Don W wrote in
m:

Those little rubber impellers are good for a
lot longer than most people think as long as your running them in
clean water.

Only problem is they get hard long before they fall apart. They are
stuffed into a cavity they don't really fit into bent over sideways like
that. They, eventually, take the shape forced into them, especially if
they sit idle quite a bit (like 90% of the boats you see). The blades on
one side sit forced over further than the blades on the other side. When
rotated, these hardened blades don't make proper contact with the walls of
the oblong cavity, so they leak their little load of water back to the next
blades coming up. Pressure drops, but not enough to hurt anything,
really.....until it's too late.

You can help, by the way, if you'll get them wet and just rotate the engine
once a month while it's stored to squish up some different blades and let
the ones squished up last month relax.