Enclosed buoyancy compartments on wood/plywood boats

Discussion in 'Boat Design' started by laukejas, Jun 27, 2019.

  1. laukejas
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    laukejas Senior Member

    Hi,

    I would like to know your opinions on enclosed buoyancy compartments on wood/plywood boats, especially the issues of the trapped moisture, ventilation, and the life expectancy of a such a boat compared to a boat without such compartments. Lately, I've been considering to design and build a racing sailing dinghy with a double bottom and open transom. This seems to be a growing trend among most of the factory-made racers, like RS Feva, Aero, Melges 14, Devoti D-Zero, and some of the larger boats as well. Such an arrangement allows for a very easy capsize recovery, as the double bottom provides adequate buoyancy to automatically bail all the water out through the open transom. But factory-made boats use modern materials, while wood/plywood are still the most accessible and affordable materials to a home builder. I gather that wood is much more susceptible to prolonged moisture damage than most modern plastics, and therefore any enclosed buoyancy compartments may present a problem. Obviously, there has to be drain holes and multiple ventilation hatches so that these compartments could be (somewhat) dried out in order to prevent rot. My good friend, an experienced boatbuilder and also member of this forum, says that natural ventilation is not enough, and that if one is to expect any kind of longevity from such a boat, one must use low velocity fans for hours, every few days during sailing season. Which, as you can imagine, is not something anyone would be looking forward to.

    Previously, I had assumed that a proper drain/limber holes, as well as thick epoxy insulation of the structural members inside these compartments (5 layers at least), and natural hatch ventilation (no fans!) would be enough on it's own. It appears it may not be enough.

    This begs the question of whenever such enclosed compartments are a practical idea on a wood/plywood boat at all, given that one might not have the time, patience or facilities for an extensive care to dry out these compartments on a daily/weekly basis beyond a simple hatch unplugging.

    Therefore, I turn to you, hoping you can provide some more insight to this issue. What were your experiences with wood/plywood boats with such compartments? To what lengths did you go to keep these compartments dry? How bad was the moisture ingress, and how long did these boats last?

    One more idea suggested by my friend. Would there be any point in having silica gel bags inside these compartments, somewhere near the hatches so they could be replaced when needed? Would that make any difference to keep the moisture away from the wood?
     
  2. gonzo
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    gonzo Senior Member

    If the bottom plywood boat with epoxy waterproofing is adequate, a flotation chamber will be fine. It will not have more moisture than the bilges of a plywood boat. There are many plywood boats with flotation chambers that were painted with alkyd enamel and have lasted for decades. Coating with epoxy will provide a much better moisture barrier.
     
  3. tpenfield
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    tpenfield Senior Member

    Enclosed compartments and wood do present a problem. I often refer to wood in boats as being similar to a sponge or open cell foam. Wood wants to absorb water and propagate it wherever it can. The nice thing about wood is that the density of wood vs density of water means a lot less flotation is needed vs. a fiberglass boat. The double bottom is also going to be a flotation compartment (presumably).

    I see a lot of boats (both power and sail) being made from wood for the reasons you stated. You don't often see the longevity aspect, only the finished result. I am wondering if a 'cold molding' approach to building the hull would be an answer to the issues you raise. More complex and tedious than traditional wood construction methods, but probably better than re-building the boat after about 10 years of use.
     
  4. JamesG123
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    JamesG123 Senior Member

  5. Heimfried
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    Heimfried Senior Member

    I'm building a plywood 6 m motor kat with 4 buoyancy compartements. My solution is to seal the plywood inside (and the wood of the frame) with epoxy and leave the compartments empty, fitting in an inspection hatch which is tightly closing. I would never use foam, because it is hiding moisture (this is partially the same for empty bottles). But I'm sailing only in sheltered inland waterways, so my decisions are made regarding this conditions! Silica gel bags will help, because they are binding some humidity. (Do you know, they are to regenerate after use? Put it for some hours (e. g. over night) in a place with temperature > 100°C and it will lose its moisture and be able again to take moisture fron the air.)
     
    Last edited: Jun 28, 2019
  6. lenm
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    lenm Junior Member

    Is your friends account from an experience or just opinion?
    Interested to hear what waterproofing was used on the failing boat.
    I can't see how a well build epoxy sheathed plywood boat will fail so prematurely compared to foam core.
     
  7. tlouth7
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    tlouth7 Junior Member

    I have a Mirror dinghy which has 5 buoyancy compartments. These compartments each have a single limber hole but no inspection hatch. There have been no issues with the wood quality after ~50 years of life.
     
  8. fallguy
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    fallguy Senior Member

    As James said, most people building ply boats are using 2# marine foam in these chambers. Be careful about sealing them until after foam as some builders have experienced expansion beyond expectations and the results are spurious, raising soles, forcing hull, etc.

    My build is foam sandwich and I have about 10-12 watertights. They all have access holes for ventilation. I intend to seal them up fairly tight at ambient temps. No foam is planned. I needed them to breathe. Someone told me I could put some sort of membrane seal on them that would allow air to pass and not water...boat is white color; not too worried.
     
