Flying Scot hull record alternatives

Discussion in 'Fiberglass and Composite Boat Building' started by Josephc, Nov 20, 2018.

  1. Josephc
    Joined: Nov 2018
    Posts: 6
    Likes: 0, Points: 1
    Location: Ashevilke

    Josephc Junior Member

    I have a flying Scot that needs most of the balsa replaced. I have cut out the old sole and removed the rotten balsa. What can I use that's not balsa for this repair? I have no interest in keeping the boat original and want to fix it on the cheap. if I were to just add epoxy/glass to the floor to make it stiff enough to leave out the balsa how much build up am I looking at? Alternatively could I replace the balsa with end grain pine? I realize this would be a bit heavier.
    Thanks!
     
  2. rwatson
    Joined: Aug 2007
    Posts: 5,868
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    You would be best to use a lighter timber like Western Red Cedar. Weight of heavier glass isn't desirable.
    If you are lucky enough to live somewhere near a supplier of plantation grown Paulownia (Kiri) you would have a rot proof timber almost as light as Balsa but as strong as Cedar.
    Even strips of a lightweight marine ply would be better than most pine.

    Glass wouldn't need to be as thick as the Balsa, but it would need extra "ridges" to ensure stiffness.

    Assuming you left the outer layer of glass intact, you could get away with a layer of 300 gsm over that, and lay a series of half rounds of PVC pipe on the surface, and glass some more 300 gsm cloth over them to build "riblets".

    You could assess how much stiffness you get simply by standing on the finished section. If it bends, it needs "more". It's only a dinghy after all.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 15,184
    Likes: 923, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    It would be easier to replace with foam.
     
  4. Josephc
    Joined: Nov 2018
    Posts: 6
    Likes: 0, Points: 1
    Location: Ashevilke

    Josephc Junior Member

    Are you referring to the dense pvc coring foams that cost twice as much as balsa? If so that's out of my price range. I have some polystyrene blue board scraps but that stuff seems too weak to me.
     
  5. Josephc
    Joined: Nov 2018
    Posts: 6
    Likes: 0, Points: 1
    Location: Ashevilke

    Josephc Junior Member

    I like your pvc idea. Only problem would be lateral stiffness with this scheme. I've done the math and the pine would weigh about 3 times as much 15 vs 45 pounds for the amount I need to replace plus I already have the scraps laying about and a chop saw to cut it.

    Paulownia tree would be nice since it grows all over the damn place here but no one saws it that I've seen.

    Oh and what is 300 gsm is that a type of roving?

    Thanks for the useful feedback!
     
  6. rwatson
    Joined: Aug 2007
    Posts: 5,868
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    I don't know why lateral stiffness would suffer. Just make the diameter of the PVC equal or in excess of the current hull thickness. As long as you do a decent job of merging the "ribs" into the remaining hull material, there will be plenty of lateral stiffness. You are after compressive strength mostly. The compound curve of the flying scott gets its strength from monocoque stiffness, not a whole lot from stringers. The PVC offers NO strength gain - its just a convenient rot proof Former

    It's not clear what you are comparing what weights with what to me.

    Heck, use any old wood you like, if convenience is your thing. Why not use Balsa again if you don't care about longevity ? Pine isn't much better than Balsa for rotproofness unless you get some resin rich samples which will then make bonding a problem.

    Most decorative (non Plantation) Paulownia isn't structurally clear, as it would have branch voids, like Pine that isn't trimmed regularly in Plantations. But, if you could get a couple of trunks, mill them up roughly and cut them end grain like Balsa, you would probably get a useful rot proof product.

    Yes, just the normal equal weave cloth. For antiquated measurers, that's about 12 oz roving, double the 6 oz you would use for covering a plywood dinghy. You might find a slightly heavier weave isn't that much more expensive.
     
