Buoy Racing - Onboard Functions
Level 3 adds a number of functions for buoy racing, with an onboard PC and live GPS/instrument data. These functions can also be explored using the expanded "Simulation" function (you enter boat heading and wind values and motion is determined by boat polars).
Projected Boat Track. This function shows where the boat will go on the present heading under the effects of wind and current. In the example below the boat is headed on a reach along the rhumbline to mark "38" (red line). Due to the ebb current (flowing west), the actual track of the boat will follow the dot-dashed line. Note that the data strip along the left is expanded from the Level 2 version (upper green portion) to include computational results (lower yellow portion).
Time to Mark. This function calculates the time to reach the mark on the present heading. In general, the boat will miss the mark by some distance (the "crosstrack error"), shown on the datastrip as XTK (0.3 nm in the example below). Time to the (nearest approach to) the mark is reported as TTM on the datastrip. The actual boat track to nearest approach is shown by the dashed line, which in general is curved due to the result of current. This differs from the "Projected Track Function" (above) in that it requires an active mark and performs time and distance computations.
Time to Layline and Boat Laylines. The "Time to Layline" function operates when a layline is displayed for an upwind or downwind leg. It calculates the time to reach the layline on the present heading (TTL), and adds time along the layline from that point to calculate total time to the mark (TTM). As shown below, a dashed line extends from the boat to the intercept with the layine (also curved due to the result of current). This differs from the "Super-If" Function" in that it is based on the boat's actual heading and not the optimal heading based on the polars. It also differs from the "Boat Laylines"function (black dotted lines below), which show where the boat will go (including effects of current) if sailed at the optimal tacking angle (recomputed along the track to take into account the current contribution to true wind). Like the TTM function above, these functions are automatically refreshed as wind and boat position change.
Compute Reach to Mark. As shown above, current will alter the path of a boat headed on the rhumbline so that it will miss the mark (reaching leg). This function computes a corrected heading that compensates for the effect of current. Selecting this function from the "Buoy Racing" menu brings up the form shown below, enabling you to select a computation either from the boat or a mark, to another mark. The function returns a current-corrected heading (contrasted to the rhumbline bearing). In the example below, the bearing to the mark is 20 degM, whereas the corrected heading is 27degM. The solid red line shows the track of the boat on the corrected heading, contrasted to the result of the "Time to Mark" calculation for a heading along the rhumbline. Like the Super-If function (for corresponding layline calculations), the "Compute Reach" function can be applied to situations where there is no underlying current model by calculating with user input for wind and current (the "Constant Current" option). This function can be used as you are approaching an upwind or downwind mark to compute the most effective heading on a subsequent reaching leg.