Balsa to foam

Discussion in 'Fiberglass and Composite Boat Building' started by Blueknarr, Jun 25, 2018.

  1. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    I'm repairing a boat with a 1/2 inch of void and occasional rotten balsa below waterline. Which foam would be most equilivant?
     
  2. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    The compression strength of Balsa varies a lot, from about 5 to 20 mpa https://www.auszac.com/pdf/Balsa_wood_Properties_Guide.pdf

    If its below the waterline, I guess you would have to go towards the higher ratio, say about 12 mpa, say
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    Depending on your locations , you might be able to find some sources of plantation grown Kirri or Paulownia, which is a stronger wood, with similar weight to Balsa, that starts at about 20 mpa, and is rotproof.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Rotproof ? I like the sound of that ! Is paulownia sold as end-grain squares, like balsa ?
     
  4. rwatson
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    Location: Tasmania,Australia

    rwatson Senior Member

    It's actually got the same strength as western red cedar, so you don't have to end grain it. Its as rotproof as Huon Pine, which is really saying something. But it grows quite quickly, in plantations.

    In Australia, the best source is Port Phillip Plantation shutters. The Bowdidge marine website link is on that page. he uses it extensively.

    Paulownia - Marine Applications http://www.paulowniatimber.com.au/marine.php
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Paulownia certainly appears to have definite advantages in boat building, especially where weight is critical. One wonders why it has been relatively unheard of, till recently.
     
  6. rwatson
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    rwatson Senior Member

    Those in the know have been pushing it for over fifteen years.

    You can find posts from me back to 2009 on this site alone, but people just don't pick up on it.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Yet it doesn't seem to be readily available as a plywood, why is that ?
     
  8. rwatson
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    rwatson Senior Member

    Plywood would be a waste of time.

    As far as I can tell, the price per square meter is pretty close, and trying to peel a log that is already strong enough and waterproof enough and attractive enough would be a waste of time. With the Kirri, you can simply edge join it, and it's a solid sheet.

    They have been using it as surfboards for years now, solid planks, edge joined with just a light oil covering. It's better with a light glass coat for boat work, as it is a bit soft to run up on a beach or a trailer continuously, as you might expect.

    When you think about it, plywood is just a sneaky way of making sh&^house wood look good with a layer of heavier wood on each side, covering knots and imperfections, and having to do cross laminations to make it strong enough. Kiris only downside is that it's not quite as flexible as WRC, not so bendy as the Cedars, so you can't really take it down to say 4mm and get it to bend like some plywoods.

    They will mail you some sample coupons quite cheap, so you can get a feel for it yourself, but I would buy a few decent lengths to get the real feel for it. It planes and cuts really well, and doesnt have a strong silica content, so it doesn't blunt saws so easily, and isn't as bad for you to breath the fine dust particles.
     
  9. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    I'll pass on the kiris for this project. There is quite a bit of shape to this hull. I don't want to break the inner skin trying to force inflexible wood up to it.

    I am surprised that it was originally balsa cored. It is an overgrown twin motered jetski. I am not surprised by the rott as the previous maintainer's repair of choice was silicone. It was brought to me on Thursday to buff out some "stress cracks" so it could be used over the weekend. They were informed that it couldn't be made safe for many weeks; and no repair estimate until exploratory surgery revealed full extent of damage.

    As a technician not an engineer, I usually rebuild as originally done. However, I don't think balsa was best choice originally, so would prefer synthetic core or solid. Would one of you certified engineers give me usable advice?
     
  10. rwatson
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    rwatson Senior Member

    Well, for a start ,you don't need to "bend inflexible wood" at all. Just cut the Kirri into squares like the balsa was. You can just epoxy them in place, and then glass over them like Balsa if the bends are too tight.

    If you still decide to go for foam, just ring up your local foam supplier and say you are replacing balsa, and get their recommendations. At least you know what the maximum compressive strength is for Balsa to check their figures.

    Its that easy.
     
  11. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    My bad. I meant to thank you for providing the compressive strength of balsa in your earlier post.

    I'm two miles away from a regional distribution center for a national composites supplier. Unfortunately the local sales team is focused on aerospace and doesn't want to understand the maritime aspects their product line. My assigned sales rep is almost as useful as a unicycle missing a wheel.
     

  12. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Can't see anything in Colorado on a first glance , but if you want a something better than balsa, here a list of Kirri or Paulownia contacts, Merchants and growers, in the USA . Some on California, a short truck load away.
    American Paulownia Association http://paulowniatrees.org/links.htm
     
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