  7. Josephc
    Joined: Nov 2018
    Posts: 6
    Likes: 0, Points: 1
    Location: Ashevilke

    Josephc Junior Member

    I think your idea is becoming clearer to me. The pvc would run port to starboard and just make a tube shape for strength. I was picturing fore to aft. So in theory I could use anything to make these voids split bamboo cardboard etc?

    I was comparing the weight of balsa to pine.

    Thanks again for the help.
     
  8. philSweet
    Joined: May 2008
    Posts: 2,414
    Likes: 240, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Just go solid GRP. Support the hull, or whats left of it, as well as possible to preserve the original shape. Don't be shy, it going to get ugly in there. Take a wrecking bar and the biggest firming chisels you have and hog out the old core. Grind with an big wheel with 36 grit and light pressure. It doesn't have to be pretty, but it does have to be smooth. Test with a small putty knife. It has to be smooth to the point that a putty knife doesn't hang up anywhere. Then start your layup. Use Gerr's Boat strength for scantlings, but a minimum of 1/8" additional regardless of the calcs. I'd use cheap $40/gal laminating epoxy and 1208 or 1708 biax for most of the hull layup.

    I assume that's Asheville? I live in Fletcher.
     
  9. Josephc
    Joined: Nov 2018
    Posts: 6
    Likes: 0, Points: 1
    Location: Ashevilke

    Josephc Junior Member

    Damn I left my copy of Gerr's at the office! You wouldn't happen to have the chart handy?

    I've got the hull mostly cleaned out already. Where can I get this 40 a gallon epoxy? Mostly I've purchased stuff from US composites which is going for $60 a gallon. I worry the amount of epoxy needed would even out to buying the balsa.

    I'm hoping to take this lady down to lake Julian before it gets to cold.

    I should add the boat set on a trailer for 10 years with the rotten core so the hull shape is fairly distorted from its original design I assume.

    Thanks
     
  10. rwatson
    Joined: Aug 2007
    Posts: 5,868
    Likes: 301, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Yup, the PVC is just a former. Its one of the easiest precision shaped non-rot materials to use. If it wasn't a budget job, I would have suggested foam strips. but that gets expensive.

    Any former that is biologically based is a potential rot hazard. Cardboard could weaken and collapse when wet out prior to cure.

    Don't restrict yourself to one direction. Some smaller diameter "crisscross" formers, to form a grid would be good value.

    The idea of supporting the hull while laying up from Phil is a good suggestion. As you press work onto the surface, you can distort the hull in unexpected ways.

    Before you lay up the first single layer of cloth, you could run some heavier cloth strips onto the hull where you expect to put the re-inforcing ribs and let them cure.
    This would reduce hull distortion from pressure while building.

    Re Phils layup suggestion DBM1208 is a 12oz double bias glass (which is what I suggested) but with 3/4 oz chop mat stitched to it. and 1708 is 17 oz. per square yard, and the mat backing is 8 oz. per square yard, so the total weight is 25 oz

    Say you have one square yard to fill, 1708 would use 25 oz of rezin at least, so 25 + 25 oz would be around 4 pounds, or 2 kilos, but you would be relying on the matt for stiffness. Some matt has binders in it that make it not work for Epoxy Resin, so just be carefull. I personally never use matt with Resin on lightweight craft because the stiffness to weight benefits are low, as well as soaking up expensive Resin.
     
  11. gonzo
    Joined: Aug 2002
    Posts: 15,184
    Likes: 923, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Using mat for bulking (as a core) will stiffen a laminate and would have little or no difference with using cloth or roving. However, the cost and speed is less.
     

  12. Josephc
    Joined: Nov 2018
    Posts: 6
    Likes: 0, Points: 1
    Location: Ashevilke

    Josephc Junior Member

    Thanks for all the advice! Just to sum it all up. If I wanted to just use grp to replace the balsa how thick of a coat would I need and what cloth, mat or roving would be best? The current hull is between an eighth and a quarter inch( 3 to 6 mm).

    Thanks again for the help
     
    Last edited: Nov 25, 2018
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.