A saildrive combines engine and drive in a single leg that passes through a large opening in the hull. The rubber diaphragm sealing that opening is one of the most critical seals on the boat.
Critical points
- Diaphragm
- The rubber seal between hull and leg. Makers recommend replacement at set intervals in years; the date should be in the service record.
- Leg casing
- Aluminium; corrosion moves fast where the paint is damaged. The wrong (copper-based) antifouling can also cause corrosion.
- Anode
- The saildrive has its own anode; once spent, corrosion goes straight to the leg.
- Gear oil
- Milky oil means water in the leg; the source is usually the shaft seals.
Service scope
- Oil check and change. Colour and any water noted.
- Shaft seals. Renewed if water is getting in.
- Anode. Renewed; connection checked.
- Paint. Damaged spots repaired with a system suited to aluminium.
- Diaphragm. If due, renewed with the engine lifted; the largest job, and one to plan.
What we often see
- Spent anodes and low protection: Saildrive and shaft anodes largely wasted; protection measured with the reference electrode is inadequate.
Why a saildrive is sensitive
A saildrive leg is an aluminium part in direct contact with seawater and one of the most valuable metal parts below the waterline. Aluminium wastes fast when it meets other metals or stray current. So anode, paint and the electrical system directly decide a saildrive's life. A well-maintained saildrive runs for many years; a neglected leg can mean an expensive replacement.
Warning signs
- Milky gear oil or a rising level.
- Lifting paint, white powdering or pitting on the leg.
- An anode wasting very fast, or not at all.
- Damp or water around the diaphragm inside the boat.
Two saildrives on catamarans
A catamaran has one saildrive in each hull and they are assessed separately. Their service histories, diaphragm dates and anodes often differ. On the sea trial each engine is tried under load on its own; if one behaves differently from the other, the difference appears in the report.
Why records matter
Diaphragm replacement date, oil changes and anode renewals should be in the service record. On a saildrive without records, the diaphragm's age is unknown; the survey report then notes it as "not verified" and recommends replacement as a budget item.
At lay-up
When the boat is hauled for winter, the saildrive's gear oil is changed and checked for water, the anode renewed and damaged paint repaired with a system suited to aluminium. Doing this each winter keeps the leg in good condition and turns a large job into small routine ones.
Folding propeller
Many saildrives carry a folding or feathering propeller. Its opening and closing, play in the gears and blade damage are checked ashore; on the sea trial the propeller is seen to open properly in astern. A propeller that opens slowly in astern can make close-quarters manoeuvring difficult.
Every saildrive finding is read in the report together with the boat's cathodic protection readings, because the leg's condition can often only be explained alongside the boat's electrical system and anodes. A spent anode and a stray-current reading together tell a different story from either one alone.
A regularly serviced saildrive is a dependable drive for many years, and its service record adds to the boat's value at sale.
Neglect is what makes it expensive.