On Tue, 23 Dec 2008 11:54:08 -0600, Vic Smith
wrote:

On the surface it looks like sailors have no mechanical aptitude, and
are getting suckered by "accepted practice" regarding engine systems.
But I don't believe that, so I'd appreciate some instruction from the
sailors here as to why this "diciness" about engine cooling exists.
I started to say these "plastic" impellers suck, but unfortunately it
appears that might be praising them.

Impellers need to be replaced on a regular schedule. 300 to 500
operating hours is about right in my experience.

In their favor, impeller pumps are compact, efficient and self
priming.