Outboard Boat Engines: How Do They Work?

An outboard boat engine is a propulsion system for boats that is commonly used in the marine industry. It is designed to be installed outside the boat on the transom, which leaves more room in the interior of the boat. They are the most common motorised method of propelling small watercraft. Outboard motors are commonly used for fishing, recreation, and light commercial inshore boats. They are also used to power small craft such as jon boats, dinghies, and canoes.

Outboard vs. Inboard

Outboard and inboard engines are two of the three types of engines used on powered boats. The third type is the stern drive engine, which combines features of both outboard and inboard engines.

Outboard Engines

Outboard engines are mounted on the transom of the boat, outside of the boat's hull. They are used to both power and steer the boat. Outboards are self-contained units that include an engine, gearbox, and propeller or jet drive. They are the most common motorised method of propelling small watercraft.

Outboards are available from tiny 2-horsepower kickers to 600 horsepower, but for family boating, the range is typically 90 to 300 horsepower. They are generally cheaper, lighter, and more efficient than inboards. They are also easier to service, as the entire engine is outside the boat. Outboards can be easily removed for storage or repairs, and they are self-draining. However, they are vulnerable to theft, a risk rarely suffered by inboard engines.

Inboard Engines

Inboard engines are located inside the boat's hull. They are four-stroke automotive engines that have been modified for use on the water. These engines power a drive shaft that is connected to a propeller. Inboard engines do not steer the boat; instead, they have a rudder or rudders located behind the propeller and controlled by a steering wheel.

Inboard engines are almost always diesel, which offers ruggedness, reliability, and fuel economy. They are also less prone to corrosion-related issues in salt water, as modern engines have improved corrosion resistance and can be equipped with a closed cooling system.

When deciding between an outboard or inboard engine, there are several factors to consider:

• Power: Outboard engines now offer big horsepower, but this comes at a high cost. Inboard engines generally offer higher horsepower, and stern drives (which combine features of both inboard and outboard engines) are available from 200 to 430 horsepower.
• Maintenance: Outboard engines are typically easier to service and repair than inboard engines, as they are located outside the boat and do not require working in cramped spaces.
• Fuel Efficiency: Inboard engines tend to be more fuel-efficient than outboard engines, as they are based on automotive engines.
• Space: Outboard engines take up space at the back of the boat, which may be desired for seating or storage. Inboard engines, on the other hand, are located inside the hull, providing more room for activities like wakesurfing.
• Cost: Inboard engines may offer better value for money in terms of performance, as they weigh less and have a more efficient weight distribution.

Ultimately, the decision between an outboard or inboard engine depends on the specific needs and applications of the boat owner.

Types of Outboard Motor

Outboard motors are a popular choice for small boats as they are light, powerful, and modern outboards are extremely quiet. They are self-contained propulsion systems that include an engine, gearbox, and propeller, and are affixed to the outside of the boat's transom. Outboards are used to both power and steer the boat. They come in a variety of sizes, horsepower, and power sources, and different types are suitable for different uses.

The first type of outboard motor is the electric outboard motor. These motors are self-contained propulsion units for boats, with most having 0.5 to 4-kilowatt direct-current (DC) electric motors, operated at 12 to 60 volts DC. Electric outboard motors are advantageous as they do not use permanent magnets, resulting in less maintenance and increased torque at lower propeller speeds.

The second type is the gasoline-and-oil mixture two-stroke engine. These were historically the most successful type of outboard motor due to their inherent simplicity, reliability, low cost, and light weight. However, they produce increased pollution and louder noise due to the high volume of unburned gasoline and oil in their exhaust.

The third type is the gasoline-only four-stroke engine. These engines offer lower pollution, noise reduction, increased fuel economy, and increased torque at low engine speeds. They have been sold since the late 1920s and continue to be developed by companies such as Honda Marine Group, Mercury Marine, and Yamaha Marine.

The fourth type is the diesel engine. These tend to be large, heavy items suitable for workboats and very large RIBs. Diesel outboards are rarely found on leisure craft.

Finally, there are propane outboard motors, which offer lower emissions and do not require a choke once the system is pressurized.

Pros and Cons of Outboard Motors

An outboard engine is a propulsion system for boats that is affixed to the outside of the transom. They are the most common motorised method of propelling small watercraft and are used to both power and steer the boat.

