Parts of an Outboard Motor and How It Works

Parts of an Outboard Motor and How It Works

A typical gasoline outboard motor has three main sections: the powerhead (the engine under the cowling), the midsection (the driveshaft housing and the brackets that mount the motor on the transom) and the lower unit (the gearcase and propeller). The engine turns a vertical crankshaft, a driveshaft carries that rotation down through the midsection, and gears in the lower unit turn it 90 degrees to spin the propeller.

This overview describes typical gasoline-outboard components; cooling and gear-selection hardware vary by model. Honda’s BF2.3 uses forced-air cooling with no water pump, and a centrifugal clutch rather than a conventional forward/neutral/reverse shift. Check Honda’s BF2.3 component details and your exact model’s manual before identifying service parts.

Outboard motor parts at a glance

Section	Main parts	What they do
Powerhead	Cowling, engine block, cylinder head, pistons, crankshaft, flywheel, fuel system (carburetor or fuel injection), ignition, recoil starter or electric starter	Make the power
Midsection	Driveshaft and housing, exhaust passage, cooling-water tube where fitted, clamp bracket, swivel bracket, tilt lock, trim pin, steering arm or tiller	Carry the power down, mount the motor and let it steer, tilt and trim
Lower unit	Gearcase, forward/neutral/reverse gears where fitted, propeller shaft, water pump impeller and water intake on water-cooled models, anti-ventilation (cavitation) plate, skeg, anodes	Turn the drive 90 degrees, drive the propeller and pick up cooling water
Controls	Tiller handle with twist-grip throttle, shift lever and kill switch, or a remote control with cables and a steering system	Operate the motor

Powerhead

The powerhead is the engine itself, under the removable cowling. A four-stroke outboard works much like a car engine laid on its side, with valves, a camshaft and an oil sump, but its crankshaft stands vertically. Fuel is delivered by a carburetor or by electronic fuel injection: Suzuki describes its DF15A, DF20A, DF25A and DF30A as using battery-less electronic fuel injection, which runs without a battery.

The flywheel sits on top of the crankshaft. Magnets in the flywheel pass a stator (charging coil) to power the ignition and, on many motors, to charge a battery and run electronics. Honda’s descriptions of the BF15 and BF20, for example, point to their charging system for running devices while trolling. Starting is by a pull (recoil) starter, an electric starter, or both.

Midsection

The midsection connects the powerhead to the lower unit. Inside it are the driveshaft, the exhaust passage and, on water-cooled models, the water tube that carries cooling water up to the engine. On many outboards the exhaust leaves under water through the propeller hub. On the outside are the mounting parts:

• Clamp (stern) bracket: clamps or bolts the motor to the transom. Read what a transom is and how to mount a motor on it.
• Swivel bracket: lets the motor turn to steer and tilt up.
• Trim pin and tilt lock: set the running angle and hold the motor up. See our outboard trim and tilt guide.
• Rubber mounts: isolate engine vibration from the boat.

Lower unit

The lower unit (gearcase) is the part below the anti-ventilation plate. It holds the gears that turn the drive 90 degrees and select forward, neutral or reverse, the propeller shaft and, on most motors, the water pump with its rubber impeller. The gearcase is filled with gear oil that needs changing at the intervals in the owner’s manual; our outboard motor maintenance checklist explains the lower unit oil change.

• Water intake: on water-cooled models, screened openings that let the pump draw in cooling water.
• Anti-ventilation (cavitation) plate: the flat plate just above the propeller. It stops the propeller drawing in air from the surface, and its height relative to the bottom of the hull is how you check that the shaft length is right.
• Skeg: the fin below the propeller that protects it and helps the boat track straight.
• Anodes: sacrificial metal blocks that corrode instead of the motor’s own metal.

Cavitation plate or anti-ventilation plate?

Both names describe the same plate. Strictly, ventilation is air drawn down from the surface into the propeller, while cavitation is vapor bubbles forming on the blades because of low pressure. The plate deals with ventilation, so anti-ventilation plate is the accurate name, but cavitation plate is the name most boaters use.

Propeller: diameter and pitch

Propeller sizes are written as diameter x pitch, both in inches. Diameter is the width of the circle the blade tips sweep. Pitch is the distance the propeller would move forward in one turn if it did not slip. A higher pitch lowers engine RPM and suits speed if the motor can still reach its rated RPM; a lower pitch raises RPM and helps acceleration and heavy loads. Choose a propeller that lets the engine reach the full-throttle RPM range given in the owner’s manual with your usual load, using the manufacturer’s propeller chart or selector. Our boat propeller pitch guide explains pitch, slip and blade choices in detail.

How an outboard motor works

1. Fuel and air burn in the cylinders and push the pistons, which turn the vertical crankshaft.
2. The crankshaft turns the driveshaft running down through the midsection.
3. Gears in the lower unit turn the drive 90 degrees; the shift selects forward, neutral or reverse.
4. The propeller shaft spins the propeller, which pushes water backward and the boat forward.
5. To steer, the whole motor pivots on the swivel bracket, so an outboard boat needs no rudder.
6. On water-cooled models, the water pump draws water in through the lower unit, sends it up through the engine to cool it and back out, including the small telltale stream you can see.

Tiller handle and kill switch

On a tiller outboard, the tiller is the arm on the front of the motor that you hold to steer. It usually carries a twist-grip throttle and often the shift lever and kill switch. The kill switch is connected to a lanyard clipped to the operator: if the lanyard is pulled, the engine stops. Attach it before starting. Remote-steering outboards are controlled from a helm with a steering system and a combined throttle and shift lever instead.

Shop by size: small and portable outboards, 15 HP, 20 HP, 25 HP and 30 HP outboards, or by brand: Suzuki, Honda and electric outboards.

Frequently asked questions

What are the three main parts of an outboard motor?

The powerhead (engine), the midsection (driveshaft housing and mounting brackets) and the lower unit (gearcase and propeller).

What is the lower unit on an outboard?

On a typical gasoline outboard, the lower unit is the gearcase at the bottom of the motor. It contains the gears, the propeller shaft and usually the water pump, and it is filled with gear oil.

What is a cavitation plate on an outboard?

It is the flat plate above the propeller, more accurately called the anti-ventilation plate. It keeps the propeller from drawing in surface air and is the reference point for setting the motor’s height.

Do outboard motors have alternators?

Most outboards use a stator (charging coil) under the flywheel rather than a belt-driven alternator like a car’s. Whether it charges a battery, and how much, depends on the model, so check the charging output in the owner’s manual.

What is a tiller on a boat?

On an outboard boat, the tiller is the handle on the motor used to steer it and usually to control the throttle. On a sailboat, the tiller is the lever attached to the rudder.

Related: what is a transom on a boat, outboard trim and tilt, 2 stroke vs 4 stroke outboards, inboard vs outboard, outboard motor maintenance, all guides.