How often should I service my boat motor?

Many petrol outboards need servicing every 100 running hours or once a year, whichever comes first; marine diesels commonly need routine servicing every 100 to 250 hours or annually, depending on the model. Follow the maintenance schedule for your exact engine and drive: low hours do not cancel calendar-based servicing requirements, and electric motors have their own schedules.

Check engine details and history

Find the model, serial number and operating hours, then compare the correct owner's manual with the last service invoice. This tells you what is due. An engine oil change alone is not a complete service.

Model and year matter, even within one brand. The Yanmar YM-series schedule supplied here includes a 250-hour or annual maintenance interval, separate checks at 50 hours or monthly, and longer-term tasks. Do not apply that schedule automatically to an older Yanmar 4LHA or another diesel.

• Known history: calculate the next service from both the date and recorded hours.
• Unknown history: arrange a baseline service now, covering the engine and any gearbox, saildrive, sterndrive or outboard lower unit.
• New or rebuilt engine: check for an initial running-in service. Some engines require early oil and filter changes, so the normal annual interval may not yet apply.

Service the engine and drive

Four-stroke petrol outboards: a typical annual or 100-hour service covers engine oil, the oil filter where fitted, lower-unit gear oil, lubrication, fuel-system checks and inspection of plugs, anodes and cooling components. Replacement intervals for plugs, filters and the water-pump impeller vary by model.

Two-stroke outboards: there is no four-stroke-style sump oil change, but the gearbox, fuel system, ignition and cooling system still need maintenance. Check the oil-injection system where fitted and use the specified two-stroke oil.

Inboard diesels: expect engine oil and filter changes, fuel-filter servicing, water-separator checks, belt inspection and attention to the raw-water pump. Coolant, valve clearances and exhaust components have separate intervals. Some engines have internal cooling-system anodes; others do not.

Drives and gearboxes: follow their own schedules. Servicing the engine does not mean a sterndrive's bellows, universal joints or drive oil have been checked. Saildrive diaphragm replacement is a separate, model-specific requirement.

Use the specified oil grade, filter and tightening torque. Drain plugs and filter housings do not share a universal torque setting.