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kersh said..
has anyone ridden the 640?? Interested to know how it compares to the Mach 1 / 2 or the 840.
I've ridden the 640, 840 and Mach1-550.
The 640 roll turns way better than the 840. The 640 needs about 1.5 kts more water speed to lift off vs the 840, and will go 2 to 3 kts faster all the time, assuming all else is equal. Once up, if in under powered conditions, the 640 has a noticeable drag-hump to get over, which is very normal for smaller foils. The 640 has a happy foil speed range between 16 and 23kts, where you feel very low arm load to pull it along. A stiff mast is essential else none of this feel stiff will mean anything... It turns great and has superb pitch control with the 220 tail using a 1.25deg shim and 2/3rds of the tail winglets cut off. It's a very fun foil with a good combo of glide and turning ability. The root of the foil is fairly stiff and the tips can flex a bit which I think makes it feel controlled and also smooth. In foil cross section it is a thin foil with a relatively full entry. I think the full entry makes it pretty good in aerated water. All the naish v2 HA foils all have some sweep aft in plan view so they are pretty smooth in turbulent water, and the 640 is the best. The 640 hits a drag barrier at 24 kts. I assume some people could push it to 28.
The Mach1-550 is a nice foil. The M1-550 likes the smaller D180 tail. To take off, the M1-550 needs about 1.5 kts more water speed than the 640. When nicely powered up, reaching or downwind, the 550 is +2 kts faster than the 640. The 550 drag barrier comes in at 25 or maybe 26 kts. I never did over 27 on it. The 550 turns great, with the right tail shim angle. The weakness with the 550 is that it will usually hold a ventilation bubble if you breach a tip. You can sand more twist into the tips to make it better. You can flat breach the 550 and it'll reattach flow easily when it goes back into the water. But beware of defects in the factory clear-coat paint.