Seawater cooling system

The various seawater cooling system arrangements on a boat other than the main propulsion engine are essential. Seawater cooling is used throughout a vessel in a variety of auxiliary systems that support comfort, hygiene, and essential onboard functions. Although each system performs a different task, washing down decks, feeding a desalination plant, cooling air-conditioning units, or flushing toilets, they all rely on the controlled movement of seawater through hull penetrations, pumps, strainers, hoses, and overboard discharges. Because these systems bring seawater directly into the boat, they must be designed with rigorous attention to reliability, safety, and corrosion resistance. A failure or leak in any seawater circuit has the potential to flood compartments, damage equipment, or cause system downtime at critical moments.

This technical overview examines the auxiliary seawater cooling system used on modern sailing yachts, excluding the raw-water cooling systems associated with the main propulsion engines, although the AC generator is very similar in design and function.  It focuses on the design and operation of water maker intakes, air-conditioning seawater cooling circuits, and saltwater toilet flushing systems, AC generators, and deck washdown systems outlining their components, flow characteristics, failure modes, and maintenance requirements.

Seawater Intakes and Seacocks

Every auxiliary seawater system begins at a thru-hull fitting and seacock. These components serve as the controlled entry point for seawater and protect the vessel from uncontrolled flooding.

Thru-Hull Materials

• Marine bronze: Highly corrosion resistant and long-lasting
• Composite (e.g., Marelon): Immune to dezincification and corrosion
• Stainless steel: Less common due to pitting and crevice corrosion risk

Seacock Requirements

• Easy to operate under load
• Resistant to marine growth
• Installed with backing plates and proper bonding
• Must be exercised regularly to avoid seizure

Because all auxiliary seawater systems rely on dependable flow, correct sizing and placement of thru-hulls are essential. Intakes are positioned below the waterline in low-turbulence areas and often use external scoops or strainers to improve flow stability.

Raw-Water Strainers and Filtration

Before seawater reaches any pump or sensitive equipment, it passes through a raw-water strainer, which removes debris such as seaweed, shells, sand, and marine organisms.

• Seawater cooling system strainer features
• Transparent bowl for visual inspection
• Stainless steel or durable polymer mesh baskets
• O-ring seals preventing air ingress
• Removable caps for quick cleaning

A clogged strainer is the number-one cause of flow loss in many seawater cooling system equipment. Strainers should be installed above the waterline if possible, or in a location where they can be cleaned without significant water ingress.

Seawater Cooling System –AC Marine generators

Marine generators are like main propulsion engines in that they typically rely on a seawater cooling system to remove heat from both the engine block (via a heat exchanger) and the exhaust system (via water injection and mufflers). Although similar in principle to main-engine raw-water cooling, generator seawater systems have specific design, layout, and operational considerations due to their smaller size, continuous runtime, and often more confined installations.  Below is an outline of all major elements and processes in a generator seawater system.

• Seawater Intake and Thru-Hull Arrangement components:
• Bronze or composite thru-hull fitting
• Dedicated generator seacock (not shared with other systems)
• External scoop strainer (optional based on installation)

Installation Notes

Intake should be located in an area of stable water flow, usually forward of the generator space and below the static waterline.

• Isolation via a dedicated seacock is essential for safe servicing.
• Raw-Water Strainer (Primary Seawater Filter)

Function.  Removes debris such as weed, shells, or sand to prevent clogging of downstream components. The main features are:

• Clear inspection bowl
• Stainless or composite mesh basket
• O-ring seals to maintain airtight suction

Maintenance. Strainer must be cleaned frequently, especially when the generator is used at anchor or in shallow, fouled waters. A blocked strainer is one of the most common causes of generator overheating.

