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Current: How fast is too fast?

Discussion in 'Advanced Scuba' started by ADeadlierSnake, Sep 10, 2012.

  1. ADeadlierSnake

    ADeadlierSnake DIR Practitioner

    # of Dives: 200 - 499
    Location: Seattle, WA
    150
    22
    Pretty basic question, so I dont need a very lengthy answer. Ive dealt with current enough times, but theres really only been a couple of instances where ive actually seen the current chart for a dive with current beforehand, so I still dont really have a reference point for what is too much. This is starting to become important as I begin to plan current sensitive dives.
    Actually, I would like the answer broken into two parts...

    A) How fast is too fast (undiveable) for the kicking dive team?
    B) How fast is too fast (undiveable) for a dive team with scooters? Lets say Gavins.

    I dont expect an exact answer, so just throw me a ballpark estimate if you want (e.g. "1.5-1.8 kts")

    Thanks!
     
    beaverdivers likes this.
  2. dumpsterDiver

    dumpsterDiver Banned

    # of Dives: 2,500 - 4,999
    Location:
    9,003
    4,674
    You have to take in many other considerations. Is it a drift dive? Is it a wreck dive or where there is a lot of structure to hide behind? How far do the divers need to move in the current? what gear are the divers wearing (a warm water diver with a little or no suit and a single tank can handle more current that a diver in a dry suit with doubles). Also, what are the consequences of screwing up? Will the team just drift off and get picked up by the boat or will they be blown offshore into a ship channel with no boat or means of recovery? If they get blown down along shore are there other safe exits, or will the divers be bashed against a vertical wall of rock if they don't make the designated exist point.
     
    beaverdivers likes this.
  3. live_2_dive

    live_2_dive Contributor

    # of Dives: 100 - 199
    Location: Ft Walton Beach, FL
    170
    25
    these are questions that I personally don't think there is a solid answer to. like ^^ said, there are so many variables that go into it, you really can't assign a cookie cutter number to it.

    Just keep one thing in mind...if you EVER have a doubt that it "might" be too much current, its too much. The result of a misjudged current can be as harmless as being back on the boat/shore tired, but as much as being swept out to sea and not being found. There is NO dive that is worth your life, so if you aren't sure about it, don't risk it
     
    beaverdivers likes this.
  4. rhwestfall

    rhwestfall Woof! ScubaBoard Sponsor

    # of Dives: 200 - 499
    Location: "La Grande Ile"
    17,111
    20,825
    A component of OW classes (and recreational fun diving) here includes drift dives in the Niagara River (the Upper River - above the falls). This is a fairly constant 2.5 kt velocity, in which you are not going to swim against it. The dives are done as either shore dives, or boat dives. Quite a bit of fun.

    Never seen a scooter here....
     
    beaverdivers and Dr Dog like this.
  5. Teamcasa

    Teamcasa Sr. Moderator ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: Near Pasadena, CA
    12,121
    442
    Any current I have to work hard kicking against. Anything (estimated) over 1.5kts becomes a drift or a thumb.
     
  6. lowviz

    lowviz Solo Diver ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Northern Delaware or the New Jersey Turnpike
    7,400
    4,366
    I can only speak to "A" via my training and experience:

    "A strong swimmer in full public safety diving gear may, at best, be able to swim at a top speed of one knot, but soon be exhausted"

    Which brings me to one of my favorite lessons from PSD training:


    Lie face down in water shallow enough that you can just plant your hands onto the bottom and arch your back to break the surface and breathe. Head down in the water while breathing from your main reg. Step one: Go to your secondary regulator and note how long it took.

    Step two: Buddy is standing next to you, one hand on your tank valve. (to lift you if necessary) Start: Pats you on head, -your signal to start finning AS HARD AS YOU CAN FOR ONE MINUTE. Buddy taps you on head again after exactly one minute. Go to your backup secondary regulator. How long did it take. Try it. Shows the effect that exhaustion due to current has on you...
     
    Doppler, henryrendleman and Jax like this.
  7. Gilldiver

    Gilldiver Contributor

    # of Dives: 500 - 999
    Location: Northeast US
    1,770
    139
    People over estimate the current that they can keep up with all the time, ask a Lifeguard sometime. A diver striped for the islands can work for a while in 1 knot, the "a while" being dependent on their physical fitness. Unless you swim a lot, like every week or more, swimming in 1 knot for more than 15 minutes is about max with a tank on. A diver in doubles, stages and a dry suit will last less than 10 minutes at 1 knot. more like 5 minutes at most. Such a diver will also have a SAC well above 1.

    I have held on in a bit over 2 knots of direct current and it was about all I could do. At this speed your mask can start to be flooded if it get sidewise to the current and looking directly into the current and depress your 2nd stage diaphragm enough to make it free flow. Above 2 knots and this will occur. Ask some of the cavers what it is like to drag themselves through a restriction at times.

    To get a good idea of what you can do, 1 knot is 1 nautical mile per hour = 2025 yards = 1851 meters or 37 length of an Olympic pool in an hour. So, next time you are in such a pool with gear, swim as fast as you think you can for 15 minutes and calculate the distance travels as a fraction of a knot. You may be surprised at just how low that fraction may be as if you have done less than 9 lengths you are under 1 knot swimming speed.

    By the way, most schools and YMCA's have 25 yard/meter pools, so double the lengths etc.
     
    beaverdivers and lowviz like this.
  8. redacted

    redacted Guest

    I have done 2kt plus drift dives. They are not a problem as long as there is some shelter features.

    My wife and I tackled what seemed like only a 1kt current on a southern reef in Bonaire. No structure to shelter and rest behind. It was a lot like work.
     
  9. danvolker

    danvolker Dive Shop

    # of Dives: I'm a Fish!
    Location: Lake Worth, Florida, United States
    5,884
    3,012
    With the right plan, I can't imagine an ocean current that would even remotely be of concern.
     
    lowviz likes this.
  10. live_2_dive

    live_2_dive Contributor

    # of Dives: 100 - 199
    Location: Ft Walton Beach, FL
    170
    25
    These are the key words here. When it comes down to it...know your abilities, know your dive buddy's abilities, and have a plan.
     

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