Which prop?

[Timber]

I have a 20 ft. Lund Alaskan dual console with a 90 hp Yamaha four stroke. The Yamaha bulletin says I should get 38 mph at 5800 rpm. All I can get is 25 mph at 5800 rpm. I have a Maxum bow mount trolling motor, three batteries, and a T8 kicker. Should I expect more, and what pitch prop should I be using?

[joshb]

it would be easier to tell what is going on if you stated what pitch you have on right now.

[Timber]

My boat was at the lake, so had to wait to get there to see what prop I had. It's a 13 1/2 x 15. The prop in the Yamaha bulletin was a 13 x 19. Would that make a difference?

[Surface Tension]

I think you should be getting a little more out of that setup. 25 mph, is that using a GPS or speedometer. I'm also wondering if your tack is correct. If you running at optimal RPMs then a different pitch prop isn't where you want to be looking to tweak, yet.
Where are the batteries situated?
Is the bow plowing or is it high?
Whats the hight of the cavitation plate in relation to the keel?