Secondary bonding to roven woving

Discussion in 'Boat Design' started by dlpanadero, Mar 18, 2016.

  1. dlpanadero
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    dlpanadero Junior Member

    I am replacing/rebuilding the stringer system in my 15' Pathfinder skiff. The attached photos show the bow area up under the front deck. I have removed the old stringers and have sanded the area down to prep for rebuilding the stringers. My question is, how will I achieve the best bond to the woven roving of the hull when I glass in the new stringers? In the photos I tried to show that the woven is very textured and 'bumpy' to the touch. It has high spots and low spots (sorry, I know there's probably a more proper terms for this). So, do I sand down smooth that woven roving on the hull to ensure a good secondary bond? It feels strange thinking about taking a 60 grit (or coarser) sandpaper to the hull and sanding those fibers down smooth.

    http://s1268.photobucket.com/user/dlpanadero/media/20160318_162404_zpsb22xar5j.jpg.html?

    http://s1268.photobucket.com/user/dlpanadero/media/20160318_162431_zps9zdydcqg.jpg.html?o=2

    http://s1268.photobucket.com/user/dlpanadero/media/20160318_162334_zpsuynaummt.jpg.html

    *Also, sorry how do I imbed pictures right into my post, instead of pasting a web link?
     
  2. Jamie Kennedy
    Joined: Jun 2015
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    Jamie Kennedy Senior Member

    Under "Quick Reply" you can "Go Advanced"
    From there you can "Insert Image" using the mountain scene icon.
    From their you insert the URL of your image, which you can get by cutting and pasting it from right clicking on your online image and viewing "Properties"
    But it doesn't seem to work from Photobucket. Not sure why.
    [​IMG]
     
  3. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I am not sure what you are planning to use for stringers, but I would scuff and vacuum the heavy Roving and then clean it with acetone, and then lay down some Matt as the first step in your stringer layup. The Matt should conform well to the heavy Woven Roving. As for scuffing up the surface of the Woven Roving, not sure, maybe a wire brush or wire brush wheel. It would be a good idea to do a test first, in a small area, maybe a 1 foot length of test stringer with a handle built onto it, and see how easily it peels away. You could test different ideas and see what works.
     
  4. dlpanadero
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    dlpanadero Junior Member

    Thanks for the reply. That's a good idea to do a test strip to see what kind of a bond I get on the Woven. *Also I just realized in the title I typed "Roven Woving" instead of "Woven Roving".. woops!

    Would you lay one big piece of Mat over the whole area, that covers everything (i.e. the hull and the old stringers) before starting to rebuild? Or just lay Mat over the bumpy Woven Roving of the bare hull? Thanks again
     
  5. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I would only put mat over the area the new strings are going. I would grind away any traces of the old stringers first, but not to the point of eating away at the woven roving.

    I can't tell from the photos. Is the heavy woven roving part of what you are calling the bare hull? (I presume yes) Or part of the old stringers? ( I presume not. I presume the old stringers were some sort of wood beam covered in glass, and the wood eventually rotted and the old stringers needed to be replaced. There might be some residual glass from the sides of the 'top hat' fibreglass going over the stringer. Some of this might be in the low areas of heavy roving, but probably well bonded and best left alone. Just needs to be scuffed up, and cleaned with a shop vac and acetone.
     
  6. ondarvr
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    ondarvr Senior Member

    60 grit won't do the job, use at least 36 on a grinder. You need to sand it down so that 100% of the surface has been sanded, which means it will be smooth at that point.

    You only need to use mat where you are actually laminating, no need for an addition layer anywhere, you just use it as the first layer of glass when putting everything down. And you don't put it down one layer at a time and let it get hard, you can put down several layers at once, the exact amount is determined by your skill, the exact resin, and the ambient temperature.
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Yes I should have been clear on that. The wetted out matt provides a nice squishy layer between the prepped hull and whatever comes next by way of new stringer, but ideally it all goes on before any of it sets so that it can all set and dry together. This not only avoids wait time, but also prevents creating layers that might more easily delaminate even if you take some extra time to reprep them. There are times though, in some operations, where some things do need to stiffen up before you can add the next stage. There is still usually a window of opportunity however, where it has stiffened up enough for you to proceed, but is still tacky and green enough that you don't need to reprep. I am not entirely sure, but I think you still need to be careful not to disturb the partially cured material, as agitating it before it completely cures will weaken it.

    So what do you have in mind for new stringers?
     
  8. dlpanadero
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    dlpanadero Junior Member

    That is correct - the Woven Roving is the bare hull, and the other material you see in the photo is the edge of the old stringer laminate that is sitting on top of the woven. I will want to glass past that original edge when I lay in the new stringers, that's why I was not sure how to best going about prepping that heavy Woven of the hull to lay new glass on. Seems like a hell of a lot of grinding to get it down smooth? Is this OK seeing that I'll be grinding/smoothing out the bare hull?

    The OEM stringers were actually foam cored with relatively light laminate laid over. This stringer delamination/failure was prevalent in these pre-1999 Pathfinder tunnel hull skiffs. I think they were actually those pre-fabbed Prisma foam stringers. As far as what I'm planning on using for the new stringers, I've been hemming and hawing over that for awhile. I'm not dead set on anything yet. I have considered nidacore. Or just shaping some foam and glassing over. I also know you can order those same Prisma foam stringers and pretty much just lay them right in. That would cut down a lot of labor it would seem. Any suggestions?? I would like to get a plan set in stone so I can really get to work and start executing. I've spent as much time looking at it and pondering than I have working on it. Thank you guys again for the insight and advice.
     