• Raw-Water Pump (Impeller Pump)
• Most generator sets use a rubber impeller pump driven by the generator’s engine. The main features are:
• Draws seawater through the strainer
• Pushes water through the heat exchanger

Key Characteristics

• Self-priming (within limits)
• Sensitive to dry running
• Requires periodic impeller replacement
• Should be mounted below the heeled waterline to ensure consistent priming
• Failure Indicators
• Overheating generator
• Weak seawater discharge overboard
• Blackened or worn impeller vanes found during inspection

Seawater Cooling System - Heat Exchanger

Transfers heat from the closed-loop coolant circuit into the seawater.  The construction is typically as follows:

• Typically, a tube-and-shell design
• Coolant inside tubes, seawater in the outer chamber
• Sacrificial anodes (zincs) protect against galvanic corrosion
• Common Degradation Issues. Cleaning is required periodically using acid solutions compatible with generator materials.
• Internal scaling
• Marine growth
• Blocked tubes
• Zinc erosion

Water Injection Elbow (Exhaust Mixing Elbow)

Mixes seawater with hot exhaust gases exiting the generator, cooling them instantly before entering the exhaust hose. The main design features are:

• Must be corrosion-resistant (stainless, cast iron, or bronze)
• Injection must occur downstream of the exhaust manifold
• Restriction at the elbow is a common overheating cause

Warning:  If seawater fails to reach the injection point:

• Exhaust hose may melt
• Generator compartment may fill with smoke
• High risk of fire or backflow water ingress

Seawater Cooling System -

Waterlift Muffler and Exhaust Hose

Waterlift Muffler

• Collects and cools exhaust/water mix
• Prevents seawater backflow into the generator
• Requires correct installation height relative to exhaust outlet and generator exhaust manifold

Exhaust Hose

• Must be rated for high-temperature wet exhaust
• Routed with proper rise and anti-siphon protection
• Must avoid low points where water can pool excessively

Seawater Cooling System - Overboard Discharge

• Visual indicator of cooling-water flow
• Typically exits above the waterline
• Should produce a strong, rhythmic flow when generator runs

Fault Conditions

Weak or intermittent flow suggests:

• Blocked strainer
• Failing impeller
• Air leaks on pump suction side
• Heat exchanger blockage
• No flow requires immediate shutdown.

Seawater Cooling System - Anti-Siphon Systems

This is to prevent seawater from siphoning back into the generator via the cooling loop.

Main Components

• Anti-siphon valve or vented loop
• Positioned at highest point of raw-water plumbing
• Must be serviced regularly to prevent salt-induced sticking

Failure Symptoms

• Rising exhaust-water levels in muffler
• Water intrusion into generator cylinders (hydrolock risk)

Common Problems and Symptoms

Overheating. The possible causes are:

• Strainer clogged
• Impeller worn
• Heat exchanger restricted
• Exhaust elbow blocked

Low or No Discharge Overboard

• Suction air leak
• Pump losing prime
• Collapsed intake hose

Water in Generator Cylinders

• Failed anti-siphon valve
• Incorrect exhaust hose routing
• Muffler installed too high or low

Safety Considerations

Because the system connects the ocean directly to the generator compartment operators must visually confirm seawater flow whenever the generator is running.

• All hoses must be double-clamped
• Seacocks must remain accessible
• Components must withstand vibration, heat, and corrosion

Seawater Cooling System - Washdown Systems

Washdown systems supply high-pressure seawater to clean anchors, chain, decks, fish boxes, and working surfaces. Washdown pumps are typically high-pressure, moderate-flow devices operating in the 20–40 psi range. Diaphragm pumps dominate because they are self-priming, tolerate dry running, and handle minor debris. The main components are:

• Seacock and strainer
• Washdown pump (diaphragm, impeller, or centrifugal)
• High-pressure hose and outlet
• Optional accumulator tank for flow smoothing

Operating Considerations

• Position pump close to intake to reduce suction losses
• Use reinforced hose to prevent collapse under suction
• Install foot switches or protected bulkhead outlets to avoid accidental spraying

Common Failure Modes. Regular flushing with fresh water extends pump life and reduces salt accumulation.

