I have a 14ft mirrocraft, 25hp F25 Yamaha 4 stroke 5-6krpm range (new) and here is my issue.
9 7/8 x 11 1/4 works fine with me or me and the dog. I get 5800-5900rpm. (29-30 mph) Add wife and it drops to 5450rpm.
9 7/8 x 10.5 with me + dog + wife it goes right into the rev limiter (6150rpm)...