I wonder on the legality for use in racing in a lot of classes. 

I think Dylan Fletcher has put some feedback somewhere online but that would be for Moth sailing, haven't seen any reports from Hiking boats as a training aid other than the RYA video

The heel and pitch is nice. And the frequency and accuracy of the gps make it much better for analysing sped lost through manoeuvres. 

However, for straight line I think I'd want the wind sensor too, because speed is irrelevant without know true wind angle and true wind speed. 

Each class is different. There is no issue with this under RRS, but class rule prohibit certain other things. The 800s have recently changed our class rules to specifically allow GPS, so long position, speed and vmg aren't be shown. 

However, it is getting increasingly difficult to police this. Most of these rule were written before gps units and action camera's were readily available and they were partly to stop an expensive arms race for bulky yacht style nav equipment. But now the equipment is small and readily available some of that argument goes away and you're left with the purist argument about the merits being about sailing by 'feel'. 

As I've said before, just being able to see speed is pretty much useless, so I wouldn't worry about that in races (spending £800 just to chase a target speed is waste of cash in my view too). Pinging locations is more of an issue for racing, as people can get a big advantage in not having to see transits on a start. But if you start getting VMG, TWA, AWA, TWS then that will take some of the sailing by feel out as people start heading for target numbers. I think classes will need to have a discussion as it will get more difficult to police as this technology gets smaller. 

Of course, what screws all this up is tide (in terms of sailing to gps speeds). 

I am seriously considering buy this (including wind sensor). I would use it for starting practice, pinging imaginary line then sailing by feel and transits, then using the data collect for post sail analysis. How close was i to the line, how quickly did I accelerate. 

It would also be good for manoeuvres in terms of ground lost. Tying that back to heel during the tack / gybe. 

In my tidal location I don't think it would be much good for solitary speed tuning yet. 

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The Sailmon Maxx does the above, but it's handily logged and visualised on the app out of the box. You also have the option of getting the wind sensor, which looks like it would be small enough to fit most dinghies.  But... I wouldn't fancy sticking it in the mud at halying!!! 

At the moment I still think a camera for £30 is pretty epic as a training tool for boat handling. At 60fps it's got a pretty good data collection interval. The water your floating on, assuming you get the horizon in shot, works as a very good spirit level. You can see the how consistent the rate of turn is by tracking object on the horizon. You can watch for steering correction on exit. Time how long it take to go from full power to full power. 

The only thing missing, is when you see a 'good' tack or gybe, you don't have the speed data to back it up. I have tried with my bike GPS (there's a video somewhere), but it doesn't have the resolution in the data to truly mark the tack as 'good' from data alone. 

Wind sensor would be great but I’m sceptical of the accuracy (happy to be persuaded though!).

One of the important things to do is to sample data at a fairly high rate and then filter it down to something usable. There’s a lot of noise if you look at speed etc 10 times a second (speed can easily vary by 0.5kt from one sample point to the next).

I think the wind speed wouldn't have to be absolutely accurate, as long as it was precise. I guess we're all going to see different windspeed at the top of different sized rigs. And that might not be the true value for mid way down the rig. But as long as X knots on day Y was X knots on day Z, then it would be useful.