Prop Suggestions

I have a 1984 Boston Whaler Sport 15CC with a 1984 Mercury 50HP (4-CYL). It currently has a 13 pitch prop that allows the RPM's to go above the suggested operating range (5000-5500). The 13 will allow RPM's around 5800-6000 WOT. I get 30MPH with the 13, but thought I should get more. Seems like it would be great for a heavy load, but feels like it cuts/slips the water too much in my case, only increasing RPM and no MPH.

Can anyone with a similar set-up offer what they get with either a 15 or 17 pitch prop?

i would go to a good prop shop and have the 13 repitched to a 15...no sense keeping the 13....i think the 15 will do the job but if not you only have to buy one prop when you go 17...these 4 cyl 50,s always seem to turn a 15 right....17 will probably give you better mileage but are slow out of the hole with any kind of load..you may also check the whaler website or give them a call...seems to me i have saw some whaler rooms on the web also...you are limited on how much a prop can be changed also...you can go from a 13 to a 15 but not to a 17...a 15 can be changed to a 17 but not to a 19......

Thanks. The 15 is actually what I was leaning towards. I know there are some scientific calculations somewhere, but I believe I once read something that stated that a 2" increasae in pitch will decrease WOT RPM's 200-300. This would make it fall in a good category for me.