Raw Water Strainer Buying Guide: Sizing, Mounting and Keeping Your Engine Cool
Quick answer: Pick a raw water strainer with ports at least as large as the hose and seacock it connects to, so it never restricts flow. Mount it between the seacock and the pump, ideally above the waterline where you can see the basket and open it without flooding the bilge. Bronze and heavy-duty composite bodies hold up best. Check it before every run, and keep the lid O-ring in good shape, because a leaking lid sucks air and starves the pump.
Every raw water system on a boat, whether it is engine cooling, a generator, air conditioning, a washdown pump or a livewell, pulls in whatever is floating nearby: seaweed, eel grass, jellyfish, plastic bags, shell bits and sand. A raw water strainer catches that debris before it reaches the pump impeller and the cooling passages. A clogged intake overheats an engine fast, so the strainer is one of the most important and most overlooked pieces of plumbing on the boat.
Step 1: Know the two kinds of strainers
| Type | Where it goes | What it does |
|---|---|---|
| Inline sea strainer (basket strainer) | Inside the boat, between the seacock and the pump | Catches debris in a removable basket you can see and clean |
| Scoop or intake strainer | Outside the hull over the intake thru-hull | Keeps large debris out and helps force water in at speed |
| Small inline strainer | Freshwater and small pump lines | Protects small pumps from grit |
Most boats with inboard engines, generators or AC should have both an external intake strainer and an inline basket strainer.
Step 2: Size it for flow
A strainer smaller than the hose it serves will choke flow, especially as debris builds up in the basket. Match or exceed the inside diameter of the seacock and hose, and check the strainer's flow rating against what the pump needs. For engine cooling, follow the engine maker's intake size.
Step 3: Choose the material
- Bronze: tough, long-lasting and the traditional choice below or near the waterline.
- Reinforced composite: corrosion-free and lighter, good quality options exist from major brands.
- Clear bowl or lid: lets you check the basket at a glance. Make sure it is rated for the location.
- Basket: stainless or Monel baskets last longer than plastic in heavy use.
Step 4: Mount it where you will actually check it
- Install between the seacock and the pump, with the shortest practical hose runs.
- Mount above the waterline when possible so you can open it without water rushing in. If it must sit below, always close the seacock before opening.
- Leave clearance above it to lift the basket out.
- Double-clamp every hose connection with all-stainless clamps.
- Support the strainer with its bracket so the hoses do not carry its weight.
Step 5: Maintain it
- Look at the basket before you start the engine, every trip.
- Clean it more often in grassy or debris-heavy water.
- Inspect the lid O-ring and replace it if flattened or cracked. An air leak at the lid can stop the pump from priming.
- If the engine runs warm, check the strainer first, then the impeller.
When you clean the strainer, check the impeller too. Our impeller guide shows what to look for.
Common strainer mistakes
- Mounting it out of sight. A strainer hidden behind the engine never gets checked until the alarm sounds.
- Undersized strainer. A small strainer on a large intake restricts flow even when clean.
- Opening it below the waterline with the seacock open. Water will pour in fast. Close the seacock first.
- Ignoring the lid seal. A cracked or dry O-ring lets air in and the pump loses prime.
- Single clamps on the hoses. Raw water intake hoses are below the waterline. Double-clamp them.
Keep a spare lid O-ring and basket on board. They are small parts that solve big problems.
Before you order: quick checklist
- Inside diameter of the seacock and hose the strainer will connect to.
- Flow the pump or engine needs, from the maker's spec.
- Space above the strainer to lift the basket out.
- Mounting location above the waterline if possible.
- Matching hose barbs or pipe thread adapters, and all-stainless clamps.
- A spare lid O-ring and basket for the boat.
Replacing an old plastic strainer that has gone cloudy and brittle is a smart upgrade. Bowls that crack can leak below the waterline and let the bilge fill without warning.
Our picks
All in stock at Boat Supplies Direct.
Inline raw water strainers
Raritan Raw Water Strainer for engine and generator intakes. For larger systems, the bronze Perko 3/4 in IPS Intake Strainer, 1-1/4 in, or 1-1/2 in.
External scoop strainers
Perko 3-1/2 x 2-1/2 in Bronze Scoop Strainer or the larger 5 x 3-1/4 in Scoop Strainer.
Small pumps and washdown
Jabsco Pumpgard In-Line Strainer, 3/4 in Barb.
Clamps for every connection
Trident Marine 316 Stainless Hose Clamps, 7/8 to 1-1/4 in, 10-pack.
Frequently Asked Questions
What does a raw water strainer do on a boat?
It catches seaweed, debris and marine life in a removable basket before they reach the pump and engine cooling passages.
How often should I clean my raw water strainer?
Check it before every trip and clean it whenever you see debris. In grassy water, check it during the day too.
Where should a raw water strainer be installed?
Between the seacock and the pump, ideally above the waterline and where you can easily see and open it.
What size sea strainer do I need?
One with ports at least as large as the seacock and hose, and a flow rating above what your pump needs.
Why is my engine overheating with a clean strainer?
Check for an air leak at the strainer lid, a worn impeller, a blocked intake thru-hull, or a clogged heat exchanger.
Do outboards need a raw water strainer?
Outboards have their own intake screens on the lower unit. Inline strainers are for inboards, generators, AC, livewells and washdown pumps.
This guide is general information. Follow your engine and pump manufacturer's requirements and use a qualified technician for below-waterline plumbing.
