I have a 24SSL, Express Tube, 6 strakes, Mercury 115 (4-stroke), current prop is a Mercury Black Max 13.75 x 15P. I get 35 MPH at 6400 rpm. WOT rpm range is 5800-6400, so I’m at the very top end. The problem I hope to fix with a new prop is when we run a full boat (6 kids and 6 adults that think they are kids), I can barely hold 20 mph in a straight line pulling a tube. There’s no way to whip the tube out in a turn, as the boat drops to around 15 mph in a turn. The kids intentionally slip off the back of the tube and starting kicking to help push, it's that bad. Had I known last summer what I know now (thanks to some of you guys), I would have taken note of the max engine rpm in those conditions, but I’ll just have to guess it’s ~4500 rmps. Maybe. Give or take 500. That’s a wide range, but I really don’t know. I also know now that I was probably lugging the engine in those conditions and that’s not good either. I’m willing to sacrifice all the top end speed I can to be able to pull a tube with a full load and get a few more mph and the rpms back up. I’ll still have the other prop if I ever want to get the speed back, so I’m willing to trade as much top end as needed for more torque, hoping it will squeak out a few more critical MPH under heavy load, which could make a huge difference since I’m right on the bubble between suck and fun. I’m wanting to try Merc’s new Spitfire 4-blade pontoon prop. I don’t know what size to buy, as there’s very little info online to help me. They only make 6 sizes that would work with my size motor. The “pontoon” sizes are: 13.8 x 10P 13.8 x 11P 13.8 x 13P They have three “regular” sizes (I don’t know if there’s much special about the pontoon props or if it’s mostly marketing): 13.4 x 15P 13.0 x 17P 12.5 x 19P I’ll eliminate the 10P, 11P, 17P and 19P props as bad ideas (unless someone thinks otherwise), so I’m left with two choices: 13.8 x 13P or 13.4 x 15P. Which one should I get? Pretty sure this will be non-returnable, so I hope to get it right the first time. Thanks in advance for any comments anyone has.