I have a Yanmar 3GMD Diesel engine in my sailboat. We start the engine at the dock and motor out to the lake for about 1/2 hour total time. The motor pumps water just fine. We sail for a couple of hours then when we start the motor to return to our dock, the motor does not pump water. I replaced the impeller last fall when we first encountered this problem and also replaced the thermostat. Someone said that I got a "weak" impeller, another person said that there is an "air leak" in the system. I am at a loss.