Cooling water is the cheapest thing in the engine room and the most expensive to get wrong. Untreated or badly treated water corrodes liner jackets and cylinder heads, scales heat exchangers until temperatures creep up, and pits pipework from the inside where nobody sees it until a leak appears. A cracked liner seal or a leaking cooler turns into oil in the jacket water, water in the sump, or seawater in the freshwater system, and each of those can take an engine out of service. Almost none of it is sudden. It builds over months, which is why the weekly water test and the heat exchanger differential readings matter more than any single maintenance job. This guide covers the whole system: how central cooling is arranged and where the heat goes, the chemistry to hold and how to test for it, dosing and top-up discipline, diagnosing contamination from a rising chloride or an oily sample, cleaning and pressure testing heat exchangers, pump and ancillary checks, and the seawater side that most fouling problems start on. Treat the figures here as typical control ranges, because the engine maker's manual and the chemical supplier's data sheet decide the limits for your ship. To keep test results and cleaning records with the equipment rather than in a folder on the desk, try Marine Inspection free.

Test weekly, trend always, top up with the right water

Most cooling water failures are visible in the test log months before they are visible in the engine. The habits on this page are cheap. The repairs they prevent are not.

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Weekly jacket water test card
Nitrite (as NO2)Typically 1,000–2,400 ppm
pHTypically 8.3–10.0
ChlorideCommonly max 50 ppm
AppearanceClear, no oil film, no rust
Level and top-upQuantity and water type used
Ranges vary by product and engine maker. Work to the data sheet for the inhibitor you actually carry.

Know where the heat goes

Most modern ships use central cooling: seawater cools a central freshwater circuit, and that low temperature circuit cools everything else, while a separate high temperature circuit cools the engine jackets. Understanding which circuit a temperature alarm belongs to saves a great deal of guessing.

Seawater circuit
  • Sea chests, high and low suctions, strainers
  • Seawater pumps, usually duty and standby
  • Central coolers, plate type on most ships
  • Anti-fouling and anti-corrosion protection
  • Overboard discharge and vacuum arrangements
Fouling, erosion and corrosion live here. Everything downstream depends on it.
Low temperature freshwater
  • Charge air cooler, low temperature stage
  • Lubricating oil coolers, main and auxiliary
  • Air compressors, alternators, auxiliary equipment
  • Three-way temperature control valve
  • Expansion tank and venting
Controls the temperature of the air and oil that the engine depends on.
High temperature jacket water
  • Cylinder jackets, heads and exhaust valve cages
  • Jacket water cooler and preheater
  • Freshwater generator, using jacket heat
  • Circulating pumps and thermostatic control
  • Expansion tank with the treatment dosing point
The circuit that needs chemical treatment and weekly testing.
Know the normal figuresRecord the usual inlet and outlet temperatures and pressures for every cooler at a fixed load. A cooler slowly losing performance is obvious against those numbers and invisible without them.

Treatment chemistry and control ranges

Jacket cooling water is treated to stop corrosion, scale and cavitation damage. Nitrite-borate inhibitors are the common choice for marine engines, with molybdate-based products used on some systems. Do not mix inhibitor types, and do not use oily inhibitors, which coat the cooling surfaces and reduce heat transfer.

ParameterTypical control rangeWhy it mattersIf it is out of range
Nitrite as NO2Commonly 1,000–2,400 ppm, with routine maintenance targets often quoted around 1,440 ppmForms the passivating film that protects the metal surfacesLow: dose per the product data sheet and retest. High: stop dosing, no further benefit and it wastes chemical
pHCommonly 8.3–10.0Slightly alkaline water protects the metal. Acidic water attacks itLow: dose alkalinity control as the supplier specifies, then retest
ChlorideMost engine makers recommend a maximum of 50 ppmChlorides break down the protective film formed by the nitriteInvestigate for seawater ingress at once. Until it is back below the limit, guidance is to keep nitrite near the upper limit
HardnessAs low as practical, which is why distilled or demineralised water is usedHardness forms scale on the hottest surfaces firstUse distilled make-up water and treat as the supplier specifies
Appearance and oilClear, no film, no rust particlesOil insulates surfaces and feeds bacterial growthFind the leak path before topping up and dosing again
Glycol, where fittedPer maker, with inhibitor compatibility confirmedFreeze protection changes heat transfer and chemistryConfirm the inhibitor is compatible before mixing anything
Nitrite is toxicTreated cooling water must never reach the potable water system. Keep dosing equipment, sample points and hoses separate and clearly marked, and be particularly careful around the freshwater generator and any temporary connections.

The weekly test routine

Testing is quick, so the discipline is in doing it the same way each time and writing down the result even when nothing has changed. That is what turns individual readings into a trend.

