It is a well-known fact that boat engines do not break down, they break down - corrosion and sub-zero temperatures cause them the most trouble during the winter break. This makes it all the more important to preserve them properly for the cold days - this applies to both diesel and petrol engines.
There are now two ways to prepare engines for "hibernation": You can either do the work yourself or have it carried out by a specialist company.
As a rule, we recommend the second option; at least within the warranty period, it is advisable to commission an authorised workshop with the winter preservation. If you want to winterise your engine yourself, follow the procedure described on the following pages.
Refer to the motor manufacturer's operating instructions for winterising
In addition, you should always refer to the engine manufacturer's operating instructions, which once again describe all the necessary work. The manufacturer Mercury even offers step-by-step instructions using a scannable QR code on the engine cover and a video platform on the topic.
If you do not have the operating instructions for your own engine, manuals from the aftermarket can often help; however, depending on the type of engine, they may only be available in English in some cases. It is best to start the conservation work on the engine when the boat is still in the water.
Inboard
Whether diesel or petrol engine with or without dual-circuit cooling - the steps are similar
Oil change
The engine oil, oil filter and the oil in the reversing gear must be changed. This requires a suction pump (permanently attached to the engine on larger diesel engines) with a suitable probe and filter tensioner. Warm up the engine beforehand so that the oil is thin and can be pumped more easily into a discarded oil or reserve canister.
At the same time, loosen and unscrew the oil filter. To prevent spillages, have a small bowl and cleaning cloth ready. Before fitting the new filter, check that the rubber seal is correctly seated and apply oil to the contact surface with your finger. Important: The oil filter may only be screwed on hand-tight (25 Nm), otherwise it may not come loose again. Refer to the operating instructions to find out which oil may be filled into the engine.
Fuel systems
The first step is to empty the water separator. To do this, hold a small bowl under the separator and open the drain plugs until no more water comes out. Then, to be on the safe side, replace all filters and filter inserts. Diesel engines must then be bled; the operating manual explains how to do this. Then run the engine (petrol and diesel) and check that everything is tight again.
Corrosion in the fuel systems of diesel and petrol engines is protected against with so-called fuel additives, which also dissolve deposits and bind moisture in the tank. Everything together is then burnt normally. These additives are added to the fuel tank before the winter work begins and the engine is left running.
The additives do their preservative work. There are special additives just for diesel or petrol engines, but there are also others that are suitable for all engines. They are available from boat accessory shops and specialised workshops.
V-belt
The V-belts should be checked at least once or twice a year - winterising is the best time to do this so that you don't forget to check them in the spring. To check, press on the centre of the V-belt; the belt should not give way by more than about 10 mm, otherwise it will have to be retightened.
If the belt is frayed or damaged in any other way, it must be replaced, because if the V-belt is defective, the battery will not be charged via the alternator, the seawater pump for the cooling system will also fail, or the power steering pump will not work.
Cooling system
Winterising work on the cooling system is best carried out when the boat is already on land, as otherwise water can easily penetrate the boat. Even today, most built-in petrol engines only have one cooling circuit: seawater cooling, also known as single-circuit cooling.
This circuit (sea water filter, pump, gearbox, heat exchanger and exhaust system) must be thoroughly rinsed with fresh water. Either leave the engine running with the flushing connection, or disconnect the cooling water intake hose from the sea filter or bottom valve and connect it to the tap water (domestic water supply) using a garden hose.
Ensure that there is always enough water running so that the seawater pump does not run dry. Now allow the engine to warm up for about 15 minutes until the thermostat has opened. The entire cooling system must then be drained - the instructions can again be found in the operating manual. Caution: Drain taps and plugs can be contaminated with rust and dirt. For this reason, care must always be taken to ensure that the water drains away.
After rinsing, we recommend preserving with a mixture of water and radiator antifreeze, as used in cars, or a mixture of water and a special preservative oil.
These mixtures are sucked in by the pump at idling speed (for example from a bucket) and run completely through the cooling system - a job that should be done as quickly as possible after draining so that the thermostat is still open and the liquid flows through the entire cooling system.
The mixture of oil and water is not frost-proof and must therefore be drained again afterwards. The mixture of water and glycol, on the other hand, should spend the winter in the cooling circuit to provide optimum protection, provided it is frost-proof to at least -25 °C.
If you have a dual-circuit cooled engine, you must also check the internal circuit. This is filled with coolant, just like in a car. A frost protection spindle is used 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 case, the engine must run for a few more minutes (then check the antifreeze again).
The cooling system check also includes checking the impeller. To do this, first unscrew the cover of the seawater pump. Pull out the impeller using the appropriate extraction tool or water pump pliers, rinse with fresh water and check for damage.
