Unsinkable boats realy?

Discussion in 'Boat Design' started by tom kane, Oct 12, 2014.

  1. PAR
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    PAR Yacht Designer/Builder

    You're assumption is not based in the dynamics involved, in a down flood situation, nor does it take into consideration the results of having floatation in appropriate locations, to maintain a swamped, yet stable condition. Simply put, if you hole your boat the last thing you want is it rolling over, because you placed it's floatation too low, as now you're faced with an inverted boat, that will require you to cling to whatever you can, instead of relatively calmly sitting in a swamped cockpit waiting for someone to see you.
     
  2. Steve W
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    Steve W Senior Member

    Correct, however, the placement of floatation is very dependant on the type of boat and what you expect out of the floatation. For example, the Macgregor 36 cat i had could carry its buoyancy low down because of the inherent straddle stability of a cat but needed to also to carry a lot of foam all the way to deck level so the boat would also float reasonably high if capsized in the absence of buoyant arched beams. A keelboat can also carry its buoyancy low as its keel will keep it upright, on my sons boat the entire area below the berth tops is full of pour foam, the berth tops themselves are 1" pink foam with a layer of glass cloth each side in lieu of plywood so the only area below the waterline that could even be punctured and can hold water is the foot area about 6.5ft x 2ft so not much free surface area. Additional benefits are nice warm berths, well insulated from the cold waters of Lake Superior, a quietness you don't get any other way, and we can lift or block it with impunity, something you don't get with air tanks btw.
    I saw a 35ft sailboat in an English magazine that was competing in an ostar some years back that had a very flared hull which apparently was partitioned off inside to provide water ballast tanks but if you dumped them it would be buoyancy in a good location in an emergency. I don't know what Toms boat is but it sounds like it may be a small powerboat which as you say would require its floatation higher up although they almost always have it under the sole also.

    Steve.
     
  3. gonzo
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    gonzo Senior Member

    Sadler boats are filled with foam and supposed to be unsinkable. I have seen photos of them swamped on purpose to test them, and it works. However, if there were to be major hull damage and some of the foam floated away, they could sink.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Agreed Steve, but (again) the location of the floatation is easy enough to figure out and make conform to small boat standards. The idea is a fully swamped condition, where the boat can still be used, to contain the (all of) crew with relative safety and tolerable stability and trim, assuming reasonable equipment and loading, before the event.
     
  5. tom kane
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    tom kane Senior Member

    Unsinkable boats realy

    I want my flotation material to stop my boat from sinking immediatly not waiting until it is half sunk before it takes over and I prefer a dry bed.
    A boat floats because it is full of air and if water intrudes the air is displaced by water unless there is flotation material low down in the hull to stop that happening.
    A flotation ring at gunnel height would be nice.
     

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  6. PAR
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    PAR Yacht Designer/Builder

    Simply do a stability study Tom and things will quickly line up for you, with a sole full of floatation.
     
  7. Ilan Voyager
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    Ilan Voyager Senior Member

  8. Ilan Voyager
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    Ilan Voyager Senior Member

    I do totally agree with you and Van de Stadt thought the same thing more than 50 years ago. Van de Stadt is the creator of the polyester yacht building in Netherlands and his designer's career spanned over 40 years with many excellent boats. He knew how to design boats...
    Tom you have to make stability calculations or you may have bad surprises. The small boats (sail and power) with foam only under the floors (and same time decks) have imperatively the floors above the flotation line by at least 3 inches thus can evacuate immediately the water. As they cannot be filled the problem of stability full of water does not exist.
    I know well as I made several small power boats based on this principle, but the water (from a wave for example) must be evacuated in the 3 seconds (open transom for example).
    These boats have an enormous gain of lateral dynamic stability if the gunwales have a high volume like a RIB. Make the Bonjean curves of such a boat caught by a wave on the side (and adding to the hydrostatic effect, the orbital effect of the movement of the water molecules) and you'll understand immediately..
     
