Laminating a Rudder's Trailing Edge...WTF?!?

Discussion in 'Fiberglass and Composite Boat Building' started by CatBuilder, Sep 22, 2011.

  1. Steve W
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    Steve W Senior Member

    Daquiri, those properties you give are pretty meaningless in this context, what you need to compare is a bog mix with wood dust to a bog mix with microballoons because you cant fillet with epoxy alone but if you could, by majic, run a 2" fillet of epoxy alone it would be increadibly weak and brittle, it NEEDS the inclusion of some sort of filler powder. As Par has said, some blend of silica/milled fiber is going to be the strongest (and heaviest), what Cat is using is what Kurt has apparently decided gives adequate strength for the lowest weight, a wood fiber/silica blend is going to be somewhere in between and would be a good choice also. Rotting out of your fillets is a non issue,its not like you are going to sand your fillets and then leave them without a coat of something, on a composite boat your are going to paint it. I pretty much do what Par does filletwise so it is interesting to see what Kurt specifies. We all have our favorite goo mixes and they all work, i have only used wood flour on plywood builds that get finished bright but i actually like them a lot, i dont use floor sweepings but use pine or maple dust i purchase, maple dust makes a beautiful, smooth mix without the need for silica to make it thixotropic, one of the only addatives that does. Another fiberous addative i like a lot is the milled PE fibers that Systems Three sell, also doesnt need silica to make thixotropic, i never use the cotton fibers but used to love the asbestos fibers that West used to sell many years ago, glad they stopped selling those though.These are just my opinions also.
    Steve.
     
  2. daiquiri
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    daiquiri Engineering and Design

    All agreed. Hoyt has asked about using wood shavings as thickener. I've simply stated that I wouldn't use it for structural bonding but it's ok as a fairing compound, since there are better thickeners than wood for adhesive bonding purpose. Colloidal silica or microfibers are an example of better fillers. West System 403, for example, is made of cotton flock microfibers. Cotton fibers alone have a very high tensile strength (around 400 MPa, against 70 MPa of wood) and it's elastic modulus comes pretty close to that of epoxy resin (8 Gpa). That's a part of explanation of why it is a successful thickener for structural bonding and fairing.

    But, I have to stand corrected for my previous evaluation of the colloidal silica + microspheres mix as an epoxy thickener for structural bonding (kind of what CatBuilder has used). The attached informative sheet from Gurit indicates that combination as a very strong structural filler, even with a modest percentage of colloidal silica - in spite of the fact that microbaloons alone are indicated only for "non-demanding" bonds.
    Citation from the brochure: "Colloidal silica is also used with microfibres to produce a mix suitable for use as a high-strength, non-sagging structural adhesive, particularly for non-absorbent materials such as grp".

    I'd like to know if there is a technical literature available with graphs and number-backed comparisons of various filler combinations.

    Cheers
     

    Attached Files:

  3. SamSam
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    SamSam Senior Member

    There are different kinds of force and strength, compressive, tensile and shear. Your chart refers to fillers, but that's not what you're doing. You're constructing the trailing edge of the rudder, it won't be encapsulated with or supported by anything. If I'm seeing this correctly, It will be attached to the rounded edge of the rudder, of maybe 1/4" diameter and trail back 3/4" or so to a knife edge. The micro fibers give you much more tensile and shear strength than cabosil and balloons. C & B has compressive strength, but you don't need that here. It will be brittle like window glass. Milled fiber will give you all the compressive strength you need here while tremendously increasing the tensile and shear strength. It will be more like laminated fiberglass than window glass.
     
  4. PAR
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    PAR Yacht Designer/Builder

    I think we're over thinking this again.

    Wood flour is quite different then the dust you get from a belt sander or router. Wood flour is hundreds of times smaller particles, which work quite well, though they can be heavy, as they absorb resin like a hooker does factory employees on a pay day. Wood flour's weight can be mitigated to a degree with spherical fillers. The most common use for this filler is in brightly finished portions of a build. West System 406 is silica, cotton flock and wood flour. The wood flours role is as a coloring agent. I only use hardwood's for wood flour BTW.

    West 405 is an example of using the fillers you need, for the desired results. I often use two or three materials in a thickened mixture, not just one. In almost all mixtures you can assume you'll need some amount of silica (cab-o-sil, etc.) to control viscosity. Then you have a primary and secondary adhesive goals. These could be a strong fillet with appropriate color or a moderate glue line with fairly easy sanding after cure, etc., etc., etc.

    After a while everyone develops "favorites", which you'll notice when reordering supplies. I use a lot more milled fibers, silica and 'glass spheres then I do phenol balloons, wood flour, cotton flock etc.

    For this specific application, I'd probably use an epoxy saturated single braid polyester line, set in a shallow groove along the leading and trailing edge of the blade. I would use milled fibers to key it to the laminate and 'glass spheres, because they're light and preferred underwater, instead of balloons. As to the single ounce I'd save by skipping some element of a fairing or fillet mixture, I wouldn't be concerned, unless it was a trend in the whole build. The polyester line will add a bit of weight to the blade, but it greatly improves the strength of these delicate edges on impacts.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Thanks PAR, your posts are always very informative. You're a living textbook about woodworking. :)
    CatBuilder, do you have more pictures of finished rudders, glassed and everything? It could be great if we could see more closely what does that trailing edge look like now, before fairing and sharpening it (which might even be unecessary, like I said before).
     
