Salt and Scale in Raw-Water Cooling: Flushing Habits, Descaling and When It's a Shop Job

Check your owner's manual for which parts see raw water: the whole engine (most outboards and many older sterndrives and gas inboards), or just a heat exchanger, oil and fuel coolers and the exhaust (closed-cooled engines and most diesels). Note where the flush connection is, if you have one, and what the manual says about using it. Not every outboard has a flush port, and Yamaha notes that outboards with two water inlets need a special adapter for muffs.

Scale builds slowly, so the signs are gradual:

• The temperature gauge reads a little higher each season at the same speed, or the alarm starts going off at high RPM or in hot weather.
• The outboard's telltale ("pee") stream gets weaker or spits.
• The engine runs cooler than normal, or takes ages to warm up. That's a thermostat or poppet valve sticking open.
• On closed-cooled engines, the running temperature rises over months while the coolant level is fine.

Write down your normal temperature and RPM in your BoatMend log once a month. A slow creep is easy to miss without notes.

Before blaming scale, check what's cheaper:

• Intake grates on the gearcase or the raw-water strainer: clear weeds, mud and barnacles.
• The telltale outlet: Yamaha notes that insects such as mud daubers love to nest in it. Clear it gently with thin wire. Also remember that some outboards don't show a stream at idle, only once RPM rises a bit.
• The water pump impeller: Yamaha suggests inspecting it every year, in salt or fresh water. A worn impeller looks just like scale from the helm. If the old impeller is missing vanes, the broken pieces have gone up into the cooling passages and can block them just like scale; they often turn up at the thermostat housing. Find them before you fit the new impeller.
• Thermostats: Honda recommends having the thermostats and water pump serviced every year or every 200 hours when you run in salt or polluted water.

See the engine overheating guide for the full checklist.

This is the habit that prevents most scale. Mercury says an outboard or sterndrive used in salt water should be flushed and rinsed after each use, and also after brackish, polluted or very silty fresh water. Honda gives the same advice and explains that flushing prevents deposits that can clog or corrode the water passages, sensors and thermostats. Do it as soon as you can after hauling out, before the salt dries.

Many newer outboards have a garden-hose fitting on the lower cowl. Yamaha's method: connect the hose to the flush attachment of the non-running outboard, turn the water on full and let it run for 15 minutes so it circulates through the whole cooling system. Mercury says its flush fitting works with the boat on a trailer or in the water and to follow your manual for the exact procedure. BoatUS notes that some makers give a water pressure range or tell you not to open the tap fully, so check your manual. Keep the engine in the position the manual says (often vertical) while flushing and afterward so it drains.

Then rinse the outside. Mercury's Flush Connect article gives a few cautions owners miss: lower the outboard to vertical before hosing the cowl, because water sprayed on a tilted cowl can get into the air intake at the back; with the cowl off, let water run gently over the powerhead (no spray nozzle or pressure washer, which drive water into electrical connectors) and keep it away from the air intake and alternator; then let it dry before closing up.

If your manual calls for muffs or a flush bag, the engine runs, so:

• Remove the propeller first.
• Turn on the water until it squirts out around the muff cups, then start the engine in neutral at idle only. Yamaha says to keep enough pressure that some water always leaks from under the cups.
• In a flush bag, the water level has to be above the water pump; Yamaha says about an inch above the lower unit's split line.
• Watch the telltale. No stream means stop the engine now.

Yamaha suggests 15 minutes. Turn off the engine first, then the water.

Mercury says MerCruiser sterndrives need flush muffs unless they have an accessory engine flush kit, and that MerCruiser engines with SeaCore freshwater cooling should still be flushed to clear salt from the heat exchanger and exhaust. Mercury's FAQ adds that the engine must be running whenever a MerCruiser flush kit is used, that Bravo, inboard and towsport kits work in or out of the water, and that the Alpha kit works only with the boat out of the water.

A flush bag or barrel works for outboards because their water pump sits in the lower unit. Engines with the raw-water pump on the engine itself (many Bravo, Volvo Penta and inboard setups) need hose pressure to push water to the pump, so use muffs or the maker's flush kit instead. Inboards and diesels usually flush through a fitting at the raw-water strainer or seacock. Follow your manual: with an inboard you must not flood the engine through the exhaust, so never leave the water running with the engine stopped.

Honda suggests flushing with its own Salt Terminator product. Other makers sell or approve similar salt-removing flush additives. Use one only if your engine maker approves it, mix it as the label says, and follow with fresh water if the label tells you to. A good additive helps, but it doesn't replace the fresh-water flush itself.

Once an engine is running warm from scale, the fix depends on the engine:

• On outboards and raw-water cooled gas engines, a dealer may circulate a descaler the engine maker approves, remove the thermostats and covers to clean passages, or replace badly blocked parts.
• On closed-cooled engines and diesels, the heat exchanger is the usual target: the end caps come off, the tubes are cleaned and the gaskets replaced. Badly scaled exchangers often go to a shop or radiator shop for cleaning and a pressure test.

The existing BoatMend advice stands: don't pump acids or household cleaners through your outboard on your own. Let a dealer decide what it needs.

Some owners descale heat exchangers, oil coolers or A/C condensers themselves, using a product the equipment maker accepts. Star brite's directions for its marine descaling fluid show what a typical job involves:

• Isolate and drain the circuit so no raw water can get in, and flush it with fresh water first.
• Circulate the descaler in a loop from a bucket with a small pump, or let it soak (Star brite says 4–8 hours).
• Don't seal the system. Descaling gives off gas, and Star brite warns that the pressure can cause damage.
• Drain, dispose of the liquid as local rules require, and flush thoroughly with fresh water.
• Star brite recommends replacing all sacrificial anodes afterward.

Star brite's label lists phosphoric acid and warns that the product is corrosive. Wear gloves and eye protection, and keep it off paint and gelcoat.

• Strong acids and homemade mixes (muriatic acid, household lime removers, vinegar "recipes" from videos). The PBO expert warns that small outboards are aluminum, which strong acids dissolve, and that a DIY mix is not recommended.
• Any product your engine maker hasn't approved for the cooling system. It can damage seals, hoses, sensors or the anodes inside the engine, and can void a warranty.
• High pressure. Don't connect a pressure washer to a flush port or force water in harder than the manual allows.
• Running the engine on a descaler without the full cooling water supply the manual calls for.

Run the engine with proper cooling water and check the telltale stream, the temperature at your usual RPM, and every hose and gasket you disturbed. Scale sometimes hides tiny leaks or worn spots, so look again after the first trip. Write the new normal temperature in your log.

Flush after every saltwater trip, inspect the impeller each year, and have the thermostats looked at on the schedule your manual gives (Honda's is every year or 200 hours in salt water). These cheap habits are what keep scale from becoming a shop bill.