All, I have a Yanmar 3GMF engine that has a water pump with a mind of its own. Several weeks ago my wife took the boat out with some girlfriends and heard that heart-sinking hollow sound coming from the exhaust.......no water coming out!!! She beat it back to the slip she'd just left and in the process, the impeller was destroyed. I bought a replacement impeller (actually three of them) and installed it. The upper hose to the heat exchanger was of course dry so I filled it and stuck it back on. The next day she tried again with her friends and no water again. I rechecked the impeller and refilled the hose and all was fine. We motored around the marina for 45 minutes or so and returned to the slip. Fast forward two weeks and yesterday she along with the same gals went for another glorious sail with no engine cooling problems at all until hours later as she was dropping the main prior to returning to our slip. The engine fired right up but no water, AGAIN!!! To her credit, she sailed our boat back into the slip and all returned to land safe and sound. As a start, I'm tempted to replace all the hoses especially and including the factory formed one from the pump to the heat exchanger but including the one from the seacock to the strainer and from there to the pump. Has anyone had a similar experience and can anyone shed light on how to solve this perplexing problem? It sure has me scratching my head. Thanks in advance, Glyn Judson, E31 hull #55, Marina del Rey CA