Blue and Yellow approach one another on a windward leg. As Yellow bears away to pass Blue's stern, Blue luffs. Yellow attempts to bear away more but there is contact between the port quarter of Blue and the bow of Yellow. There is a Y-flag. What should the call be?

Provided the umpires are satisfied that Yellow was keeping clear before Blue luffed, Blue broke rule 16.1 by changing course without giving Yellow room. Penalize Blue.