• 16 parts
  • Checked September 2026

Parts of a Boat Propeller

The propeller from behind, one blade cut across, and the parts that hold it on the shaft.

Parts of a Boat Propeller

Name Date

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

  1. Hub
  2. Blade
  3. Blade tip
  4. Blade root
  5. Leading edge
  6. Trailing edge
  7. Diameter
  8. Blade face
  9. Blade back
  10. Cup
  11. Pitch
  12. Propeller shaft
  13. Splines
  14. Washer
  15. Propeller nut
  16. Cotter pin

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 boat it drives

All 16 parts on the diagram

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Hub and blades 9

  1. Hub The center of the propeller. The blades are fixed to it, and it is bored through to take the shaft.
  2. Blade The blades are set at an angle, the pitch. Turning them then pushes on the water and drives the boat.
  3. Blade tip The blade's farthest reach from the hub's center. It separates the leading edge from the trailing edge.
  4. Blade root Where the blade joins the hub.
  5. Leading edge The edge next to the hub's forward end. It leads into the flow when the propeller drives the boat forward.
  6. Trailing edge The edge next to the hub's aft end. It is the one nearest you when you look at the propeller from behind the boat.
  7. Blade face The pressure side of the blade: the aft side, facing the stern. In the cut it is the straighter lower line.
  8. Blade back The suction side of the blade: the forward side, facing the bow. In the cut it is the curved upper line.
  9. Cup Cup is a small lip or curl on a blade's trailing edge.

Diameter and pitch 2

  1. Diameter Twice the distance from the hub's center to a blade tip. It is the first number given in a propeller size.
  2. Pitch How far forward one turn would move the propeller. That is with no slip between blade and water. It is the second number in a propeller size.

On the shaft 5

  1. Propeller shaft The engine turns it, and the propeller turns with it. It is greased before the propeller goes on.
  2. Splines They fit the inside of the hub. That passes the turning of the shaft to the propeller. They are checked for wear and damage.
  3. Washer It sits on the shaft between the propeller and the propeller nut.
  4. Propeller nut It is tightened onto the end of the shaft. It is then lined up with the hole in the shaft for the cotter pin. Also called a castle nut.
  5. Cotter pin Its ends are bent over so the nut cannot work loose. A new one is fitted each time the propeller goes back on.

How this list was compiled: Every name checked on 2026-09-30 and kept only where two independent sources use it. Blade terms: a propeller maker's terminology page (https://www.miwheel.com/terminology/) and Wikipedia, Propeller, text only; cup, diameter and pitch also in a propeller seller's sizing guide (https://www.propellerdepot.com/blogs/default-blog/how-to-read-propeller-sizing-pitch-diameter-cup-more). Mounting hardware: the owner's manuals of two outboard makers, read as archive.org full text (https://archive.org/details/manualzilla-id-7318703 and https://archive.org/details/manuallib-id-2596588: propeller shaft, washer, propeller nut or castle nut, cotter pin), with Wikipedia for the splines. Licence: own work

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

How it works

The parts of a boat propeller in three drawings: the propeller seen from behind the boat, one blade cut across to show its two sides, and the propeller taken off its shaft.

Read the full guide

The blades

A propeller is a hub with blades fixed to it. Each blade joins the hub at its root and ends at its tip. The leading edge meets the water first and the trailing edge lets it go; many blades have a small curled lip there, the cup. The side facing the back of the boat is the blade face, and the side facing the front is the blade back.

How it is held on the shaft

The propeller slides onto the splines of the propeller shaft. A washer and the propeller nut go on after it, and a cotter pin through the nut and the shaft keeps the nut from working loose.

  • Prop is short for propeller, so the parts of a prop are the parts shown here.
  • A propeller size gives the diameter first and the pitch second.

Questions and answers

What are the parts of a propeller called?

A hub in the center with blades fixed to it. Each blade has a root, a tip, a leading edge, a trailing edge, a face and a back. On the shaft, a washer, a propeller nut and a cotter pin hold the propeller on.

What is the difference between the face and the back of a propeller blade?

The face is the pressure side of the blade, the side turned toward the stern. The back is the suction side, turned toward the bow.

What do the numbers on a boat propeller mean?

Diameter first, then pitch. Diameter is the width of the circle the blade tips sweep. Pitch is how far the propeller would move forward in one full turn if there were no slip.

What is cup on a propeller?

Cup is a small lip or curl on a blade's trailing edge.

What holds a boat propeller on?

On an outboard, the propeller nut, which is tightened onto the end of the propeller shaft over a washer. A cotter pin goes through the nut and a hole in the shaft, and its ends are bent over.

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