Short-range Tactics I: Best Tack Function
Advantage has unique functions for short-range tactics, such as a windward-leeward course. These functions can either be used live, with an onboard PC connected to GPS/instrument data, or in simulation, to prepare for a race. The "Best Tack" function compares total time to an upwind (or downwind) mark for two alternative sequences of tacks: (1) starboard tack to port layline, then on port tack to mark along layline, or (2) port tack to starboard layline, then on starboard tack to mark along layline. For zero current, or uniform current, total time to the mark is identical for either sequence of tacks, but for non-uniform current the times can be very different. In such circumstances identifying the optimal sequence can give you a significant advantage. [Our "Best Tack" function is sometimes called "Super-If" to distinguish it from the "What-If" function of other software that can only compute with uniform current - thereby missing the tactical advantage of the optimal sequence.]
In order to use Super-If, you need to have the boat token onscreen, up or downwind of the active mark at which laylines are displayed. This causes a question-mark button [?] to appear on the right side of the lower red status bar. Click this to bring up the Super-If form (see below), and click again to close it. Click 'Calculate' with the default values of the parameters to perform the Super-If computation, with the results shown below. The form directly gives the time advantage of the optimal tack, plus times on each tack to the laylines and mark. The track of the boat on each tack is shown, to the intercept with the layline (with the optimal tack in red). Note that these tracks are in general curved due to the effects of non-uniform current (the tracks assume that the boat at any point sails at its optimal tacking angle, for the (current-modified) wind measured at that point).
The 'Enter Data' option on the Super-If form enables it to perform the simple "What-If" calculation. If there is no underlying current model for the venue, this mode allows you to input wind and current values and estimate time to each layline and the mark. You can type in values for current and wind and use 'Calculate' to explore various assumptions. In the example above, where there is an underlying current model, the form initially shows average current components calculated from the model. [The exact Super-If calculation is based only on the current model and employs the same wind values used to calculate the current-corrected layline.]
The 'Enter Data' (or "What-If") mode will never produce accurate results with non-uniform currents. The graphic below shows the result for the same situation as above. The straight red lines show the corresponding "What-If" laylines, which differ considerably from the exact laylines, and the intercept times and distances to the laylines are quite different, as is total time to the mark. As is necessarily the case for this computation, both sequences of tacks result in identical time to the mark.
USING 'BEST TACK' FUNCTION FOR PRE-RACE TACTICAL ANALYIS
The "Super-If" form allows you to explore in advance what tactics will work best in a venue for different combinations of wind and times in the current cycle. The favored tack sequence can save you minutes, in a single leg, and will be different for the same ground wind conditions, at different times of the current cycle, or for different wind conditions at the same time of the current cycle. The first example above gives a starboard tack advantage of almost 1 minute for a modest flood, and the next example below shows the result for the same wind and location, but during the ebb. The starboard tack is again favored, with almost the same time advantage differential (1:06). During slack water, of course, both tacks will take the same amount of time.
In this location, for an assumed northerly (NNW) wind direction, one can derive a simple rule of thumb: sail upwind to the left side of the course during either ebb or flood. This illustrates the general point that the preferred tack sequence is unrelated to current direction. If the current was the same at each point, but in one case flowing to the right across the course, and in the other to the left, there would be NO favored tack. The time difference arises solely from the DIFFERENCES in current point to point, not from the principal direction of flow.
MORE EXAMPLES: the examples below are for the same times and locations as those above, but with more easterly wind (from NNE). The first is at the same time of flood current shown in the first graphic above, but with the opposite result: now the port tack sequence is (somewhat) favored.
The example below corresponds to the same ebb current as the third graphic, and again has the opposite result: now the port tack sequence is favored, not starboard, and by a lot (3:31). These examples illustrate that there can be large differences even with fairly modest non-uniform currents, and that the tactical advantage of a given tack sequence is often not apparent.
To prepare for sailing in a venue with non-uniform currents, you should pick representative times (at max ebb or flood, or so many hours before or after high or low tide, and do calculations for a variety of wind speeds and directions. In general, current effects will be more pronounced for lighter winds, and the time differentials will depend on the type of boat (the above examples were calculated with J35 polars, with a predicted boat speed of about 6 knots at the assumed wind speed of 10 knots). In this way you can develop a table to summarize all possibilities, and also use the program to make printouts for a given race day (with different wind assumptions) to take along on the boat. Of course, if you sail a boat with an onboard PC, these computations are even more effective "live," using actual wind measurements by the boat instruments.
The examples above show that the effects of non-uniform current can be very significant, even where currents are not very large and do not vary a great deal over the venue. In venues where currents are large, and vary sharply over the distances involved in a leg, the time difference between port and starboard tack sequences will be much larger. The example below is taken from San Francisco Bay, on a day of relatively modest current for that venue (the currents, show by the blue streamers, do not exceed 2 knots). Due to the rapid variations in current from point to point, however, the port tack advantage is more than 6 MINUTES (6:20), or about 25% faster than the starboard tack approach.