A "True" code zero still rates as a Spi and fits the 75% rule, but generally within Cat circles, we talk about a C0 as a light sail (usualy maid from rip-stop nylon type clock and not "headsail" cloth) that DOES NOT measure as a Spi as it fails the 75% rule.

As for when it wortks. Fetching in light winds, and tight reaching in other conditions up to an upper wind level.

V70's use them to great effect and a number of boats use them as a matter of course. The Wetra Tri has one that fails the 75% rule. Some people also call "non 75%" C0's "screachers".

The X16 uses one, but I have no data on how well it is optimised.


There are a number of teams who are working on C0 projects at present on various other platforms.