yamaha F15 prop size

[htdawg]

Hi,

I have a 1999 yamaha F15 4 stroke long shaft with a 9 1/4 10 1/2 prop on it. could you tell me what size prop i could put on it to get a little extra top speed out of it? without killing the motor
p.s
its on a 13 foot fiberglass fishing boat

thanks
htdawg