SEE BELOW DETAILS OF OUR SPECIAL DEAL FOR PARTICIPANTS IN THESE RACES .... |
| San Francisco Bay is famous for
its strong, variable tidal currents, which are always a factor in Bay racing. During the
SAP 505 World's (and 505 North Americans), maximum currents in the Bay will exceed 5
knots. Currents in the 505 venue (Berkeley Circle) are not as strong, but vary
sharply in magnitude and direction from one point to another across the course. The venue
lies in the eastern Bay, where incoming current from the Golden Gate splits into two
portions. One portion flows north, up to San Pablo Bay and the Sacramento River; the other
flows south into the South Bay (down to San Jose). The pattern in the image below, taken a
half-hour before maximum flood at the Golden Gate, shows a strong northerly flow at the
left, and current dividing to a southerly flow at the lower right. Currents in the middle
and upper right are flowing to the east, at right angles to the primary directions of
flow. A similar split occurs during the ebb cycle. As a result, even when currents
generally are less than a knot at any given point, there can be a 2-knot differential in
current flow from one part of the racing venue to another. As shown below, currents in the
venue can shift laylines significantly, and result in preferred routes to a mark. All
sailors need to be aware of these effects to be successful in SF racing. |
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| Public information on San
Francisco Bay currents is minimal, and limited to isolated points scattered at random
across the Bay. The distance between these (NOAA) points, and their uneven distribution,
render them useless for interpolation and the point to point current predictions needed
for tactical computations. Our company ("Local Knowledge") has developed a
detailed, hydrodynamic model of the Bay, incorporating all available public data and many
local measurements, improving every year since 1995. We have supported many winning racers
over the past decade, and still are the ONLY source of detailed, continuous current
information for San Francisco Bay ( and for many other venues worldwide). The example below (during the 505 North Americans) dislays an alternative
to the "vector" current option shown above, using streamers of constant length
to show direction and numerical values for current strength at selected points. NOTE:
these points are unrelated to the resolution of our numerical model, and are chosen simply
to give a quick overall impression of current variations. The user of our software can
define additional display points at any desired locations (see the yellow current boxes
added by the user). The graphic (using 505 polars and assumed wind of 8 knots) illustrates
the effect on laylines and tactics due to the variable current. The solid blue lines
emanating from Mark "4" are the laylines as they would be in the absence of
current (zero-current laylines). The magenta dots show the actual laylines when current
effects are included. In addition to shifting the laylines, the non-uniform current
requires a preferred sequence of tacks in order to reach the Mark in minimal time (for
zero current, or uniform constant current, all tack sequences take exactly the same amount
of time). In this example, sailing first on starboard to the port layline is more than 1
MINUTE FASTER than sailing on port and taking the starboard layline to the Mark (a
significant time differential compared to the approximately 20 minutes to reach the Mark).
Note also that, in general, laylines and boat tracks are curved as a result of sailing to
optimal tacking angles in the non-uniform current.
Our software allows you to study currents, laylines and
optimal tacks and simulate boat motion at any day and time (with different assumptions as
to wind, time of day, mark placement, etc.) in advance, to develop a feel for the venue,
and enable you to make printouts to take along on the boat. |
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| The example below, during the
505 Worlds, shows a similar effect, at a different assumed wind direction and mark.
In general, changes in the laylines, and in the preferred tack sequence and time
advantage, will vary significantly depending on the wind, date and time. For small boats,
our software is best used to study many possibilities in advance and work out some rules
of thumb so you know how to adjust to conditions on a given day. Large boats with an
onboard PC can calculate these effects live, with position and wind values taken
continuously from the instruments, as in ongoing Fastnet. |
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| SPECIAL OFFER
FOR PARTICIPANTS: Normally, we sell copies of our Advantage software, complete with charts
and incorporating the current region of your choice, at various Levels of operation,
ranging from $400 for Level 1 to $1,900 for Level 3. The current model and displays,
current-corrected laylines and basic navigation features are included in Level 1. To
perform the comparative tack calculation requires Level 2 (at $900) or above. HOWEVER, for this race we are offering a LIMITED LICENSE, to use Advantage
Level 2 from the time of purchase until August 31 for only $150. You have full use of the
program for the period necessary to compete in San Francisco. The program is available for
download from the internet, and payment can be made via Paypal, so you can be up and
running in a matter of minutes. Use the email link below to contact us with questions
and/or to place an order. Other links connect to additional examples and information on
our current models and unique tactical software. |
FOR MORE INFORMATION ABOUT OUR UNIQUE "BEST TACK" FUNCTION,
CLICK HERE |
EMAIL LOCAL KNOWLEDGE |
LOCAL KNOWLEDGE HOMEPAGE |
ADVANTAGE RACING SOFTWARE |