GPS for VMG on dinghy

I have been looking into getting a GPS so I can work out the best VMG upwind (in a Laser). Some of the Velocitek units and the more expensive Tacktick seem to be able to do this

I have reservations about GPS systems on dinghys.  i have seen one advertised recently that allows you to pin the ends of the start line and in the run up to the start give you feedback as to how far you are off the line - to me thats too much outside assistance.

Originally posted by Neptune I have reservations about GPS systems on dinghys.  i have seen one advertised recently that allows you to pin the ends of the start line and in the run up to the start give you feedback as to how far you are off the line - to me thats too much outside assistance.

Quite apart from the question of whether you think pre-start, it's a great idea on a one or two person boat to be fixated on an instrument instead of the boats around you. Crunch, sorry didn't see you mate, I was looking at my GPS.

With differential GPS you can certainly get more than enough velocity accuracy, but such units are more expensive/complex. Normal open service single-frequency units like the velocitek etc will give you an accuracy of 0.1 kn at best and you get into needing smoothing - the infamous GPS speed spikes. GPS is reasonably useful on a yacht for seeing the effect of trim, but even then the speed varies quite a bit with the waves.

Dave 101 "My question is that are the measurements accurate enough to pick up small speed changes due to sail trim, e.g. outhaul tension, sail angle of attack?
The simple answer is NO! Details like that are not available even from two boat training. On the water you are sailing within a system which is chaotic in two dimensions with a crew which can also vary in many aspects. And feel is very much a part of good performance. So results will only ever be approximate. But, you can still gain macro type info from training with a GPS unit like a Velocitek. For example you will be able to deduce what macro settings are good and what are bad up and downwind after analysing a reasonable amount of data obtained while sailing, (and you will have to try and remember what the wind strengths were and the set ups were at the time). But that is not too difficult; I have done it. And it will certainly help your tacking, gybing and mark rounding. It is very easy to work out just how much time you lose on these from a Velocitek graph. And you know what they say. More time is lost from poor tacks on the beat than from a badly set up boat.
Lastly, put GPS into the Search function above as this subject has been done many times before.