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Raw-water strainers are most often associated with the cooling systems of inboard engines. Every boat that brings aboard outside water to cool the engine, whether the raw water flows through the engine or through a heat exchanger, needs a strainer to prevent grass and other solids from reaching the pump.
Other Applications
All other raw-water intakes on a boat should also have a filter in the line. This obviously includes the cooling water inlet for an auxiliary generator or onboard air conditioning. Less obvious, perhaps, is the necessity of an upstream strainer for a refrigerator heat exchanger, a live well, or a deck-wash pump. And the benefits of a strainer for the intake line supplying the head and the raw-water spigot in the galley are nearly always overlooked. Marine toilets often stink because of grass lodged in the inlet-water passage under the rim, and a filter eliminates this problem. The merit of filtering water flowing into the galley sink shouldn't require explanation.
Installation
Plumbing a raw water filter is simply a matter of inserting it into the line connected to the seacock. Bronze filters typically have threaded inlet and discharge ports that you will need to fit with appropriate bronze tailpieces (hose barbs). For leak-free connections, be sure to wrap the tailpiece threads with Teflon plumber's tape before screwing them into the ports. On some makes of plastic filters, integral molded hose barbs rather than threaded ports minimize their leak potential, but this does require you to match the filter to the hose.
Raw water filters must be fastened securely to a bulkhead. The mounting location should be as close to the seacock as practical while still allowing for easy servicing of the filter. An above-the-waterline location places less demand on the filter as far as keeping the ocean out of the boat, and it simplifies cleaning. Be sure, however, that the filter is designed for above-the-waterline installation.
The hose connection from the seacock to the filter should be as short and straight as possible. The outlet hose to the pump should likewise not have extra length or bends. Be sure the seacock is connected to the inlet side of the filter-often identified by an arrow pointing on the direction of flow. Use double hose clamps on both the seacock and the filter, even if the filter is above the waterline.
Cleaning
The easier a strainer is to clean, the more often it will get cleaned. For this reason, my personal preference is for strainers with spin-off caps and lift out baskets. No tools should be required to service the filter, and if it is mounted above the waterline, it won't even be necessary to shut off the seacock to clean out the basket.
Most modern strainers meet these criteria, but a lot of old strainers must be disassembled to gain access to the screen. If your boat is equipped with such a strainer, you will be doing yourself and your engine a favor by replacing it with one that can be cleaned in half a minute.
Where the filter is located below the water line, be sure to close the seacock before servicing. Open the filter and dump the basket and/or clean out the bowl. If you clean the filter often, a rinse of the screen/basket will usually be sufficient, but if it hasn't been cleaned in a while, use a brush on the screen to remove hardened deposits.
Reassemble the filter, taking care that all gaskets seat properly. Reopen the seacock and operate the pump the filter protects to make sure the bowl refills. Check for leaks.
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