Building Flotation Compartments (w/Epoxy & Fiberglass)

Discussion in 'Fiberglass and Composite Boat Building' started by useragentseven, Mar 21, 2019.

  1. useragentseven
    Joined: Apr 2017
    Posts: 40
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: Orlando, FL

    useragentseven Junior Member

    Now, I know that water-tight compartments are normally placed in appropriate areas of a vessel to provide bouyancy, and prevent the vessel from sinking.

    I have recently noticed some catamaran manufacturers boasting that their cats have 'floatation compartments' designed into their hulls, and bridge-decks for the event of a capsize.

    I'd like to experiment with the 'flotation compartment' idea, and try to create a water-tight/air-tight compartment with epoxy and fiberglass.

    I'd like to build the box out of marine-grade playwood, and glass over it. For strength, I'd like to seal all the inside edges of the compartment with epoxy fillets (including the 6th side, if possible).

    Can anyone tell me what is the best method(s) for attaching the 6th side (the top of the box), and making sure it is a strong, secure, air-tight/water-tight seal? o_O

    Thanks! :D
     
  2. JamesG123
    Joined: Mar 2015
    Posts: 655
    Likes: 74, Points: 28, Legacy Rep: 10
    Location: Columbus, GA

    JamesG123 Senior Member

    Just use foam.

     
  3. Ike
    Joined: Apr 2006
    Posts: 2,208
    Likes: 169, Points: 63, Legacy Rep: 1669
    Location: Washington

    Ike Senior Member

    Actually most "flotation compartments" don't necessarily have six sides. They are usually built into the hull or under things such as seats. Building them into the bow only requires one watertight bulkhead. Under seats they can be a box, with the seat as the top, or go all the way to the deck below the seat. On the sides of the hull they can be a box with the overhead (the deck) as the top, and the lower deck as the bottom. Use your imagination. They need to be firmly attached to the hull. If the lift they provide is only under a deck or cabin top, they may just lift the deck or cabin top and leave the hull behind.
     
  4. Dejay
    Joined: Mar 2018
    Posts: 314
    Likes: 41, Points: 28
    Location: Europe

    Dejay Senior Newbie

    Sure! It's simple. On the opposite side of the 6th side (1st side?) you saw a hole you can fit your hand in. Then you can attach the 6th side and make good fillets. Easy!

    You're welcome! Always happy to be helpful :p
     
  5. useragentseven
    Joined: Apr 2017
    Posts: 40
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: Orlando, FL

    useragentseven Junior Member

    OK, so, I could make the hole, let's say a square shape, and reach inside and build out the inside with fillets (sounds good).

    But, then how would I seal the hole to make the compartment air/water-tight?
     
  6. Dejay
    Joined: Mar 2018
    Posts: 314
    Likes: 41, Points: 28
    Location: Europe

    Dejay Senior Newbie

    That's easy as well! You just cut a hole say in the... oh wait I see the problem now ;)

    I'm sorry haha. Honestly I've contemplated the same about making long box beams. Maybe there actually is a trick to this. The only thing I can think of is just either glue it without a fillet, or make a "hanging" fillet between the sides and a temporary polyester or melamine board, wait till it hardens and then glue the sixth plate in. But since you'd be expecting relatively equal pressure from the outside I don't think a fillet would add much strength. Maybe choose the fillet-less side of a compartment as the one that gets the primary water pressure so the forces acting on it don't require a fillet.

    Or just use foam. No condensation either.
     
  7. JamesG123
    Joined: Mar 2015
    Posts: 655
    Likes: 74, Points: 28, Legacy Rep: 10
    Location: Columbus, GA

    JamesG123 Senior Member

    If you are building a compartment large enough to bother with finishing out, then you should also make it a "dry storage" space. Your hole should be round and once you finish slopping and slathering epoxy around, you close off the compartment with one of these:
    [​IMG]
    That way you can use your "flotation space" to store stuff.
     
  8. fallguy
    Joined: Dec 2016
    Posts: 1,802
    Likes: 98, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    You can also flange the top of the box, then relief and radius the top of the box, glue the lid on and glass tape only the outside top of the box.

    There is really no benefit to glassing the seam from the inside.

    Rumor has 12 oz mat backed tapes are sort of the minimum for watertights.
     
  9. useragentseven
    Joined: Apr 2017
    Posts: 40
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: Orlando, FL

    useragentseven Junior Member

    Althought the access hatch is a great idea, that would finish off the compartment nicely, and allow for storage, I wouldn't want the integrity of a multi-ton vessel to depend on an 1/8" inch thick plactic cover. I want an air/water-tight compartment that is completely, and permanently sealed.

    Any ideas? Methods? Techniques?
     
  10. JamesG123
    Joined: Mar 2015
    Posts: 655
    Likes: 74, Points: 28, Legacy Rep: 10
    Location: Columbus, GA

    JamesG123 Senior Member

    Foam.

    If your multi-ton vessel is depending on a single compartment for buoyancy then....
     
  11. Dejay
    Joined: Mar 2018
    Posts: 314
    Likes: 41, Points: 28
    Location: Europe

    Dejay Senior Newbie

    Or foam core + insulation. 100m² of 20mm foam core would provide almost 2t of displacement. You could also insulate a few more walls with even thicker XPS foam.
     
  12. useragentseven
    Joined: Apr 2017
    Posts: 40
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: Orlando, FL

    useragentseven Junior Member

    I would like to try to avoid using as much foam as possible, as foam can add up and get quite expensive quickly when using it throughout a vessel as a core or fill material.
    Air is free, however, and I want to completely enclose/seal multiple compartments throughout the vessel for air/water-tight buoyancy.

    Just interested to see if there are any industry professionals in here who may know how to properly sealing a water/air-tight compartment, and what kind of seal is an acceptable strength for a multi-ton vessel?

    Thank you guys, who have helped with your input, and I don't mean to minimize your help/contributions. Sorry.
     
  13. Dejay
    Joined: Mar 2018
    Posts: 314
    Likes: 41, Points: 28
    Location: Europe

    Dejay Senior Newbie

    XPS rigid foam costs about 137€ per cubic meter (tonne) of displacement.

    You're right it's probably a rather rare special use case where this makes sense on a boat. I plan to insulate part of my boat as a permanent living space. But that should make it absolutely unsinkeable.
     
  14. JamesG123
    Joined: Mar 2015
    Posts: 655
    Likes: 74, Points: 28, Legacy Rep: 10
    Location: Columbus, GA

    JamesG123 Senior Member

    Industry professionals don't use wooden boxes for their flotation. They use foam. Foam for intentional bulk buoyancy spaces, foam as the core material in their composites, and foam to fill all the tiny nooks and crannies of unuseable space within a boat that add up to "unsinkable".
     

  15. useragentseven
    Joined: Apr 2017
    Posts: 40
    Likes: 0, Points: 6, Legacy Rep: 10
    Location: Orlando, FL

    useragentseven Junior Member

    I'm only experimenting with plywood as a quick and dirty way of supporting the fiberglass I have on hand.
     
Loading...
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.