James Ashworth
Why Winterising Matters More Than You Think
Every January, boatyards across the country take calls from owners who have discovered their engine is ruined. The cause is nearly always the same: water left sitting in the wrong place when the temperature dropped below zero. Water expands as it freezes, and inside a narrowboat engine that expansion has nowhere to go. It splits hoses, cracks cylinder blocks and punches out core plugs. A cracked block is usually the end of the engine, not a repair job, so an afternoon of preparation now is worth far more than the cost of a replacement.
Your narrowboat has two separate water systems to think about. The raw water side draws canal or river water in to cool the engine, and it must be drained completely. The freshwater or closed circuit circulates coolant and antifreeze around the block, and its strength needs checking. Add the fuel system, the electrics and the engine bay itself, and you have a straightforward job that most owners can do alongside the boat in a morning.
Draining the Raw Water Side
This is the step that saves engines. Water sits in the strainer, the pipework, the pump, the exhaust injection elbow and the silencer, and it is usually the silencer or the elbow that splits first.
- Close the seacock before you touch anything.
- Slacken the hose clips and drain the sea strainer, then clean the gauze basket and refit it.
- Disconnect the lowest point of the cooling pipework in the engine bay and let it run into a container.
- Remove the drain plug on the raw water pump if it has one, and turn the engine over by hand a few turns to push out trapped water.
- Drain the exhaust injection elbow and the water trap silencer. If the silencer has a drain plug, open it, and tip the boat slightly stern-down if you can.
If your boat is raw water cooled — no heat exchanger, just canal water straight through the block — you cannot simply drain it. Fill the system with a 50:50 antifreeze mix instead, run the engine briefly to circulate it, then leave it in place over winter and drain it in spring.
Antifreeze and the Freshwater Circuit
On a heat exchanger or keel-cooled engine, the closed circuit should already contain antifreeze, but its strength fades over time as it is topped up with plain water. A 50:50 mix of glycol antifreeze and water protects down to roughly minus 35 degrees Celsius, which is far more than any British winter will throw at you.
Test the concentration with a refractometer, which is more accurate than the old floating hydrometers and costs very little. If the reading is weak, drain and refill with fresh coolant of the type the engine maker specifies. Do not mix different coolant chemistries, and do not assume a two-year-old fill is still doing its job. Coolant also carries corrosion inhibitors, and these wear out long before the freezing point becomes a problem. Renewing it every two or three years keeps the inside of the block clean.
Oil, Filters and Fuel
Change the engine oil and filter before you lay the boat up rather than in spring. Used oil contains acids and combustion byproducts that sit against bearings and bores for months, quietly doing damage. Warm the engine first so the oil flows freely, drain it fully, fit a new filter and refill to the mark on the dipstick. If you run the engine afterwards to circulate the new oil, remember to drain the raw water side again.
On the fuel side, drain any water from the sedimenter or water trap, and change the fuel filter if it is due. Fill the tank to the brim before winter. A full tank leaves very little room for air, and less air means less condensation forming on the cold steel walls. Add a diesel conditioner or biocide at the same time to keep the dreaded fuel bug at bay, and check that the filler cap seals properly.
Batteries, Belts and the Engine Bay
A flat battery can freeze and buckle its plates, while a fully charged one shrugs off hard frost. Charge everything to 12.6 volts or above, then either remove the batteries to a dry shed or leave them aboard on a smart trickle charger. Smear the terminals with petroleum jelly or silicone grease and label the leads so you know exactly where they go back.
Check the alternator and water pump belts for cracking and glazing, and set the tension correctly, or slacken the alternator and remove the belt if you prefer to keep it supple. Give the engine bay a good clean: spilled oil and damp rags hold moisture against metal and encourage rust. Leave the ventilation grilles open so air can move through, and plug the exhaust outlet with a rag or a wooden bung to keep rain and rodents out. Write yourself a note on the throttle lever so you remember to remove it in spring. A couple of turns on the stern gland greaser will not go amiss either.
Your Winterising Checklist
- Close the seacock and drain the raw water strainer, pipework, pump, elbow and silencer.
- Raw water cooled engines: fill with a 50:50 antifreeze mix.
- Test the closed circuit antifreeze and top up or replace as needed.
- Change the engine oil and filter before layup.
- Drain water from the fuel system, fill the tank and add conditioner.
- Charge batteries, protect terminals and connect a smart charger.
- Check and adjust belts, or remove them.
- Clean the engine bay, plug the exhaust and leave ventilation open.
If the boat stays in the water and you plan to visit over winter, running the engine up to full temperature for twenty minutes every few weeks does no harm and helps keep condensation at bay. Just remember to re-drain the raw water side afterwards if a hard frost is coming. Look after your engine through the cold months, and it will fire up first turn when the ice finally melts.
