Outboard motors are found on all styles of boats – pontoons, flat skiffs, salt-water fishing vessels, river runners, and lake boats. They are also propelling boats of many different sizes. An outboard motor is a self-contained piece of equipment that consists of the engine, gearbox and propeller.

The outboard motor propels the boat and provides steering control so you can direct the boat to follow a desired path. Most are mounted on the boat’s transom, so the engine is easy to reach for maintenance and repair purposes. The outboard motor can be tilted out of the water for staying in trim (running angle), cruising shallow water or protecting the motor during trailer transport. The transom is the vertical section or cross section at the rear of the boat (stern), and is designed to hold one or more outboard motors.

The main types of outboard motors you’ll see on boats are the following:

• Two-stroke – The original and thus older outboard motors were two-stroke engines. Motion is produced by burning gasoline with oxygen in a combustion chamber or cylinder. The exhaust exits the outboard via an exhaust port in the cylinder. They are called two-stroke because there is a compression stroke to compress the fuel, followed by an explosion stroke to create power.

  The cylinders have sliding pistons that move back-and-forth in the cylinder. The piston rod turns a crankshaft which converts the reciprocating (back-and-forth motion) into a rotary motion. The crankshaft turns a long driveshaft that runs down the motor’s spine into a gearbox at the bottom. The gearbox then coverts the rotary vertical motion into a horizontal spinning motion which in turns powers the propeller.

  Two-stroke engines with carburetors are the older of the two-stroke engines. The carbureted two-stroke engine may not comply with local environmental regulations on emissions in many areas.

• Fuel Injection two-stroke –In the Direct Fuel Injection (DFI) model, fuel is injected directly into the cylinder. This is opposed to a carbureted system that requires air to carry the fuel into the cylinder. An Electronic Fuel Injection (EFI) model uses a computer or electronic control system to manage the flow of air and fuel. Neither DFI or EFI uses as much fuel as the regular two-stroke engine because of efficient fuel delivery. The very newest models have innovative technology, like Evinrude’s E-TEC engine. This outboard has an engine management module that injects the exact amount of fuel into the cylinder. The Engine Management Module generates its own electricity, has a computer-controlled fuel injection system, and has very low hydrocarbon emissions.

• Fuel Injection four-stroke – The four-stroke outboard motor works with one compression stroke followed by a return stroke and one exhaust stroke followed by a return stroke. The Direct Fuel Injection and the Electronic Fuel Injection systems work as just described. The four-stroke outboard motor uses valves instead of two ports that open and close. The design requires gear and chain mechanisms, making it a more complicated setup.

  The four-stroke motor with fuel injection is more fuel efficient and environmentally friendly than the two-stroke, though the newest fuel injection two-stroke innovations may make that statement outdated. Even the older DFI four-stroke engines put out few emissions, which is one reason they were developed. Compared to the older two-stroke engines, the four-stroke is quieter, heavier and larger.

• Sterndrive (inboard/outboard) – The sterndrive is a combination of an inboard engine and outboard motor. The sterndrive is also called an outdrive. The motor is under the transom, while the drive unit is under the swim deck. The motor is connected to a drive unit that is both propulsion and transmission, so when the steering wheel is turned, the entire drive unit turns. The sterndrive keeps the full swim platform unencumbered but the transom must be large enough to accommodate the motor so cockpit space is lost.

Of course, there are many components in an outboard motor besides the ones already mentioned. Here are a few of them:

• Flywheel – helps to maintain a steady engine speed by building up momentum as the engine accelerates

• Starter motor – engines are started electrically or via a pull cord attached to the flywheel

• Carburetor – combines fuel with air to make the explosive gasoline-fuel mixture; only older two-stroke engines use carburetors to manage the fuel-air mixture

• Spark plugs – ignites the fuel once in the carburetor

• Fuel pump – responsible for pumping the fuel to the carburetor

• Anti-cavitation plate - located over the propeller, it reduces the wearing of the propeller’s surface due to cavitation; cavitation is the process in which a moving propeller churns up air and engine exhaust in the water, forming bubbles that burst and can cause propeller deterioration

• Jet drive outboards – there is no lower unit below the hull so the boat can maneuver through shallow waters; it’s a standard outboard with a jet pump instead of a prop; outboard motors with 25 or more horsepower can be converted to jet drive by installing aftermarket jet pumps

The two-stroke engine has fewer moving parts compared to the four-stroke because it only takes two strokes to intake, compress the fuel-air mixture, ignite the mixture, combust, and exhaust and uses port holes instead of valves, gear and chain. A stroke refers to the piston’s full travel in the cylinder. In a two-stroke, the end of the combustion process and the beginning of compression strokes happen at the same time. The same is true for intake and exhaust. The two-stroke motor is a favorite of fishing men and women because its lighter and can move a smaller boat quickly. However, it is also noisy and definitely not environmentally-friendly, unless you have a state-of-the-art DFI or EFI two-stroke.

The four-stroke motor needs four strokes to complete intake, compression, ignition, combustion, and exhaust. It’s quieter than a two-stroke outboard and more fuel efficient.

It’s a little difficult to compare the two-stroke and four-stroke engines today because of advances in technology. The newest four-stroke engines can accelerate as fast as the lighter two-stroke units. Technology has radically improved outboard motors in both categories. For example, Honda Marine developed Variable Valve Timing and Lift Electronic Control (VTEC) technology that offers a broader, flatter torque curve and smooth power delivery to improve power, torque, and fuel efficiency.

Need Outboard Motor Repair?

If you need repairs, you can search for an outboard motor mechanic near you. Mariner Exchange lists repair companies in most major port cities in the United States.