  6. hoytedow
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    hoytedow Carbon Based Life Form

    Thanks, guys, for the answers to my question.

    Bear in mind that the wood dust is pressure-treated and highly resistant to rot.
     
  7. hoytedow
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    hoytedow Carbon Based Life Form

    Would wood flour be more akin to the dust collected in a random orbital sander?
    I have plenty of that too.
     
  8. AndrewK
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    AndrewK Senior Member

    While it would be nice if the typical low end epoxy resin that most of us use in boat building had greater elongation it still is not as brittle as some think it is.
    CatBuilder, grab one of your infusion resin lines and see how much flex it can take. Flex some shorter lengths and say 1m long (close to your ruder depth). Also repeat the same after you post cure some at say 60'C. You will see that even the ambient cure will be fit for your resin/silica trailing edge. Post cured exercise will demonstrate the benefit of even modest post cure temperature. For some resin systems the benefit is very significant.
    The trailing edge having silica will be better than the neat resin.
     
  9. CatBuilder

    CatBuilder Previous Member

    I can take some more pictures tomorrow of the trailing edge.

    I would actually like to put the final build of the staggered trailing edge on ahead of time, because I don't want to have to haul the boat and remove bottom paint to do so if I get a lot of hum.

    The designer says the staggered trailing edge will eliminate the hum (I saw Eric Sponberg say this in another thread recently as well).

    One thing that's absolutely baffling me right now is that my designer says that "all the white fillers absorb water." "Use the brown for anything below the waterline."

    Gurit and PAR say the opposite.

    This stuff is just crazy. No wonder I've had such a hard time with it. Nobody can agree on anything and there is no definitive source of information out there.

    Maybe they all work "well enough" and none cause any real problems, other than theoretical??

    "I'd like to know if there is a technical literature available with graphs and number-backed comparisons of various filler combinations" - Daiquiri

    I'd like to see this literature as well. I've been looking for it since starting my build. A man can dream...
     
  10. CatBuilder

    CatBuilder Previous Member

    You're absolutely right. I can take a filled resin line and turn it right around in a circle without the epoxy breaking. I have to really kink it to get the epoxy to snap.
     
  11. AndrewK
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    AndrewK Senior Member

    Hoyte, one thing to keep in mind is that wood dust can be toxic and a lot worse for you than silica. In your case treated timber?? I would seriously do more research before doing this.

    Regarding cove sizing and strength, the only article I have seen was specific for plywood, T join. Data showed that the magic number for a silica cove radius is 2.1 x ply thickness. If the radius was <2 the joint failed, if above 2.1 the substrate failed.
    It also showed that the silica cove was ~ 25% stronger than a microsphere one.
     
  12. CatBuilder

    CatBuilder Previous Member

    Interesting. My cove radius is 4x the ply thickness, bulkhead T join.

    I can definitely attest that removing my temporary bulkheads was pure hell.

    I had very small bits of a cove on one side of the ply only, just 4 inches long max. Each temporary bulkhead had maybe 2 or 3 of these. I lifted the hull easily by these temporary bulkheads and had quite a time trying to break them out later.

    The bulkheads had to be pried out, pushed toward the bow or stern. Even then most of the time I had to cut the bulkhead out, then grind off the leftover piece of plywood and temporary cove.

    I guess that was a good test.
     
  13. PAR
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    PAR Yacht Designer/Builder

    I think what your designer is saying relates to round or spherical fillers, not calcium metasilicate (West 404 and partly in Cab-o-sil) or silica, both of which are literally stone, Okay actually they are some of the minerals that make up rock or at least the calcium is, while the silica is literally quartz in it's common form. Silica in particular is a non-porous and quite diverse 3 dimensional chain of microscopic particles. Calcium metasilicate is a crystal and also non-porous. Both of these materials are highly used in many industries and well understood, physically and chemically.

    Wood flour is surely resin absorbing, as is cotton flock and micro spheres. Except for wood flour all of these are white. If properly mixed into the resin they become saturated and can't absorb moisture. This said, and again I don't use spheres below the LWL, because they can absorb moisture. It's possible, some not perfectly mixed wood flour could absorb moisture under the LWL, but this would be an exception to the rule and assumes improper procedures. The same could be true for cotton flock, again this is akin to wetting out 'glass fabric and intentionally leaving white spots visible in the process. Given reasonable precautions and good procedures, all the fillers mentioned except spheres will completely wet out and become part of the cured matrix.
     
  14. AndrewK
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    AndrewK Senior Member

    CatBuilder, are your coves 4 x radius or 4 x diameter ? I guess 4r would be practical for very thin material but consume lots of material for 20+mm panels.

    I agree with PAR that your designer is saying no only to white microspheres below the waterline. The reason for this is they are mineral based, brown ones are organic. Mineral ones are broken down by water so eventually you have a matrix with water filled microspheres. Brown ones being plastic do not suffer this.
    I dont know if this is really a problem with epoxy, one of the local builders to very early use epoxy resin and foam has always used Q-Cell in entire build with no problems.
     

  15. CatBuilder

    CatBuilder Previous Member

    Radius is 4x the ply thickness. 12mm ply with 48mm radius circle object used to drag out the fillets.
     
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