Is this thing ratified by the International Speed Sailing Council?http://cgi.ebay.com.au/solar-speed-watch-for-sailboard-dingy-or-surfboards_W0QQitemZ320457481658QQcmdZViewItemQQptZAU_Sport_Surfing?hash=item4a9cc11dba
Is it a GPS unit? A question was asked if everything is included. The seller answered everything was including a propeller.
I figure it's some sort of log (propeller) that you toss in the water and it's attatched to a wrist strap, there must be a wire that records revs of propeller and converts to knots. Probably worth attaching a treble hook to the propeller. 
Ahhh... you young blokes. No perspective from history.
Actually it was the pre-GPS generation!
The SpeedWatch was the best thing ever for speedsailing before GPS came along.
It was the first device that I know of that enabled the sailor to get instant speed feedback while sailing.
I actually still have two if these things tucked away somewhere in my speed sailing museum, but the one advertised is a later model that I have not actually seen in real life. This model had the great ability to save the top speed on the display (Like the max speed on the GPS). I would have LOVED to have one of them! There was another brand called SpeedMate that was sold in the USA which also had this capability. (I think they were both made by the same company in Europe - SWATCH).
The device worked by receiving magnetic pulses from a small propeller attached to the fin. The receiver/display had to be placed within about 50cm of the propeller so we attached them to the rear of the board. I used to attach them to the Velcro on the back foot strap (they have a Velcro type 'hook' pad on the back of the unit).
They had a couple of minor problems:
- They were bloody expensive. In todays terms think at least x4 times cost of GT-31!! 
- The display unit could get ripped off in a crash. Luckily, they float.
- You had to make a very small hole in the trailing edge of the fin to attach the prop. (Have a look at all my old speed fins! )
- The prop created some drag and was not necessarily accurate. It was difficult to quantify, but on one of the few occasions I ran timing gates while also using the SpeedWatch, the gates gave me a knot or two faster speed than I was seeing on the display. I saw 38 knots (peak) in the SpeedWatch in around 1990, and have always wondered how close I really got to the magic 40 knots (14" needle, 3.7m Wild Winds speed sail, 35-40 knots of sand filled wind).
The company that distributed the USA SpeedMate used to hire them out a dozen at a time for speed competitions. We actually had some correspondence about bringing a batch out to Aus. for a comp at the Pit in the early '90's. It looked a bit too expensive so it fell through.
All I can say is: Thank god for the GPS!!
Oh, and you could also attach a wind prop to them to use them as an anemometer. This photo was taken at the 1989 Sandy Point speed trials on the day that Mal Wright exceeded the existing world record, and yes... the figure is Knots!
Oh, and another big problem was that it was all too easy to lose the prop. Once you did that the thing was pretty much useless. Hmmm.... I used to have a couple of spare props stashed away........ 
same thing like very popular speed reader Navman Yellow M300
but, no decimal places, so you can ride 38 knots and you never know
is this 38.1 or 38.9
and, of course not any connector on navman so you cannot export any of data
ahhh ninetees.... ahhhh
I had one of these. The way you avoided losing the propellor is by drilling a an anchor hole in the fin...extreme!
