Life Vest Hydrodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by DouglasEagleson, Sep 15, 2016.

  1. DouglasEagleson
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    DouglasEagleson Junior Member

    This is a topic about life vests. Would anybody be interested in this non-boat design question. It is like boat equipment theory though.

    My topic concerns the capacity to stay on the surface with a life vest in a location of a downward shear of current. It is obvious that a vest will not necessarily ensure survival in surf. But I am concerned with innocent looking flat water that has downward shear currents.

    Any interest?
     
  2. PAR
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    PAR Yacht Designer/Builder

    You're over thinking this. A vest design doesn't consider current, just buoyancy. Adding in all the variables involved in pressure drops, annular flow, various Reynolds numbers, etc., doesn't really offer much but a bit of convolution to a relatively simple concept premise. Now, if a special situation was being studied, such as getting caught in a dam's suction pipe, well then, you'd have something to work on.
     
  3. Petros
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    Petros Senior Member

    life saving devices such as PFDs are regulated by the Coast Gaurd in the USA. You can not use any device that does not have a USCG approval. They set the standard for life jackets, you can not even alter or repair it, say to add a pocket, without being in violation of USCG regs.

    There is no point is discussing that the standard is inadequate and how to fix it. The only way to fix it is to get the USCG to change their standards, which is not likley going to happen. Like most regulations, they are determined by lobbying by the very large corporate interests.

    A brilliant friend of mine who used to run a small company making specialized mountaineering and climbing equipment, but also loved fishing, considered making a custom PFD to suite his needs. He gave up on it when he discovered he would get fined by the CG if he was caught using a home made PFD (even if it was better than production ones). I told him to just get a cheap "horse collar" approved life jacket (made in a communist factory for about one dollar) and throw it in the bottom of his boat. You only have to have one on the boat. He was so disgusted with the idea he never even wanted to try.

    It is the golden rule in action, those with the gold, make the rules.

    Nothing will get better with the current regulatory process in place, and not likely that will change anytime soon.
     
  4. Ike
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    Ike Senior Member

    Sorry to disagree Petros. I spent much of my life in the USCG boating Safety program. Part of that was PFD regulation (by the way they have dropped the PFD designation. It's now lifejackets, life vest or life saving device.) Yes, the industry does have input but that's it. So do Labs like UL and South West Research or Amanna, and other organizations like ABYC, the Red Cross, Canoeing associations, and many other boating organizations, as well as the National Boating Safety Advisory Council and even lobbies like BoatUS.

    You also have the law wrong. You must have a USCG approved lifejacket or vest of the appropriate size for each person on board the boat, but you can have any type of lifejacket you want to use as well (in addition to the USCG approved devices) and use them. This is common practice on ocean racing sailboats because admittedly many approved devices don't have built in harnesses that sailors like. So they use ones from Scandanavian countries.

    Yes getting a device approved does take some doing. It has to be submitted for evaluation, tested by an approved laboratory (usually UL but there are others) and demonstrate it's effectiveness, as well as it's durability.

    There are now six different types: One of which is a special use. These have been specifically approved for use on canoes, others for use by waterskiers and wakeboarders and so on. Anyone can submit a device. It may not get approved, but then it might if it has the right amount of buoyancy, meets the material specs, the straps can resist the specified pull, and so on. In fact we were always looking for new designs. And yes it does take a while. If your friend was planning to actually manufacture these it would be considered. If he wanted just one for himself then it wouldn't. But he could use it, as long as he had an approved device in his boat.

    Also many approved lifejackets have pockets and D rings for attaching flares, whistles, lights, PLBs, and so on.
     
  5. Ike
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    Ike Senior Member

    In fact in the late 90's we had a lifejacket design competition. If my memory serves me some teenager girl won it. A quick search also shows that in 2011 there was another one. See http://bdept.cgaux.org/wp/?p=190. The winner was a flotation device that looks like a T-shirt.
     
  6. Petros
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    Petros Senior Member

    Ike,

    that is good to know, but that is what I stated, to put a cheap USCG approved life jacket in his boat, and just wear his home made one. I do not recall that he wanted to produce them to sell, if so it would have only been less than ten a year.

    All of the gear him and his wife made was specialty made to order. He made crazy stuff, like the first big wall tents, that hung from a cliff and had a trampoline "floor" for climbers on multi-day climbs. Not a lot of demand for such items, but there were only a few companies making them at the time.

    He was of the mind-set that he did not need nor want government approval for the gear he made and sold. So he likely would have just decided against it.
     
  7. SukiSolo
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    SukiSolo Senior Member

    Not forgetting that in Europe, devices will be CE approved, which may or may not be similar to US (or USCG) standards. For all that I'm fairly sure that for experimentation, design development and trialling you could claim exemption with complete validity. You can with experimental hulls etc at least here in Europe afaik, done it a few times.

    Regarding the OP's question, surely flat water with downward shear would simply indicate the need for more newtons buoyancy? to resist the 'pull'? As long as the main balance ie chin up to breathe attitude is maintained it should be simply increasing by the desired percentage to counter the 'suck'. Obviously the material bulk could become more of an issue - as in another thread, there are times and situations where too much buoyancy is a bad thing as well as bulk and snagging points on the garment. It could also be designed so that at certain pressure ie as a body was sunk a certain depth, that the drop in pressure could trigger additional buoyancy such as a compressed air bag/charge/compartment. Pretty tricky to engineer in moving water and bouncing around I'd guess.

    If I may ask the OP, is it in an offshore swimming situation you envisage? Or some vortex such as a sinking vessel?
     
