Velocitek created the world's first GPS-based VMG tool back in 2006, the S5. VMG was also a prominent feature on the Velocitek SC-1, the ProStart’s direct predecessor. When several other companies around the world tried to copy the SC-1’s success they also included a VMG feature on their products. Velocitek’s tag line even used to be “Velocity. Made Good.”
The reason why we decided to drop VMG as a feature on the ProStart is that we couldn't find any professional sailors who used this feature while racing. The main reason the pros don't like to look at VMG when they are racing is that the boat's momentum makes it difficult to use these measurements to find an optimal heading.
As an example, imagine this:
You are on a broad reach and then you decide to try pointing lower. Initially your speed will not change very much because of your boat's momentum. The indicated VMG will jump up (same speed as before, better angle). You are happy; the instrument seems to be telling you that soaking low was a good idea.
As you hold your course steady at this new angle, your speed will eventually drop and, as you are falling off a plane, the instrument will finally come around to telling you the truth: this new, deeper angle was, in fact, a very bad idea.
The trouble is that by now you are losing boats like crazy and you're going to have to head way up to get planing again; the damage is done.
The above notwithstanding, these measurements can be useful for training, especially when you don't have other boats to sail against. A GPS VMG instrument can help you figure out what the optimal settings and angles are for your boat in different conditions. It's just that these things need to be figured out before you are actually racing.
Our goal with the ProStart was to make an extremely easy to use instrument that only incorporated the features that pro sailors use when they are racing. There is definitely a good justification for selling a VMG product as a training tool but the feature did not fit with our brief for the ProStart.