That prop nut should be tight. Loose is not good. Sometimes hard to do with a castle nut and cotter key. So, the problem is not with the prop. Check your setup again. Is the engine shaft length the same as the transom height of the boat. (both boat and engine are 20" models?) For your testing purposes, it's best to have the tilt rod as you have it- in the last set of holes, closest to the transom.
