first thing you are running too big a prop with that motor and boat. you should be running a 24 raker or a 24 tempest for a 3 blade, since I dont run 4 blades I dont know them but I would drop a pitch. you should see your rpms go up and you should also see some speed, the battery has nothing to do with it . Now you also need to post your rpms, water pressure and if you have a jackplate or not and also your prop shaft to pad measurement to get a better help , also you since this is not really a motor issue try posting with the added info on the stratos boards