buoyancy and safety issues

Discussion in 'Boat Design' started by fabrice, May 25, 2012.

  1. fabrice
    Joined: Mar 2009
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    fabrice Junior Member

    Hi, it's been a while !
    I need your insight to help me figure how to handle a big buoyancy debate in our paddling federation these days.
    I paddle in 6 men va'a (outrigger canoes). They are 44 ft long, 23 in large, with closed compartiments at bow and stern. the ama (side hull) is about 13 ft long.
    That buoyancy is enough to maintain the hull half flooded after a capsize recovery.
    Some recent accidents called for a change in our regulations : The federation officals claimed that an extra buoyancy under the seats is mandatory to minimize the water volume inside the hull.
    As I agree to think that extra buoyancy could help when a compartiment is flooded (leaking very rare), I beg to differ when it comes to the water volume.
    My feeling is that if the hulll is half flooded in normal conditions, the extra buoyancy under the 6 seats will just raise the water level, keeping the same volume.
    The hull flooding occurs when you turn back the capsized hull, and the only way in my opinion to keep water from entering would be to add floatation to the sides and under the deck(the hull would be higher on water thus bailing less water when turning)
    Or ... to recover from the ama side, pushing the ama under water and raising the hull above the water (a hazardous way to go with a 330 pounds boat I guess :D )

    Thanks for your help
    P.S. : If I'm wrong, feel free to say so and explain, I'm here to learn and understand !
     
  2. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    My understanding is that the volume of water to be bailed out is that which is inside the boat at the point where the last piece of the edge of the boat breaks surface ie the cross hatched area in the attached sketch.

    Anything added, which is anywhere in the cross hatched area which displaces the water would reduce the volume of water to be bailed even if it did not raise the level of the boat eg some material of the same density as water.

    If putting something buoyant anywhere in the cross hatched area the edge of the boat would break surface earlier and with less water in the hull.
     

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  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Fabrice,

    When you say:
    Not being familiar with the boats you mention, you first have to establish how the water can gain entry into the hull hull to flood it.

    Is the boat an open cockpit type, thus waves can wash over the hull onto and inside the hull? Or are you, like on a traditional paddling canoe "sealed" into the hull which prevents any ingress of water?

    We first need to understand what mechanism are at play first to establish what is their thinking.
     
  4. lumberjack_jeff
    Joined: Oct 2010
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    lumberjack_jeff Sawdust sweeper

    Agree.

    Also, anything which reduces the amount of water taken aboard lowers the waterline (raises the freeboard) at the point of capsize and thus increases the angle of heel necessary to make the boat to take on more water.

    So, additional buoyancy reduces volume of water taken aboard during a capsize event, and reduces the likelihood of taking on more water while bailing.

    Some designers simply strap pfd's under the seats.
     
  5. Submarine Tom

    Submarine Tom Previous Member

    Fabrice,

    I believe I understand your description of what is happening.

    Of course the foam under the seats will help as when the boat floods in the re-righting procedure less water will fill due to the displacement of the foam. The water level inside should not change appreciably (although it will be slightly less due to the buoyant effect of the foam during flooding). But, you will have as much less water to bail as the foam displaces. So, only the foam that displaces water will be benificial.

    To more easily visualize what I am saying, imagine you filled in the entire inner volume of the boat with foam. When you recovered from a capsize, no water would remain because the foam would displace it all. What you have described is simply a lesser version.

    So, I am sorry to say I think you are mistaken, but I may be wrong.
     
  6. fabrice
    Joined: Mar 2009
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    fabrice Junior Member

    Thank you for your answers.
    Latestarter describes fairly well the water entry issue.
    The canoe has deck holes just like a kayak, and bails in during the recovery turn.
    I tried it yesterday with my one man, same design : 4 inches of water inside when recovering from capsize, in a 12 inches high hull.
    My point is : If I displace the water under my seat, it will be the same volume, just higher in the hull.
    It's not like the water was flooding the hull entirely (then it would be obvious that any amount of foam will push water out !)
    The only improvement in my opinion, is sticking the man holes above the surface before the turn.
     
  7. latestarter
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    latestarter Senior Member

    The way you are looking at it would be correct if you put the foam in after the boat was flooded.

    If the foam is put in before the hull is flooded (in some part of the cross hatched part of my previous sketch) the water can not get to that part of the hull.

    The foam is not pushing the water out, it is stopping it coming in.

    ETA However if the situation is more like the sketch below, where the space under the seat is not in the cross hatched area, then you would be right, it would only raise the level of the water but not reduce the volume. Putting foam in this cross hatched area would help.
     

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    Last edited: May 26, 2012
    1 person likes this.
  8. fabrice
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    fabrice Junior Member

    Latestarter your sketch is brilliant !
    This is exactly the situation.
    The only solution resides in foaming inside the walls of the hull, specifically the right one (the ama being always on the left)
    As you stated, we need to stop the water coming in.
    QED.
     
  9. latestarter
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    latestarter Senior Member

    I was thinking where was the most effective position within the cross hatched area of the previous sketch to place the foam.
    Filling it all up would give the position in sketch A with the edge of the deck hole, well out of the water.

    With this arrangement the new point where the edge of the deck hole comes clear of the water would be as sketch B. The horizontally hatched area would not be effective but might help stability.

    In theory it could be calculated what volume of foam to add to support the whole boat so that the hull came up dry.
    In practice for reasons including wave action, getting the centre of buoyancy of the foam in line with the centre of gravity of the boat measured along the length of the hull and longitudinal stability it is difficult.

    A possible disadvantage in adding the foam is that it might take more effort to turn the boat back over as it will be floating higher than before.
     

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  10. fabrice
    Joined: Mar 2009
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    fabrice Junior Member

    your last sketches are a good view of my thoughts.
    Nevertheless, I had completely forgotten what comes as your last point :
    Eventually, the boat will be harder to turn back, you're right.
    It's a big issue as it is already difficult to do so in rough seas, and takes a bit of practice.
    Thank you for puting that in light.
    Not to mention a comfort issue : va'a hulls are very skinny, so we can paddle very close to the hull side. A large width of foam won't do.
    It's interesting to mention that the more we search a solution, the more it seems obvious that the thousand years old design is probably the best !
    Considering the ability to bail out fast !
     
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