Engines

How to Buy a Boat Engine

Choosing the right type of propulsion system for your boat is very important. Its weight and horsepower will both have an impact on the performance of your vessel. If your vessel is underpowered, its engine will work hard continually and will provide poor performance. Additionally if your vessel is overpowered, it may exceed the safe operating speed that was designed for the vessel.

Outboard Motors – 2 stroke

The definition of an outboard motor is a detachable engine mounted on outboard brackets on the stern of your boat.  Today’s outboard motors range in horsepower from 1.0 HP to over 300 HP per engine.  Outboard motors can be used in all types of waters.
There are two general types of outboard motors; 2-stroke and 4-stroke motors.

2-Stroke Benefits

• Generally better acceleration out of the hole and at top end
• Overall, excellent power-to-weight ratio
• Basic models are simple by design
• Generally lower priced than a 4-stroke (carburated 2-Stroke only)
• No need to change the oil
• Generally weighs less than a 4-stroke
• Usually better fuel efficiency than carburated 2-Strokes (comparable to 4-stroke)
• Usually quieter than carburated 2-stroke (comparable to 4-stroke)

Outboard Motors – 4 stroke

One noticeable difference between 2-stroke and 4-stroke engines is weight of a similar horse powered engines. Four-strokes tend to be heavier than a 2-stroke engine of comparable horse power. Two-stroke engines use a petrol and oil mixture, while a 4-stroke burns petrol. A 4-stroke engine is more like a car engine – it includes an oil filter and generally has lower torque than 2-strokes.

4-stroke Benefits

• Low emissions
• Strong top end and good acceleration
• Excellent fuel economy
• Quiet operation
• Engine management systems
• Electronic ignition systems
• The oil is added to the engine rather than the fuel, allowing for clean and efficient fuel consumption

Diesel Inboard Engines

Diesel engines rely on compression to power the engine. Compared to a traditional petrol engine, the engines are similar by design, as they have crankshafts and cylinders and pistons; however, the fuel systems are completely different and more complex in a diesel engine.
Diesel engines range in size and horse power. Diesel engines are widely used in other parts of the world.  The main reason that diesel engines are not used in smaller boats is weight. The engine in general weighs more than a gas engine. However, they are used in larger vessels because of their ability to produce torque. In general diesel engines run at lower RPM’s than a traditional gas engines.

Benefits / Notes

• No carbon monoxide to worry about in cabins or on the back of boats
• Excellent torque
• Long life expectancy
• Low running costs
• Majority of the engines weigh more than a traditional gas engine
• Non-explosive fuel

Petrol Inboard Engines

While similar to what is under the bonnet of your car, gas engines used in marine applications are modified to make them marine engines. Petrol inboard engines range in horsepower from 90 HP to over 1000 HP per engine and are used in a variety of engines from tow sport boats to large cruisers.
In an inboard engine configuration, the engine sits amidships, with a drive running through the bottom of the boat to a propeller, and a separate rudder used for steering. A transmission is often used to transfer power from the engine to the propeller. Exhaust is passed through the stern of the boat. Inboards are also common for tow sports, such as waterskiing and wakeboarding, as they allow the propeller to be brought forward of the back of the boat, providing an area for platforms to help skiers enter and exit the boat.

Benefits/Notes

• Simple drive system can lower maintenance
• No means to trim propeller
• Low running costs
• Quite and out of the way

Jet Boat Engines and Propulsion

These propulsion systems have the advantage of having no propeller to cause potential danger to people in the water and marine life. They are usually inboard engines that take in water that flows through a pump powered by an impeller. The water is then discharged at high pressure through a nozzle that propels the boat forward. The nozzle swivels to provide steering to the boat. Most personal watercraft use jet drives.
Note: When power is not being applied, a jet driven vessel loses its steering because it is the stream of water that steers the boat. Keep hands, feet and hair away from the pump intake and do not operate in shallow water.

Stern Drives

A stern drive consists of an engine and drive connected to one another through the transom of a boat. A stern drive is sometimes called an inboard/outboard, reflecting its design. It is designed so that its engine is inside and enclosed by the boat, while the propulsion system (out drive) is outside of the boat and in the water.
The out drive can be trimmed – moved up or down – and has a propeller attached to the end of it to propel the boat forward and reverse. This is also the main steering system for the boat as it can turn from side to side. Because the engine is in the boat, rather than being supported by the transom as in an outboard, larger engines with greater horsepower can be used.
Stern drives can come in a variety of different functions depending on their intended use; drives are available with one or two propellers on a single stern drive. They can also be used in tandem, with two drives on a single boat, each with single or multiple propellers.

Benefits/Notes

• Allows for ease of control and steering in forward or reverse
• Can be used on petrol or diesel engine packages
• Relatively low maintenance
• Quiet and efficient
• Good value

Size Matters

When you’re choosing the appropriate power for your boat, consider the size and weight of the boat and remember that weight includes your passengers, fuel and gear.
A good rule of thumb is to come as close as possible to the maximum horsepower that your boat is rated for.
Both weight and horsepower will have an impact on the performance of your vessel. If your vessel is underpowered, your engine will work harder than it needs to, possibly leading to more routine maintenance.  The boat will also not perform to your expectations, which may leave you unhappy with the boat.  This may also lead to higher fuel consumption.
Additionally, if your vessel is overpowered, it may exceed the safe operating speed that was designed for the vessel. Manufacturers and dealers will all have a recommended power package for each and every boat that they manufacture and sell.