Sterndrives are a popular form of propulsion in the powerboat market. While popular, they require a fair amount of maintenance, their care and maintenance can keep a DIY boater very busy.

The main factors to be aware of are salt water corrosion, lubrication and regular inspection of the bellows, the condition of which is vital to prevent water from entering into the hull.

Bellows inspection

There are three types of bellows – for the exhaust, drive shaft and gear cables – and they all need careful inspection. With the boat out of the water, these are the things to check and do:

• Some bellows can be partially covered up by protective caps or steering helmets and these will need to be removed to make a proper inspection.
• Check for signs of cracking in the rubber. If the rubber is becoming brittle it is time to replace the bellows.
• Look out for any marine growth and remove any barnacles, which are very sharp and can easily damage the rubber.
• Check the condition of the stainless steel clamps.
• Check inside the hull for any signs of water ingress.
• Check for any damage that may have been caused by fishing line wrapped around the propeller. This will entail removing the propeller.
• Replace bellows if there are any signs of damage.

Surveyor’s tips

• As a rough guide, the bellows on your sterndrives should be replaced every two years. Some manufacturers will state that their bellows will last for longer than this. However, I feel that it is best to err on the side of caution and replace them every two years. At a minimum, properly inspect the bellows yearly for any signs of damage or cracking.

Corrosion inspection

Sterndrives, as with saildrives, have aluminium housings and these remain in the water when the boat is afloat. This means that if they are used in salt water they must be protected from galvanic corrosion by sacrificial zinc anodes and well maintained paint layers. Here is what to do:

• Check the owner’s manual for the location of all anodes.
• Check the condition of the sacrificial anodes and replace them when they are 50% wasted. If unsure how much 50% is, buy spares so you can double-check.
• Remember never to paint anodes with antifouling as their surfaces must be in direct contact with water to be effective.
• Carefully check the condition of the painted surfaces of the sterndrive before applying any antifouling. If any repainting is necessary, this must be done first (see below: Painting a sterndrive).
• Make sure the whole sterndrive is free of any standard antifouling bottom paint that may be used for the rest of the hull below the waterline.
• Apply copper-free antifouling paint suitable for aluminium outdrives as recommended by your sterndrive manufacturer. These paints should contain copper thiocyanate rather than copper oxide.

Surveyor’s tips

• It is always a good idea to check the electrical continuity between the anode you have fitted and the items that you are trying to protect. This can be done with relative ease, by setting a multi-meter to its “continuity” setting, and touching one lead firmly to the anode, and the other lead firmly against the stern gear you are trying to protect. You may have to scratch away at the anode and stern gear to make sure that both leads are touching good metal.
• If you are switching a vessel from freshwater to saltwater (or vice versa), it is worth checking if the anodes you currently have fitted are correct for the type of water your vessel will be moving to. Magnesium anodes tend to perform better in fresh water, whereas zinc anodes tend to perform better in saltwater. You might have to ask local suppliers for the best option if your boat is moored in brackish water.

Painting a sterndrive

The key to all boat painting and varnishing work is to prepare the surfaces thoroughly. Anything less than this will result in wasted effort and frustration when the job has to be done again. Sterndrive surfaces are fiddly and uneven, so sanding back is best done by hand – a good supply of 80, 120 and 240 grit aluminium oxide wet and dry paper and a wire brush will be needed. These are the steps involved:

1. When the boat is hauled out, ask the yard to give the sterndrive a thorough pressure wash.
2. Before doing any sanding, make sure that all barnacles are removed. Use a plastic scraper for this if needed.
3. If the paintwork is in good condition and there is no sign of underlying corrosion, then wet sand using 120 grit paper to provide a key for new topcoat.
4. If there is any loose or flaking paint or any sign of bubbling, this must be removed. Use 80 grit wet and dry paper for this, removing all signs of aluminium oxide in the process. A wire brush may be needed on badly pitted surfaces.
5. Rinse thoroughly with fresh water – a high pressure hose is best. Wipe dry with an old towel.
6. Apply an etch primer. This paint is designed to bond itself to the substrate and contains acid so the use of rubber gloves and protective gear is essential.
7. Apply two coats of antifouling primer. Allow to cure and dry between coats according to the paint manufacturer’s instructions.
8. Lightly sand back any paint drips or imperfections using 240 or 400 grit paper.
9. Apply two coats of antifouling paint.

Lubrication and other checks

• Check the condition of the cabling.
• Check the hydraulic hoses for steering and trim, making sure they are not cracked and their connections are all in good condition.
• Check hydraulic system fluid levels and if necessary trace and repair any leaks.
• Check gearbox oil condition. If this is milky in colour, then this indicates a seal has failed, allowing water ingress. The shaft seals will need to be replaced.
• Check and grease all grease points off the drive as recommended in the sterndrive instruction manual.