17.5 knots this after noon showed on the gps while under spinaker, uni rig, on one hull with the boards half way up, in about 15 knots of wind. Every time I tried to heat it up more, I would stuff the bow and slow down to 15 knots. I think in 20 knots of wind, I could get it to plane if on both hulls boards up.
How far back on the boat were you? I imagine you obviously had your foot in the strap... but were you leaning back also? How much do you weigh? I have hit the same number several times but it is hard to maintain it with any significant chop (more nose diving).