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    Re: Wind & Current Navigation
    From: Trevor Kenchington
    Date: 2003 Apr 16, 22:58 -0300

    It may sound silly but think you are asking the wrong questions.
    Once you slipped your boat's lines, or at least once she was clear of
    her slip and fully exposed to the tidal stream, that stream had no
    motion relative to your boat's frame of reference because the water _is_
    the frame of reference and the boat moves with it. You then have two
    motions to care about, aside from whatever forces you may apply with
    screw, rudder and sails (if any): The first is the effect of the wind
    pushing the boat through the water. Unless you have light winds and
    strong tides, that won't be materially affected by, e.g., a
    weather-going stream increasing the relative wind by a knot or two. The
    second motion is that of solid objects (like slips) and anchored boats
    moving through the water as a result of the water moving over the seabed.
    Of course, if your boat's stern was in the full flow of the stream and
    her bow was sheltered from it, life would be more interesting. Also,
    there is some small delay while moving water accelerates a boat that was
    previously static relative to the seabed. But for most practical
    purposes, boats and ships move relative to the water and are carried
    along with it when it moves.
    You asked:
    > How can you determine the relative strength of a current compared to
    > the wind and determine the outcome of the two?  I know how to
    > mathematically add two vectors, so it is not the actual math that I am
    > asking about, but rather, how empirically does one determine the
    > relative forces involved?
    Again, I don't think that is a useful way of considering the problem.
    > How does one weight the effects of wind and weight the effects of
    > current?
    In mathematical terms, I guess I would give the current an infinite
    weight and look at how the wind moves the boat through the water.
    > How do the size, shape, and mass of the boat and the hull, and the
    > superstructure (masts, flying bridges, etc.) effect these relative
    > forces?
    All of those factors have large effects on how much movement through the
    water a given wind will produce -- as does the relative bearing of the
    wind's eye. You just have to learn how your boat responds.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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