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You can't really use a symmetrical (pole) spinny as a genoa, at least not if you expect to make good anything above a beam reach, and then only if it's not blowing hard, which would heel you too much.
With an assym (bow or bowsprit) spinny, a really flat one (code zero or one) can be sort of a genny, might get you up to almost a close reach. But a baggy one like a Code 3,4, 5, is just going to be too "full" and curvy to be a jib. Jibs are flatter, hence have a much narrower angle of attack at the luff, so can point higher without backwinding and collapsing.
also, reaching with spinnakers entails a lot of side force, and not that much forward lift, unlike a jib. And you can get lee helm as a result. Also they can backwind your mainsail, whereas a jib (reaching or above) will complement your main by directing and accellerating the windflow over the outside of the main, generating more lift/speed. Spinnys want to take you where the wind is blowing towards (which on a reach is leeward). Jibs want to give you lift that helps you go into the wind.
And you can't tack a spin, it'll invariably end up caught behind the headstay on the new "tack".
Last edited by nolatom; 10-14-2011 at 02:35 PM.