48hp evinrude lacking power

Hey guys! I am new at this whole boat thing and I was wondering if someone can help a poor boat newbie. I bought this boat a month ago and have had it out on the water several times. at first it ran pretty good once we got the kinks worked out but now it seems to have lost power and can barely tow a tube with one person on it. I know when all my family gets out on it we are at the weight limit for the boat and pulling a tube is harder on a boat but it seems that it should be going faster. Even the ducks are moving faster up the river! It seems the problem started when we were out on the river and we hit some shallow water and the prop ran the bottom of the river till I could get out of it. when we pulled the boat out of the water the prop was banged up quite a bit. since then it has not run right. should i change the prop or is there some kind of adjustment that should be made. also where should the motor be set in the water to get the most speed? my tilt is manual and it has numbers on it. thanks for your help!