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Essential saildrive checks for your boat

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Originally published May 17, 2021

Theo Stocker - Yachting Monthly

Maintaining a saildrive is just as important as caring for the engine itself. Dennison Berwick explains how to give your stern gear some much-needed attention

Essential saildrive checks for your boat

The season draws to an end, and whether laying up for winter or not, thoughts turn to the annual engine service and getting the boat ready for a period of reduced activity, writes Dennison Berwick.

For most of us this will include an engine service, whether you do it yourself or pay someone to do it for you, changing the oil, impeller and filters is all part of the annual cycle.

But how much attention do you really pay to your stern gear?

This all-important connection between your engine and the world outside of the hull is crucial, not only for ensuring your engine can provide smooth, reliable thrust for when motoring, but it also provides a critical watertight seal for one of the largest holes in the hull.

Look for manufacture date marks to give the age of the saildrive diaphragm

Look for manufacture date marks to give the age of the saildrive diaphragm

Saildrives and shaft drives vary slightly as to the critical moving parts that can fail, but you certainly wouldn’t want your prop shaft sliding out or a saildrive diaphragm giving up the ghost.

These are the cruder mechanical and easily visible areas to check, but more important is that the inner workings of the gear box and the drive shaft’s lubrication are all working smoothly with oil or grease where it should be, and no water where it shouldn’t be.


Regular maintenance is especially important with a saildrive as they are less forgiving than in-line transmissions for any lack of attention.

Best practice is to check the saildrive fluid level before every engine start-up, along with the engine oil.

This is more important in saildrives than regular gearboxes because water can leak unnoticed into the lower unit and water, especially salt water, can seriously damage the lower unit through corrosion and by preventing lubrication – presence of water means the absence of oil.

Check your saildrive fluid level and viscosity on a regular basis

Check your saildrive fluid level and viscosity on a regular basis

If seals fail, corrosive raw water, (salt or fresh) can enter under pressure into the lower unit, mix with the gear oil and cause extensive damage.

Seals can only be checked with the vessel out of the water; so while afloat the saildrive’s oil dipstick should be checked regularly for signs of emulsification – water mixed with oil looks ‘milky’ or ‘like mayonnaise’.

The gear oil should be changed once a year or according to the manufacturer’s recommendation.

Seal of approval

If the vessel is out of the water, check for fine fishing line or netting caught in the gap between the propeller and saildrive anode or housing.

Line can damage the oil seals and rubber hub fitted in many saildrive propellers.

A man checking a gaiter on the hull of a yacht

Make sure the gaiter is firmly adhered to the hull

The diaphragm sealing membrane between the hull and the saildrive leg, and prevents water coming into the boat, should last 5-10 years depending on servicing.

Replacing it is typically a dealer-only procedure.

Corrosion and anodes

Saildrives can be quickly eaten away by corrosion because their aluminium casing is galvanically very active, much more active than a bronze through-hull or an exposed steel keel.

In addition, the aluminium housing of the lower unit has only a slightly lower electrical potential than materials typically used as anodes (including aluminium) so the margin of protection is small.

Replace the anode with the right size and shape for your saildrive so it bonds properly

Replace the anode with the right size and shape for your saildrive so it bonds properly

Any compromise in protection is likely to allow corrosion. Use the correct anodes for the vessel’s location – magnesium only in fresh water, zinc in salt or brackish water and aluminium in all types.

Magnesium will be consumed too quickly in salt water; zinc is too inactive in fresh water.

Don’t mix anode metals or assume that anodes installed by the previous owner or dealer are correct for the current location.

A saildrive may have three anodes – on the upper unit inside the boat, the lower unit/leg and on the propeller cone.

Replace each when they are 50% consumed (or likely to be during the season).

Saildrive anodes are sized to protect only the saildrive and the original propeller; installing a feathering (bronze) prop with greater surface area can increase the cathodic load on the anode –so protection will need to be increased or the anode will likely need to be changed more often.


Paint coverage is part of a saildrive’s anti-corrosion regime.

Any scratches increase the area of metal to be protected by the anodes, which will be consumed faster.

Stern gear being checked over on a boat

Check the paint for scratches as any exposed metal will corrode quickly

Best practice is to touch-up any damage or scratches immediately.

Use two-part epoxy sealer/paint if the original manufacturer’s paint is not available.

Two-part underwater epoxy paint is available so a boat does not need to be hauled for touch-ups.

Use only an antifoul paint that has been formulated specifically for aluminum outboards, saildrives or sterndrives.

Avoid paint with (cuprous) copper oxide as this will promote galvanic action.

It’s also worth giving the gaiter a quick glance, as this protects the top of the saildrive leg and the diaphragm from damage and helps create a smoother flow.

If repair is necessary, either reglue or replace.

How the pros service a sail drive

We went along to Berthon boatyard in Lymington to watch how the professionals do a full saildrive service

A man servicing a saildrive
1. Loosen the locking screw and then remove the propeller cone and bushings.

Slide the propeller off and remove the drive rings and bushing behind it, making sure to clean out any mess behind.

Use a rag to remove all the old oil so you just have clean metal.

Check raw water intakes while servicing a saildrive

2. Check raw water intakes are free of fouling as these help cool the leg.

Carefully remove any debris stuck in the holes.

Use a screwdriver to remove the saildrive drain plug

3. It’s now time to drain the oil from the leg.

Inside the boat, remove the sail drive dipstick to prevent suction, then unscrew the drain plug.

If this has been painted over, it may require some force – a screwdriver (above) on which you can use a spanner will help give you that leverage if needed.

An engineer taking an oil sample
4. Boatyards like Berthon will take oil samples to track any trends that may indicate wear or developing problems – DIY sampling kits are available.

A saildrive will contain roughly two litres of oil, so have a bucket ready to collect it.

Change o rings, drainpug and washers on a saildrive during a service

5. Having a system that is completely protected from water ingress is key, so change the drain plug, sealing O-rings and washers, then pressure test the oil system if the model allows.

top up oil levels during a saildrive service

6. Filling the leg up from the bottom is preferable as it will drive air bubbles out of the system, helping you put enough oil in to prevent cavitation.

Use a pressured container, such as a weed sprayer, connected to a hose and glue a tapered nozzle (such as one from a sealant tube) to the end.

Have someone inside to keep checking the oil level and top up the last bit of oil from the top.

You’ll lose a little oil as you replace the drain plug, so top up the last little bit from above with a large syringe.

Make sure you oil your stern gear during a service

7. When it comes to reassembling, make sure the anodes are checked for condition and replaced if less than 50% wear for a normal season’s use.

If in doubt, a new anode is worth the peace of mind.

You can then grease the shaft and splines.

Morse cable attachment on stern gear on a boat

8. Check the morse cable attachments and that the gear selection and neutral button are greased and all work correctly.

It’s a simple check to do, but a broken linkage or incorrect alignment can cause serious problems out on the water.

Tightening a boat's propeller

9. With the new bushings in place, replace and tighten the propeller and cone, using a drop of loctite on any locking screws to guard against vibration.

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