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Diving and Seamanship

Discussion in 'Basic Scuba Discussions' started by Akimbo, Jun 12, 2014.

  1. Eric Sedletzky

    Eric Sedletzky Great White

    # of Dives: 500 - 999
    Location: Santa Rosa, CA
    Who can tell me what the difference is between a:


    Is it in the rigging? or the boat itself?
    First one to answer correctly gets a browny button.
  2. Rich Keller

    Rich Keller Solo Diver

    # of Dives: 5,000 - ∞
    Location: Long Island NY
    It is a little bit of both. This site will explain it better.

    Popular Types of Sailboats Illustrated and Described in Detail
    Wingy and Eric Sedletzky like this.
  3. FinnMom

    FinnMom Divemaster Staff Member

    # of Dives: 500 - 999
    Location: Finland
    Subs are also different in that even the really big ones are "boats".

    As I recall from my years in (the fine seafaring city of) Savannah, a yawl is any group of 2 or more persons.
    Last edited: Dec 20, 2014
    Bob DBF likes this.
  4. Rich Keller

    Rich Keller Solo Diver

    # of Dives: 5,000 - ∞
    Location: Long Island NY
  5. Akimbo

    Akimbo Lift to Freedom Volunteer Staff Member ScubaBoard Supporter

    Ask any oceanographer or navigator. Most people grossly overestimate swell height and current speed. Both of these professions need to know accurately what they are.

    Current speed is a lot easier now that most boats have GPS with speed displays. Of course there can't be any wind when the boat drifting. A measured 5 knot surface current is described as screaming by most divers. The problem is that high surface currents and no wind are not that common. Scientists have GPS recording instrument$ now that break it down to a tenth of a knot.

    NOAA's National Ocean Service Education: Currents: How Are Currents Measured?

    I was on an oil production platform (fixed to the seabed) in the North Sea and everyone was convinced the swell was over 100' high -- until someone pointed out that the lowest deck was at 60' above MLW (Mean Low Water) and the seas were barely slapping the bottom of the compartments. Then we started observing the height markings painted on the platform legs. Wave height, or trough to peak distance, actually averaged 56'. Glad I didn't put any money down that day!

    It isn't that easy to measure swell height on a small boat. The best way, if there are no fixed platforms around, is to really steady yourself and sight with one eye at something fixed very close to the shoreline. You can calculate your eyeball to the waterline distance pretty accurately so keep climbing the boat until the peak of the swell just blocks your view when you are in the trough. All that is great if your point of reference is close to perpendicular to the swell and you aren't rolling so bad it makes it impossible.

    On the other hand, just using our first impressions of current speed and swell height makes for better sea stories. :wink:
  6. Hank49

    Hank49 Solo Diver

    I've learned that there are two ways of measuring wave heights. Normal, and Hawaiian.

    Hawaiians tend to estimate wave height to be about 1/3 of what 90% of people would estimate it at. Triple overhead surf on the North Shore is about 8 feet. This has some merit because the Hawaiians say they measure the wave from the back, not in front where it's sucked up a lot of ocean as it breaks. Pipeline and Teahupuu…(that Tahitian break) actually suck up enough water to where the flat in front of the wave is below mean sea level.

    Here in Belize we do get some pretty strong currents in the cuts of the barrier reef during tidal change, and very calm water. I'll drift and check my GPS speed next time I'm out. And yes, I believe we do over estimate the speed. There is no way I could make headway in a 5 mph current. Even with my trust carbon fiber fins. :D
  7. Akimbo

    Akimbo Lift to Freedom Volunteer Staff Member ScubaBoard Supporter

    My understanding, perhaps incorrectly, is the Hawaiian method was for wave height (breaking near shore) and the "Normal" method described by NOAA is for swell height. Basically one is for surfers and one is for mariners. Does anyone use the Hawaiian method offshore for swell height?
  8. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    Wouldn't the two methods give the same numbers for offshore waves, but differ for waves close to the shore?
  9. Akimbo

    Akimbo Lift to Freedom Volunteer Staff Member ScubaBoard Supporter

    The Hawaiian method produces estimates that are about a third to a half of more common method used by mariners. Mariners use the trough to peak measurement.

    Last edited: Aug 31, 2015
    cipherbreak likes this.
  10. Seamutt

    Seamutt Captain

    # of Dives: I just don't log dives
    Location: Tampa
    This has been a fun thread. Have not been here in a long time. Thanks.

    One thing that got me out of my shell was the current speeds. This always make me smile when I hear these. A number worth knowing is an Olympic men's freestyle swimmer, full sprint on the short 50m heat swims at 4.5 knots for 21.5 seconds. I'm a pretty good swimmer and with jet fins I can't make that.

    Wave heights are all over the place but in general most people who dont spend alot of time at sea (more than a week a month) over estimate waves up to 8 ft and under estimate them over 8 ft.

    I would also like to throw in the slippery-hitch as a useful knot and it just a square knot with one bow, like when you tie your shoe and pull one side all the way through instead of having two loops you have one. It jams less frequently.

    The Bowline is the king of knots. Learn this one from either hand and around something and then again around you. 4 different ways you can need it.

    Keep up the thread, big fun.

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