Why Your Boat’s Raw Water Strainer Keeps Filling With Air

A raw water strainer is a simple component with one job: trap debris before it hits your raw water pump. But if you keep opening the lid and finding air inside — or see bubbles swirling while the engine runs — something’s wrong. Air inside the strainer means your system is either pulling in air or failing to push it out, and both can stop water flow or burn up your impeller in minutes.

What a Strainer Should Look Like in Operation

When everything’s working correctly, the strainer housing should stay full of water during engine run time. There may be a few small bubbles when priming, but these should clear quickly. If air continues to collect inside the bowl, or you notice poor water flow from the exhaust, that’s a red flag.

Common Causes of Air in the Strainer

• Loose or damaged lid gasket: Even a tiny gap around the strainer lid can let air in under vacuum, especially if the gasket is cracked, dry, or misaligned.
• Threaded fittings not sealed properly: NPT threads on the seacock, strainer base, or hose adapters may allow small air leaks even if they don’t drip water.
• Cracked strainer housing: UV damage, over-tightening, or freeze damage can cause hairline cracks in the bowl — air leaks in, but no water leaks out.
• Collapsing or cracked intake hose: A soft or rotted hose may look fine from the outside but let air enter under suction.
• Pump mounted too high above the waterline: If the raw water pump sits higher than the water intake, gravity may allow air to collect and stay trapped in the system.

What to Look For

1. With the engine off, check that the strainer is full — if it slowly drains, you likely have an air leak upstream.
2. While running, watch the strainer bowl. If you see constant bubbles or swirling air, suction is pulling in air from somewhere.
3. Try lightly greasing the lid gasket. If air bubbles stop during the next run, the lid is likely your issue.
4. Inspect all threaded fittings and hose clamps — reseal or tighten if anything feels loose or brittle.

Fixes and Prevention

• Replace the lid gasket yearly, or at the first sign of drying, cracking, or flattening
• Use PTFE paste (not just tape) on threaded fittings for a better vacuum seal
• Inspect the bowl for cracks or haze — replace if damaged or yellowed
• Replace old intake hose with marine-grade, wire-reinforced hose that won’t collapse under suction
• If the pump sits above the waterline, install a priming valve or loop to help purge air at startup

Conclusion

If your strainer keeps filling with air, the system can’t create reliable suction — and that puts your entire cooling circuit at risk. The problem might be as simple as a bad lid gasket or as complex as poor system design. Either way, constant air in the strainer means something’s leaking. Fix it early, and you’ll avoid overheating, pump damage, or complete loss of cooling flow when you need it most.