2008 SF Big Boat Series

Currents in San Francisco Bay are very strong, and vary dramatically in both strength and direction over very short distances. If you will be racing in the Big Boat Series (or any race in the Bay), a knowledge of SF Bay currents is essential. During the BBS the flood will range to 3 knots and the ebb to 4 knots. Our "Local Knowledge" current model has been developed and improved every year since 1997, and is the only PC-model that provides detailed, point-by-point continuous current predictions for the Bay. The "LK" model is incorporated in all levels of our Advantage Racing Software. With Advantage Level 1 you can study currents in advance, see the effect on laylines at the BBS marks and make printouts to take along on the boat. Better, with Level 2, you can perform unique tactical computations as you race that exploit the nonuniform current distribution. Our new "Level 3" goes even further, with even more powerful functions and features.

The graphic below shows a typical upwind approach to the "Blackaller" buoy, against a strong flood. The quickest (optimal) route is shown in red, and takes 16:32 to complete from the boat position at the right. Advantage is unique in calculating other routes for comparison, enabling you to get a feeling for just how "optimal" the red route may be. The times to complete each route are shown in the box, color-coded to match with the route. For this example (which assumed 1D35 polars and wind at 10 knots from 250 true), the times range to 24:22 for the yellow route, a difference of some 8 minutes. You can step along the solution with the arrow keys at the bottom of the box, to see the predicted position of the boat on each route (black dots). The DTG column shows distance to the destination from each dot.

Advantage has other unique functions, including its "Super-If" (or "Best Tack") function, which compares alternative sequences of tacks to the mark. Every other program has a "What-If" function, that assumes uniform and constant current. This assumption is not only unrealistic in a venue such as SF Bay, it is incapable of determining the tactical advantage of a route that exploits nonuniform current. When currents are nonuniform, laylines are not straight, but curved (red dots in graphics, compared to zero-current blue laylines), and a particular sequence of tacks can take a lot less time than the alternative. In the example below, a starboard tack to the port tack layline is 4:32 faster (out of total time 10:10) than a port tack to the starboard tack layline. The curved lines show the track taken by the boat on each tack, which assumes the boat at each intermediate point sails at its optimal (target) upwind angle. Note that current affects the "true" wind experienced by the boat, as well as altering its net velocity due to "set" and "drift." If you use a program such as DFW (or Expedition) with "What-If" in SF Bay, you will miss the optimal tack sequence. Plus, the predicted times and distances to the laylines will be very inaccurate, even with a source of current input. (And without a digital current model, tactical computations in SF Bay are pretty much worthless.)