15 knots = 25 feet per second. Five boatlengths = 90 feet.

About four seconds.

HOWEVER - I'd bet that F18 was anticipating the tack. They had no rights as the clear-astern boat on port. A hail from the F16 gives the F18 plenty of opportunity to keep clear as the F16 sails her proper course by tacking for the mark. The fact that the F16 made it to the mark first indicates to me that all is right with the universe - good job, Gill.