I have twin Lehman 120s.The engine with the heater and water heater runs at exactly the same temp as the other engine. You guys need to realize we are talking about a heater the size of a shoe box. Commercial guys in the north have used the heaters in all of their boats for decades. We had one of the heaters in our last boat and used it for five years. Geez guys these are boats with big heavy engines, be not afraid!