1Sample properlyDraw from the dedicated sampling cock on the running system, not from the expansion tank. Flush the cock first, and let the sample cool before testing.
2Test the full setNitrite, pH and chloride as a minimum, using the kit supplied with the inhibitor and following the tablet or reagent method exactly.
3Record the numbersValues, date, engine, who tested, and any dosing carried out. A tick in a box is not a record.
4Compare with last weekA single reading tells you the state. The direction of travel tells you what is happening.
5Send samples ashorePeriodic laboratory analysis checks what the onboard kit cannot, including metals in solution that indicate where corrosion is happening.
6Keep a sample point on every engineEach engine cooling system should have an accessible sampling cock, so a problem can be traced to the machine it belongs to.

Dosing, filling and topping up

Most chemistry problems are caused not by the dosing but by the top-up. Water lost to evaporation should be replaced with distilled water, because the treatment stays behind when water evaporates. Water lost through a leak or a drain-down took the treatment with it, so it should be replaced with treated water and the level of inhibitor checked afterwards.

Filling a clean system
  1. Flush after cleaning until the water runs neutral, then drain
  2. Fill with distilled or demineralised water, leaving room in the expansion tank
  3. Prepare and add the inhibitor solution at the initial dose in the data sheet
  4. Top up to the working level and circulate, commonly for at least 24 hours
  5. Test and confirm the values before considering the system settled
Routine dosing
  1. Test first. Dose against a measured value, never by habit
  2. Work out the addition from the product's dosage table, which is normally given per cubic metre of system volume
  3. Add through the dosing pot or the expansion tank as the supplier specifies
  4. Circulate, then retest after the period the supplier gives
  5. Record the quantity added, the system volume used in the calculation and the result
Know your system volumeEvery dosage calculation depends on it. Find the figure in the ship's documentation, write it on the dosing instructions at the tank, and use the same number every time so results are comparable.

Reading the test results: what contamination looks like

The value of testing is diagnostic, not administrative. Each of these patterns points somewhere specific.

What you seeWhat it usually meansWhat to do
Chloride rising week on weekSeawater ingress, commonly a leaking cooler or a seawater-cooled componentFind the leak by isolating sections, keep nitrite high meanwhile, plan a pressure test
Nitrite falling steadily with no dosing changeWater loss and top-up with untreated water, or inhibitor consumed by active corrosionCheck for leaks and top-up practice, then investigate the system internally
pH drifting downContamination, exhaust gas leakage into the jacket water, or depleted treatmentDose as specified and look for a combustion gas leak path
Oil film on the sampleLeaking lube oil cooler, liner or head seal, or a cracked componentIsolate coolers in turn, check sump level trends, pressure test the suspect item
Water in the lube oilCooler leak in the other direction, or a jacket leak into the crankcaseStop dosing guesses and pressure test. Check oil analysis and purifier operation
Expansion tank level risingCombustion gas or air entering the circuit, or a cooler leaking inCheck for gas at the tank vent, test for combustion products, investigate liner and head seals
Expansion tank level fallingExternal leak, evaporation, or a leak into an oil or seawater sideTrace the loss before topping up repeatedly, and record every top-up quantity
Brown or black sampleCorrosion products, or bacterial growth where oil has contaminated the waterLaboratory analysis, then a cleaning and re-treatment plan
Water tests, dosing and cooler cleaning in one recordCapture the weekly result, the quantity dosed and the cooler differential at the machine, offline, and see the trend without rebuilding it from notebooks.
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Heat exchangers: watch the differentials, then clean

Coolers lose performance gradually. The way to catch it is to log inlet and outlet temperatures on both sides, and the pressure drop across each side, at a fixed load. A rising temperature difference at the same load, or a rising pressure drop, means fouling somewhere.

Plate coolers
  • Compare the plate pack dimension with the maker's figure when closing up
  • Open, clean and inspect on the maker's interval or on rising differential
  • Check plates for pitting, cracking and erosion, particularly at the ports
  • Renew gaskets as specified. Do not reuse deformed gaskets
  • Keep the plate order and orientation exactly as marked
  • Pressure test both sides after assembly before returning to service
Shell and tube coolers
  • Clean tubes mechanically or chemically as the manual allows
  • Check tube ends for erosion, which is where seawater velocity bites first
  • Inspect and renew sacrificial anodes, and record the condition found
  • Check the water boxes and coatings for corrosion and blistering
  • Pressure test after tube work, and plug or renew leaking tubes as approved
  • Record how many tubes are plugged, since capacity falls as the count rises
Chemical cleaningUse approved products, follow the contact time, protect gaskets and anodes, and neutralise and flush thoroughly afterwards. Dispose of the spent solution as the ship's procedures and MARPOL require.
After any cooler workPressure test, vent properly, confirm flow, then watch the temperatures and the water test for the following week to prove nothing was disturbed.

