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.
John,
Proper course plays NO part in this situation.
F16 needs to COMPLETE the tack before the F18 needs to take avoiding action in order to "tack at will".
As stated by others, best bet is to either pinch up to drive the other boat away - it will not take much to force them away, or drive them low (by backwind of YOUR sail) more to make the "tack and gain rights" a sure thing; OR drive a little to leeward by footing a little to create more time to make sure the tack is complete and so rights are gained with time to FORCE the F18 to duck or tack.
It is best to anticipate this happening and try and make the other boat tack away before YOU want to tack.