.
    Mahogany – Established as excellent marine plywood, good epoxy adhesion.
    Sapele - Good reputation for boatbuilding.
    Spanish Cedar - Lightweight, Relatively poor impact strength.
    Douglass Fir – Fair impact resistance, possible issues with checking.
    White Birch - Good impact resistance.
    Red Birch – Good impact resistance.
    Hickory - Best impact resistance, poor workability, poor epoxy bonding.

    Starboard claims that the natural resins in the Australian pine they use prevent the pine from absorbing an excess of epoxy and raising weight. Hickory and birch are both about 50% more dense than pine, but test with approximately twice the impact strength of the other veneers with equal thickness.

    Mahogany is likely the most similar to the Gaboon backing and so might make the best choice to preserve consistency.

    Shrinkage with moisture changes is another consideration. The panels will be asymmetric and so I want to do all I can to avoid panel warping as a result of moisture shrinkage or expansion.

    I’m looking for advice (pros and cons) on which wood veneer to use and also general comments/suggestions on my planned panel construction. Thank you in advance for your replies.
     
  2. rasorinc
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    rasorinc Senior Member

    Best wood vineer

    If you are going to use a 2 ply veneer you want a very durable wood. Do not use Hickory http://www.connectedlines.com/wood/wood31.htm
    You have to assume water will eventully get to the 2 ply veneer so look at cypress, western and eastern red ceder. Mahogony is good as is teak.
    The durability of the wood should be more important to you than hardness. You can use Xynole cloth on the bottom for more hardness say 2 layers of tape on the keel which will protect you when beaching. The above woods are extremly durable. Also Black Locust is very durable but hard to find and it is very hard wood and would be my choice for a long lasting boat. Do you have a link to your supplier of wood 2 ply? Ironbark (eucalyptus) is durable and Hard--see Glen-L list of boat building lumber. http://www.glen-l.com/wood-plywood/bb-chap5c.html If you post where in the US you are I might have other links I can provide you.
     
  3. sabahcat
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    sabahcat Senior Member

    They used foam/nomex and exotic skins for weight reduction.
    Skinning with wood after the fact would go some way to negating the weight saving.
    If worried about dings, why not save time and a heap of money and just use thin ply from the start, no core?
     
  4. Timothy
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    Timothy Senior Member

    I have a wood decked Starboard Isonic and a new carbon Isonic. The wood one is fine after five years. The new carbon one already has had impact related problems. I would say you are on to something with the wood skins. I have noticed that many of the kite boards use paulownia but perhaps just as a core.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    TJ, you've made some statements that are different than what's employed in standard practice or accepted in usual engineering respects. What level of engineering experience, particularly composite and marine related do you have?
     
  6. TJ Cameron
    Joined: Mar 2012
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    TJ Cameron Junior Member

    PAR-
    And which statements in particular are not accepted in the usual engineering respects? I am a P.E. in mechanical engineering with only a little bit of composite and marine boat building experience. (mostly repairs) I see that as an advantage because it seems that a lot of people in the field have trouble thinking outside the box. I am not impressed by the poor durability of ultralight beach cats and think we can do better. Starboard has shown us one way. My web search turned up a smattering of builders using ply/foam/ply for big boats for similar reasons as my own. My proposal differs only in that the layup may be asymmetric which may admittedly have warpage issues.

    I am counting on you all to fill in for my lack of composite and marine experience. ;)

    Racing beach cats are usually in the water for less than 4 hours at a time and so wood durability of epoxy coated veneers may not be an issue. Okoume allegedly has poor rot resistance but is still widely used for marine ply. Starboard's wood sailboards with pine seem to hold up well for many years. (See Timothy's post)

    sabahcat-
    I've looked closely at both thin ply and carbon reinforced cedar strip boat building methods. Neither method can meet both my my stiffness goals and my weight targets at the same time.

    rasorinc-
    My two-ply veneer supplier with AA veneers and reasonable prices is veneersupplies.com. They also provide a choice of parallel or cross-ply backing.

    I've narrowed my veneer wood choices to four, from least to most dense.

    1. Clear pine
    2. Spanish cedar
    3. Cherry
    4. Walnut

    These woods all have relatively high impact strengths compared to their densities. All allegedly have some degree of rot resistance. Walnut is about the same density as mahogany and sapele but has higher impact strength. Most cedars have very low impact strengths. Black locust is unavailable and eucalyptus is three times the price of the others.

    Comments?

    This web site: http://www.mcilvain.com/hardwoods/walnut/ has density, impact strength, max. crushing strength and sharpy hardness for most of the woods under consideration.

    I will probably wait to make a final decision and will base it on meeting the target weight of the boat.

    Because the boat will not sit in the water for long and the veneers will be epoxy and urethane encapsulated, impact strength and density will probably take priority over rot resistance.

    The reports I have read say that Xynole cloth soaks up a huge amount of resin, so not good. I am considering 4 to 8 inch wide skid plates of 4.0 oz. S-glass over the hull keels to protect against abrasion.

    Thank you all for you comments. Keep them coming.
     

  7. idkfa
    Joined: Sep 2005
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    idkfa Senior Member

    Bonding all those layers will take lots of epoxy, and to bond the veneers, it should be thickened too to ensure contact (on top of the wet-out coat). Think the weight would be quite significant.

    How about using a high density foam core instead of the usual 5lb. At 1/4 thickness a 10lb core would probably still be lighter than the epoxy needed to bond the wood veneers? just a thought.

    guesstimate about 15lbs of more weight in foam, versus weight of additional epoxy and wood veneers?
     
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