I have a 94 5.7L Mercruiser with bravo one outdrive. Sometimes my prop just slips and spins basically stopping all forward movement. What can be the cause of this? Should I replace the prop, it's a 3 blade 14 1/2X19? It also never planed today when I took it out, I had more people on the boat but still it wouldn't plane with them in the bow. Any help would be much appreciated.