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Estimating Current

Discussion in 'Basic Scuba Discussions' started by Silty Sam, Nov 25, 2019.

  1. The Chairman

    The Chairman Chairman of the Board

    # of Dives: I just don't log dives
    Location: Cave Country!
    55,855
    23,204
    113
    I estimate current by the buoys I see. It helps if you're familiar with the dive sites in question. If they are under water, I'm probably staying in the boat. If I am unsure, I'll hold on to a line secured to the vessel and splash. If it's too much, I'm pulling myself back in.
     
  2. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Philadelphia and Boynton Beach
    10,780
    6,211
    113
    I estimate current by how much reef I cover. Even when I am flying, I rarely cover more than about half of Boynton Beach reef, that would be about a knot and a half. I'm not sure most people know how fast that actually is, very fast
     
  3. Mustard Dave

    Mustard Dave Rebreather Pilot

    # of Dives: I just don't log dives
    Location: Manchester, UK
    546
    376
    63
    Get a chart of the area. It will give you the speed and direction according to the time before and after high water. If you know your tide times, you can easily work out the rate of current.

    When we are using a shot line, we have the usual weight on the bottom end, and a small A-0 buoy at the top. We then have a further 2 metres of line tied to a larger buoy.

    When the current is running, the force against the larger buoy is going to push it away from the position of the weight. This force is enough to pull the smaller buoy under water. When the smaller buoy begins to pop up, you know you have slack water.
     
  4. Dizzi Lizzi

    Dizzi Lizzi Manta Ray

    # of Dives: 50 - 99
    Location: California
    962
    841
    93
    Great question, thanks for that and all your answers.
     
  5. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    8,012
    5,707
    113
    You actually trust the charts?
    If you trust the charts, why do you need to do this?
     
  6. BRT

    BRT Giant Squid

    10,881
    6,784
    113
    A knot an a half is about 5 knots in scuba speak.
     
    couv, nolatom and scubadada like this.
  7. seeker242

    seeker242 DIR Practitioner

    # of Dives: 200 - 499
    Location: Pompano Beach, FL
    1,163
    689
    113
  8. Steelyeyes

    Steelyeyes Manta Ray

    # of Dives: 200 - 499
    Location: Redmond Wa
    555
    422
    63
    Learned that one the hard way. My wife and I were diving with a dive outfit I won't name in and undisclosed location. The first attempt was on a wreck we'd been on four other times the previous year. It had four buoys on it. When we showed up this time one was missing.

    I thought, "Someone is stealing the buoys".

    The boat operator and the DM decided to cancel and head to another site. This one we'd been to before. It had two buoys, one was "missing". Damn thieves. We moored, deployed the current line from the buoy that was left and splashed. The current was ripping. With a full BCD I was pushed under water hanging on the current line. My wife aborted the dive. I should have because it was a waste of air. We hit the wreck in low viz, drifted bow to stern, fought our way back to the buoy line and made our ascent. Blew through air like crazy, didn't see much, arm was sore for a couple days from hanging on that current line.
     
    The Chairman likes this.
  9. aldertyler

    aldertyler Nassau Grouper

    71
    79
    18
  10. Mustard Dave

    Mustard Dave Rebreather Pilot

    # of Dives: I just don't log dives
    Location: Manchester, UK
    546
    376
    63
    Yes, I do trust the charts; I plan many dives in UK tidal waters, where diving on slack is essential. Tides can be predicted over 100 years in advance.

    The reason for the trick with the buoy is because while the charts are correct, you have to allow for some error:

    • The charts give direction and rate at specific points on the chart; you select the position closest to the dive site. As you move away, there is going to be some variation.
    • The charts give values for springs and neaps; there are two weeks between them, during which the rates are going to change.
    • The information is given at one hour intervals before and after high water; it is not stated to the minute.
    The charts are going to give you an approximate time; you decide the exact time by monitoring the conditions on site. You could do the latter at any time, but expect a wait of up to six hours if you don't use the charts and tide tables.
     

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