With that KV motor on 6cell Nimh cells you should be in good shape. Can you post a pic of your setup in the hull? The only thing I can think is possibly you have a lot of resistance in your connectors, your wires are very long or your boat is running very wet and because of the drag your amp draw is high. I run that ESC in my rigger pulling over 100 amps and I never had a problem. Did you try running the hull for a minute and checking the temps? If they are fine after a min run for 2 min and check again. Maybe 4 min at a time is just too much for your setup.