It is best to reinstall the impeller, including the new seal, in the spring and store it in a dry place until then. If the impeller pump is in the Z-drive, we recommend having the inspection carried out by a specialist. The strainer in the sea water filter must also be removed and cleaned.
Air filter
Air filters in petrol marine engines consist of wire screens as standard and are called flame dampers. Modern diesel engines filter the air more finely - using either paper or foam filters. Always replace paper filters. Wire screen and foam filters, on the other hand, are only cleaned with petroleum ether and blown out with compressed air.
Ignition system
In winter, the ignition cables, distributor and distributor cap are wiped clean and dry and then thoroughly sprayed inside and out with a contact spray with water-displacing properties. The latter also applies to the plugs of modern, electronically controlled engines. Any necessary replacement of wearing parts is postponed until spring.
Tips
Pistons and cylinders
Pistons and cylinders can be protected against corrosion by removing the air filters and spraying a protective oil such as WD-40 m into the intake manifold or carburettor while the engine is running. The engine should then stop running. This step is therefore best carried out 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 do not require any further treatment. Clamps and connections are checked for tightness and corrosion. If they are loose or loosened, 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, winterising work is quick and easy to do yourself
To check the engine for leaks and damage, it must be clean. The easiest way to clean it is with cold cleaner. To do this, place a plastic tub under the outboard motor to collect the washing liquid.
If you notice 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 also applies if the engine has only been in contact with salt water for a short time or not at all.
If it does not have a separate flushing connection or you do not have a special flushing clamp, it is placed in a sufficiently deep container - such as a rain barrel or masonry bucket. Attention: Remove the propeller due to the risk of injury.
At the end of the flushing process, spray anti-corrosion oil into the intake duct or carburettor while the engine is running. Then remove the engine from the water tank or disconnect the hose. If you want to do something good for your engine, let it run briefly in a mixture of water and antifreeze/corrosion protection.
It is then turned by hand to press the last of the water or water/frost/anti-corrosion mixture out of the water pump. For engines with electric starters, pull the quick stop so that the ignition is interrupted.
The spark plugs are then checked and - if present - the carburettors are drained and the fuel filters cleaned (screen) or replaced (paper insert). The propeller shaft is then cleaned and greased. Fastening parts such as the split pin, castle nut and shear pin are checked for damage and replaced if necessary.
Also check the protective anodes on the outboard motor; you can find out where they are located in the user manual, which also shows the lubrication points on the motor and tells you which grease to use. Waterproof grease is also used to lubricate moving parts such as the steering, throttle, gears and engine mountings.
Then spray the motor block, the electrical system and the motor shaft with anti-corrosion oil. We recommend changing the gearbox oil regardless of the operating hours. To do this, open the drain plug on the gearbox housing above the spur and the inspection screw in the area of the cavitation plate and drain the oil into a drip tray.
A white, milky colour indicates water in the oil, while silvery oil indicates a mechanical defect. In both cases, a specialist must be called in. If everything is in order, replace the sealing ring on the drain plug and inspection screw and press the transmission oil (tube) through the drain hole into the transmission until it emerges from the top of the inspection hole. Now insert the inspection screw, then screw the drain shut.
If you have a newer Evinrude engine, you only need to know the correct switch position for winterising. The engine will then winterise itself automatically.
Tip
Rust protection
First wash the outside of the engine, gearbox or sterndrive with cold cleaner, protecting the bilge from the "washing liquid" with a flat plastic tray and cloth. The easiest way to dispose of rags and collected liquids is at the local recycling centre or pollutant bus. Then repair any damage to the paintwork with the original paint and spray all moving parts, engine, levers and cables with corrosion protection, such as WD-40 or Brunox Bootfit.
Z-drive
Fresh oil and grease ensure smooth running
Once the boat is on land, the Z-drive is started. Drain the old oil (remember to use 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, silver-coloured oil indicates mechanical damage to the sterndrive; professional help is required in both cases.
The easiest way to fill in the new oil is through the drain hole. Using an oil tube or special pump, press the oil into the gearbox until - depending on the model - it emerges from the top of the inspection opening or is between the minimum and maximum markings on the dipstick. Then screw in the screw plugs fitted with new sealing rings (first at the top, then at the bottom).
Grease nipples and moving parts of the steering system must be lubricated. The operating instructions indicate where the lubrication points are located and which grease should be used. Carefully check the bellows, water intake hose, electric cable, power trim cylinder and hydraulic hoses for damage.
Finally, don't forget to check the anodes on the drive and bracket (seized anodes are a good sign - they are working!) and replace them if necessary.
Literature for reference
This article appeared in BOOTE issue 11/2016 and was revised by the editorial team in October 2023.
