As others have mentioned, you have an issue there. Either a spun prop hub or something mechanical in the driveline. Check the prop hub first, otherwise take it back to where you bought it.
BTW, that 115 should be pushing that boat 35+ with a 17-19 pitch prop. I ran an Sportfish 1850 with a Yamaha 115 4 stroke and would push 38 with an 18 pitch SS prop.