Pumps, valves and controls

The cooling system is only as good as the flow through it. These are the items that quietly reduce flow or hide a problem.

Cooling pumpsBearing condition and noise, mechanical seal leakage, coupling alignment, impeller and wear ring clearance, suction and discharge pressures against the baseline
Standby pump startingTest the automatic start on low pressure regularly and record the result. A standby pump that has never been proven is not a standby pump
Strainers and filtersClean on differential pressure rather than the calendar, and look at what comes out. Shell, weed or scale each point somewhere different
Three-way and thermostatic valvesCheck that they actually modulate. A stuck valve gives stable temperatures until conditions change, then a sudden alarm
Expansion tank and ventingCorrect level, working vent, no continuous gas, and the dosing arrangement clean and clearly labelled
PreheaterElement and thermostat condition, and the circulation pump. Preheating protects the engine at start and matters most on auxiliaries

The seawater side, where fouling starts

Marine growth, silt and shell reduce flow before they block anything completely, and corrosion at high flow velocity attacks pipe bends, cooler tube ends and fittings. Guidance commonly limits flow velocity in seawater and freshwater lines to around 3 metres per second for this reason.

Sea chests and strainersClean on a planned cycle and after any silty or shallow water passage. Inspect gratings and blanking arrangements.
Growth preventionWhere a marine growth prevention system is fitted, check anode or dosing operation and current settings, and record them.
Sacrificial anodesIn coolers, water boxes and sea chests. Record the condition and the replacement date, because they protect everything downstream.
Erosion pointsBends, tee pieces, valve seats and tube inlets. Look for the polished, scoured appearance that means the protective layer is being stripped away.
Pipework and coatingsInternal coatings, rubber linings and pipe supports. Leaks in cooling lines often start as a small weep that is painted over.
Ballast and port operationsSilt drawn in during shallow water or ballast operations ends up in the coolers. Plan strainer cleaning around those passages.

Troubleshooting matrix

SymptomCheck firstLikely causes
Jacket water outlet temperature risingCooler differentials, flow, thermostatic valve, loadFouled cooler, low flow, stuck valve, engine overload
Charge air temperature highAir cooler differential, LT circuit temperature, seawater sideFouled air cooler, warm LT water, fouled central cooler
One cylinder head running hotLocal flow, venting, depositsBlocked passage, scale, air pocket
Pressure falling in the circuitPump, strainer, leak, levelPump wear, blocked strainer, external leak
Temperature swings under stable loadControl valve, sensor, air in the systemSticking three-way valve, faulty sensor, poor venting
Repeated top-ups neededLeaks, drains, gland leakage, oil and seawater sidesPump seal, cooler leak, cracked component
Cooler cleaned but performance unchangedThe other side of the same cooler, and flowFouling on the untreated side, low pump output, wrong valve line-up

Records that prove the system is managed

Cooling water work sits inside the machinery survey arrangement, and a surveyor will usually ask for the water test log before anything else.

Water test logWeekly nitrite, pH and chloride values per system, with dates and the person who tested
Treatment recordProduct used, quantity dosed, system volume, and the result on retest
Cooler recordsCleaning dates, condition found, plates or tubes renewed, tubes plugged, pressure test results
Laboratory reportsPeriodic analysis with the recommendations and what was done about them

Frequently asked questions

How often should cooling water be tested?

Weekly is common practice for jacket cooling water, with periodic laboratory analysis alongside. The supplier's programme and the engine maker's manual set the requirement for your ship.

What nitrite level should jacket water be kept at?

Commonly between 1,000 and 2,400 ppm as NO2, with routine maintenance targets often quoted around 1,440 ppm. Always work to the data sheet of the inhibitor carried on board.

Why does chloride matter so much?

Chlorides attack the protective film the nitrite forms, so corrosion accelerates. Most engine makers set a maximum around 50 ppm, and a rising value usually means seawater is getting in.

Should we top up with distilled water or treated water?

Losses through evaporation are replaced with distilled water, because the chemical stays in the system. Losses through leakage or draining are replaced with treated water, then the levels are checked again.

How do we know a cooler needs cleaning?

By trend, not by calendar. A rising temperature difference or pressure drop at the same load is the signal. That is why the readings are worth logging at a fixed reference condition.

What records will a surveyor expect?

The water test log, treatment records, cooler cleaning and pressure test records, and any laboratory analysis. See our main engine overhaul guide and turbocharger guide, since cooling problems show up in both.

Keep the weekly test where the trend can be seen

Marine Inspection records water tests, dosing, cooler cleaning and pressure tests at the machine, offline, links them to the equipment and running hours, and builds the survey pack from the same data.

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