Re: 97 115 mariner
I've got a 96 mariner 115, it has a WOT of 5280. I'm turning a 21 pitch 12.75" quicksilver prop and can get up to 50mph pushing a 18' deep V with 2 people. You should be able to view the numbers around the inner hub of your prop. Look for numbers on the outer side of the prop nut. It should have the diameter and pitch. If it's worn off, and 5000 is all you're getting at WOT, I'd say you need to pitch down to get your RPMs up a bit. I had a problem with mine and after re-propping found that I was losing one cylinder due to a shorted plug wire. Propped back up and found about 15 more MPH. You can rule that out by running your motor on the muffs at night with the cover off, it'll look like the 4th of July. It was kind of hard to see at first, the plug wire dry rotted between it and one of the two wire holders on the waterjacket cover. Good luck.
