Personally I think momentary (display) speeds on a GPS are highly undependable. The minimum average time should be 10 seconds. I think your 24 seconds is small enough to get a good average speed reading. 17.38 knot (European Inter-17 singlehanded no spi) is the current top score.

What were the winds and sea conditions, on what course did you achieve the top speed (reach, broad reach?)

Thanks,

Wouter