Do you have the present prop and rpm on the pontoon?Without these specs It is difficult to know the best prop.I would say an 11in pitch is in the ball park.I would try it with the present prop just to get some specs to work from.
whatever prop you finally go with, make sure it will twist at 5900+ with your normal load of people and coolers or you may get to chat with me in the ymaha forums about detonated pistons.
You need to determine whether your engine has the 1.85:1 or 2.33:1 gear set in the lower unit. 1.85 units require a 12-1/4 x 9 prop. 2.33 gear set units can run the 14 x 11. Using the 14 on the other engine will cause it to be severely lugged.
Thanks for the replys, I am not sure about my gear ratios. I suspect that the T50 has the lower ratio & the F50 uses the higher?
How do I determine the gear ratio on my OB?