Boat Winterisation: How to Prepare Your Diesel Engine for Winter

With technical input from Jeremy, founder of Parts4Engines.

If you're laying your boat up for winter, it's well worth giving the engine some attention before the cold weather sets in.

Water left in the cooling system can freeze and cause serious damage, while old oil and moisture can lead to problems when you come to start the engine again in spring.

Whether your boat is coming ashore or staying afloat, here are seven things we'd recommend checking to help protect your diesel engine over winter.

1. Change the Engine Oil and Oil Filter

Before laying up your boat for winter, it's a good idea to change the engine oil and oil filter. Used oil can contain moisture, acids and other contaminants, which are better removed than left sitting in the engine for several months.

What to do:

• Warm the engine briefly if it's safe to run it, then switch it off and allow the oil to cool enough to work safely.
• Drain or extract the old oil and replace the oil filter.
• Refill with fresh engine oil of the correct grade for your engine.
• Check the gearbox oil while you're at it, particularly for signs of water contamination, which can give the oil a milky or gravy appearance.

2. Protect the Cooling System with Antifreeze

This is one of the most important jobs when winterising your diesel engine.

Water left in the cooling system can freeze and expand, potentially cracking the engine block or damaging the heat exchanger and other components. And that's an expense you can do without!

Many marine diesel engines use raw water (seawater or river water) for cooling. Some are cooled directly by this water, while others use a heat exchanger and have a separate freshwater cooling circuit. The two systems need different treatment.

Raw-water cooling system

Any water left in the raw-water circuit needs to be drained or treated against freezing. This includes water trapped in the pump, cooling passages, heat exchanger or exhaust system.

• Close the raw-water inlet seacock before disconnecting any hoses.
• Follow your engine manufacturer's instructions to drain the system or, where recommended, circulate a suitable marine winterisation antifreeze through it.
• Make sure the antifreeze concentration provides adequate protection for the temperatures you're expecting.
• Check that all parts of the system where water could remain are treated.
  

If the procedure involves running the engine, make sure it has an adequate cooling-water supply. Never allow the raw-water pump to run dry as this will damage the impeller.

Freshwater cooling system

If your engine has a separate freshwater cooling circuit, check the coolant level, condition and antifreeze concentration.

Many engines use a 50/50 mixture of antifreeze concentrate and suitable water, although this varies depending on the engine and coolant specification. Some products also come ready-mixed.

It's also worth checking when the coolant was last changed. Some types need replacing every couple of years, while others last longer. Your engine manual will tell you what's recommended.

You can find our range of antifreeze and coolant products here.

Always collect and dispose of used antifreeze responsibly, and avoid discharging it into the water.

Do you need to keep your boat engine warm in winter?

Not necessarily. The important thing isn't keeping the engine warm, but protecting it from freezing. If it's been properly winterised for the temperatures you're expecting, you shouldn't need to keep it continuously heated.

3. Prepare Your Diesel Fuel for Winter

Diesel fuel can deteriorate when left unused for long periods, particularly if water gets into the tank.

Moisture can encourage the growth of bacteria and fungi, commonly known as ‘diesel bug’. This can clog fuel filters and cause problems when you come to start the engine again in spring.

Before laying up the boat:

• Check for signs of water or contamination in the fuel.
• If you're storing diesel for several months, consider using a suitable fuel treatment to help protect it.
• Follow the product instructions, including whether the treatment needs circulating through the fuel system before storage.
  

Should you leave your diesel tank full over winter?

There's some differing advice on this. Keeping the tank topped up can help reduce condensation on the side walls of the tank, but modern diesel can also deteriorate if stored for too long, particularly if it contains biodiesel.

It's worth considering how much fuel you typically use and how long the boat will be laid up, rather than automatically filling the tank.

We stock Liqui Moly Marine Diesel Protect, which is designed to help protect diesel during storage and prevent microbial contamination. Always check the product's suitability and follow the instructions on the label.

4. Inspect Hoses, Belts and Other Engine Parts

Winter is a good time to give your engine a proper once-over. Look for anything showing signs of wear, damage or corrosion, particularly parts exposed to heat, vibration and salt water.

Things to check:

• Hoses: Look for cracks, splits, bulges or signs of leaks.
• Drive belts: Check for cracking, fraying, glazing or incorrect tension.
• Raw-water pump impeller: Check its condition and whether it's due for replacement before the next season.
• Hose clips and connections: Look for corrosion, loose fittings or signs of leaks.
• Engine and exhaust: Check for oil, fuel or coolant leaks, along with any visible corrosion. Bear in mind that cast-iron and aluminium exhaust outlets can also corrode internally, even when they look fine from the outside.
  

