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  • Writer's pictureDieter Peschkes

Winterising a Marine Engine

Updated: Nov 1, 2023

One of the biggest cause of hold ups on a boat delivery is due to engine problems, poor maintenance cost time and money. However with a bit of basic maintenance most problems can be prevented and save you money, here is OceanTrax Boat Delivery guide to winterising a marine engine.

When decommissioning your boat for the winter, it is important that you winterise your vessel to prevent problems before you launch in the spring. Winterisation will vary depending on the climate, type of vessel and the construction material. The basic areas to focus on are engine, rig, hull and interior. In this blog we will focus on the engine

FUEL FILTER Do this before topping up your tanks for the winter, if you find an issue and need to clean your tanks it will be easier with less fuel in and there will be less waste or contaminated fuel. Starting with the fuel pre-filter, a large clear plastic bottle cut in half is ideal for this job as you can drain the fuel and in some cases catch the filter in the bottle where you can examine the condition of the fuel. Inspect the filter for signs of heavy sediment or any signs of the diesel bug which will show here as a sludge or jelly like substance. If you find an issue you can make a decision on what action to take, such as tank removal and cleaning or a fuel polishing service.

To read our full guide on the diesel bug click here Once you have refitted filters you will need to bleed the system, some fuel filters may need to be filled with fuel from a can or if space is tight that clear plastic bottle cut down again, just make sure it is clean.

OIL FILTER Next, run the engine up to temperature, this will insure all air is cleared from the fuel system, you will be ready for an oil change, this so any corrosive acids formed as product of combustion do not remain in the engine over the winter. The cleanest and easiest way to do this is to use a vacuum pump. A vacuum pump has its own self contained reservoir and tube that is inserted into the dipstick. This makes the whole process very clean and helps prevents spills. Remember if there is a spill the oil will be hot.

Change the oil filter. Carbon, swarf and other combustion products build up in the filter and start to restrict the flow, so changing once a year is a minimum requirement but should be done as per manufacturer’s recommendations. This is a messy job and access can be tricky so have plenty of rags to hand. You will need a good filter wrench, one that doesn’t damage the filter, and to minimise mess use a plastic bag to put around the filter to catch the oil and filter. Old engine oil is full of carbon and is hard to clean, so use a barrier cream on your hands or have swarfega handy. Alternatively you could wear latex gloves but remember the filter will be very slippery. Replace with a new filter; make sure the rubber seal is correctly positioned on the new filter and a small amount of oil is smeared on the rubber seal to prevent it catching and becoming unseated when tightening up. Refill the sump with clean oil of the correct grade for your engine. Tip: for a long boat delivery it is always good to have spare filters onboard

FILL YOUR TANK This is to reduce condensation in the tank over the winter, during spring and Autumn. When the days are warm and the nights cold, condensation forms in tanks which provides the ideal conditions for the diesel bug to thrive, especially with the increased use of biodiese. OceanTrax has recently had several cases where boats have stood for sometime for sale and the diesel bug has taken hold requiring costly repairs. and can considerably delay a boat delivery. If you suspect signs of the bug, then this would be a good time to add a fuel additive to keep it under control. After filling, check the fuel filler cap rubber seal and use a light smear of Vaseline around the thread to give a better seal to stop any rain water over the winter entering the tank. This also makes it easy to open in the spring.

FRESH WATER SYSTEM Check the fresh water cooling level and top up if necessary. If you have topped up with just water all season, it is also important to ensure that you have the right ratio of water to Anti-freeze. Anti-freeze or coolant doesn’t just protect your engine from the cold but more importantly acts as a corrosion inhibitor that protects the internal water- ways within the engine. If the ratio is incorrect, you might like to drain the system and top up with the correct mixture. Manufactures will recommend that the system is drained at a particular service interval. Remove the salt water pump impeller and tie to the side of the pump. It is important that everybody who may start the engine knows this is removed, especially if a boat delivery crew are moving your vessel. If the impeller remains in one position for too long then it might become deformed, possibly crack and become less effective and fail. You should always change your impeller annually at least.

GENERAL MAINTENANCE Relax or remove the drive-belts. Leaving them in one position for several months may cause them to deform and crack. In the spring we strongly recommend that you replace them if there are any signs of wear. Signs to look out for are black belt dust on the engine and engine bearers – this means the belt is slipping. Clean the engine and inspect it from all angles looking for leaks in the fuel, water and oil systems, corrosion on electrical connections. Check all jubilee clips for corrosion and chaff on any hoses or wiring.

Also make sure the bilges are clean. Clean bilges make it easy to spot faults early. This is useful if a boat delivery crew will be moving your vessel as problems show up early can be dealt with quickly. Finally after cleaning the engine, coat it with duck oil or corrosion block to prevent any corrosion. Pay particular attention to the drive belt pulleys as these must be kept free of rust, or when the belts are replaced they will wear down quickly.

It is a good idea to block off the exhaust and if possible the air inlet to the engine. This can be done with tape or a wooden bung and prevents moisture from getting into the engine. Don’t forget to remove before you launch in the spring.

BATTERY TYPES There are essentially three types of marine batteries: AGM, gel cell, and lead-acid. By far the oldest, least expensive and most common type is lead-acid. In these batteries, lead plates are suspended in a solution of sulphuric acid, called an electrolyte. AGM and gel batteries each also have an electrolyte, but instead of being in a liquid form, it is retained as either a jelly (gel cell) or in an acid-saturated fiberglass mat (AGM). These batteries require less maintenance and there is no need to top up the electrolyte as with lead-acid batteries.

BATTERY MAINTENANCE Check levels of electrolyte when the battery is fully charged and topped up with distilled water if necessary. Ensure to take suitable care whenever inspecting battery levels. You may consider using a battery hydrometer to obtain an accurate reading of your battery’s health. This simple tool sucks up a small amount of electrolyte and measures the specific gravity of the fluid. This offers an extremely reliable reading of your battery’s health. Check terminal connections. Make sure posts are clean and free from any white powder. Any poor connections will result in poor charging and extra resistance in circuits.

Isolate your batteries when you leave the boat. (Tip if a boat delivery crew are picking up your vessel leave a note detailing the location of the isolator. It makes our lives so much easier, especially if we arrive in the dark!) By turning off the battery master switch, you’ll ensure that there are no parasitic loads depleting your batteries. Properly installed bilge pumps and carbon-monoxide alarms are wired directly to the battery, so turning off the isolator will not stop these working. Clean the tops of the batteries as part of routine servicing. A dirty battery can self-discharge to ground, given enough dirt and the right circumstances.

A lot of newer vessels are fitted with battery chargers which maintain your batteries throughout the winter months. However you must ensure there is plenty of ventilation to stop hydrogen gas build up. If your vessel does not have a fitted battery charger there are plenty of smart battery chargers available which run off a mains outlet. Finally if you know a boat delivery crew will be moving your vessel please leave a list of the preventative maintenance you have carried out and what needs to be re-commissioned before setting sail, it makes our lives a lot easier!


Owner & Manager of Oceantrax Yacht Deliveries


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