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Leighbreeze said..
Thanks for the post Jonathan.As we have found out that the Apple Watch on waterspeed app can vary up to 4kts less than my mates Garmin GPS watch.It was doing my head in being neck & neck with my mate then discovered my Iwatch was 4 kts less speed.
So quite a discrepancy.Anyone else discovered this issue?
The question is whether the Garmin read too high, or the Apple watch too low. Overstating top speed in spikes is more common than understating by several knots. One way to find out is to look at speed over longer stretches and compare it to the top speed; the difference should be just a couple of knots. If you use apps or k72.com, you can look at 5x10 second average speeds. You can even eyeball the speed graph, here's and example:

Typical speeds were about 11-12 knots, top speed was a bit over 16 knots. Tops speeds were around 16 knots twice, and often around 15 knots, indicating that the top speed is real. If you have just one or two peaks that are a few knots above everything else, it's very likely to be an artifact.