How to clean a raw water strainer

Close the seacock, open the strainer lid, lift out the basket, clear it, check the lid seal, refit, open the seacock, then start the engine and confirm water at the exhaust. It takes five minutes, it requires no tools on most installations, and in silty or weedy water it is the most frequently useful thing an owner can do.

The strainer sits between the seacock and the raw water pump and catches what would otherwise reach the impeller and the heat exchanger.

How often

More often than the manual says, if you are in the wrong water. The interval is set by where you boat, not by the engine:

Weedy or silty water — check before every trip. The Gippsland Lakes and Moreton Bay are examples where this is routine.
Shallow sandy water — frequently. Sand reaches the basket continuously.
Big tidal ranges — Mackay, Darwin and Broome keep sediment suspended, and southern intervals do not apply.
Clean deep water — at service intervals is generally adequate.

The procedure

Close the seacock. You are opening the seawater circuit below the waterline. Confirm the handle actually moves and actually shuts.
Have rags ready. There will be water in the bowl.
Open the lid. Most are hand-tightened; some have a clamp or a knob.
Lift out the basket.
Empty and rinse it. Fresh water if you have it aboard.
Look at what came out. It tells you what your intake is drawing — weed, plastic, sand, shell, growth — and whether the problem is occasional or constant.
Check the bowl for debris that got past the basket, and clear it.
Check the lid seal. A perished or displaced O-ring lets the pump draw air instead of water, which is a much harder fault to diagnose than a blocked basket. Replace it if it is flattened or cracked.
Refit the basket the right way up, and the lid hand-tight. Overtightening distorts the seal.
Open the seacock. Writing this on a note near the ignition is not excessive.
Start the engine and confirm water at the exhaust within seconds.
Check the strainer for leaks and air bubbles once running.

The two things that go wrong

Forgetting to open the seacock. This destroys an impeller in under a minute and is one of the commonest self-inflicted marine engine faults.

The lid seal. An air leak at the strainer lid produces intermittent overheating with an apparently clean circuit, and owners chase it for months. If you can see bubbles in the strainer while running, that is the answer.

What a full basket means

A basket that is full every trip is telling you about the water, not about the strainer. On a workboat operating permanently in that environment, it is one of the strongest arguments for keel cooling.

Frequently asked questions

How often should I clean the raw water strainer? Set by your water rather than the manual. In weedy, silty or sandy conditions, before every trip.

What happens if I forget to reopen the seacock? The impeller runs dry and is destroyed within a minute. It is one of the most common self-inflicted faults.

Why is my engine overheating with a clean strainer? Check the lid seal. An air leak there produces intermittent overheating with an apparently clean circuit.

Should there be bubbles in the strainer? No. Bubbles while running mean air is being drawn in, usually at the lid seal or a hose joint.

Do I need tools? On most installations, no. The lid is hand-tightened.