Buying a Boat Engine

Many boats, particularly small yachts and trailer sailers, are sold without an engine. While this makes sense – after all, it’s not totally necessary to have horsepower when you have wind power – an engine can be invaluable in an emergency, help with boat manouverability, and will certainly prove useful if the wind dies off halfway through a trip!

There’s more than one type of engine to choose from, however, and choosing the right one for your boat is important. Your vessel’s performance will be impacted once the engine’s weight and horsepower are taken into consideration. Choose an engine with a power rating that’s too low and your engine will be under continual strain. Conversely, choosing a very powerful engine can be dangerous, and may mean your boat exceeds the speed it was designed to operate at. There are two different types of engine most commonly purchased – outboard motors and inboard engines. Propulsion systems are also available, though less common.

Outboard Motors

Outboard motors are totally detachable, and the smaller models are often removed from boats while they’re not in use. They’re mounted on brackets on the stern, and typically range in power between 1.0 horsepower to over 300. Outboard motors are usually either 2-stroke or 4-stroke.

2-Stroke benefits

  • Simple to operate
  • Lighter
  • Quieter
  • More fuel efficient
  • Better acceleration on the whole
  • Good power-to-weight ratio
  • Lower price (on average)
  • No oil changes required.

4-Stroke benefits

  • Electronic ignition systems
  • Low emissions
  • Engine management systems
  • Oil is added to the engine (as oppose to the fuel, as in a 2-stroke) meaning fuel is used more efficiently and cleanly.

Inboard Engines and Stern Drives

Inboard engines are more complicated to use than outboard motors, as they require complicated installation best left to a professional. Typically the engine sits in the middle of the boat, with a shaft connecting the engine to the propeller on the outside. Waste gases are passed through the exhaust pipe at the boat’s stern.

Inboard engines are the preferred option for use in watersports such as wakeboarding (provided the engine is powerful enough) because the propeller usually isn’t right at the back of the boat posing a potential hazard to those trying to board.

Diesel Inboard Engines

Diesel inboard engines are similar in design to traditional petrol engines, and are typically only used in larger boats because of their weight. They’re capable of producing superior torque comparative to petrol engines, however, and tend to operate at lower RPMs.

Diesel inboard engine benefits

  • Don’t produce carbon monoxide in or around the boat
  • Fuel isn’t explosive
  • Provides excellent torque
  • Long-lasting
  • Cheap to run.

Petrol Inboard Engines

Petrol inboard engines share many similarities with car engines. However, they’re also modified in several notable ways. They tend to range in horsepower from about 90 to over 1000.

Petrol inboard engine benefits:

  • Lighter than diesel engines
  • A simple drive system often means less maintenance
  • Cheap to run
  • Quiet.

Propulsion Engines

Propulsion engines are the safest type of engine to use in watersports applications, as they have no propeller. The engine operates by pumping in water, then spraying it out under high pressure via a nozzle. The nozzle moves and rotates in order to provide steering.

These types of engines are most commonly seen in jet skis and personal watercraft, and are rare in other boat types. One drawback of a propulsion engine is that steering is instantly lost when power isn’t being applied.

Propulsion engine benefits

  • Great steering control in both forward and reverse (many engines, particularly inboard ones, are difficult to use in reverse).
  • Relatively low maintenance
  • Quiet
  • Safe
  • Efficient.

For courses on maintaining your inboard or outboard engine, view NZ Coastguard’s website pages on inboard engine maintenance and outboard engine maintenance.