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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 quarters.

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

------------------------------------------------
Tim Whelan
timwh@pobox.com
http://www.tgw.net
------------------------------------------------

On Wed, 16 Apr 2003 18:08:42 -0700, Dan Allen  wrote:

> 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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