Steve: Yamaha tested this combo on 8-21-02 with a 15 1/4 X 17 SWS prop. See Yamaha's test here:
http://www.yamaha-motor.com/assets/prod ... -GRW-B.pdf
The next day, Grady tested this same combo (probably same boat?) with a 13 3/4 X 19 M prop. See Grady's web site for performance specs.
This is pure speculation, but I'm guessing that Grady didn't like the fact that the first (Yamaha) test gave a top end just under 40 mph. So Grady changed props the next day and touched 40 mph.
Looking at the two - and assuming all else is equal - I like the higher cruising speed and greater fuel efficiency at 4,000 rpm of the Yamaha test more than I like the slightly higher top end of the Grady test.
On my boat I have a 15 1/2 X 17 M SWSII, but I have an F250. For what it is worth, I get 27 mph at 4,000 rpm and a top end of 42.7 mph at 6,000 rpm with the boat lightly loaded and a half tank of gas.
Rob