I have a 97 15hp mercury outboard that i bought last year. I noticed that whenever i get to about half throttle, it'll sound like the motor is hitting the bottom, but i'll be in deep water. it's like a thumping sound and the motor kind of shakes like i hit a rock or something. could this just be the prop coming out of the water? yes, i do have it trimmed all the way down in case you guys ask. i've checked it out and everything is in working order. i noticed sometimes it doesnt do it. could it be that it's digging too far in the water when i take off?