Reserve buoyancy in a small open craft, where to place?

Discussion in 'Boat Design' started by Flotation, Jan 12, 2020.

  1. Flotation
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    Flotation New Member

    I'm trying to find out where the best location is to place reserve buoyancy in a small (6m ~20ft) open craft.

    I can rationalize the placement high, near the gunwales or low near the bottom of a boat but i can't find a good answer or discussion online to help find a definitive answer.
     
    Last edited: Jan 12, 2020
  2. bajansailor
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    bajansailor Marine Surveyor

    How about both high and low combined?
    You will need buoyancy compartments in way of the bow and stern, and ideally they should be as deep as possible.
    How 'open' is your boat?
    Can you fit (for example) a couple of transverse seats buoyancy boxes in way of the middle of the boat? Or watertight seating along the hull sides?
    Is this for an existing boat, or a new design that you want to build?
    Do you have any drawings or photos of the boat?
     
  3. Flotation
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    Flotation New Member

    Last edited: Jan 12, 2020
  4. DCockey
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    DCockey Senior Member

    Floatation foam/tanks in small boats needs to be placed so that the boat will remain upright if flooded, preferably with the occupants sitting inside. To accomplish this the floatation needs to be outboard and not too low.
     
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Since your question is quite open and general, I'd chime in to suggest Flotation be designed to serve some additional function besides sitting around taking up otherwise useful space.

    Lower Flotation could also serve to block water (and muck) from settling into places where it can't be easily completely cleared. (always been a pet-peeve of mine that car floors can't just be hosed out with cleaning solution to perfect hygienic, the way I stripped my cargo van's floor to bare metal with removal rubber mats, but I digress)

    Upper Flotation could also serve as paddling for either seating AND/OR fenders. Buy a bunch of these and rig them so when on the inside of boat they are a row of hammock seats along the gunnel, but can be flipped over to the outside of boat to become fenders. https://www.amazon.com/Stearns-3000...ting+throw+cushion&qid=1578844395&sr=8-2&th=1
     
  6. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I suggest covering the upper 2ft of outer hull with a couple dozen of these "eyes". That will allow layers/levels of 6" poodle-noodle with PVC pipe inserts to be added as conditions warrant (also serve as fenders). The Eyes will also come in handy to secure covering tarps a good 2ft below the gunnel so nothing blows up under them. https://www.amazon.com/Stainless-Installation-Standard-Tapping-M4-10PCS/dp/B0784SC2RX/ref=sr_1_2?keywords=boat+eye&qid=1578845112&sr=8-2&th=1
     
  7. philSweet
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    philSweet Senior Member

    You are in Canada, and are straddling the 6 meter limit. Under six meters, you fall under Canadian construction standards for a monohull. At six meters, ISO 12217-2 comes into play, and is a good idea anyway, since it is about sailboats.

    See here - sections 4.3 and 4.4 - Construction Standards for Small Vessels (2010) - TP 1332 E - Transport Canada https://www.tc.gc.ca/eng/marinesafety/tp-tp1332-menu-521.htm#wb84

    and here - http://jsaf-anzen.jp/pdf/ISO_12217-2_2013(E)-Character_PDF_document.pdf

    The minimum required by these regs is just that, the minimum, and it is often the case that additional flotation is desirable. But start with what is required to meet the performance requirements. And if you want to add more, be sure that it doesn't have the side affect of causing the flooded vessel to become less stable when flooded.
     
  8. BlueBell
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    BlueBell Ahhhhh...

    Welcome to the forum.

    Why?
    Just to keep it from sinking?
    Positive floatation is a good thing.
    I would look at filling in the forepeak with foam.
    And/or below the gunwales, inside.
     
  9. Flotation
    Joined: Jan 2020
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    Flotation New Member

    Thanks for the welcome!


    As for the why, I'll mostly be sailing, motoring and rowing in secluded waters but occasionally a!so visit more open lakes. It's mostly for safety reasons in those more open waters. The thing will not be really stable even with the moderate to small sail plan I'm thinking of. It has a rather narrow beam.
     
  10. bajansailor
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    bajansailor Marine Surveyor

    Re an aluminium version of a typical Dutch 'punter' as per the photo in your link above (copied below) - being aluminium rather than timber, how authentic does it have to be?
    Re having a narrow beam, will the length / beam ratio be similar to the timber boat?

    I think you will find that she will stiffen up quickly as she heels, due to the flare in the topsides amidships. Up until the upper chine is immersed, and you really don't want to be pushing her much further than that.

    Does it HAVE to have the same layout as the timber boat, or could it have larger enclosed buoyancy compartments forward and aft?

    And if the crew are 'hiking out', you could have side decks (rather than just gunwhales) which would be a wider area to sit on (more comfortable) - and with vertical coamings on the inboard sides you can have enclosed buoyancy compartments here as well.

    Dutch Punter.jpg
     
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  11. Flotation
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    Flotation New Member

    Preferably I'd like to keep the general shape of the hull intact without adding to many details to distract from it's shape.

    Lenght to beam ratio wil approximately be the same for efficiency and rowabillity reasons. Rowing will be done from a removable bench (a simple board sitting between frames.)

    The fore and aft deck could be a little bigger, making room for flotation chambers there.

    Floatation chambers in the bottom could add self-draining capabilities but I'm starting to think those would make it not want to float upright when flooded.

    I don't intend to do a lot of hiking out and the design will probably be to narrow to add side decks to sit on without ruining the "punter" aesthetic.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    It is good from the perspective of keeping the boat upright, to have the flotation material reasonably high up, but if you are in cold water, it is a risk to the crew who may be unable to stay reasonably dry.
     
  13. DCockey
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    DCockey Senior Member

    But it is better to be sitting in an upright swamped boat than to be submerged in the water next to an overturned boat.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    You won't last long immersed in cold water to any degree, inside or outside the boat, ideally the crew should be able to remain above water.
     

  15. bajansailor
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    bajansailor Marine Surveyor

    Even if you build in aluminium to the exact shape of the wooden boat in the photo above, you could still have side buoyancy compartments without losing much 'working space' in the cockpit, as the hull has a fair bit flare and tumblehome.
    This would give you narrow side decks - a bit wider than the gunwhales - and easier to sit on (if required, eg in stronger winds) than a 'traditional' gunwhale.
    Ok, it will not then look so 'traditional' - but it will be safer.
     
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