Re: Merc impeller
Mike, I'm glad you asked that question, because it is seldom addressed, and I'm sure it has been asked by many. The owner's manual will always list the impeller as an item needing replacement at certain set intervals.<br />There are very good (and seldom-challenged) reasons for this.<br />First, the impeller is made of neoprene, and sourounded by a harder material,plastic in some models, metal in others). The pump housing usually has a removeable plate under it, that also is sujected to wear.<br /><br />Second, <br />Water is not allways as clear as it looks, especially shallow and salt water. All bodies of water we putt-around in have small particles of stuff that is harder than neoprene that wears the impeller out. But an impeller is rarely made of anything else (rubber or carbon-rubber formualtions have been used but....),this is because of the fact that an impeller functions as two different classifications, and it's pump housing is not perfectly round.The impeller must flex.<br />Thirdly, the pump is a Dr Jekyl/Mr.Hyde.<br />When idling, the pump is classified "positive-displacement" type, and the vanes are in contact with the walls of the pump housing. This delivers a set volume at a set pressure, to allow water to circulate the entire cooling circuit, and overcome the spring pressure of the thermostat.<br /><br />As thge motor is throttled-up, these vanes 'tuck-in' and no longer contact the wall of the pump housing. If this was the case at idle speeds, the impeller could have been made of a harder,stiffer material. and would last much longer ,but--- The pump is now classified as "Centrifugal" type.<br />This type of pump delivers much more volume, but allows pressures to be governed by how much demand the cooling circuit can handle(what the cooling system can take without blowing the head gaskets), while minimizing wear on the pump itself,and assuring maximum cooling capabilities.<br /><br />"Fourthly", neoprene and other rubberised materials have a nasty characteristic... They get brittle over time. They also harden and can assume the concentric shape of the housing they are in, if not used often. This is what marine techs call "taking a set". When the impeller takes a set, it will work fine for an indeterminate amount of time----then fragment inside the housing.<br />And the final reason to "replace the impeller every two seasons whether it needs it or not"... is, the impeller will look prefect, and operate just fine,until it does not. When this occurs on the water, with all the helmsman has to concentrate on, there will be little indication of impeller failure until the motor starts to seaze-up. By that time, seriious damage has occured.<br />Even if the helmsman saw the water temp indicator climb, and shuts-down, he and his mates are shut-down, and in most cases,stranded.<br />If he shut it down when the overheat alarm went off, he shut it down (who knows how far from port), because it overheated..Overheated is ungood. Not to mention, that when an impeller grenades in the housing, the pieces go into all areas of the colling circuit they can get too...Costly.<br /><br />An excellent question. I hope everyone with a boat learns this answer.
for that explanation. I learned a little more about my motor today!