I have a 1996 Mariner 115 hp out board . I have it on my 1850 Crestliner Sportfish. It has been pushing my boat at around 35 mph GPS speed. Yesterday for half the day at Erie it would only run about 25 mph. Then all the sudden bang back to 35 mph. Today took it down to the river it tops out at 25-26 again. Anyone have any suggestions to point me in the right direction to figure out why.

How does the prop look?

Prop looks good , thought maybe it was slipping but its still keyed in.

If the rpms are the same it could be a spun hub. If the rpms are down it could be the fuel or a number of other things. Check the compression and check or change the fuel filters for starters.

Paint a line across the back of the hub, run it a while and then look to see if the line is still straight across. If it is not the center hub has spun. About a 40-50 dollar fix.

I agree with fishfordollars, it sounds like a fuel problem. I have the same problem sometimes with my 250. When it happens I put some dri-gas in my gas tank and let it sit for a while and it works fine. Water in your gas tank can be a pain.

Good luck,

Falcon