Raw Water Strainer
The raw water strainer is an essential component in the cooling systems of small marine diesel engines commonly found on yachts and pleasure craft, including Yanmar, Volvo Penta, Beta Marine, Nanni, and Perkins-based engines. Their role is simple but vital: prevent debris from entering the raw-water pump and downstream cooling components. A functioning strainer protects the impeller, heat exchanger, oil cooler, exhaust elbow, and entire cooling circuit from marine contamination such as weed, shell fragments, silt, and marine growth.
Raw water strainers are vital in marine diesel cooling systems, positioned at the seawater intake before the pump. They filter debris such as seaweed, sand, and shells, ensuring clean water enters the circuit. By preventing blockages, they protect pumps, hoses, and heat exchangers, maintain efficient heat transfer, and reduce overheating risks. With removable baskets for easy cleaning, strainers provide reliable protection, extend engine life, and minimize costly downtime
Raw Water Strainer Function
In a raw-water-cooled or heat-exchanger-based cooling system, seawater enters through the hull via a seacock. Before reaching the engine’s raw-water pump, usually an impeller-type pump, the water must pass through a strainer. Here, a perforated metal or plastic basket captures debris while still allowing unrestricted flow. The cleaned water then proceeds to the raw-water pump, which pushes it through the heat exchanger and into the exhaust elbow for discharge. Raw-water strainers must offer both high flow capacity and easy maintenance. Transparent lids and tool-free access are common features that allow quick inspection before trips and during troubleshooting.
Raw Water Strainer Designs
Several well-known marine equipment manufacturers produce raw seawater strainers, each with variations in construction, filtration capacity, and maintenance features.
Vetus. Vetus strainers are widely used on small to midsize yachts. Their popular models, such as the Vetus FTR series, feature clear screw-top lids and removable cylindrical baskets. They are lightweight, corrosion-resistant, and available in multiple sizes to match engine pump output.
Groco. Groco produces heavy-duty bronze strainers designed for long service life. Models such as the Groco ARG and SSC series offer robust construction, large filtration surface area, and quick-access lids secured with clamping rings. These are preferred for vessels operating in debris-prone or commercial environments.
Perko. Perko offers traditional bronze strainers with stainless steel baskets and gasketed lids secured by wing nuts or clamps. Their rugged bodies and simple layout make them easy to maintain, and replacement parts are widely available.
Racor / Parker Marine. Racor, known for fuel filtration systems, also manufactures durable seawater strainers. Their strainers often feature reinforced housings and fine mesh baskets ideal for sensitive cooling systems.
Jabsco and Johnson Pump. These brands supply compact inline plastic or composite strainers often fitted in small auxiliary engines. Their screw-off clear caps and small footprint make them ideal for tight engine spaces.
Raw Water Strainer Types
In-line plastic strainers. Used on smaller engines where flow demand is moderate. These lightweight units typically include clear lids for instant visibility and easy removal.
Top-loading canister strainers. This style (common among Vetus, Groco, Perko) offers greater filtration surface and more rugged construction. Ideal for cruising yachts and higher debris environments.
High-capacity metal strainers. Bronze or stainless strainers with large baskets, commonly found on commercial or offshore vessels. They deliver maximum durability and resist corrosion and impact damage.
Engine-manufacturer-specific strainers. Yanmar and Volvo Penta installations sometimes feature factory-supplied compact strainers. While functional, many owners upgrade to larger aftermarket units for improved reliability.
Raw Water Strainer Installation
- Mount above the waterline where possible to enable basket removal without flooding.
- Maintain vertical orientation so debris settles at the bottom of the basket.
- Ensure straight, short hose runs between the seacock and strainer to prevent air locks.
- Allow physical access from above for fast inspection—a key requirement when diagnosing overheating.
- Match strainer size to pump flow rate to avoid unnecessary restrictions.
Raw Water Strainer Maintenance
Regular maintenance ensures the strainer performs its job efficiently and prevents cooling failures.
Routine Inspection. Check the basket visually before each passage. Look through the lid for leaf, grass, or plastic buildup.
Basket Cleaning. Shut the seacock (if below the waterline), remove the lid, and lift out the basket. Rinse with freshwater and use a soft brush to remove lodged debris. Replace damaged or corroded baskets immediately.
O-Ring and Gasket Care. The O-ring or gasket is critical for sealing. Lightly lubricate with silicone grease. Replace if flattened, cracked, or swollen.
Check Fasteners and Lids. Plastic screw-top lids (Vetus, Jabsco) should be hand-tight only. Bronze strainers with clamp-down lids (Groco, Perko) must be tightened evenly using the manufacturer’s recommended method.
Annual Structural Inspection. Look for signs of bronze dezincification, plastic embrittlement, or cracking around hose barbs. Replace suspect components.
Raw Water Strainer
Troubleshooting
Engine Overheating. A clogged or blocked basket is the most common cause of overheating. Inspect for weed, mud, or jellyfish remnants. Ensure the seacock is fully open and the intake is not blocked externally. LESSON LEARNED! when cleaning jellyfish out of strainers suggest you wear rubber gloves in case of sting. Been there and done that!
Air Leaks (Loss of Pump Prime). Air infiltration at the strainer lid or hose connections reduces raw-water pump efficiency. Test the lid seal, inspect hose clamps, and check for cracks in the housing.
Air Bubbles Inside the Strainer. Indicates suction-side leakage. Re-seat the lid, replace the O-ring, and inspect hose fittings.
Water Leaking from the Lid. This suggests a worn or poorly seated gasket. Replace and re-lubricate, ensuring correct fit.
Recurrent Debris Accumulation. Frequent clogs may indicate issues with the through-hull—such as barnacle buildup or operating in high-silt environments. Installing a larger Vetus FTR or Groco ARG strainer can provide increased filtration area.
Sea Water Pump Impeller Damage. If the basket becomes perforated or misplaced, debris may reach the pump and shred the impeller. Always verify basket integrity after cleaning.
Raw Water Strainer Practical
Tips
- Carry spare gaskets, lids, and baskets for your specific manufacturer.
- Upgrade to a larger-capacity strainer when cruising in weedy regions.
- Label the seacock for quick shutoff during maintenance.
- Avoid overtightening plastic lids.
- Inspect strainer function immediately if an overheat alarm sounds—it is the fastest onboard troubleshooting step.
- Flush the system with freshwater regularly in silty environments
Raw Water Strainer Summary
Raw water strainers are essential in marine engine cooling systems, filtering debris like seaweed, sand, and shells from incoming seawater. By preventing blockages and protecting pumps, hoses, and heat exchangers, they ensure steady cooling flow. Regular inspection and cleaning of strainers safeguard against overheating, reduce wear, and extend engine life, making them critical for reliable vessel operation. Check your raw water strainer and make sure it is clean.