I am trying to downrig with my 50 hp merc.I am currently using a 13p 10 1/4"?nd I have to rev pretty high to maintain 3 mph.I have a 15p 10 1/4" and a 15p 10 3/4" spare props.Does anyone have an idea which,in theory,will drop my rpms the most or will a 15p raise my rpm to obtain the same speed?