Now that the end-of-season oil-change is done, it’s time to winterize your diesel’s cooling system. There are a few different cooling setups out there, including straight raw-water cooling and dry exhaust used in conjunction with keel coolers, but the great majority of diesel-powered pleasure boats use so-called freshwater cooling systems with heat exchangers. In this system, the “fresh water” is actually coolant/antifreeze. While freshwater cooling systems vary, the basics are the same: Raw water, sucked in from outside the boat through a strainer and impeller pump, circulates through a series of tubes in the heat exchanger, then goes overboard along with the exhaust gases from the engine. Meanwhile engine coolant passes in and out of a reservoir and circulates around the raw-water tubes, being cooled in the process.


An environmentally friendly combo: Rydlyme to clean the heat exchanger and West Marine Pure Oceans antifreeze to winterize the engine cooling system.

The freshwater/coolant side of the heat exchanger is a closed loop with little to go wrong unless there’s a leak or the coolant is so old that it loses its working properties. The raw-water side of the heat exchanger, however, needs to be watched with a paranoid eye, and it’s this area that needs to be filled with non-toxic antifreeze for the winter. Letting raw water freeze inside your cooling system is a costly mistake.

Before getting to the winterization, though, consider the state of the heat exchanger. As good as the main strainer may be, raw seawater contains a menagerie of minute beasts and minerals—especially calcium carbonate, the stuff shell and scale is made of—that will infest the narrow tube structure, causing a sclerosis that will eventually cause engine overheating. The raw water side is also subject to corrosion, and it will have a zinc anode that needs to be changed at least once a season (some say once a month).

So, before winterizing, it’s a good idea to attend to that side of the heat exchanger. It can be done at spring commissioning, but it’s nice to give the engine a clean slate for turnkey operations when the weather warms. Also, while both projects—cleaning and winterizing —can be done while the boat is in the water, they’re best done when the boat is ashore, to minimize the release of chemicals, even relatively benign ones, into the water.


Heat exchanger sclerosis is evident at the end of a six-month season.

The ideal way to clean the exchanger is to take it out of the boat and give it a proper descaling and, if necessary, a careful cleaning out of the tubes. However, I’ve found that using a descaler with the heat exchanger in place, while it might not spit-shine every tube, keeps things in shape for several seasons before the whole thing needs to come out.

There are different liquid descaling products available. Most are extremely caustic to work with, tough on the environment, and probably should not be used often on metal surfaces. In tests at Practical Sailor we found that Rydlyme Marine, which is milder, non-toxic, non-corrosive, and biodegradable, works just as well if left in place longer—say 15 minutes, as opposed to 3-4 minutes for harsher products. The Rydlyme won’t hurt hoses, seals, gaskets, impellers, or other soft parts, and it can be handled and disposed of safely.

You can buy or build a pumping system that bridges the raw water intake and the heat exchanger’s outlet and circulate the descaler solution in a loop, but I’ve had success just filling up the raw water side of the exchanger with Rydlyme, letting it percolate out to the transmission oil cooler, and leaving it for a while. Important tip: first remove the zincs from both the heat exchanger and transmission oil cooler, and replug the sockets with empty brass zinc holders. Descalers that dissolve calcium carbonate also dissolve zinc – very quickly and fizzily.


When using a descaler, even a mild one like Rydlyme, make sure to remove any zincs from both the heat exchanger and transmission oil cooler (shown here), and replug the sockets.

I let the brew sit while doing other chores, then drain everything carefully and rinse with fresh water. The rig I use is simple: a length of clear hose with a funnel on top and a threaded nipple at the other end that screws into the heat exchanger zinc hole. But there are so many variables involved in engines and installations that this may not work for everyone.

Next, the rinsing and winterizing phase. (Anyone else reminded of Austin Powers?)

1. Make sure the zinc sockets are plugged, either with new zincs or the brass plugs from old zincs. Shut the seacock off at the raw water intake (if you’re in the water), and remove that end of the intake hose.

2. Fill a clean bucket with fresh water and have more fresh water standing by. Insert the intake hose into the water in the bucket. If the hose isn’t long enough, add an extension with a smooth coupling and hose clamps, or use a smaller bucket down lower in the bilge and be prepared to refill it on the fly. Obviously this takes some pre-planning.


A collapsible bucket can sometimes get into difficult places. A better solution would be to lengthen the raw-water intake hose with a smooth coupling and hose clamps, and insert it into a proper 5-gallon pail.

3. Have someone else start the engine to draw the fresh water through the raw water side of the system. If you’re holding the intake hose, make sure it doesn’t vacuum itself to the bottom of the bucket.

4. Flush liberally. The exhaust water should run clear.

5. Turn off the engine. Rinsing done.

6. Now fill your bucket with undiluted, non-toxic, water system antifreeze like West Marine’s Pure Oceans brand, which is an environmentally safe propylene glycol formula. It’s important NOT to use a toxic ethylene glycol solution of the kind used in automotive antifreeze (and in the closed loop of your boat’s cooling system). You will have to estimate the amount of antifreeze to have on hand. Our 96-hp Isuzu takes a bit more than three gallons, but I always keep another gallon or so handy. If you have extra, you can use it in other water system winterization projects.

7. Again insert the raw-water intake hose into the bucket and have someone else start the engine. As the antifreeze is taken in, watch the overboard exhaust. When you see the colored antifreeze come out there, your system is protected. Kill the engine, reattach the intake hose to the strainer, and, if the boat is in the water, open the seacock so that the strainer will drain when the boat is hauled.

There are more winterization chores, but now the four biggest ones for the engine are done – fuel treatment, oil change, heat exchanger cleaning, and cooling system protection. Time for a hot-buttered rum.

But first, here are five winterizing mistakes to avoid.

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Written by: Doug Logan
Doug Logan has been a senior editor of since 2010. He's a former editor-in-chief of Practical Sailor, managing editor and technical editor of Sailing World, webmaster for Sailing World and Cruising World, contributing editor to Powerboat Reports, and the editor of dozens of books about boats, boat gear, and the sea.