Don’t Forget to Flush

There are two rooms on a boat that should always have plumbing for proper flushing — the head and the engine room. While not flushing the head is merely inconsiderate, not flushing a raw water–cooled engine is irresponsible. Salt or brackish water can cause corrosion damage, and freshwater marine organisms can cause clogs. In either case, you may experience performance loss and, eventually, engine failure.

Do you have a raw water–cooled inboard or sterndrive engine — or two? Here’s how you can install a three-port valve to simplify flushing your engine(s) while it’s in the water.

Similar to their automotive counterparts, marine engines produce heat while converting fuel into mechanical energy (motion). Automobiles use air through a radiator to cool their engine, but boats use water from their environment.

When the engine is running, an impeller inside the water pump turns and draws in “raw water” and circulates it through the engine (an open cooling system) or through a heat exchanger on the engine (a closed cooling system).

Usually, to flush an open cooling system, you disconnect a hose from the water pump, connect a separate freshwater hose and then reconnect the original hose.

Intake Take Out
Locate your engine’s water pump and identify the raw-water hose. Follow it to where it is connected to the raw-water source. Inboard engines typically use a through-hull, while sterndrives draw water through the outdrive. If the hose is connected to a shut-off valve, be sure it is closed. Remove the clamps at each end of the hose, and then remove the hose. Use this hose to find a replacement and to size the valve and fittings.

Important: If you do not have a shut-off valve, be sure to plug the raw-water source to keep water from entering your boat.

My 32-foot Bayliner Conquest has twin sterndrive engines. Raw water travels from the outdrive to the inside of the boat via a stainless steel tube. A flexible hose is used to deliver water from the tube (picture 1) to the water pump (picture 2).BW_04-2014_DIY_02BW_04-2014_DIY_03

Cleaning Boosters

Barnacle Buster ( and Salt Away ( are supposed to clean your cooling system better than flushing with fresh water alone. You can find them both at West Marine (
Be sure you research products and carefully read their instructions before flushing your engine with them.

Tee Time
Use a T-shaped, three-port ball valve (picture 3) to connect hoses for raw-water and freshwater sources. It has two inline input ports and one output port. The water sources will be connected to the input ports. The output port will be connected to the intake on the water pump. The water flow can be switched from raw water to fresh by moving the valve’s handle.BW_04-2014_DIY_04

I went to Icon International in San Pedro, Calif., to gather parts for my flush system. My parts list included:

20 feet of 1¼-inch wire-reinforced hose ($200)

2 bronze 1-inch three-port valves ($60)

6 1-inch-by-2½-inch nipples ($40)

20 stainless steel hose clamps ($60)

Thanks to Armando for patiently helping me.

I used Teflon tape on the nipples’ threads before connecting them to the valve (picture 4). Then I cut sections of hose to connect the raw-water source to one of the valves and the water pump to the output. I used some miscellaneous leftover hose for the freshwater input. I used two hose clamps on each end of the hoses.BW_04-2014_DIY_05

Once everything was connected and leak-free, I mounted the valve to the inside of the transom (picture 5).BW_04-2014_DIY_06

Let it Flow
During normal engine operation, the valve’s handle should be positioned to allow raw water into the cooling system.

The engine should be running when you are ready to flush it. Place a bucket of fresh water in the bilge, and put the freshwater input hose into the bucket. Move the valve handle completely to the freshwater position. Otherwise, you can still be drawing in raw water.

Monitor the water level in the bucket. Shut off the engine before the bucket is empty, or you may damage the water pump’s impeller. To flush longer, place a hose from a freshwater source into the bucket. The flow from the source should maintain a constant freshwater level in the bucket. Once you have sufficiently flushed the engine, shut it, and then the freshwater source, off.

To finish, return the valve back to the raw-water setting. Be sure the handle is completely moved, or you could damage the impeller.

Important: Always check the position of the valve before starting the engine.

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