VMG TACTICAL DISPLAY
Our "VMG" tactical display is unique. The idea is to factor in both wind and current variations into a display that shows you at a glance where you can make the best "velocity made good" towards the mark. The display uses four circles of increasing diameter to show how VMG varies across the course. There are two types of applications for this display: short range tactics in buoy racing and to analyze long distance races. In the example below (upwind against adverse current) VMG is best along the right-hand (starboard) layline, and poorest up the center. Unlike routing and other calculations which take longer to compute and are concerned with the overall path to the mark, the VMG display shows you locally where you can make the best progress, going to the right or left. This can be very useful if your primary focus is on your nearby competitors rather than the optimal route to the mark, and is easy to see at a glance.
Often, you can pick out the optimal route simply by linking up the locations with the largest circles, while still focusing locally on your competitors. The image below shows the same situtation with the routing solution, using the new, Level 3 "W/L" routing option that maximizes accuracy for upwind and downwind legs. The optimal (red) route to the starboard layline is about 4 minutes faster than the black route up the middle. Note that VMG is better yet if you go beyond the layline (to reaching angles), but this takes extra distance.
The second application for the VMG display is to analyze races that take many hours, or days, to complete. Ocean races utilize wind prediction files (wind models) for routing, and typically you can only look at the wind distribution at a single time. This can be confusing, since you will reach different parts of the chart at very different times. The VMG display estimates how long it will take the boat to reach a given location, and computes VMG at that time. This enables you to see a glance where conditions will be favorable at any point in the race. The examples below are taken for the Newport to Bermuda race, using a wind file we provided for the 2007 double-handed race. The VMG display factors in both wind and our Gulf Stream current model for that race. The two graphics illustrate the very different conditions experienced for different boats. The left-hand graphic is for a Benetau 40.7, and the right-hand is for Blue Yankee (an RP66), whose additional speed enables it to reach the middle of the course faster and benefit from good wind pressure there while it lasts.
The left-hand image below shows the wind distribution at the time of the start (on 6/9/2007), and the right-hand image the distribution at the time the Benetau 40.7 approaches Bermuda (on 6/13/2007). By this time the strong winds to the west of Bermuda are gone, and the better pressure is to the north and east. The VMG display covers these changes (and current effects as well) in a single image.