A radar gun will only give a "one instant fastest time" mary, and basically what is needed is an average time over a set distance. If a time was recorded with a radar gun in an instant that a boat, for what ever reason, had suddenly accelerated to a way faster speed than it would normally be capable of, and the "gun" recorded that "one off" exceptional speed, it would be an "aboration" and just not truly representative of the potentual of that craft over any greater distance.
