Sailing techniques - using GPS to optimize sailing
This material is covered in Chapter 5 of AP. Some of these drawings are from Chapter 20 of "The Weekend Navigator" by Bob Sweet, which is one of the texts for the AP Class.
A challenge for Sailors is to optimize performance when sailing into the wind.
If you sail at a higher angle to the wind (say 50° instead of 45°), your boat will have higher boatspeed, but will be travelling "further away" from the destination. If you head closer to your destination, your boat speed will drop.
Racers use polar diagrams and sophisticated computers to select the optimum sailing angle.
However, by using your GPS to read-out "VMG", or "Velocity Made Good", you can adjust your boat heading for optimum course and get similar results.
This technique also works when sailing downwind. You can improve boatspeed by sailing at a slight angle - say 20°- 30° from downwind - rather than "dead" downwind. This also makes the boat easier to steer. But of course, you increase the distance travelled. Using VMG you can determine how far to sail off-the-wind but arrive to your destination quicker
Polar Diagrams
A Polar Diagram shows, for a constant wind speed, the corresponding boat speed at different sailing angles (heading vs the wind). In this example, with a wind speed of 12 kts, the boat sailing at 45° to the wind has a speed of 6.0 kts. At a 50° sailing angle, it has a speed of 7.0 kts.
In Drawing 2, the boat is sailing at a higher angle to the wind than in Drawing 1 -- approximately 50° to the wind (255° - 205°) . The vessel's performance at this angle results in a speed (SOG) of 5.6kts, higher than 45°(Drawing 1). Even though the boat is not heading as close to the destination as in Drawing 1 (BRG 255°), VMG (3.6) is higher. You are making more progress toward your destination by tacking at this higher angle, and would be able to reach the destination quicker.
The next two charts show a vessel tacking into the wind. By tacking at 50° to the wind, the vessel has higher boat speed and higher VMG. By sailing and tacking at 50° to the wind, the boat will reach its destination quicker.
On the final tack (below), the boat is heading directly toward the destination. The VMG will be equal to the SOG (speed over ground).
By properly adjusting your sails for the current wind conditions, and monitoring VMG and boat heading, you can maximize your performance when sailing to your destination.
In Drawing 1, your heading (COG) is approximately 45° (250° - 205°) to the wind (your destination BRG). Your boat speed (circled in red) is 5 kts, and your VMG (circled in blue) is 3.5.
The next three charts ("Using GPS Velocity Made Good") help explain the concept of boat speed, sailing angle, and velocity made good. They compare boat speed, boat heading, and VMG at three different headings. The GPS screen compares your heading (COG), boat speed (SOG), and velocity made good (VMG). (This is a Garmin "highway" screen - your GPS may look different.)Your destination (active waypoint) is directly upwind, i.e., the wind is coming from 205°
To use this technique, your destination must be the "active waypoint" in your GPS.
In Drawing 3, the boat is sailing at a an even higher angle to the wind - approximately 55°. As a result, it's speed (6.0) is higher than Drawings 1 and 2. However, it has a lower VMG, due to the geometry -- the boat is not heading as close to the destination (BRG 260°)as in Drawings 1 and 2,
Therefore, in Drawing 3, the boat would take longer to reach the destination quicker. The optimum sailing angle (50°) is in Drawing 2, which gives the highest VMG.
This technique can be used to optimize downwind sailing, too. Make your destination the active waypoint, and sail directly toward it (sailing "dead downwind"). Sailing downwind is the slowest point of sail. Now, head up slightly, 10°, 20°, or 30° from dead downwind. As you head up, your boat speed will increase, but you will be sailing a longer distance to the waypoint.
The key is to check your VMG. As long as VMG at these angles increases, compared to sailing directly downwind, you are better off sailing at this increased angle.
In addition, sailing at a slight angle to the wind is more comfortable, easier to steer, and has less chance of an accidental jibe.
Multihulls are known for this approach. They will sail as much as 45° from downwind, but their boatspeed will increase more than enough to compensate for the extra distance sailed.
