Velocity Made Good is a concept that combines your speed and direction towards a given waypoint to get you there as fast as possible. In navigational terms, a waypoint is a fixed position on planet earth using Latitude and Longitude. For short course racing sailors, the marks of the course are the waypoints. For offshore sailors, they might use a series of waypoints to get to their end destination (the last waypoint). VMG is defined as the combination of direction and speed that gets you to a waypoint in the fastest manner possible and is a key component in race strategy. Strategy is defined as what course you would take to get from one point to another in the absence of any other boats or obstacles.
To explain VMG think about this example: If you wanted to get to a windward mark and you had the magical ability to sail directly upwind at 10 knots, your VMG would be 10knots. If you travelled in the direct opposite direction from the mark at 10 knots your speed would be negative 10 knots. If you moved sideways at 10 knots your VMG would be negative 1 knots (or thereabouts) as you are essentially moving away slowly from your waypoint objective in terms of angle. If you simply didn't move at all and stayed in place your VMG would be neither positive or negative. It would be zero as you are not moving in any direction.
Sailors of course cannot travel directly upwind because their sails don't fill and the boat won't move. Sailors have to sail at approximately 45 degrees off of the wind in order to get the sails to fill and the boat to move forward at the best speed. Pinching sailors who sail high but slow have poor VMG. Sailors who sail low with sails stalling would have worse VMG as they are going low and slow. Sailors are trying to find the ideal upwind angle to maximize their boat speed. We call this sailing on angle where the wind is flowing perfectly around their sails to maximize power.
In wavy conditions your VMG can improve by sailing slightly lower. You can actually sail a better course angle because the boat is moving faster through the water and is not slipping sideways. In flat water you can increase VMG by gaining speed and sailing higher. The idea being to get fast and then try to keep that speed at a higher angle.
Remember that speed is different from Velocity Made Good. It is only one component of VMG. The other is direction. When sailing upwind, you are either on starboard tack or port tack. Depending on where you are on the course, one tack will take you closer to the mark than the other. That is the tack with better VMG from a direction perspective which is the second element of VMG. When you look upwind and you see that you are sailing closer to the mark on starboard tack compared to the port tack, the starboard tack is what we would call the "long tack" and it would have better VMG compared to the port tack. In this example we would call port tack the "short tack". You still have positive VMG but it won't be as high as the long tack. So if two boats are both sailing upwind at 6 knots the boat on the long tack will have a higher VMG compared to the boat on the short tack as it is sailing a closer angle to the mark.
Determining the exact VMG number is impossible in sailing without GPS instruments that are constantly calculating the combination of your speed and direction towards a mark (waypoint). When you see America's cup sailing, you often see an iphone attached to a tactician's arm which has crazy changing numbers. It is telling the tactician if their VMG is increasing or decreasing depending on their speed and angle towards a mark. This helps them optimize their boat's angle to get the best combination of speed and angle towards the mark or make a decision if the other tack has a better VMG.
In Lasers, we are not allowed to use instruments like this so we have to rely on our senses to make the best VMG decisions both upwind and downwind. It takes years of practice to become fast in different conditions but anyone can tell if one tack is taking them closer to the mark compared to the other. Sail the long tack or the long gybe first as it will have much better VMG compared to the other.
If there were no changes in wind direction and boats are moving at the same speed, they both would get to the mark at the same time. At some point the long tack becomes the short tack when you hit the layline. If you sail the short tack first and hit the layline, you then tack and you are now sailing the long tack. So both boats have to sail a long tack and a short tack and presuming they are sailing at the same speed they will get to the mark at the same time. So why the long tack first?
The answer of course is that the wind doesn't stay the same. It shifts and you often can't predict how it will shift. There are times when even the best sailors are unsure how the wind will shift during the upwind leg. If you sail the long tack first and there is a shift in either direction you have options. If there is a header you can tack and take advantage of the shift and sail better VMG and a new long tack. If you get lifted you get to sail closer to the mark which also improves VMG and still keeps you on a longer tack where you continue to have options.
If you sail the short tack first you are quickly cutting down your options. Yes you will gain if there is a persistent shift but that's the only time you will make a gain. Sailing is a game of consistency and taking advantage of unknown wind changes. If you always sail the long tack you have options to take advantage of changes. If you sail the short tack and get out into a corner first you are cutting down your options. Only sail the short tack if you have high conviction of an advantage. You have to have a great reason to sail the tack that doesn't take you closer to the mark.
Sail the long tack to keep your options open. Only sail the short tack if you have very high conviction that there is a wind or current pattern that is persistent that will give you an advantage.