Raw water impeller

While cruising one's senses are heightened and sight, sound and sight are improved from the time one was within the rat race. When did you get up in the night if a door banged closed or the wind rose. The same can be said about sound. I believe one becomes very aware of the sound of your yacht at anchor, sailing and under way. Malua has a 50 hp diesel engine which has performed very well in the Mediterranean. I designed the exhaust system to handle sailing in flat and rough water.
When we were motoring towards Previsa I noticed a change in the tone of the exhaust and looked over the side to find the amount of water coming out of the outlet had reduced substantially.

After anchoring I investigated and found the raw water impeller blades had disintegrated. Having a replacement in the store I was able to install a new impeller immediately. I now have to get a new impeller from the local supplier.
Some people believe the impeller should be replaced annually. I think this is good but unnecessary. It depends on where you sail. While sailing in Venice there was a lot of sand and mud. This caused the impeller to wear faster. One should have a grill on the through hull and a filter soon after. These will catch larger items but not sand and mud.
How do you know when the impeller has gone?
Listen and watch the raw water exhaust outlet.  If you don't notice the raw water the temperature gauge should tell you or a sound alarm will (if you have one).