  8. DouglasEagleson
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    DouglasEagleson Junior Member

    To: SukiSolo

    My interest is in tidal current zones and rivers. I live near the Potomac River in Gaithersburg, MD and it has an understood danger zone just above the Great Falls. But my concern is how far upriver do undertows exist.

    The calm surface to 4 knot currents deceives people danger wise. The bottom is full of rock ledges and ridges.

    I also shutter at tidal currents along the east coast inlets. In Maine it only gets worse as you go north. Obviously a tidal bore is dangerous, but what about a smaller precursor to the bore wave.

    The goal for me would be exactly as you suggested. A special addon automatic inflator with "X" pounds of lift. "X" would be determined by a test in a small dam rotor current or a rapids trap spot. Traps as a term is something I would like to devise.
     
  9. DouglasEagleson
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    DouglasEagleson Junior Member

    Pragmatic Solution- my judgement

    With out going into true hydrodynamic theory for the undertows here is my decision.

    type V life vest 22 lbs.
    inflatable 34 lbs


    Get a Stearn's as the basic US approved vest.
    Stearns I650, VR Versatile SAR Flotation Vest Type V (PFD Vests)
    25# foam type V

    then get an inflatable over-jacket of non-US approval.
    iso EN399 28kg 60lbs

    http://www.lifejackets.co.uk/categories/75/performance-gas-lifejackets
    Crewsaver Ergofit 290N Extreme Automatic Lifejacket with Harness, Light and Sprayhood

    Except the automatic inflator won't function right for a kayaker. I would have the senses to pull manually. I put my old plywood Duet-25 to rest on the shores of the Chesapeake bay a long time ago. It is a real listed shipwreck now. I retreated to the kayak world after it. So I will put in a testing trip to the Potomac River in my old Klepper. I will make a dummy adult and tow it around looking for traps to study. I will start with a type V vest only.

    Obviously I can not go into the Falls danger zone. I will start by plotting the low side of danger.
     
  10. Ike
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    Ike Senior Member

    It is good you are thinking about this. The Great Falls of the Potomac is , if not the the most dangerous, certainly one of the most dangerous stretches of water in the country. On the surface it is very deceiving (been there several times). My son who is a paramedic, trained there for swift water rescues with the fire department.

    Frankly, my reaction is the same as the Park Service, stay away!. Don't go near the falls. But some people don't heed the warnings and every year some die. Unfortunately these same types won't wear a lifejacket. But it would be good if there was a device that was available to kayakers and others who need such a device.
     
  11. gonzo
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    gonzo Senior Member

    Current, regardless of velocity, won't have any effect on floatation. If the water has a downwards velocity, as in a waterfall or a large water-whirl, extra floatation won't help at all.
     
  12. JSL
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    JSL Senior Member

    You can come up to British Columbia, we have some good 'passages' to test your products. Sechelt Rapids (Skookumchuck) gives quite a ride - full flood or ebb you can get 17+ knots with a good 6' over-fall. For more timid types, try Hole in the Wall, Seymor Inlet, etc.... still a good ride.
     
  13. Petros
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    Petros Senior Member

    I am inclined to agree with gonzo here, the currents suck down logs, trees, leaves, even boats and people.

    near where I live two local rivers come togther in a city park, every year around graduation time some teenager about to graduate from high school drowns within feet of his or her buddies on the rocks on shore.

    The surface does not look rough, but where the two rivers come together there is a very large and slow moving rotor, coupled with a deep bottom and usually submerged logs or a log jamb where people get sucked under and get stuck.

    I do not even understand the attraction to go swimming in the ice cold June water. I suspect alcohol or some other drugs involved. The high school kids go down to the river one night about the time of the high school graduation. THey have a party on the river bank in the park, someone gets the bright idea to go swimming, they strip their cloths off and dive in, usually several follow, than one disappears below the surface and they are not found until late the next day by search and rescue personnel. They are trapped under the surface. It is almost predictable.

    I have sat on the bridge and watched the currents there, without drastically altering the rivers, there is no way to stop it. There are warning signs, and a growing number of memorials for these kids that died way too young. I can not imagine the grief the families experiance. My own daughter went to the same small town high school.

    You would think they would learn. The sign, the memorials, the collective memories does not stop teenagers who think they are invincible, especially when it is graduation time.

    It would be an interesting experiment to get a properly weighted dummy and throw it in the river, and do it again with different amount of auxiliary flotation (life jackets) to see if something makes a difference with the dummy. It will not help with the living type of dummies, you can not force people to behave responsibly.
     

  14. SukiSolo
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    SukiSolo Senior Member

    Thanks for posting in more detail the design aim. I would agree with Gonzo, if you have a real downward vortex it will move everything with it. If you have almost a two tier situation, with an undisturbed water layer on top of a downward pull, you might get some benefit, but this must be a rare occurence. I do commend the design intent, so please take some credit for thinking about the subject.

    We also have some interesting places in the UK, quite a few (swimmer) deaths this summer in tide rips off Camber Sands (a relatively benign looking beach). For nice whirls, the mouth of Loch Etive near Oban gets pretty interesting on a spring tide! I've seen drunk idiots swimming the tidal Thames at Putney (low tide) after a few beers(!) and end up in an ambulance or worse, and that is generally not too dangerous IF you know what you are doing. Ah! the Christmas spirit....

    The one thing additional buoyancy would do is get the body back up to the surface faster after it is out of the downward undertow. Maybe that is enough? It's probably a bit like kayakers encountering a stopper - best way is to get out of it, and having the knowledge of how to achieve that. The golden rule that I always like to remember, is the sea (river/lake) is master, we enjoy ourselves on it but it can turn on you at any time.
     
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