Plywood, fibreglass, carbon for centreboard project

Discussion in 'Fiberglass and Composite Boat Building' started by polarhiker, Feb 22, 2018.

  1. polarhiker
    Joined: Jan 2015
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    polarhiker Junior Member

    I'm working on a new centreboard for my 24' catamaran. It's my forth lamination project, so I'm still very much learning.

    The plan is to use BS1088 marine plywood core, two halves of 18mm (actually 17.2mm), CNC milled to NACA 0009 profile. This is a very thin board for my comfort, so I wanted to add more strength to the plywood. Currently the core is already milled so I cannot change it.

    I'll start with my questions and then provide more details on the project and how I ended up there:

    - I read that to add strength you have to match materials with similar elongation at break. From here it follows that laminating plywood with fibreglass does not give you full strength, because fibreglass will stretch much more than plywood (5-9% elongation at break vs 1-2%). Your part will then only be as strong as your core, which will break first. So basically, fibreglass is only used to protect, not reinforce. Is this correct?

    - To add strength to the plywood, you'd need to use carbon fibre which has similar maximum strain as plywood. Is this a good choice?

    - If I do go with carbon reinforcement, is it worthwhile to add a few layers of fibreglass on top to protect it? Going all carbon might be expensive, but on the other hand I'm worried about thermal expansion / delamination or any other problems when using different materials in a single laminate.

    - Any other advice you could give is greatly valued!

    More context:

    My centreboard is 2m (79") long and 380mm (15") wide. I wanted to use NACA 0011 profile initially, but BS1088 marine plywood is only supplied in 18mm standard thickness before it goes to 25mm which is much more expensive to both buy and mill. To add to that, the supplier took advantage of all the leeway in the standard and provided actual thickness of 17.2mm, which effectively means NACA0009 for my width.

    My boat is not designed for unaided capsize recovery, so I'm not planning to have three 200lbs guys in wet clothes jumping on the centreboard. Still, sailing on the Great Lakes where gusty winds are common, such a thin profile does not give me comfort, and I wanted to reinforce. For reference, my sail area (unreefed) is 16 m^2 (160 sqft) and underwater portion of the centreboard is 1570mm (62") long. Even though it's a catamaran, it's not crazy fast. I would start reefing when we hit between 7-9kts close hauled depending on seas.

    I haven't worked with carbon before, so I'm wondering what's the best way to do this would be, and whether carbon is really the best choice.

    Also I'm based in Toronto; while I usually try to store the rudders and the centreboard in heated space for the winter, it would be better for the parts to be able to withstand large temperature swings if one season I'm not able to store them properly.

    Any advice is appreciated!
     
  2. TANSL
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    TANSL Senior Member

    You could consider that the plywood only serves to give form to the centreboard, that it does not contribute to the global resistance and, therefore, to calculate the necessary glass so that he supports all the efforts that are exerted on the centrebooard.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Plywood is not a good choice for an appendage. It's only 2/3's as stiff as a solid timber laminate and tends to fail in repeated "rolling shear", because of the way the plywood is constructed. It's also quite heavy compaired to other options.

    If plywood thicknesses are causing issues, consider using multiple layers, rather than a single layer. This places more veneers in an appropriate orientation and an extra glue line too, both making the board stiffer. You can also "cant" the plywood layers within the multi layer plywood arrangement, further improving the boards modulus.

    Consider inert materials, possibly those that don't inherently need additional reinforcement or abrasion protection. Cossa would be a first choice, though there are several other board construction methods, that show considerably more merit than a single piece of plywood.
     
  4. TANSL
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    TANSL Senior Member

    PAR, I think everything you say is very correct but according to the OP
    the remaining options are very few.
     
  5. Zilver
    Joined: Nov 2007
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    Zilver Junior Member

    On daggerboards usually unidirectional carbon is used at around 1/3 of the chord of the profile (measured from the front of the profile). Here is an example of the use of the uni carbon
    Making a Dagger Board http://smalltridesign.com/Trimaran-Articles/Construction-tips/dagger-board.html
    You might try to use a router to make a shallow rebate where the carbon uni is placed, else you have to fair the whole board again and that might be a waste of all the cnc cutting. here you can see an example
    April | 2014 | F36 #005 Ravenswing – Trimaran Building https://cartersboat.com/2014/04/
    Good luck, Hans
     
