a sensor for measuring a rate of liquid flow through a liquid strainer on a marine vessel;

a sensor for measuring an operational speed of a device on a marine vessel that utilizes the strained liquid;

a central processing unit configured to:

receive at least one measurement from the liquid flow rate sensor;

receive at least one measurement of operational speed from the operational speed sensor; and

compare a relationship of the measured liquid flow rate and measured device speed to a predetermined reference relationship of liquid flow rate and device speed at an equivalent device speed to determine a degree of blockage in the liquid strainer

and further comprising a removable magnet/paddlewheel assembly within the liquid flow sensor and a valve adjacent to the liquid flow sensor, wherein the magnet/paddlewheel assembly is not removable when a handle on the valve is oriented in a valve open position.