How to Winterize a Marine Diesel Inboard (Sailboat or Trawler)

Run the engine (under way, at the dock in gear lightly, or ashore with a hose or bucket feeding the raw-water intake) until it reaches normal temperature. On the hard, keep it in neutral: the cutless bearing is water-lubricated and the raw-water pump must never run dry. Warm oil drains better and carries more of its contaminants with it. While it runs, look for leaks, listen for odd noises and note anything to fix over winter.

BoatUS and Westerbeke both say to top off the tank: a full tank leaves little air space, so less condensation (water) forms over winter, and Practical Sailor adds that condensation can rust a black-iron tank. Add a biocide to stop bacteria and fungus growing in the fuel. Westerbeke also recommends a fuel stabilizer or conditioner; BoatUS says a stabilizer isn't strictly needed for diesel if the engine won't run over winter. Follow the product label (more is not better) and make sure the additive is compatible with your filter/separator. Engine makers don't all agree here: Yanmar's FAQ says it doesn't advise any fuel additives and recommends keeping the tank full and draining water instead, so if your engine is a Yanmar (or your manual says the same), check with the dealer before adding anything. Practical Sailor notes that fiberglass tanks of unknown resin may be better laid up empty; ask your builder if unsure.

Drain any water from the primary filter/water separator bowl, and change the primary and engine-mounted secondary filter elements if they're due. Bleed air from the fuel system as your manual shows. Then run the engine 5 to 10 minutes so treated fuel fills the filters, lines and injection pump, and check every fuel fitting for leaks. Any fuel leak or strong diesel smell in the bilge: stop and fix it before going further.

Westerbeke is clear: don't leave old oil in the sump over winter. Used oil holds acids and combustion by-products that attack bearings and other internal parts while the engine sits. Practical Sailor explains that sulfur in diesel fuel forms acids, which is why the change matters more on a diesel. Most boat diesels are pumped out through the dipstick tube. Replace the filter (a thin film of oil on the new gasket), refill to the dipstick mark with the oil your manual specifies, run briefly and recheck. See the diesel oil guide for the full job.

Westerbeke's lay-up list includes draining and refilling the transmission (and V-drive, if fitted), then running and shifting forward and reverse once or twice and rechecking the level. If you don't change it every winter, at least check the level and look for milky or burnt oil. The diesel transmission oil guide has the details.

Test the coolant's freeze protection with a tester. Westerbeke recommends a 50/50 mix of antifreeze and distilled water year round, which it says protects to about -40°F; colder climates may need more. If the protection is too weak, drain some coolant from the block and add a stronger mix, then run the engine to circulate it and test again. If the coolant is due for a change, this is the time. Never use pink RV antifreeze in the closed side: Mercury reserves it for the raw-water side only.

Close the raw-water seacock. Westerbeke says to check the pencil zinc in the heat exchanger and clean or replace it before the antifreeze run. Open the raw-water strainer, clean out the basket and check the lid seal. BoatUS warns that strainers left full of water can freeze and crack, so they need antifreeze or draining too.

With the seacock closed, take the intake hose off the seacock (or the strainer outlet, per your layout) and put it in a bucket of clean fresh water. Have your helper ready. Start the engine and let the pump pull the fresh water through to flush salt out. Stop the engine before the bucket runs dry. Some owners use a fresh-water flush fitting instead, if one is installed.

Fill the bucket with propylene glycol antifreeze mixed as the label says for your winter low (Westerbeke suggests slightly stronger than you need). Westerbeke and Mercury both accept propylene glycol "RV" antifreeze for the raw-water side; it's the less harmful choice. Start the engine and let it draw the antifreeze in. Practical Sailor's test: keep going until the water coming out of the exhaust is the color of your antifreeze, then shut down before the bucket empties so the system stays full. As a rough guide, Practical Sailor says one gallon of concentrate (two gallons mixed) covers engines up to about 40 hp, plus extra for long hose runs to the muffler. Catch what comes out of the exhaust; don't let it go overboard.

Can't run the engine? BoatUS describes a no-run method: with the seacock closed and the pump outlet hose reconnected, pour a 50/50 propylene glycol mix into the disconnected cooling-water discharge hose with a funnel until it won't take more, let it sit a few minutes, then open the raw-water drain plugs and drain. It leaves a protective film and freeze-protects water trapped in low spots. Your manual may show its own version; follow it.

If you pumped enough antifreeze that the exhaust ran colored, the muffler should be protected. BoatUS and Practical Sailor both suggest draining the water-lift muffler canister anyway if it has a drain plug, so there's less liquid in it to freeze. Reinstall the plug right away so it isn't forgotten in spring. Never leave the exhaust hose or muffler full of plain water.

Westerbeke says to take the impeller out of the raw-water pump for the winter (catch the antifreeze that comes with it), inspect it, buy a replacement and a cover gasket if needed, and leave it out until spring. BoatUS explains why: the vanes take a set if left bent in the housing all winter, and antifreeze can swell some rubbers, so rinse the impeller. Tape a note to the pump and the ignition: "IMPELLER OUT."

Westerbeke suggests a clean, lightly oiled rag in the air intake opening to keep moist air out, but never push it out of sight, or you may need a mechanic in spring. The exhaust thru-hull can be closed the same way. BoatUS adds crankcase and transmission breathers and tank vents to the list, on a dry day. Don't seal a fuel tank vent completely: if you cap it, puncture the cap so pressure can escape. Write every plugged opening on a list and tag the ignition.

Fogging a diesel (optional): some owners spray fogging oil into the intake for long lay-ups. BoatUS says to turn the engine over slowly by hand to spread it, and never with the starter, even with the stop control pulled: a diesel can start and run on fogging oil, and you can't shut off that fuel. If your manual doesn't mention fogging, skip it.

Westerbeke says to lubricate the shift and throttle controls so they move freely (BoatUS's trick for cables you can't pull out: tape a small oil-filled bag around the upper end of the housing and let the oil work down), and to leave a pull-type stop lever in the RUN position over winter. On the hard, it suggests disconnecting the prop shaft coupling, checking the coupling set screws, and checking alignment in spring after launch. If the boat stays afloat, BoatUS suggests tightening a traditional stuffing box so it doesn't drip, with a tag to readjust it in spring.

Westerbeke's rule: if batteries stay aboard, keep them fully charged so they can't freeze; if you can't, take them home. A fully charged battery resists freezing far better than a flat one. Turn the battery switch off, clean the terminals, and use a proper marine charger or maintainer if the boat has shore power. Check stored batteries every month or two.

Westerbeke suggests using the lay-up to look over belts and hoses and order spares (filters, zincs, impeller, belts) before spring. Write the date, engine hours, fluids used and every "remove before start" item in your BoatMend log.