  6. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    Your assumption that the elongation at failure has to be the same or the you will not obtain an advantage from adding a laminate to the outside of your board is not correct. While the system would be OPTIMIZED with that approach, it ignores the fact that the addition of a laminate on plywood substantially increases the stiffness of the overall part. That increase in overall stiffness substantially reduces the stress in the core and the net result is a much greater overall strength. In your case the high stress is acting to bend the board and adding layers of laminate to the maximum thickness area as shown in the links shown by Zilver above do that and it makes the board much stronger. The maximum strength and stiffness is obtained by adding the laminate at the furthest distance from the neutral axis (the centerline of the foil) and the laminate is less effective the closer you get to the centerline. Because the stiffness is higher in the laminate, the stress is higher in the laminate for the same deflection, so what you are doing with the laminate is transferring load to the laminate and taking load out of the core. While you can go all the way to a lot of laminate and a foam core, there is nothing wrong with adding strength to the system by selectively adding laminate to strengthen the plywood.

    It doesn't take all that much glass or carbon fiber to substantially stiffen and strengthen a piece of plywood. While it is not necessarily the most effective way to do it, laminating the entire board to encapsulate the board is probably a good idea to keep the wood from getting wet and adding some scratch protection. You could pretty easily mill the max thickness area of the foil down to a flat that was 6 inches wide and then laminate layers of increasingly narrower unidirectional tape along the length of the board, refair the shape and then wrap the entire thing in 6 or 8 ounce glass and that would likely easily double the strength of the board and substantially stiffen the board at the same time. If you are worried about hitting something then the overlay could be done on an angle to spread impact loads over a larger area.

    So although it isn't a optimum structure, it would be much stronger that it was before you added the laminate. And yes if it fails you'd fail the core, but the objective is to make it strong enough so that it doesn't fail and an overlay of laminate may do that just fine.

    To give you an example of what a laminate can do for a piece of plywood here's a table. While this is just showing the effect on thin plywood, but it does illustrate how a laminate can improve a piece of plywood and that's what needs to be done in this case.

    Glass isn't as strong or as stiff as carbon, but it will likely do just fine for what you want to do. If you have 1/4 of an inch of laminate at the max thickness and then overlay the entire board with 8 ounces of glass for protection I doubt you'd ever break the board. If you want to really overkill it, you could add another layer or two over the entire board and you probably could have a couple of guys standing on it and it wouldn't break.

    To do it right you could calculate the stiffness and strength by assuming a cross section of only the laminate with a shear tie and then using a beam calculation figure out what it would take to support an appropriate load and if you ignore the contribution of the wood you'll be way overdesigned.

    glass over plywood.jpg
     
  7. polarhiker
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    polarhiker Junior Member

    Thanks all! This is great advice.

    So fiberglass will definitely be adding quite a bit of strength, but I also see others choosing CF and putting glass on top. I'll follow suit.

    Making a groove or flattening manually will be problematic in my case, because of taper. My planform is "quasi elliptical", and the taper preserves profile ratio (meaning it's the same naca profile along the entire length). I'm afraid I will lose that smooth surface as soon as I touch it.

    My next design will have a groove for sure, but then again I might get a proper lumber as well.

    So probably best in my situation is to lay CF over entire width even though it won't be effective at the edges, and reduce the fraction of length and width covered with each layer. I can then add 2-3 5.8oz glass layers and fair any steps still visible.

    On Feb 22, 2018 07:24, "Boat Design Net" <no-reply@boatdesign.net> wrote:
     
  8. PAR
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    PAR Yacht Designer/Builder

    No amount of carbon or sheathing will prevent the rolling shear failure the plywood brings to the table (assuming reasonable sheathing/reinforcing schedules), so if the board is modestly to highly loaded, this will be the failure point, while the 'glass/carbon sheathing will contain the broken bits of plywood when it does fail.
     
  9. polarhiker
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    polarhiker Junior Member

    I haven't seen this before, so went to Google... trying to figure out if I'm "doomed". I'm fine with the part being not as long-lived as solid wood (that's a trade-off I made going with ply); but would not want it to break on the first day either.

    From what I read, rolling shear is worst when two things to happen:

    1. Non-negligible load on *core* (as opposed to centreboard overall)
    2. Repeated bend direction change (e.g. frequent tacking)

    (2) is less of an issue as we usually cruise rather than race courses, so I would stay on the same tack for a while.

    Can (1) be mitigated / reduced by minimizing tip deflection using carbon? What is the range where you can avoid the threshold of rolling shear?
     
