How to Inspect and Clean a Raw-Water (Sea) Strainer

Make a list: engine (one per engine), generator, A/C, washdown, head intake. Note which ones sit below the waterline. BoatUS suggests labeling every seacock by its job and posting a simple diagram aboard, so anyone can find them in a hurry. Tie or stow a soft plug near each through-hull.

Turn off the engine and wait for it to cool; the exhaust and hoses can be hot. Switch off the breakers for any pump that draws through the same strainer (A/C, generator, washdown) so nothing starts while the lid is off.

Turn the handle across the hose (a lever-type seacock is closed when the handle is at 90° to the hose). BoatUS says to be sure the seacock is closed before servicing a strainer below the waterline. Close it even when the strainer is above the waterline; it keeps the hose from draining and refilling. If the seacock won't move, stop here and see the seacocks guide; don't force it.

With a flashlight, check the bowl: weed, shells, sand or plastic in the basket; a stream of bubbles (an air leak); cracks, cloudiness or crazing in a clear bowl; green or pink corrosion on bronze parts. Note any drips around the lid or hose clamps.

Put a rag and bucket under the strainer. Unscrew the top or loosen the wing nuts or lugs by hand (use the maker's wrench or a strap wrench only if it's stuck). A little water will spill. If water keeps flowing in, the seacock isn't fully closed: put the lid back on right away, then deal with the seacock before going further.

Lift out the basket and dump it into the bucket (not overboard into the marina if it's full of oily gunk). Rinse it. If it hasn't been cleaned in a while, BoatUS suggests brushing the screen to remove hardened deposits. Look for holes, torn mesh or heavy corrosion. Perko uses 316 stainless baskets and GROCO offers stainless, Monel or non-metallic ones; a worn-out basket of any kind should be replaced, because debris goes straight through a hole. Wipe out the sand at the bottom of the bowl.

The O-ring or gasket under the lid is what keeps air out (and water in). Pull it out, clean the groove and the sealing face, and look for flattening, cracks, nicks or a stretched ring that won't stay in its groove. Replace it if in doubt; seal kits are cheap. Many owners apply a light film of lubricant to the O-ring and threads; use what your strainer maker recommends.

Look closely at the bowl and housing for hairline cracks, especially near the threads and mounting feet. A cracked bowl below the waterline is a flooding risk: replace it before the boat is left alone. Squeeze the inlet and outlet hoses for soft or crunchy spots, and check that the clamps are stainless and tight. BoatUS recommends double clamps at the seacock and at the strainer where the hose barb is long enough, and warns that failing to secure fittings like a sea strainer is one of the top causes of sinkings in its claim files.

Seat the basket all the way down so water can't bypass it. Set the O-ring squarely in its groove, then thread the lid on by hand, making sure it isn't cross-threaded. Snug it firmly, but don't crank on a plastic lid or bowl; overtightening can crack it or pinch the seal.

Open the seacock slowly. On a strainer below the waterline, the bowl should fill. Watch the lid, bowl and hose ends for a minute. Any drip means the seal isn't seated: close the seacock, open the lid and reseat it. BoatUS's last step is the same: reopen the seacock, run the pump the strainer protects, make sure the bowl refills and check for leaks.

Start the engine (or switch the A/C or generator back on) and watch the strainer and the cooling-water discharge. You want a full bowl, steady water at the exhaust or outlet, and a normal temperature gauge. A stream of bubbles in the bowl, a bowl that stays half empty, or weak discharge after cleaning usually points to an air leak at the lid seal or a hose clamp. Shut down and fix it; an engine pulling air can overheat just like one with a clogged strainer. If flow is still weak with a clean, sealed strainer, check the impeller (see the impeller and overheating guides).

Is the blockage before the strainer? A BoatUS article gives a quick check for a below-waterline strainer: with the engine off and the basket clean, crack the seacock open for a moment with the lid off and a bucket ready. You should see a steady stream of water coming in; close it again right away. Little or no flow means something is blocking the intake: a plastic bag sucked against the hull (someone has to dive or the boat has to be hauled to clear it) or sea grass packed in the hose between the seacock and strainer. With the seacock closed, loosen the clamps, pull that hose and look, then reassemble and test. If the seacock won't close fully, don't open the lid at all; call a pro.

In freezing climates the strainer has to be drained or filled with non-toxic antifreeze. BoatUS warns that a strainer left full of water can crack and possibly sink the boat. When you winterize the engine, generator or A/C, make sure antifreeze reaches the strainer and the hose from the seacock; Xylem notes that the inlet hose is the part owners most often miss. Use only non-toxic propylene glycol, never automotive antifreeze. Close the seacock and hang a tag on the helm that says so.

Before you start the engine or switch on any raw-water pump after storage, open its seacock and check that the strainer basket is in place and the lid is tight. Xylem calls this the most easily forgotten step: a pump running against a closed seacock can ruin its impeller within moments.