Hull (Bottom) Thickness-Does It Matter?

Discussion in 'Wooden Boat Building and Restoration' started by Rollbar, Aug 24, 2018.

  1. Rollbar
    Joined: Jul 2018
    Posts: 12
    Likes: 1, Points: 3
    Location: Florida (West Coast)

    Rollbar Junior Member

    Hello, soon (in a few months) I'll be building a Spira Albion 19' (first time builder).
    Plans call for a 5/8" on the bottom of the hull and I was wondering if it is worth using 3/4" instead/just wanting a stronger or thicker hull.
    If not maybe just the front part of the hull with 3/4" for beaching/longevity etc. and fair it down to meet the 5/8" for the rest of the hull.
    Is it worth it since I'll be glassing the hull (6oz)-2 layers maybe)?
    Thanks for your help/opinion,
    JB
    http://spirainternational.com/study/AlbionStudy.pdf
     
  2. Blueknarr
    Joined: Aug 2017
    Posts: 657
    Likes: 100, Points: 43
    Location: Colorado

    Blueknarr Senior Member

    welcome to the group

    Congrats on biting the bullet and starting your first build.

    3/4 plywood will be much harder if at all possible to flex into the bow's shape.

    It will be impossible to scarf two thickness of plywood and have them flex smoothly.

    5/8's is pretty stout for a 19 footer.

    Adding extra fiberglass is an easier way to achieve greater impact and abrasion protection.

    Good luck on your build.
     
  3. JamesG123
    Joined: Mar 2015
    Posts: 655
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    Location: Columbus, GA

    JamesG123 Senior Member

    Anything that can penetrate or make 5/8" fail is probably going to make 3/4" fail too. And like "Bluekarr" mentions, its pretty hard to work with unless your design is faceted from flat sheet.
     
  4. Rollbar
    Joined: Jul 2018
    Posts: 12
    Likes: 1, Points: 3
    Location: Florida (West Coast)

    Rollbar Junior Member

    Ok thank you very much.
    Been reading up on a lot of posts to learn.
    Thanks again,
     
  5. flame
    Joined: Mar 2003
    Posts: 19
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    Location: Laxenburg/AT

    flame Junior Member

    Spira Albion is a Dory, so the bottom is more or less flat with a skight bend towards the bow, that should be manageable
    the sides are sitting on the upper surface of the bottom, so that doesn't need the plans to chage, another pro
    the center of gravity will not move significantly due to the additional weigth, so impact will be minimal
    weigth will be a bit higher ... you feel it when you carry the hull to the water with your buddies, but you'll manage

    the question really is: cui bono? what forces are you expecting from below (or above ... through the false floor) that would call for 3/4" ... if any the bottom will slide on something ... for that it doesn't matter if 5/5" or 3/4" .... price?

    how's going sofar with your build?
     
  6. romeomikehotel
    Joined: Feb 2019
    Posts: 17
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    Location: Ft Lauderdale

    romeomikehotel Junior Member

    I believe Spira spaces his frames nearly every 2’.
    If speeds were in the 55-65 range, would 3/4 be a significant advantage over 5/8?
     
  7. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Nope.
     
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  8. romeomikehotel
    Joined: Feb 2019
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    Location: Ft Lauderdale

    romeomikehotel Junior Member

    Would that opinion change if the requirements were to run 55-65 through 2-3’ chop. Assuming some slamming now and again at speed?
     
  9. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Seriously ? 55-65 (mph ?) in that boat in 2-3 feet chop, completely unrealistic, don't worry about the boat breaking, the passengers would be medivac jobs.
     
  10. romeomikehotel
    Joined: Feb 2019
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    Location: Ft Lauderdale

    romeomikehotel Junior Member

    Lol no not in that boat.

    Just hypothetically. Assuming a boat that’s not a flat bottom.
    Thinner ply is easier to work with and likely saves around 100lbs if a hull bottom is 5/8 instead of 3/4.

    But is that 100lbs and ease of use a worthwhile trade off in an extreme use environment.
     
  11. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    100 lbs sounds like a lot for 1/8" of ply, but it depends on how close spaced stringers and bulkheads are, as to whether it would help, the main thing would be to have a nice stiff bottom, rides better, and over time ply does lose a little bit of stiffness with repeated slamming loads.
     
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  12. romeomikehotel
    Joined: Feb 2019
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    Location: Ft Lauderdale

    romeomikehotel Junior Member

    So I was ballparking a bit. 3/4” is 68.1lbs a sheet and 5/8” is 56.6lbs a sheet.

    Spira’s larger plans call for around 7 sheets for the Hull bottoms so that puts it around 80lbs.

    Which I guess probably answers my own question, it’s probably better to use 3/4”
     

  13. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    Look at the Spira Albion study plans. http://spirainternational.com/study/AlbionStudy.pdf The Bill of Materials calls for 4 sheets for the bottom, not 7 sheets. And not all of the plywood will be used in the boat. There will be some scrap. So the weight difference will be 50 lobs or less.
     
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