  10. PAR
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    PAR Yacht Designer/Builder

    Rolling shear will happen, but predictions are dependent of cyclic rates, internal flaws in the plywood and the amount of flex the board initially offers. Can the board be made stiff enough to mediate these risks, yep, though it'll likely be way heavier than it needs to be, given materials employed and/or much more costly, than it needs to be for the same reasons.

    Plywood boards even reinforced, are generally left o budget minded and fairly small, lightly loaded designs. Your 24' cat, high aspect, well over 6' long board would suggest much higher loading, at least in as much as to warrant a different construction method, if durability is desired. Will it work, yeah it'll do, but don't be surprised if it takes a dump, before you realize. Personally, I don't see the logic behind the effort and cost, knowing it has to be heavy and/or costly to get reasonable durability, particularly when equally inexpensive methods can be used, that don't have these inherit flaws.
     
  11. rob denney
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    rob denney Senior Member

    I am pretty sure that adding material over the entirety of your machined shape will do a lot more damage to the section shape than cutting a tapered depth trench down each side and filling it with carbon (and bog if necessary), then applying a layer of +/- 45 over the entire board. Fairing the board at max width is hugely easier and less critical than shaping the nose.

    We make the trench paralell sided and deeper where the foil goes through the hull and taper the carbon layers in both directions. You can bag it or get a correct width piece of timber, brown tape it and clamp it onto the wet carbon. 200 gsm uni carbon is about 0.2mm thick, so use this to determine the depth of the trench. Keep adding carbon until it is just above flush with the surface, then sand and bog it until the curve is fair.

    If rolling sheer is a problem (Par, do you have any photos of a rolling sheer failure?), cut out the middle section of the board altogether and rebuild it with carbon flanges and a high density core/sheer web. End grain balsa or cedar, high density foam or timber, plywood on edge or multiple laminations of glass, both with the grain/fibres running at +/- 45 degrees) all work as the core. Then glue the leading and trailing edges of the foil to this and glass it at +/-45. You should do this where the board leaves the hull regardless. Given the taper of the board, I would cut a trench in one side, tape it and lay up the carbon. Then repeat. You now have the flanges, 'just' need to shape the sheer web, but this will be pretty easy using the leading and trailing edges and the carbon flanges.

    If you are worried about the strength, support the sheer web at the top and at the exit point and drive a bike/car/truck/bus over the unsupported end. If the web doesn't break, it will support double that load spread over the entire board. If this is less than the weight of the boat, you should be fine.
     
  12. polarhiker
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    polarhiker Junior Member

    Precise conformance to section shape is not as much of a concern at this performance class I think; (otherwise different foil could have been used altogether). I'm more worried about smoothness / evenness of the resulting surface, and a fairing project that follows if I get it wrong. For one thing it would need to be vac bag because timber is flat and the board isn't.

    How deep / long should the trench be? I have a flat section some of the length before the tamper begins.

    Another twist in my case is that the board doesn't actually "exit the hull". It is suspended on the frame structure under trampoline above water and is attached via two points (also above water), which do not align with the thickest section of the curve. (the board can then change angle of attack depending on the tack, which is configured via turnbuckle tension). So the point of maximum load is actually not an airfoil, and the full-width carbon should perform there. A picture is worth a thousand words...

    If trench is the way to go, where would you place it? I would think between the suspension points? But then it cannot continue onto the airfoil section, as it won't hit the thickest part.

    For the rolling shear, I spent some time on cantilever beam calculator and it promises me 5-6cm tip deflection at 150kg load on the tip depending on various parameters. I was assuming 80GPa modulus of elasticity, 1.5mm CF at the thickest place. That was CF only, but then there is some glass and the core. I'm not sure what plywood tolerance would be, but I guess it's hard to predict.
     

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  13. rob denney
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    rob denney Senior Member

    The carbon should go down the thickest part. Stiffness is a cube of beam thickness (twice as thick, 8 times as stiff). Maybe the width of the top veneer.
    It should go almost all the way to the bottom (until the ply is strong enough without the carbon) , with the tapers stepped by 100mm to the attachment point, then tapering again (probably not worth it) to the top. It the trench is flat bottomed, a clamped on plank will compact it.

    Looks like the uni would go over the forward attachment point. If the attachment is a point load, it needs beefing up with multiple tapered discs of double bias on both sides and through bolting. As does the rear one, although I doubt this one sees much load. If it does, the laminate needs to spread down into the shaped part of the board. Maybe router a rebate in the shaped part, lay up the glass, then bog and fair it.
     
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