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Subject: Wind & Current Navigation
From: Dan Allen (danallen46@XXX.XXX)
Date: Wed Apr 16 2003 - 21:08:42 EDT
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.
WHAT HAPPENED
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!
OBSERVATIONS
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?
QUESTIONS
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?
HYPOTHESIS
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.
FINAL QUESTIONS
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
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