  9. tom kane
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    tom kane Senior Member

    Archimedes would be thrilled to read this thread. Like him I make my own investigations and decisions and do not need to study anything because it has already been done.
     

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    Last edited: Oct 18, 2014
  10. PAR
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    PAR Yacht Designer/Builder

    You'll note precisely what we've been suggesting was employed on the Sherman (M4) with the DD variant applied. The CG is way under the floatation making it possible to use them wet, though 1/3 of these still swamped and went down. If the floatation was lower, as you've suggest, the moment they engaged the duplex drive, they would have flopped over on their sides and sunk.
     
  11. rwatson
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    rwatson Senior Member

    That's like saying "if someone else says something, they must be wrong, no matter how much they know"

    "5.0 Level Flotation
    FEDERAL LAW:

    "Flotation material located at the sides, as far aft and as high as possible, will help make boats with machinery located aft float level when swamped. "

    now, have you actually filled your boat up with water to make sure it doesn't turn turtle ?

    Check out this boat trying to stay afloat long enough to get to the dock, and what happens as the free water inside her starts to become significant.

     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    Rwatson, thanks for the video. They were very close to make the last roll. happily the sea was as flat as a table. At the end the pendulum effect induced by the slashing inside water was impressive.

    Tom please leave Archimedes out of the discussion (at least do you know why he started a research about displacement of water by a body? nothing to do with boats) and try to figure out the stability of a monohull, heavily foamed at the bottom, ie under the floors, filled with some water coming from a wave for example (a lot of boats have sunk because of the openings on the deck, not by holes in the hull) and ask yourself what this water (Free surface effect, carène liquide in French) will do when the boat heels just a few degrees... It does not need complicated calculations, just 3 2D drawings and figure where is now the center of gravity when heeled.
    A lot of illustrations and explanations in the web.
     
  13. tom kane
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    tom kane Senior Member

    Thank`s for you information regarding stability but I think you are all taking the need for stability beyond reason. If you google boat flotation images you can see hundreds of ideas to add flotation to boats for many different situations and some may be better than others as far as stability in bad conditions is concerned when the boat has taken on water because there was no boyancy in the right places.
    In my application the flotation would not have allowed the boat to sink into the water because there was flotation in the bottom and under the bunks in the cabin if the bottom was damaged. I could have gone to extremes and added flotation rings around the sides and other innovations to make an impressive professional job but not necessary on a classic wooden boat.

    I do not think that the boat in the video would have had much flotation material in the bottom of the hull to keep water out so it was in danger of sinking so the need to beach it became vital.

    Having spent quite some time as a gunner-driver in tanks and going into water I would not want to travel in a tank that someone has fitted a life jacket to even if it was up to high stability standards and many did sink through lack of flotation I would transport my tank on a stable barge.
    Yes I do understand the principles of stability I have been fighting stability from the time I tried to walk and even fighting for stability now.
    I do not claim to "know" anything I stand on the shoulders of Giants.
     
  14. Steve W
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    Steve W Senior Member

    Beautiful boat in that movie, hope they made it. Almost all smaller production open powerboats have floatation under the cockpit sole but unfortunately the coast guard approved foam soaks up water like a sponge as soon as the skin that is formed on curing gets damaged, the stuff is supposed to be closed cell but the term doesn't mean much apparently. Macgregor used blocks of polystyrene packed in which is 100% reliable but does not provide the rigidity you get when every in2 is supported evenly such is the case with pour foam. Does anyone know of an A/B pour foam that absolutely will not absorb water, ever?

    Steve.
     

  15. Fanie
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    Fanie Fanie

    I guess it's the same everywhere. If you're into boating you fight for financial stability, at boat eight you fight for marriage stability. If the son stole the boat and stays out all night with a girlfriend then you fight for mental stability...

    We live in an unstable world.

    There is no such thing as an unsinkable boat either. All we can do is to change the odds a bit in our favor, within your abilities.
     
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