  9. Ike
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    Ike Senior Member

    I have experience with all kinds of flotation arrangements. For small wood boats (say 13 feet or less) the most common is simply sealed air chambers. I have seen no issues with these boats getting moisture in the air chambers. Putting in a watertight access port gives you the ability to check if needed but is really not necessary. On some boats, builders have (including me) put foam blocks (not the expanding pour foam) in the chambers. But I see no real advantage in that. I have not seen any problems with longevity either. Sealing the wood with epoxy or a penetrating epoxy can keep any incidental moisture for penetrating the wood.

    On larger boats that have outboard motors the problem arises of adequately supporting the motors so the boat doesn't roll over. That is the problem with putting the flotation in the bottom of the boat. These small boats are supposed to have level, or modified level flotation, meaning they float upright when swamped. If you put the majority of the flotation in the bottom they roll over and float nice and level upside down which is not acceptable. And the number one reason for wooden boats failing flotation requirements in Coast Guard testing is inadequate flotation for the motors.

    If you need more info on this see my page on flotation Boat Building Regulations | Flotation http://newboatbuilders.com/pages/flot.html
     
  10. fallguy
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    fallguy Senior Member

    Almost all the builders I know are foaming under the sole, which would go against your desired outcome.

    In boats without a deck above the outboard height; is it even achievable?

    I might sound antagonistic, but mostly curious.

    Is desired outcome only then achievable in certain designs?
     
  11. Ike
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    Ike Senior Member

    It is. Just takes a little thought about placement of flotation. If you put it under the sole separate it into two parts, one to port, one to starboard, with a space between. In a wooden boat it doesn't take a lot of added flotation to get it to float. The only thing that really needs extra flotation is the engine. Again separate it into two components one on each side in the rear quarter next to the transom. If the boat is a rowboat you may not need any added flotation at all. Sailboats aren't required by law to have flotation but many builders put it in anyway. If the boat has a gunwale, put some on the sides under the gunwale to provide stability, or if it has athwart seats, under the seats. They are high enough above the bottom to provide stability. On the two small wooden boats I built there are air chambers in the bow under the seat and also under the seat in the stern. These provide enough buoyancy to float the boat relatively level and stable. On the 12 foot one the flotation under the seats including the middle seat provides enough to float the 2 HP outboard it is rated for. I did put in some foam just for added safety in case the chambers got water in them.

    see Boat Building Projects | Building the FL12 | Page 1 http://newboatbuilders.com/pages/fl12.html
    and Boat Building Projects | Building a Sailing Pram | Page 1 http://newboatbuilders.com/pages/Dinghy.html especially page 3

    You will notice that on the 12 foot the front and rear seats had hatch covers. That didn't work out very well and I have since made the top with out the hatch. They are now just sealed compartments.
     
  12. laukejas
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    laukejas Senior Member

    Thank you all for your replies. You have raised multiple good points. So I gather that the moisture problem in these enclosed compartments is not as severe as I had previously thought. I do have a question about the foam, though. Some of you said to use foam, some said not to. What purpose does the foam serve if these compartments are already sealed with hatches? I don't really see any way how a hatch could fail. Wouldn't that foam trap moisture and prevent ventilation, as Fallguy has suggested?

    How much sailing did that boat actually see? 50 years sounds like almost unrealistic for a wooden boat, especially one with an enclosed buoyancy compartments.
     
  13. Blueknarr
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    Blueknarr Senior Member

    Flotation foam servers multiple purposes.

    Used as structure of float chamber.
    To satisfy Coast Guard requirement that flirtation chambers need to be filled.
    Even a small vent hole near top of chamber may negate buoyancy potential of chamber
    ****Piece of mind****
     
  14. laukejas
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    laukejas Senior Member

    "flirtation chamber" sounds like the place on a boat that you would bring women to :D As a structure, you say? So it should be somehow glued to the plywood panels of the boat? Is that even possible without using expanding spray foam? As for the vent holes, shouldn't they be closed off while the boat is in the water?
     

  15. Ike
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    Ike Senior Member

    I would recommend you do not use expanding spray foam. It has issues, the worst of which is its tendency to absorb water. Yes, it is not supposed to, but has a notorious reputation for doing just that. I use block foam (or sheet foam) It's inexpensive, you can buy it at any hardware or home improvement store, and easy to cut into right sizes to fit into compartments. Of course you have to put it in before you close up the compartment. And you have to protect it from exposure to gas or other petroleum products that dissolve it. It is polystyrene, not polyurethane, and not styrofoam which is a trademarked brand name and has much larger cell structure. People often refer to these insulation foams as pink or blue (depends on the brand) but they have a much smaller cell structure and are not as prone to friability (coming apart) as styrofoam. We have all seen the little white balls floating in the water from stryrofoam. The polystyrene foams sold for insulation don't do that, and they are two pound density foam which is the right weight for flotation foam.
     
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