Pros

• Ease of maintenance and accessibility: Outboards can be removed from the boat in less than an hour and taken to any repair shop. Inboards, on the other hand, are in very small, cramped compartments that are difficult to work in.
• Improved manoeuvrability: Outboards are easier to steer and more responsive than inboards. Inboards have a larger learning curve when it comes to docking, reversing, and turning.
• Quietness: Outboards are generally quieter than inboards because they are located outside and at the stern of the boat. Inboards, being inside the hull, can produce considerable vibration and noise.
• Weight: Outboards are much lighter than inboards, which can improve performance and fuel economy.
• Tilt: Outboards can be tilted out of the water, which enables navigation through shallow waters and reduces exposure to corrosion and marine growth.
• Space: With the engine outside of the boat, there is more space for rear seating and storage below deck.
• Draft: Outboards can be trimmed or lifted out of the water, whereas inboards often need more water to operate or they will run aground.
• Fuel efficiency: Modern four-stroke outboards are more fuel-efficient than sterndrives.

Cons

• Power: Outboards are usually less powerful than inboards. However, you can have multiple outboards on your boat to increase power.
• Theft: Outboards are vulnerable to theft, whereas inboards are not.
• Weight distribution: With an outboard, a significant weight is at the aft end of the boat, which can affect performance and handling.
• Space (again): While an outboard gives you more space at the rear of the boat, you lose space at the back with an outboard motor attached.

How Outboard Motors Work

Outboard motors are a common propulsion system for boats, typically mounted on the outside of the transom at the back of the boat. They are usually powered by gasoline, although diesel and electric versions are also available. They are the most common method of propelling small watercraft.

The main difference between an outboard motor and a car engine is that an outboard motor usually has fewer cylinders, operating in either a two-stage or four-stage cycle. Instead of driving a gearbox, the motor powers a propeller. To steer a boat with an outboard motor, you simply tilt the whole motor casing so the propeller pushes the water away from it at an angle.

The top of the engine contains the outboard powerhead, which is made up of the engine block, cylinder heads, pistons and valves that make the engine run. The powerhead is connected to the midsection, which acts as a connector between the engine and the lower unit, housing the driveshaft and providing a channel for cooling water to the powerhead. The lower unit is submerged under water and contains the shift mechanisms, driveshaft, prop-shaft, gearset and other components crucial for propelling and steering the boat.

The primary components of an outboard motor include the powerhead, the midsection and the lower unit. The powerhead contains the internal combustion engine, which converts fuel into mechanical energy. This energy is transferred to the midsection, where it drives a vertical driveshaft connected to the lower unit. The lower unit houses the gearbox and the propeller. When the engine runs, it turns the driveshaft, which in turn rotates the propeller via the gearbox, propelling the boat forward or backward.

Outboard motors come in two versions: 2-stroke and 4-stroke. A 2-stroke engine is usually found in a low-power outboard, while a 4-stroke design is more common in larger motors. 4-stroke engines are more fuel-efficient, produce less exhaust, and generally require less maintenance than 2-stroke motors.

Outboard Motor Maintenance

An outboard engine is a propulsion system for boats, consisting of a self-contained unit that includes an engine, gearbox, and propeller or jet drive. They are affixed to the outside of the boat's transom and are the most common method of propelling small watercraft.

After Each Outing:

• Flush out the engine, even after trips in freshwater. Use "earmuffs" (flushing attachments) and a garden hose to flush the engine, then start it up and let the water pump do its job.
• Check the water pump's output by putting your finger through the stream of water – it should be warm, not hot. If the output is weak, shut down the engine and check for debris in the outflow tube.
• Disconnect the fuel line if storing for an extended period, and let the engine burn off the remaining fuel in the carburetor.
• Turn off the key and battery switch.
• Remove the engine cowling and check for fuel or water leaks. Consult a mechanic if leaks are found.
• Wipe down and spray all accessible mechanical components with an anti-corrosive like WD-40. Lubricate moving parts such as the shift, throttle cables, and carburetor valves.
• Replace the cowling, wipe it down, and cover the engine with a canvas or plastic cover for storage.

Regular Maintenance:

• Replace spark plugs and plug wires as needed. Check for issues by unscrewing and testing with a spark tester.
• Regularly check for water in the fuel, as this can damage components like spark plugs.
• Check the fuel line fittings to ensure they are properly seated and not leaking.
• Replace the water pump impeller every two years (or annually if used in saltwater).
• Always use fresh fuel, and drain tanks and lines at the end of the boating season.
• Periodically check the fuel line for cracks and wear.
• Check the fuel primer bulb for cracks and pliability.
• Inspect clamps on the fuel line for rust or corrosion.
• Examine the fuel tanks for damage and corrosion.
• Ensure the tank vent is functioning properly and aspirating.

By following these maintenance procedures, you can help ensure safe boating and extend the life of your outboard motor.

Frequently asked questions