Engine
The engine is the power source of a motorboat. Engines are distinguished by their installation, design, and fuel. The choice of engine depends on the boat's size, intended use, and the regulations of the sailing area.
Outboard Engine
The outboard engine is mounted on the outside of the transom. Engine, gearbox, and propeller form a compact unit that is swung completely when steering.
Advantages:
- Easy to remove and service
- No shaft penetration through the hull
- Tilts up for running into shallow water
Outboard engines are used mainly on smaller sport boats and inflatables.
Inboard Engine
With an inboard engine the engine is fixed permanently inside the hull. Power is transmitted to the propeller via a drive shaft. Inboard engines are heavier but more powerful, longer-lived, and better placed for the boat's centre of gravity. They are used on larger cabin boats and yachts.
Sterndrive and Saildrive
The sterndrive (also Z-drive) connects an internal engine with a swivelling drive leg outside the transom. It combines the steering of an outboard with the power of an inboard.
The saildrive is a vertical drive leg passing through the hull, used mainly on motor-sailers.
Two-Stroke and Four-Stroke
- Two-stroke engines are light and simply built, but run on an oil-petrol mixture and emit more pollutants.
- Four-stroke engines burn pure petrol or diesel, run cleaner and quieter, and are the standard today.
Petrol and Diesel
Petrol engines are lighter and more responsive, but require an explosion-proof engine compartment with forced ventilation. Diesel engines are heavier and more expensive, but more economical, more reliable, and safer regarding fuel storage. They are installed in most larger boats.
Cooling
Boat engines are usually cooled with seawater. With direct cooling the surrounding water is pumped directly through the engine. With two-circuit cooling the surrounding water cools, via a heat exchanger, a closed coolant circuit, which extends the engine's life.
Maintenance
Regular maintenance significantly increases operational safety. Before every trip the oil level, cooling water exhaust, fuel level, and bilge should be checked. Seasonal maintenance includes oil changes, replacement of the impeller, inspection of the anodes, and cleaning of the fuel filter.