Building a 10' Wood Duck Kayak... Help needed with wood choice

Discussion in 'Wooden Boat Building and Restoration' started by cdb5015, Jun 7, 2013.

  1. cdb5015
    Joined: Jun 2013
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    cdb5015 Junior Member

    Hello all,

    So I am building a 10 foot Wood Duck kayak... The plans call for 4mm Okoume on the hull and 3mm Sapele for the deck.

    http://www.clcboats.com/shop/boats/...ood-duck-10-light-recreational-kayak-kit.html

    My problem is that I can only come across 3mm Okoume and 3mm Sapele.

    I plan on glassing the whole thing with 6 oz cloth and epoxy.

    Would it be okay to use 3mm Okoume for the hull instead of 4mm?

    Thanks for any help.
     
  2. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    I depends on how you intend to use it.

    Okoume is pretty soft, and it can get dinged pretty easily. So long as you aren't hitting any rocks or using it in shallow water it is fine, but it doesn't handle damage well and for that reason it's really not that good choice for the bottom unless you glass it. The advantage to the stuff is that it has a very good strength to weight ratio and it makes the boat stiffer and stronger than if you used less of a heavier wood.

    Glassing it will increase the damage tolerance, but if you only do it on one side it won't help the overall strength appreciably.

    Glassing both sides will increase the strength to be equal to or stronger than 4mm, but it obviously won't be as strong as if you used 4mm and just glassed the outside.

    If you can only get 3mm and are looking for min weight, then I would glass inside with 4oz glass and do the outside with 6 oz to give you better damage protection.

    Another option would be to use 3mm sapele on the bottom and glass it on both sides. 4oz on the inside and 6oz on the outside. That would be stronger than 4mm okoume, but it will weight a bit more, but it would stand up to abuse a lot better than okoume.
     
  3. cdb5015
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    cdb5015 Junior Member

    Thanks for the reply. I have a ton of 6 oz cloth laying around. Probably enough to do both sides of the boat in it.

    I'm a bit scared to use 3mm okoume, as it seems like everyone is pretty strict on not deviating from using 4mm like the plans say. Is the 3mm really going to make the boat flimsy like people seem to say?

    Oh, and this kayak will mostly be used in small lagoons and lakes.
     
  4. alan white
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    alan white Senior Member

    Reducing the thickness of plywood reduces stiffness more than you'd assume based on the linear reduction of thickness. At 1/2 the thickness, stiffness is about a fourth.
    Therefore, a reduction of 25% would reduce stiffness by around 50%.
    Do what you can to source the 4mm stuff because it is going to have the required stiffness.
    When you build a kayak there are few construction details that aren't critical. Virtually every detail matters tremendously due to each component needing to be as light and strong as possible. A change from 4mm to 3mm is huge. I'm assuming the glass is only an option according to the designer. It's not needed for stiffness (too thin) so it's just used at the chines to strengthen the joint.
    Doing all that glassing is going to add weight.
    It's too bad plywood costs so much to ship!
     
  5. cdb5015
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    cdb5015 Junior Member

    Thank you for that reply. Very informative. :)

    The plans do call for glassing the whole outside of the boat with the 4mm plywood. But the plans use 4oz cloth. Would using 6oz cloth (I have a ton of 6oz cloth already) on the 3mm plywood make up the difference in strength?

    I have heard before that when you fiberglass wooden boats, the fiberglass is what provides most of the strength rather than the wood. Is this true?
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    'Glass the outside with two layers of 6 ounce cloth, which will return much of the stiffness and improve abrasion resistance - a big concern with a hunting boat.
     
  7. cdb5015
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    cdb5015 Junior Member

    Would I be better off using 3mm Sapele rather than Okoume? Would using Sapele over the whole kayak add more strength and rigidity than Okoume would?
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yes, Sapele is stronger and stiffer also prettier if left brightly finished. A light weight cloth on the outside is still a good idea, to protect the bottom and sides, but you only need2 - 4 ounce cloth, which is lighter and needs less resin.
     
  9. alan white
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    alan white Senior Member

    If the plans call for glassing the whole thing, do it. Whether you use the thinner plywood and add a thicker glass or try to find 4mm plywood and go with 4 oz cloth is up to you. I believe it will make the boat a few pounds heavier to use the heavier glass but the difference may not matter in your use of the kayak. Fiberglass hasn't got a whole lot of stiffness and I don't think a 25% reduction in the wood and 6 oz cloth is going to be as stiff as thicker wood and thinner glass. However, it won't matter as much if the design is overbuilt a bit. You can build a ten foot kayak as light as 20 lbs or as beefy as 40 lbs. 3mm is about 1/8" and 4mm about 3/16". Frankly, I would think the kayak could be 1/4" ply if you omitted the glass except at the seams. This would be stiff and probably lighter than the 4mm/4 oz glass called for.
    Plywood alone is stiffer than the same thickness of ply and glass. Just take a 1mm piece of thin wood and a strip of cured epoxy/glass that's about .5mm thick (1/48"?) and compare the two. The wood is orders of magnitude stiffer.
    Again, fiberglass is adding abrasion resistance but little stiffness. I don't see the point in using it to compensate for wood thickness but maybe I'm missing something.
     
  10. Yellowjacket
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    Yellowjacket Senior Member

    Ok, time for some real data that should help you make up your mind.

    Strength and bending stiffness go as a cube of the thickness. 3mm plywood has only 42% of the stiffness and strength of 4mm. Obviously that's a big hit for only 25% more material.

