• 18 parts
  • Checked September 2026

Parts of a Boat Motor

A small outboard motor clamped to a boat, then cut open to show what is inside.

Parts of a Boat Motor

Name Date

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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Write the name of each numbered part.Answer key

  1. Cowling
  2. Recoil starter
  3. Tiller handle
  4. Throttle grip
  5. Shift lever
  6. Clamp bracket
  7. Clamp screw
  8. Anti-cavitation plate
  9. Water intake
  10. Gearcase
  11. Skeg
  12. Propeller
  13. Flywheel
  14. Powerhead
  15. Drive shaft
  16. Drive shaft housing
  17. Water pump
  18. Propeller shaft

Drawn for this site on 2026-09-30 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: the parts of the propeller

All 18 parts on the diagram

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Top and controls 5

  1. Cowling The removable cover over the engine. Some makers call it the engine cover.
  2. Recoil starter A rope starter. Pulling its grip spins the engine to start it.
  3. Tiller handle The steering handle. Swing it left to turn right.
  4. Throttle grip Twist it to change the engine speed.
  5. Shift lever It selects forward, neutral or reverse.

Mount and lower unit 7

  1. Clamp bracket It hangs the motor on the transom. Also called the stern bracket.
  2. Clamp screw Tightened by hand to grip the transom.
  3. Anti-cavitation plate A flat plate above the propeller. Some makers call it the anti-ventilation plate.
  4. Water intake Cooling water enters here. It has to stay under water.
  5. Gearcase It holds the gears that turn the propeller shaft.
  6. Skeg The fin under the gearcase. With the engine off it works as a rudder.
  7. Propeller It pushes the boat, turned by the engine through the shafts.

Inside 6

  1. Flywheel A wheel on the crankshaft. Magnets in it make the spark.
  2. Powerhead The engine itself, mounted on top of the motor.
  3. Drive shaft It carries the engine's power down to the gears.
  4. Drive shaft housing The hollow leg between the powerhead and the gearcase.
  5. Water pump A rubber impeller pump. It sends cooling water up to the engine.
  6. Propeller shaft The propeller is fixed on it. The gears turn it.

How this list was compiled: Every name checked on 2026-09-30 and kept only where two independent sources use it. (1) A motor maker's owner's manual for a small four-stroke outboard, https://archive.org/details/manualsonline-id-f94bcd20-c107-4518-a821-da37fcc7a496 (main components list). (2) A second, unrelated maker's owner's manuals for two small outboards, counted as one source, https://archive.org/details/manualsbase-id-554696 and https://archive.org/details/manualzilla-id-5670948 (component identification pages). (3) US War Department Technical Manual TM 5-278, Outboard Motors, 1944, public domain, https://archive.org/details/TM5-278 (powerhead and lower unit, drive shaft, propeller shaft, gearcase, clamp screws, anti-cavitation plate, skeg, flywheel). (4) A third maker's service manual for small outboards, https://archive.org/details/manualzilla-id-6016817 (power head, drive shaft housing, gearcase, water pump, recoil starter, clamp bracket). (5) Wikipedia, text only: Outboard motor, https://en.wikipedia.org/wiki/Outboard_motor (cowling, powerhead, skeg, water intake, tiller, impeller pump). Skeg rests on sources 3 and 5; cowling on 1 and 5; shift lever and throttle grip on 1 and 2. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's motor, nothing traced. Licence: own work

How it works

The parts of a boat motor, drawn on a small outboard steered by hand. The first view shows it clamped to the back of a boat. The second cuts it open.

Read the full guide

Two halves, joined by a leg

An outboard is an engine on top and a propeller at the bottom. The engine is the powerhead, under the cowling. The part in the water is the lower unit: the gearcase, the skeg under it and the propeller behind it. A drive shaft runs down the hollow leg between the two.

How it hangs and how it steers

The clamp bracket hooks over the transom, the back wall of the boat, and clamp screws pinch it tight. The whole motor pivots to steer. On a small motor the tiller handle swings it, and the throttle grip on the end of the tiller sets the speed.

  • With the boat afloat, the anti-cavitation plate and the water intake are both under water.

Questions and answers

What are the parts of a boat motor called?

From the top: the cowling, the powerhead under it, the tiller handle with its throttle grip, the clamp bracket, the drive shaft housing, the anti-cavitation plate, the gearcase, the skeg and the propeller.

What is the bottom part of a boat motor called?

The lower unit. It is the part that runs under water: the gearcase with its gears and propeller shaft, the skeg below it and the propeller behind it.

What is the fin on the bottom of an outboard motor called?

The skeg. It sits under the gearcase, in front of the lowest sweep of the propeller, and it works as a rudder while the engine is off.

What does the plate above the propeller do?

It is the anti-cavitation plate, which some makers call the anti-ventilation plate. It helps stop air from the surface being sucked down into the propeller.

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