It is well known that boat engines do not wear out through use, but rather stand idle until they break down – corrosion and sub-zero temperatures cause them the most damage during the winter break. This makes proper preservation all the more important for the cold months – and this applies to both diesel and petrol engines.
There are two ways to prepare engines for ‘winter storage’: either you carry out the necessary work yourself, or you have it done by a specialist firm.
As a general rule, we recommend the second option; at least whilst the vehicle is still under warranty, it is advisable to have an authorised workshop carry out the winter storage preparations. Anyone wishing to prepare their engine for winter storage themselves should follow the procedure described on the following pages.
When preparing the engine for winter storage, refer to the engine manufacturer’s operating instructions
In addition, you should always refer to the engine manufacturer’s instruction manual, which sets out all the necessary procedures once again. Mercury even provides step-by-step instructions via a scannable QR code on the engine cover and a video platform. on this topic.
If you do not have the operating instructions for your own engine, manuals from specialist retailers can often be of help; however, depending on the engine type, these may in some cases only be available in English. It is best to start the engine preservation work whilst the boat is still in the water.
inboard engine
Whether it’s a diesel or petrol engine, with or without a dual-circuit cooling system – the steps are similar
Oil change
The engine oil, oil filter and the oil in the reversing gearbox must be changed. To do this, you will need a suction pump (which is permanently fitted to the engine on larger diesel engines) with a suitable probe and filter clamp. Before doing so, let the engine run for a while to warm it up, so that the oil is thinner and can be pumped out more easily into a discarded oil can or spare container.
At the same time, loosen and unscrew the oil filter. To prevent spills, have a small bowl and some cleaning cloths to hand. Before fitting the new filter, check that the rubber seal is seated correctly and use your finger to apply a thin layer of oil to the contact surface. Important: The oil filter must only be tightened by hand (25 Nm); otherwise, it may not be possible to unscrew it again. The type of oil suitable for the engine is specified in the owner’s manual.
Fuel systems
First, drain the water separators. To do this, place a small bowl under the separator and open the drain plugs until no more water comes out. Next, as a precaution, replace all filters and filter inserts. Diesel engines must then be bled; instructions on how to do this can be found in the owner’s manual. Afterwards, run the engine (petrol and diesel) and check that everything is watertight again.
Corrosion in the fuel systems of diesel and petrol engines is prevented by using so-called fuel additives, which also dissolve deposits and bind moisture in the fuel tank. All of this is then burnt off as normal. These additives are poured into the fuel tank before you start your winter maintenance and then you let the engine run.
The additives perform their preservative function in the process. There are special additives designed solely for diesel or petrol engines, but also others that are suitable for all engines. They are available from boat supply shops and specialist workshops.
V-belt
You should check the V-belts at least once or twice a year – putting the bike into storage for the winter is the best time to do this, so you won’t forget to check them in the spring. To check them, press down on the centre of the V-belt; it should not give more than about 10 mm, otherwise it will need to be retensioned.
If the belt is frayed or damaged in any other way, it must be replaced; this is because a faulty V-belt means the battery will not be charged by the alternator, the seawater pump for the cooling system will fail, and the power steering pump will not work.
Cooling system
Winterisation work on the cooling system is best carried out when the boat is already ashore, as otherwise water can easily get into the boat. Even today, most inboard petrol engines have only one cooling circuit: seawater cooling, also known as single-circuit cooling.
This circuit (seawater filter, pump, gearbox, heat exchanger and exhaust system) must be thoroughly flushed with fresh water. Either run the engine with the flushing connection fitted, or disconnect the coolant inlet hose from the seawater filter or bottom valve and connect it to the mains water supply using a garden hose.
Ensure that there is always a sufficient supply of water so that the seawater pump does not run dry. Now let the engine run for about 15 minutes until the thermostat has opened. Afterwards, the entire cooling system must be drained – instructions for this can be found in the owner’s manual. Caution: Drain cocks and plugs may be clogged with rust and dirt. For this reason, you must always ensure that the water actually drains out.
Following the flushing process, we recommend preserving the system with a mixture of water and radiator antifreeze, such as that used in cars, or a mixture of water and a special preservative oil.
These mixtures should be drawn in by the pump at idle speed (for example, from a bucket) and allowed to flow completely through the cooling system – a task that should be carried out as soon as possible after draining the system, so that the thermostat is still open and the fluid flows through the entire cooling system.
The mixture of oil and water is not frost-resistant and must therefore be drained off again afterwards. The mixture of water and glycol, on the other hand, should – to provide optimum protection – remain in the cooling circuit over the winter, provided it has a frost resistance of at least –25 °C.
Anyone driving a vehicle with a dual-circuit engine must also check the internal circuit. As in a car, this is filled with coolant. Use a frost protection tester to check whether the fluid provides sufficient frost protection. If this is not the case, some of the coolant must be drained and replaced with antifreeze. In this instance, the engine must then be run for a few more minutes (after which the frost protection should be checked again).
The cooling system check also includes inspecting the pump impeller. To do this, first unscrew the cover of the seawater pump. Pull out the impeller using the appropriate puller or a water pump pliers, rinse it with fresh water and check for damage.
It is best to wait until spring to refit the pump impeller – including a new seal – and to store it in a dry place until then. If the impeller pump is fitted to the Z-drive, we recommend having the inspection carried out by a specialist. The strainer in the seawater filter must also be removed and cleaned.
