VMG display, app ?

A fair number of GPS systems do require a waypoint or mark to navigate to before they provide a number for "VMG". What they are actually providing is velocity toward the mark, not upwind or downwind VMG and not velocity along the rhumb line. 

With these systems, if you enter the location of a mark that's upwind of you and begin navigating to it, they show the component of your course and speed that's taking you toward the mark. The result is that they display a "VMG" value that decreases to 0 as you approach the layline. That's really misleading and it's been misunderstood by a lot of folks I sail with.

If you want upwind / downwind VMG you'll either need a system with wind sensor, or a smart app that determines true wind direction as you tack. Since I wanted that and a lot more and couldn't find it in a handheld device or a GPS watch, I developed an app that provides me what I want while I'm racing. It provides upwind / downwind VMG from a good estimate of true wind direction along with VMC (velocity along the rhumb line), tacking angles, angle to the mark, start timing, etc.

It runs on fairly inexpensive Garmin watches (~$200 and up).

You can check out the Sail2WIN app on my web site, or at the Garmin App Store.

Here's a screen shot:

However, anyone I have ever talked who races has used wind direction to define VMG. And unless you plan on doing some long distance route with a motor on your dinghy then it's fair assumption to discuss VMG in reference to the OP under the most common use of the term. 

Anyway, back to the topic in hand. As this is dinghy forum, many of us will be sailing on lakes where current isn't an issue. Here GPS is actually better than traditional instruments for calculation of VMG as it includes leeway. The other thing about dinghies is we tend to sail our boats flatter and on flatter water, so a lot of the issues with calibration from heave, roll etc. are minimised. Additionally, unlike yacht, use of many electronic is illegal during racing, so often dinghy sailor would be more concerned about a system which works during training; which opens up more elaborate mounting options to get around the some of the other calibration issues. 

Once in the tide, however, it does get very messy, and that's why I've not spent out on the equipment. One of the driving factors, beyond daylight and winter storms for INEOS going to Cagliari was tide in the solent messing up their instrument readings. 

But, I don't think we're that far away. Reliable apparent wind data I think in many ways is easier on a dinghy now that instruments are miniaturised and is really just a problem with selecting the appropriate smoothing. Heading has been sorted for a very long time already. So it's just mostly about getting an accurate speed through water and leeway. 

I would probably spend £1-2k if I could get real time VMG and post session analysis for my dinghy in a tidal venue. I am already very tempted on getting what is presently available and going to other venues to avoid the current.

Gamin connect has a dinghy racer app for the smart watches (I have it on my fenix 5+) that gives VMG but it relies on a single TWD number , however, it uses an assumed tacking angle of 90 degrees. Which cause a couple of issues, firstly, I am not sure if the watch can tell the difference between pinching up 5 degrees and a 5 degree lift. If both happened without a change in speed the former should increase VMG and later leave it unchanged. 

Then there is the issue that not all boats tack through 90 degrees anyway. Then the issue of tide. So the VMG is useless for me. 

To get VMG you can plug in a wind direction, but it's next to useless. You need real time wind readings.

I looked at getting the Sailmon MAX + wind. But the wind sensor had no built in compensation for movement of the mast. Sailmon thought this wouldn't be an issue on most dinghies where you sail relatively consistently upright. And with the right amount of smoothing I think it could decent VMG readings. I know Lennon Sails were using this linked to the Clyclops load cells to give customer some solid numbers to work to for initial sail set up. 

However, there is no way to calibrate for tide in real time. So for training against numbers it still wasn't worth the money for me given where I sail. 

If you want to look at VMG post sail then Njord Analytics is probably the best and you can plug in a wind direction to get VMG then. But it's not a live display thing.