Okay, my 2 cents having looked at both boats, with a serious purchase pending. You can dump any of these boats at a gybe, but (all other things equal mass, mast position relative to center of effort, sail area) I find that how aggressively you can bear off, coupled with, foredeck hull volume, and hydrodynamic drag, and mast postion relative to center of effort can be pretty boat-specific. First, both the Cap and the Nacra are really nice boats. All this about masts snapping is baloney. Unless you raise a phone-pole on your F18, any mast will snap with a spin if not appropriately rigged for the conditions. Learn how to set your rig tension and read and repeat: big breeze, no mainsheet tension or incorrect diamond-wire setting, with any spinny= *SNAP*. If you think, with thousands of pounds of wind force pushing on it, that mast strength is solely a function of the flimsy aluminum and not the assisting stay settings and mainsheet tension, I can recommend a physics class. (Don’t be so dumb.) Having souped up the 5.5sl (and sailed most other F18’s), I can say that there is, a critical moment at the gybe when the kite goes up that is extremely boat-specific. When the kite goes up, the top of your boat wants to scoot forward, but its, draggy hulls (they are in water, a much more dense a fluid than air) are slower to accelerate and hold it back. This acceleration causes the center of effort to move very far forward until such a time as the hulls can catch up a bit and get back under the rig. Once sufficient speed is achieved on a catamaran with a spin, the leeward hull begins planing, the hull slips forward under the mast, and the center of effort moves aft again, and the process is facilitated by the lift provided by the spinnaker on the front of the boat, re-directing air both downward and aft. However, on each, hull design governs acceleration and is quite different. Based on my experience, hull design and water chop largely govern a propensity spectacular catamaran gymnastics at the gybe. At the downwind gybe, nose diving is a function of lesser volume fore, and less leeward, outward side hull curvature, and how much water is on top of the hull as the top of the boat (mast, sails) tries to go fast and the bottom (hulls) drag behind. Narrow sleek hulls and far forward CE design elements provide greater ease in the hull hunting for the bottom of the ocean in a big breeze as the CE tips fore on the boat at the gybe, with hulls pointing to the bottom of the ocean for a bit of time until planing begins. At the same time, the sleek hull features providing less overall resistance in the water (hulls more slippery) and better acceleration. Having souped up a (spinned) a nacra 5.5sl and sailed it as F18 (Second ! on day one in a big breeze, and...umm, oh yeah, dead last on day 2 in a light breeze)... the low volume hull 5.5sl (spi) with the F18 class in force 4 winds was very, very fast downwind in heavy air, especially after it got rolling, presumably because of the small hulls. Once planing, the old boat actually felt very stable. However, watching the 5.5’s low volume hulls torpedo until the boat started to plane at the gybe is a pretty “exhilarating” experience. This acceleration is not necessarily bad (I have lots of experience with the my skinny, low volume 5.5 hulls!), but does influence how hard you can (should) push the boat immediately off a gybe-I go a little deeper and more s l o w w l y). Reduced hull volume can make for a startlingly fast acceleration but managing the boat during this point, requires a bit of care, until the hulls begin planing (if it is breezy enough). Just looking at the cross-sections-one would then expect the Cap to provide better acceleration, is probably quicker to plane, but maybe a bit more likelihood of a stuffed hull during a reckless, hard gybe’. The big volume on the Nacra hulls may provide a bit more overall stability at this point, at some expense of overall acceleration. It’s a trade off. Once planing, though, the boats are about the same. As for quality, I think AHCP (Cap) makes a really nice, solid boat, but then again, it costs more than the Nacra Infusion. Bottom line? Learn your boat, avoid generalizations-all F-18’s are pretty fast.