Air filter
In petrol marine engines, air filters consist of wire mesh as standard and are known as flame arresters. Modern diesel engines filter the air more finely – either using paper or foam filters. Always replace paper filters. Wire mesh and foam filters, on the other hand, should only be cleaned with white spirit and blown out with compressed air.
Ignition system
Before winter, the ignition cables, distributor and distributor cap should be wiped clean and dry, and then thoroughly sprayed inside and out with a contact spray that has water-repellent properties. The same applies to the plugs on modern, electronically controlled engines. Any necessary replacement of wear parts is postponed until spring.
Tips
Pistons and cylinders
To protect the pistons and cylinders from corrosion, remove the air filters and, whilst the engine is running, spray a protective oil such as WD-40 into the intake manifold or the carburettor. The engine should not be run again after this. It is therefore best to carry out this step at the same time as preserving the cooling system.
Cables
Apart from a visual inspection for kinks and chafing, and a leak test, the fuel lines require no further attention. Clamps and connections must be checked for a secure fit and signs of corrosion. If they are loose or wobbly, they must be retightened. Caution: do not overtighten when retightening! Corroded or overtightened parts are best replaced with stainless steel hose clamps.
outboard motor
With the right plan, you can carry out the winterisation work yourself quickly and easily
To check the engine for leaks and damage, it must be clean. The easiest way to clean it is with a cold cleaner. To do this, place a plastic tray under the outboard motor to catch the cleaning solution.
If you discover any leaks or defects, it is best to take the engine to a specialist workshop for repair. If this is not the case, the next step is to flush the cooling water system thoroughly with fresh water. This applies even if the engine has only been in contact with salt water briefly, or not at all.
If it does not have a separate rinse connection, or if you do not have a special rinse clamp, it should be placed in a container that is deep enough – for example, a rainwater butt or a builder’s bucket. Please note: Remove the propeller to avoid the risk of injury.
At the end of the flushing process, whilst the engine is still running, spray anti-corrosion oil into the intake manifold or carburettor. Then remove the boat engine from the water tank or disconnect the hose. If you want to look after your engine, let it run briefly in a mixture of water and antifreeze/corrosion inhibitor.
It is then turned by hand to force any remaining water or water-antifreeze/anti-corrosion mixture out of the water pump. For engines with an electric starter, pull the Quickstop lever to cut the ignition.
Next, the spark plugs are checked and – where fitted – the carburettors are drained and the fuel filters cleaned (mesh) or replaced (paper element). This is followed by cleaning and greasing the propeller shaft. Check fasteners such as split pins, crown nuts and shear pins for damage and replace them if necessary.
You should also check the sacrificial anodes on your outboard motor; their locations are specified in the user manual, which also shows the lubrication points on the engine and specifies which grease to use. Water-resistant grease should also be used to lubricate moving parts such as the steering, throttle, gearshift and engine mountings.
Next, spray the engine block, the electrical system and the engine shaft with anti-corrosion oil. We recommend changing the gearbox oil regardless of the number of operating hours. To do this, open the drain plug on the gearbox housing above the tail and the inspection plug near the cavitation plate, and drain the oil into a drip tray.
A white, milky colour indicates the presence of water in the oil; if the oil appears silvery, there is a mechanical fault. In both cases, a specialist must be called in. If everything is in order, replace the sealing ring on the drain plug and the dipstick, and squeeze the gearbox oil (tube) into the gearbox through the drain hole until it emerges from the top of the dipstick hole. Now insert the dipstick, then screw the drain plug back in.
If you own a newer Evinrude engine, all you need to know to prepare it for winter is the correct switch setting. The boat engine will then prepare itself for winter automatically.
Tip
Rust protection
First, wash the engine, gearbox or Z-drive from the outside using a cold cleaner, taking care to protect the bilge from the ‘wash water’ using a flat plastic tray and a cloth. The easiest way to dispose of the cloths and any collected liquids is at your local recycling centre or hazardous waste collection point. Next, touch up any paint damage with the original paint and spray all moving parts – the engine, levers and cable assemblies – with a corrosion inhibitor, such as WD-40 or Brunox Bootfit.
Z-drive
Fresh oil and grease ensure smooth operation
Once the boat is ashore, it’s time to work on the Z-drive. Drain the old oil (make sure you have a sufficiently large drip tray) and replace it with fresh oil. Pay attention to the colour of the drained oil: white, milky oil contains water, whilst silvery oil indicates mechanical damage to the Z-drive; in both cases, professional assistance is required.
The easiest way to top up the gearbox with new oil is through the drain plug. Using an oil syringe or a special pump, press the oil into the gearbox through this opening until – depending on the model – it comes out of the top inspection hole or reaches a level between the minimum and maximum marks on the dipstick. Then screw in the plug nuts fitted with new O-rings (top first, then bottom).
The grease nipples and moving parts of the steering system must be greased. The operating instructions specify where the grease points are located and which grease to use. Carefully check the bellows, water intake hose, electrical cables, power trim cylinders and hydraulic hoses for damage.
Finally, don’t forget to check the anodes on the drive and bracket (anodes that have been worn down are a good sign – they’re working!) and replace them if necessary.
Reference works
This article appeared in the 11/2016 issue of BOOTE and was revised by the editorial team in October 2025.