Hello,

I'm the designer of the S5. I hope I can contribute to the conversation by explaining a little bit more about how our device works.

The S5 gives the magnitude of your velocity projected onto the rhumb line between the windward and leeward marks. You define the direction of this line by entering two reference points. These points are only used to define a DIRECTION. For example, you could use two points on the beach or in the parking lot, as long as one was directly upwind of the other.

Your velocity projected onto the rhumb line depends only on two things: the magnitude of your velocity vector (your speed) and the direction of your velocity vector (your heading) relative to the direction of the rhumb line. It does not depend at all on your position relative to the marks. No matter where you are on the racecourse the S5 will read the same thing for a given speed and heading.

Other GPS devices don't give VMG, they give closing speed. You can get them to approximate VMG by giving them a waypoint far upwind of the windward mark (say 50 miles). Picking the exact location of this point 50 miles upwind can be a big headache when the wind direction changes. If you just used the windward mark as your waypoint, the closing speed given by the device would not look anything like VMG once you got close to the windward mark.

To sum it up, our device tells you how fast you are moving in the upwind (or downwind) DIRECTION. Other devices tell you how fast you are moving towards (or away from) a pre-programmed LOCATION.

When we first started testing this device, we used closing speed like everyone else. Our pro team complained that it did not "feel" like real VMG and got all screwed up close to the marks. This made us figure out the difference between closing speed and real VMG. The pro team is now confident that the output of the device is the number they really care about.

I hope this helps. Please let me know if any of this doesn't make sense to you.

Thanks for reading,

Alec