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    Re: Wind & Current Navigation
    From: Dave Weilacher
    Date: 2003 Apr 17, 12:27 -0400

    My notions on this subject.
    I am an expert on entering and leaving slips in a multitude of different 
    sailboats, in all kinds of wind and current conditions.
    I put myself forward as an expert because I have screwed up in every one of 
    them in numerous ways.  Nowdays, it is a surprise when I lose control of the 
    situation but my plans still go to hell regularly.
    So here are my observations.
    Current is king.  The whole boat is going to do what the current tells it.
    Wind is queen of the bow.  While the whole boat is doing what the current 
    dictates,  the wind will take the bow off to whatever point the wind finds 
    her balance.
    Always make a plan.
    Abandon the plan the second it is doubfull.  Often the best place to go is right back into the slip.
    Most of my biggest screwups have been trying to hang on to a plan that was obviously irrelevant.
    It doesn't matter what you hit as long as you aren't moving when you hit it.  
    I know this sounds like a bit of an oxymoron but it makes sense from the helm 
    of the boat.
    My own boat is a fin keel, spade rudder, sloop that walks hard to port in 
    reverse.  The rudder will only turn 30 degrees off center.
    More often than I'd like, I end up backing all the way out of the marina.
    -------Original Message-------
    From: timwh@POBOX.COM
    Sent: 04/17/03 01:08 PM
    Subject: Re: Wind & Current Navigation
    > Hi Dan,
    I have a slip that faces east. I reverse out, and then turn south to
    proceed down the fairway. When the wind is blowing from the south or southeast,
    executing the turn to the south can be challenging. When a strong south
    wind is blowing, I usually reverse as far as I can (litterally within inches
    of the boat accross from me) and then use maximum horsepower to execute the
    turn (quickly as possible). This whole operation is complicated by the fact
    that I am in a newer marina (Elliott Bay Marina in Seattle) and the
    fairway is not very wide. I also have a full-keeled cutter that presents a lot of
    windage (staysail on a boom and a self furling jib). To date, I have
    assumed current in our marina is minimal as there is "pretty much" a full rock
    breakwater around the marina.
    However, about a month ago I left my slip when there was very little wind
    and had a hard time executing my turn to the south. I didn't think much of
    it at the time, other than to be more careful next time. However, now your
    post has me thinking there might be noticeable current running through the
    marina. The marina has entrances (fairly small) through the breakwater at
    both ends which would allow the current to theoretically flow through (east
    to west). The tides on the day I had problems were consistent with the
    theory. This theory would explain why sometimes executing "my turn" is somewhat
    hairy, while other days it is a non-event. The mathematics are beyond me,
    but I can attest that my full keel catches the current like a bugger when
    we are broadside to the current. It is definately most noticeable in close
    Bottom line, I am going to pay more attention to the water flow in the
    marina on a calm day when there is a big tidal swing and try and test this
    theory. Many of the close calls I have had sailing have been current related
    and I definately should have thought of this earlier. Thanks for your post.
    Tim Whelan
    On Wed, 16 Apr 2003 18:08:42 -0700, Dan Allen 
    > Today I went out on my boat, but leaving the slip became dicey.  This
    > experience leads to a couple of questions about determining the
    > cumulative effects of winds and currents.
    > My slip faces due south.  The 15 knot wind was coming from the SE.  I
    > released my lines and engaged reverse and pulled out of the slip, so
    > far so good.  As I entered the fairway and the boat began turning so
    > that the bow began to face East, I stayed relatively close to my slip,
    > reasoning that the wind would push me out more to the middle of the
    > fairway, and when I had turned enough, I would go forward and that
    > would be that.  This plan has worked fine in no wind, or in light
    > winds, or even winds this strong that I have encountered before.
    > However, the incoming invisible tidal current was coming from the
    > North.  It pushed me back towards the dock when I was starboard side to
    > and I narrowly missed hitting docks and boats.
    > The wind seemed to be controlling the situation.  The waves were all
    > heading NW due to the wind from the SE.
    > The current seemed negligible.  I knew that low-tide was just occurred
    > about 30 minutes earlier; the flood was beginning again, but I did not
    > think it had much strength compared to the wind, as I saw no sign of a
    > current pushing me back into my slip.
    > I was wrong!
    > The current was darn near invisible to me. The winds appeared to be all
    > powerful, but the quiet, silent, invisible currents got me again.
    > Let us say that we knew that the wind was 15 knots from SE and that the
    > current from the incoming tide was 2 knots from the North; are they
    > factored equally in how they combine to move the boat?  If they were
    > weighted equally the result would be a 13.66 knot force coming from 129
    > degrees, but this would not have blown me against the dock.  It still
    > would have blown me away from the dock, but this is not what happened.
    > A current from the North of 11 knots combined with a 15 knot wind from
    > the SE would give a total force of 10.6 knots coming 2 degrees North of
    > East.  This would have just barely begun to nudge me back toward the
    > dock, but the tidal current certainly was nowhere near 11 knots!  It
    > was more like 2 knots, perhaps 3 or 4 at the most.
    > This leads me to believe that the current needs to be weighted much
    > more than the wind, at least in my craft, but is this always the case?
    > 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?
    > How does one weight the effects of wind and weight the effects of
    > current?
    > How do the size, shape, and mass of the boat and the hull, and the
    > superstructure (masts, flying bridges, etc.) effect these relative
    > forces?
    > Let the vector "W" be the wind, and the vector "C" be the current.  Let
    > "s" be a factor for the amount of superstructure on a boat, which the
    > winds effect.  Let "h" be a variable representing the hull under the
    > waterline, which the currents effect.  It seems to me that there should
    > be some formula to determine the final speed and direction of the boat
    > under these two forces of Wind and Current, the result being the vector
    > B (for boat).  The formula would look something like:
    >        B = s * W + h * C
    > s and h obviously are determined by a variety of measures and need to
    > be further broken down.
    > Are there any rules of thumb that help one determine the sum of the
    > wind and current accurately?  How do master mariners figure this out?
    > What observations can be made to help predict the cumulative effects of
    > wind and current accurately?  In other words, how could I prevent such
    > near disasters in the future?
    > Dan
    Dave Weilacher
    .US Coast Guard licensed captain
    .    #889968
    .ASA instructor evaluator and celestial
    .    navigation instructor #990800
    .IBM AS400 RPG contract programmer

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