• Air leaks causing loss of pressure
• Strainer blockages
• Worn diaphragms or valves
• Corrosion in electrical components

Desalination (Water maker) Seawater Intake Systems

Water makers require a steady supply of clean seawater to operate efficiently. Their intake systems must be designed to minimize sediment ingestion, air entrainment, and suction losses. Design requirements are:

• Dedicated intake and seacock (never shared with toilets or AC)
• Smooth-bore reinforced hose
• Oversized plumbing to reduce flow restriction
• Intake located away from turbulence and exhaust outlets

Prefiltration Stages. These filters protect the high-pressure pump and reverse osmosis membrane.  Water maker feedwater typically passes through:

• External intake strainer
• 20–30-micron sediment filter
• 5-micron polishing filter

Critical Factors. Regular freshwater flushing is essential for long-term membrane health, especially in warm climates where microbial growth is rapid.

• Air leaks result in membrane damage due to cavitation
• Overly fine prefiltration can starve the pump
• Biofilm in the intake hose can contaminate the system

Air-Conditioning Seawater Cooling Systems

Marine air-conditioning (reverse-cycle) units rely on seawater to remove heat from refrigerant circuits. These systems typically use centrifugal pumps that must be installed below the waterline, as they are not self-priming.

• System Components
• Seacock and strainer
• Centrifugal seawater pump
• Cooling manifold (for multi-unit systems)
• Condenser coil within AC unit
• Overboard discharge

Operational Characteristics

The pump circulates seawater continuously whenever the air-conditioning system is running. Heat is absorbed from the condenser coil and carried overboard through discharge outlets. Periodic acid flushing (using mild, manufacturer-approved solutions) removes calcium and marine growth from condenser pathways.  The common issues are:

• Air locks preventing pump priming
• Marine growth inside condenser coils, reducing cooling efficiency
• Corrosion on pump housings
• Strainer blockages leading to high-pressure shutdowns

Saltwater Toilet Flushing Systems

Many marine toilets draw seawater directly for flushing. Although freshwater flushing is increasingly popular for odor control, seawater-based systems remain widespread. The main system components are:

• Seacock and strainer
• Manual diaphragm pump or electric macerator/flush pump
• Marine toilet bowl and discharge system
• Hoses leading to a holding tank or overboard discharge (where legal)

Technical Considerations

• Seawater contains organic material that can create odors when trapped in hoses
• Marine organisms entering the system can die and decompose within intake lines
• High-quality sanitation hose reduces permeation and smell

Periodic flushing with fresh water or chemical cleaners reduces biological growth and odor buildup.

Safety, Redundancy, and Flooding Prevention

Redundancy is common on larger vessels, such as dual seawater pumps for air-conditioning or additional filtration for water makers.  Because these systems open the hull to the sea, safety mechanisms must be incorporated:

• Accessible seacocks for rapid shutoff
• Properly double-clamped hoses using stainless clamps
• Non-corroding materials to prevent thru-hull failure
• Anti-siphon loops where vertical lift may cause back-siphoning
• Clear labelling for emergency operation

Maintenance Best Practices

Neglect of the various seawater cooling system often leads to reduced performance, contamination, and in severe cases, dangerous flooding.  To ensure reliable operation:

• Clean raw-water strainers weekly in fouling-prone waters
• Inspect hoses annually for softness, cracking, or odor
• Exercise all seacocks monthly
• Flush washdown and toilet intake hoses with fresh water when leaving the vessel
• Monitor pump amperage and noise for signs of wear
• Perform water maker chemical preservation (“pickling”) during extended periods of non-use
• Remove marine growth from discharge outlets

Seawater cooling system

Auxiliary seawater cooling system plays a critical role aboard modern boats, supporting deck washing, freshwater production, cabin cooling, and sanitation. Each system depends on a reliable seawater intake, effective filtration, corrosion-resistant materials, and properly installed pumps and plumbing. Though simple in concept, these systems require careful engineering and maintenance due to the corrosive, biologically active nature of seawater and the safety implications of hull penetrations. Understanding their operation and upkeep ensures dependable performance and enhances onboard comfort, safety, and autonomy for cruising sailors. The auxiliary seawater cooling system is important to look after.