If you spot anything worn or damaged, now's a good time to sort it out. There's nothing worse than discovering a split hose or worn belt just as you're getting ready for your first trip of the season.

5. Check the Diesel Fuel Filters and Water Separator

Water and dirt in diesel fuel will cause problems, so it's a good idea to check your fuel filters and water separator before winter.

Many marine diesel engines have a water-separating fuel filter between the tank and engine, as well as a separate engine-mounted fuel filter. The water separator helps remove water from the diesel before it reaches the engine's fuel system.

What to do:

• Check the water separator bowl, if fitted, for any collected water or sediment.
• Drain any water into a suitable container, following the manufacturer's instructions.
• Check the condition of your fuel filters and replace them if they're dirty or due for a change.
  Inspect the seals and connections for signs of leaks.
  

If you replace a fuel filter, remember that you may need to bleed or prime the fuel system before starting the engine again. Check for leaks once it's running.

6. Look After the Battery

Batteries don't like the cold! If your boat's laid up over winter, the battery will gradually lose some charge while sitting unused. Letting it become heavily discharged can shorten its lifespan and leave you struggling to start the engine in spring.

Before laying up the boat:

• Fully charge the battery and check its general condition.
• If practical, disconnect and remove it, storing it ashore somewhere cool, dry and protected from freezing.
• Use a suitable battery maintenance charger where recommended, or check and recharge the battery periodically in line with the manufacturer's instructions.
• Clean the battery terminals and check the cables and connections for signs of corrosion.
  

If your boat is staying afloat, be careful before disconnecting batteries. Equipment such as automatic bilge pumps and alarms will still need power while you're away.

Make sure any charger you use is suitable for your battery type, whether that's lead-acid, AGM or lithium. Some lithium batteries also have restrictions on charging in freezing temperatures, so check the manufacturer's guidance.

7. Grease and Lubricate Moving Parts

Salt water and damp aren't kind to moving parts, especially when your boat's sitting unused over winter. Over time, corrosion can build up and leave controls and linkages stiff or difficult to operate.

Before winter, give the throttle and gear controls a check to make sure they move freely. Look for signs of wear or corrosion around the linkages and cable ends.

It's also worth checking the steering system and any other lubrication points listed in your engine or equipment manual.

Where recommended, use a suitable marine grease to help protect moving parts from moisture and corrosion.

Not everything needs greasing, though. Some components require different lubricants or shouldn't be lubricated at all, so always check the manufacturer's recommendations.

Jeremy’s Extra Tip: Keeping Condensation Under Control

Damp and condensation can be a real nuisance on boats over winter, particularly when they're closed up for several months. Left unchecked, moisture can lead to mould, corrosion and damage to woodwork and upholstery.

Here's a tip from Jeremy, founder of Parts4Engines, who uses a dehumidifier on his own boat:

"I like to use a dehumidifier on my boat over winter, and it makes a huge difference in reducing mould on soft furnishings and headlinings. Damp can also get under the woodwork varnish, causing that lovely interior joinery finish to age prematurely.

If you've got shore power, I think it's well worth considering. I try to keep the cabin humidity below about 65–70%. Just watch the temperature, as some dehumidifiers don't work particularly well when it gets really cold."

If you're leaving a dehumidifier running while you're away, make sure it's suitable for unattended use, the shore-power supply is safe and there's a suitable way for the collected water to drain. Never block essential ventilation."

Even without a dehumidifier, there are a few simple things you can do to help keep damp at bay:

• Remove wet clothing and bedding, and take cushions home if practical.
• If you're leaving cushions on board, prop them up to allow air to circulate underneath.
• Leave interior lockers open where possible and check the bilge for standing water.
• Check for leaks and make sure the boat has suitable ventilation.
  

If you're leaving a dehumidifier running while you're away, make sure it's suitable for unattended use, that the shore-power supply and drainage are safe, and that your marina and insurer permit it. Never block essential ventilation.

Ready for Winter?

One last thing before leaving your boat for winter: make a note of anything you've drained, disconnected or removed. Taking a few photos can help too, particularly when it's time to get everything running again in spring.

Every engine is different, so always check your manufacturer's recommendations, especially when it comes to cooling systems and antifreeze.

If you're winterising a Perkins, Volvo Penta or Yanmar diesel engine and need filters, impellers, belts or other maintenance parts, we're here to help.

Get in touch with the Parts4Engines team if you're unsure which parts you need. Just let us know your engine model and serial number, and we'll help you find the right ones.