| Only Local Knowledge
computes currents point-to-point in San Francisco Bay, with resolutions as fine as .05 nm.
Published tidebooks and "Tide & Current" software simply repeat decades-old
NOAA data at widely-scattered points, too far apart and oddly spaced to interpolate to
where you are likely to be sailing. The graphic below shows all NOAA points near the
Berkeley Circle (the blue boxes show current magnitude in knots and the red streamers
direction). There are no NOAA points within or very near to the race venue (right-hand
side of graphic). This example is in the
Berkeley Circle at 1100 hours on 10/7/2006, during the Star Worlds. The box "58"
shows the likely location of an upwind mark, and the blue lines show zero-current
laylines, assuming typical winds of 10 knots from 235 deg (mag). When current effects are
included, the actual laylines are shown by the magenta dots. Even though current magnitude
is not large in this region, there is a significant change in direction from south to
north, and east to west, in addition to rotary effects as the current cycle progresses
(see below). In order to compute current effects on laylines and preferred routes, it is
necessary to have a predictive model which computes current everywhere, point by point. |
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| The graphic below shows
the same region and time using the Local Knowledge predictive model. The current points
with blue boxes are part of the default display (compare to above), but the user can add
more points anywhere desired, and zoom in to see more detail. The yellow boxes are custom
points added by the user. The Local Knowledge model incorporates all known public and
private data, including scores of direct, on-the-water measurements over the past 10
years. Note that while currents to the right of the laylines do not exceed a knot, there
is a directional difference of more than 90 degrees from one layline to the other, leading
to current corrections which are asymetric about the zero-current laylines and
non-intuitive. |
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| Currents in the Berkeley
Circle are ROTARY CURRENTS. The currents do not simply increase, decrease to zero and then
reverse along a given direction, but rotate counterclockwise in direction, never going
through zero. The four timepoints below show how these currents change between maximum
flood (at Golden Gate) and maximum ebb (at Golden Gate). Rotary currents are ignored in
the tidebooks and in all "Tide & Current" software. Only Local Knowledge
includes these effects in its current predictions and tactical software. |
| Berkeley Circle at 1100 hrs (max
flood) |
Berkeley Circle at 1300 hrs (2 hrs
after max flood) |
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| Berkeley Circle at 1400 hrs (near GG
slack) |
Berkeley Circle at 1700
hrs (max ebb) |
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| The non-uniform (differential) current in San Francisco Bay creates a
situation where one set of tacks can take considerably less time than another. In this
example, taking a port tack from the boat position to the starboard layline, and following
that to the mark, is MORE THAN 1 MINUTE faster than the alternative (starboard tack to
port layline). This is a large difference for a portion of a single leg (and over a
distance only 1 nm from the mark). The red line from the boat token shows where the boat
on port tack will intercept the current-corrected layline, but the actual track of the
boat will not be a straight line under these conditions, but curved. These computations
depend on the use of a continuous current model, and can only be accomplished through the
use of Local Knowledge tactical software (SEE MORE EXAMPLES). |
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If you are sailing in the
Star Worlds, you really can't afford to ignore these effects. Our "Force 2"
program allows you to display currents at any date and time of the year, as well as
current-
corrected laylines at marks of your choosing. You can print out the screen display to make
hard copies (say at 1 hr or half-hour intervals) to take along on the boat. TO ORDER:
download Force 2 for San Francisco Bay from our download page,
and obtain an authorization password for $200. Do it now and you can make printouts at
home before coming to SF.CLICK HERE TO CONTACT US WITH ANY QUESTIONS !! |