Definition
A raw-water strainer is a housing with a removable basket fitted between the seacock and the raw-water pump, catching weed, plastic, shell and debris before they reach the impeller and heat exchanger.
The strainer protects everything downstream. It is also the easiest inspection on the engine — most are transparent or open in seconds — which makes it the first thing to check on any cooling complaint.
The lid is the part that catches people out
A strainer lid that does not seal draws air rather than leaking water, because the strainer sits on the suction side of the pump. The result is weak flow with no visible blockage and nothing obviously wrong. Check the O-ring or gasket whenever the lid comes off, and do not overtighten a transparent housing.
Checking frequency
Warm water fouls faster. Through an Australian summer, and year-round in the tropics, a strainer wants checking as routine rather than annually. River berths add silt on top of growth.
Where you meet it
In pre-start checks, in every overheating diagnosis, and after any grounding or passage through weed.