  9. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I am not familiar enough with all the core choices out there today, or the boat and the exact application and environment of these stringers. The original mode of failure is always an important clue. As for prep, don't be afraid to grind down into the layup as long as you replace what you remove with something comparable in thinkness and strength before add the stringer on top of that, while it's all still wet. I think you want to avoid a situation where the hull is significantly thinner under the stringer core than the hull not under the stringer core. Then again, I think you are actually grinding on either side of the stringer, but perhaps directly under also. Not sure. Anyhow, within the roving each glass fibre with a bundle of fibres in a weave is separate, so even if you remove half a bundle, when you clean it all up and add your resin and fibres it will all be good. The important thing is to replace what you remove with something equivalent, and to do so in the usually tapering fashion, and to get a good bond. The matt you will add to help conform to the surface will not compare pound for pound with the woven roving, but whatever you add on top of that, and as part of the stringer, should make up for it. Whatever stringer system you choose, in addition to adding stiffness in bending, it may also need to some good longitudinal strength in tension, and perhaps compression, if the entire hull length is prone to flexing as it bounces from wave o wave. The other consideration, even if no wood or water prone core is present, you want to avoid situations where water will collect and remain in place, because the presence of water, in addition to adding unwanted weight, will also contribute to the eventual deterioration of polyester resin and glass fibres, or whatever composite materials you are using. Water gets into cracks, makes cracks bigger even without freeze thaw cycles, gets absorbed and reacts with resin, eventually, even if the resin is well cured, more so in warmer climates. Water can also wick its way along glass fibres reducing the bond and average sheer strength, thus weakening the structure, more strain, more cracks, more water.

    So avoid air pockets, avoid uncured resin, use enough resin for full saturation, use a stringer system that avoids corners that make it hard to avoid air pockets, use a good roller for the job, a stringer system that matches the size of roller. Wait for the right temperature conditions to be able to use a resin and catalyst percentage that gives you the working time you need while still achieving fully cured resin. Use a heat lamp when you are done to provide a post-cure. This is all mostly picked up from my brother that does this more for a living than as an occasional hobby and interest.

    Does anything sit on there stringers, like the floor above? Do any fittings go into the stringers? Do you have good access for inspection, and ventilation when required, and a good water drainage system if required. These all effect what core to use, and what you might need to do if you need to screw into a stringer, or put a water channel through a stringer.

    If weight isn't too critical, use a heavier core material that is less prone to problems, or even consider a stringer system that doesn't require a core material, or uses something like a plastic or aluminum square or round tube that the fibreglass material can go up and over. Polyester resin doesn't bond well to aluminum, but epoxy does. You generally want to avoid using epoxy though, because once you can't go back to polyester over it.

    I think I saw a good video on replacing stringers on a power boat like a pathfinder. I think he used wood, had a good system to keep the wood sealed. If I used wood for a core material, under polyester glass, I would use real wood, not plywood, and make sure the sides and end grains are good and rough. I would coat the wood with polyester before glassing over it. I would be particularly careful where I had to screw into or bolt into the wood, maybe drilling out an oversize hole and filing it with epoxy glass, and then screwing into that. Similarly, if I put a drainage hole through it, it would be a plastic or aluminum tube, epoxied into a bigger hole. Basically keep the wood well buried, and keep the glass over the wood dry.

    Maybe consider all glass stringers.

    My 2 cents. Have fun with it.
     
  10. dlpanadero
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    dlpanadero Junior Member

    Thanks so much for the response - much appreciated and really has been helpful. Nothing will be screwed/bolted into the stringers, and only 2 of them support the (very small) cockpit. I think what I'll do is a layer of Mat like you suggested for a 'base' layer, then nidacore composite material as the core, glassed over with biax. I think this will be absolutely sufficient, and I'll just do the best I can to ensure good secondary bonding to the Woven on the hull. At the end of the day, it is a 15' fishing skiff. That being said, of course I want to do the job to the best quality possible and in a way that is structurally safe and efficient. I'll give it my best shot.. :cool: Thanks again, and feel free to add anything else if you've got additional insight/suggestions
     
  11. ondarvr
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    ondarvr Senior Member

    Nidacore would be a poor choice as a core for a stringer, it's costly and offers no strength in that orientation, cardboard would be about the same. If the cost isn't an issue, then you just need to use more glass than with a wood stringer.
     

  12. dlpanadero
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    dlpanadero Junior Member

    Thanks for the input. I think I've got a substantial amount of nida-core scrap pieces left over from a project my brother was doing. If there's not enough I doubt I'll buy more to use just for stringer core. You're right it would be a waste. I've also been thinking about PVC tubes? Seems they would over a very nice medium with no sharp edges, and would allow the laminate to lie over it with minimal air spaces. If I did use PVC, would it be bad policy to just leave them open at the ends and not seal them off? *Keep in mind for this application I am speaking just about the bow area of the boat under my front deck (see pics from 1st post). The primary stringers under the floor/cockpit will have a different structure.
     
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