How to Flush a Boat Engine the Right Way

I was like a lot of other non-boaters.

I imagined that the turn-key experience I had with my car would extend to my boat.

Admittedly, I was naïve when I bought my boat. I didn’t grow up around vessels or the water so I didn’t know that there would be a lot of work to do on a regular basis.

I learned quickly that Mother Nature would be slowly, insidiously and continually trying to destroy my boat engine.

It was up to me to take care of her if I wanted to enjoy the time I could carve out to spend on the water.

However, I also learned that being proactive about caring for a boat means fewer headaches and repair bills.

Also, taking the time to do my homework paid off—I did the jobs more efficiently. Flushing my boat engine went faster than I initially thought possible, and I saw great results.

Oh, I couldn’t completely stop salt corrosion from eating away at the engine any more than I could prevent that mouse from storing sunflower seeds in my car’s starter. But I could move the supply of seeds out of my garage and I could wash away the salt regularly. Both actions, taken frequently, made a huge difference.

Really, my initial dismay at the amount of work that a boat owner assumes was tempered by the realization that I would get a lot out of it, too.

Why Are You Flushing Your Boat Engine?

There are two main reasons to flush a boat engine, and both have to do with avoiding problems:

1. Reducing the corrosive effect of salt water during the boating season.

2. Keeping the engine block from freezing and cracking during winter storage.

Salt water corrosion

Removing salt after a day on the water will improve performance and extend the life of the engine.

Since the Long Island Sound is my playground, that means fishing for saltwater treasures like stripers but also sucking in a lot of corrosive liquid as we troll.

Getting rid of that stuff and replacing it with fresh water helps the engine perform reliably, minimizes the repair bills and just makes me sleep better at night.

Boat manufacturers are actually pretty helpful in this and many new engines let you hook up a hose right to the intake so the task is as simple as watering the garden. If that’s not the case with your boat, there are inexpensive specialized tools you can use—and I’ll introduce you to some of these later on.

Surviving the winter

As for surviving the winter, well, anyone who has ever stored a bottle of anything in the freezer knows that liquids expand when converting to solids.

Physics was never so well illustrated as the day I cleaned red wine from a bottle that my brother had left to chill (and forgotten). Like that wine bottle, which broke under the pressure exerted by expanding wine, a boat engine will too crack.

A broken bottle of wine is sad, but so much easier to replace than a dead engine.

The trick to avoiding this very unpleasant problem is to run something through the cooling system that won’t freeze. Kinda why they call it antifreeze.

Of course, you’ll want to flush that fluid out of the system in the spring when you prep for long, hot days of water fun.

How to Flush a Boat Engine the Right Way

Get the Right Supplies to Flush Your Engine

The tools you’ll need depend on your engine type.

Your car engine uses a fan and a self-contained system of liquid coolant (that antifreeze again) to wick excess heat away from the engine and keep it at an optimal running temperature. Air is in plentiful supply around a car, just as water is all around a boat.

Boats can suck in cold water and pump out hot.

There’s such a thing as a closed system for inboard boat engines but that kind of system is overly complicated for an outboard. So, flushing one of these engines is as simple as running water from a hose into the engine.

For an outboard engine

An outboard engine will have either a direct hose hook-up at the intake (where water is drawn into the engine) or a port that you can cover with something that looks like earmuffs (hence the common term for these). The earmuffs (check price on Amazon here) cover the holes so that water from a hose makes it into the boat’s engine without sucking in air.

The whole process of flushing an outboard engine is easy and takes just a few minutes.

Basically, hook up the fresh water supply, whether it’s screwing the hose into the engine or hooking the hose to the earmuffs and laying those over the engine. Then, turn on the water.

Read the manufacturer’s manual for your engine because you may or may not need to run the motor as you flush.

For an inboard engine

Inboard motors can’t easily be flushed on a regular basis but, honestly, there’s a trick that’s more fun than using a hose. Consider where you moor your boat for the summer.

A quiet marina set back from the coast can be safer because, not only because will you be farther from the salt but coastal storms like hurricanes will have to spend more energy to reach you in a protected harbor. And, if you can moor close to one, a quick ride up a river and away from salt water will flush out your engine with fresh water.

At wintertime, you’ll basically substitute antifreeze for water during the flushing process. A hose will push water to the engine and ensure that there will be coolant available.

You’ll need to ensure the same kind of pressure is available behind the antifreeze so think of elevating that bucket of environmentally-friendly antifreeze to get gravity to do the work for you.

There are some cool adapters that will convert a simple five-gallon bucket (check price on Amazon) into a great dispensing system or you can buy a complete setup (check price on Amazon).

You may also want to invest in a good fogging oil (check price on Amazon) to protect your engine in the winter, which will coat engine components and protect them from the elements.

I found it was worth paying a few dollars to the marina’s mechanic to teach me where to apply the oil so that I wasn’t wasting any or missing important parts that needed protection.

Tips for Flushing Your Boat Engine

Flushing your boat engine isn’t really difficult but there’s one thing you must remember.

Never, ever run an engine without coolant.

If your earmuffs slip and water isn’t being pushed into the engine, or if you walk away from a bucket of antifreeze and it empties before you return, you’ll do bad things to your engine.

I remember when the impeller on my inboard engine got stuck and we weren’t taking on fresh, cooling water. Luckily, we noticed the smoke and stopped the engine before there was serious damage but, really, you wouldn’t run a marathon without water so why ask your engine to do it?

Also, check out all supplies before you use them. Water from a garden hose can be used to flush an outboard whether it’s in a slip or on a trailer because it won’t hurt the environment when it spills onto the ground. Be sure the antifreeze you use will be just as safe as plain water.

A lot of boat maintenance is based on the idea that an ounce of prevention is worth a pound of cure.

Waxing the boat will extend her life and beauty for years, even decades. In the same way, flushing the engine will keep engine components free from corrosion and running in top condition far longer than if you get lazy and skip this step.

Put it another way, a few minutes of flushing will be a lot easier than the hours of work you’ll have to do to buy a new engine.