Hi all,


It is. Much more than most people are aware. There is a good reason why we do not directly use civial GPS signals to control the auto pilot of aircraft.
Most handheld GPS receivers truncate the time stamp of the track data to 1s. Hence the 10s track has an 1s time error, e.g. 10% error.
Even if we would all use WGS 84 as position reference, it is not guaranteed that the software of the device will use this model for speed calculation (some use a simple sphere model for speed calculation). On top of that you will have the position error of GPS signals.
As a consequence it is very difficult to compare just the plaine speed numbers, just have a look at the numbers posted here, ranging from 17 to over 20kts. Comparing the tracks with the same software would already mitigate a huge error source...

Typically the top speed of beach catamarans is not limited by drag, but by stability (against pitchpole or heeling). Of course you need to have enough wind.
With a proper spi setup, you should get the highest speed, since the center of effort with spi is lower than without. Many sailors describe this effect as "the spi is lifting the bows".
Saying this the simplest way to make the boat faster is to use a lower rig. Fast in high winds and a sitting duck in low winds (you can call this also a Dart 18).

Cheers,

Klaus

and for those still believing in numbers:
17.5 single handed on a broad reach without spi (I didn't dare to pull it), more than one knot slower than our fastest track on a classic T.