Both the Texel and ISAF systems have the very funny setup that putting a spi on a cat-rigged boat only gets 1/3rd the hit than putting a small jib on it. This is not only wrong by a gut feeling ! It is also easily proved that this theoretically wrong. Goes like this; Biggest gains of jib are on reaches and downwinders; only a small gain is experienced on the upwind. When a spi is set the gains of a having a jib on the downwind and shy reaches are nullified by the gains made by adding a spi. So only the small gain of having a jib upwind is left. So this leads to the situation were the spi hit must be bigger than the jib hit when looking at putting either one of these to a cat rigged boat.

This is actually the situation where both texel and ISAF make the largest errors.

Luckily it isn't encountered often

Wouter