How to Identify a Failing Raw Water Strainer (and Why It Matters)

Raw water strainers protect your engine by filtering debris out of the cooling intake. Whether you’re pulling water from a lake, river, or saltwater inlet, the strainer prevents seaweed, plastic, silt, and marine life from clogging your pump or heat exchanger. But when the strainer itself fails — whether by clogging, cracking, or leaking — your engine pays the price.

What the Strainer Does

Sitting between the seacock and raw water pump, the strainer collects debris before it enters the cooling system. Most use a mesh basket inside a clear or metal housing, sealed with a gasket and secured with a screw-on lid or clamp. The system depends on airtight suction. Even a small leak or air bubble can cause the pump to lose prime, starving your engine of cooling water.

How a Strainer Fails

Clogged basket: Seaweed, sand, or mud builds up and blocks flow to the pump.
Cracked housing: UV exposure or impact damage can cause tiny cracks that leak air under suction.
Bad gasket or loose lid: An improperly sealed lid allows air to enter, breaking the pump’s vacuum.
Corroded fittings: Metal strainers may develop leaks or pressure loss at corroded hose barbs or drain plugs.

Symptoms of a Failing Raw Water Strainer

Overheating, especially at low RPM or idle
Dry or weak water flow from the exhaust outlet
Bubbles inside the strainer housing when the engine is running
Visible debris inside the basket
Water level inside the strainer dropping under load

How to Inspect the Strainer

Shut off the seacock and remove the strainer lid.
Check the basket for mud, weeds, or scale buildup — clean it thoroughly.
Inspect the gasket. It should be flexible and intact, with no cracks or flattening.
Look closely at the housing for hairline cracks — especially around the threads or near hose connections.
Check hose clamps and fittings for signs of corrosion or leakage.

After cleaning and resealing, prime the system and restart the engine. The strainer should fill completely with no visible bubbles. If air appears in the housing, suspect a suction leak.

How to Prevent Strainer Problems

Inspect and clean the strainer monthly during active use
Lubricate the gasket lightly with silicone grease to maintain a tight seal
Replace the basket or gasket if they show signs of aging or cracking
Always tighten the lid evenly — overtightening can warp the seal or crack the housing
Flush the system after saltwater use to reduce salt buildup

Conclusion

A bad raw water strainer creates the same symptoms as a bad impeller or blocked riser — but it’s a much cheaper fix. If your engine’s overheating or losing suction, check the strainer before tearing deeper into the system. It’s the first line of defense for your cooling loop. Keep it clean, sealed, and airtight — and your engine will keep running cool.