    What is pretty interesting is how much you can make up by putting only a small amount of fiberglass on a piece of plywood.

    The attached table shows that 4mm plywood with a layer of 4oz glass ON EACH SIDE (and that's important) makes the resulting sandwich as strong a 1/4 inch plywood without glass, and it's a bit lighter than the bare plywood too. You still can't get back all the stiffness that you lose with the lower thickness, but you can probably get a good bit of it back. If you compare the 7/32 bare wood with the 5/32 with 4oz glass you will see that the the sandwich is a bit stiffer and in terms of stiffness to weight it's a bit better, so it should work just fine.

    While just glassing the outside is good, you just don't get the improvement in bending stiffness if you don't do both sides, that's the trick.

    I would use 6oz on the outside to improve the abrasion resistance, and then I would do 4oz on the inside to give it the sandwich effect and keep the weight down. With that approach you should have a stiffness and strength pretty close to what was originally intended, and if you use the sapele, it can look pretty nice if it is bright finished.

    Finally, most folks use epoxy on the inside to seal the wood anyway. If you look at what the difference in weight is for a 4 oz layer of glass and epoxy as compared to the two coats it takes to seal the wood, the weight difference of the cloth is nothing. In fact, it might weigh less since you can squeegee out the cloth and get a thinner, even coat using a thin cloth, than you do with two coats of pure resin.
     

    Attached Files:

  11. alan white
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    alan white Senior Member

    It seems that the unglassed 1/4" is almost as light as, but far far stiffer than 5/32". Twice that of the 3mm glassed with 4 oz! 4 oz over 4mm approaches the unglassed 1/4".
    That means the 3mm plywood and 6oz glass will not be stiff enough, being only marginally stiffer than 3mm glassed with 4oz.
    Again, 1/4" unglassed is the winner. Look at the stiffness figures for plain 1/4 compared to double-glassed 4mm. Weight is similar and the glassed 4mm has a bit more strength but what's wanted is stiffness and the thicker plywood is impressively stiff. This is to say, if you can just paint the plywood and glass the seams only, you'll get the stiffest hull at the design weight.
    I know that I would happily forgo glassing inside and out if I could save all that labor and money. And maybe one day I might have to fix up a few scrapes and gouges but the labor wouldn't approach the original glassing of the whole hull.
     
  12. El_Guero

    El_Guero Previous Member

    The one thing you did not mention (make that two) I would like to ask are:

    1. Number of layers?

    2. What grade is the plywood?

    Your plywood/glass is acting as stiff skin, if it is over a good frame, you will get a much better product. Good is NOT the same as stiff, but sometimes close.

    If you are using this for hunting, is there a reason for the shorter & deeper draft?
     
  13. El_Guero

    El_Guero Previous Member

    PS -- the 12' or 14' should give you a lot more bang for your buck ...
     
  14. LP
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    LP Flying Boatman

    3mm is very thin for a kayak bottom. 4mm is your best solution. Most everything else is going to involve more labor and possibly weight to regain lost strength and stiffness. Another option to loading your kayak up with glass would be to run stringers or doublers to regain stiffness. A 1/2"x1/2" (or even 3/8"x3/8") stringer run full length (one each side) would add stiffness by cutting your unsupported spans by half, especially if there are framing members to give additional support to the stringers. Another option if you have some left over ply, would be to add 1-2" wide doublers laterally on 8" to 12" centers on the widest portions of the hull. Taper the fore n aft edges and the you've gained no weigh over the 4mm ply. Consider sheathing or doubling the floor area where you'll be stepping. You might find yourself stepping through a 3mm hull.

    Good luck
     

  15. Yellowjacket
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    Yellowjacket Senior Member

    There is no 3mm in the table, 5/32 is 4mm. And yes, the 4mm glassed with 4oz is less stiff than 1/4, but it is also 20% lighter. You are comparing something that is 33% thinner, and that is a bigger section modulus change than you can make up for with glassing with just 4oz of glass.

    If you want similar stiffness, strength and weight as 4mm with no glass you can very likely get there with 3mm glassed on both sides with 4oz of glass. The weight of a layer of 4oz glass on each side is 1.6 ounces per square foot. 4mm marine ply weighs 6.7 ounces per square foot, the weight of 3mm plywood weighs 5.2 ounces per square foot. So 3mm with 4oz of glass on each side is going to weigh 6.8 ounces per square foot, essentially the same. The difference is that 4mm plywood is now so thin that it can only stand 45 lbs in the bending test. If you glass the 3mm inside and out, it will be stiffer and stronger than the 4mm for the same weight without glass.

    If you use 3mm ply with 6oz on the outside that will increase the bending strength substantially for forces from the inside, and as a result you won't have to worry about putting your foot through the inside. The additional weight of the single layer on the outside of 6oz as compared to 4oz is about .6 oz per square foot, so your material is going to weigh about 8.1 oz per square foot.

    Another way to look at it is if you were going to use 4mm ply with a layer of 6 oz glass on the outside, the weight of 3mm ply with 6oz on the inside and out is going to be essentially the same (8.4 oz for the 4mm + 1 layer of 6oz, compared to 8.275 oz/sqft for the 3mm with 6 oz of glass on the inside and out), and it would be stiffer and stronger than the 4mm ply with glass on the outside only.

    While glass on the outside provides more abrasion resistance, without the inner layer of higher modulus material (glass), you won't get similar improvements in strength or stiffness.
     
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