Not enough pitch. I believe the engine will pull the load just fine turning a 13x13. Several people here rave about the Acme 541 13x12 so I jumped on the bandwagon since mine still had the original cast 14x13 prop, but I suspect those who like that prop are running significantly more weight than me or at a much greater elevation. I operate at sea level at my home ride spot and maybe 500' at the lake house.

The 515 is the same prop just drilled for a 1-1/8" shaft instead of a 1" like the 541. If you have a big block you'll probably want more pitch in a smaller/lighter boat unless you primarily surf and run very heavy. With a small block it may be a good match depending on your use.