Propulsion basics: how a boat is pushed

The lesson in plain words

1. Parts
   • The hub is the centre fitted to the shaft, the rod transmitting engine rotation.
   • On a typical shaft drive, a key and nut secure it.
   • Each blade has a root at the hub, an outer tip, a leading edge that meets the water first and a trailing edge where water leaves.
   • In ahead operation, the face is the aft-facing pressure side pushing water back.
   • The back is the forward-facing, lower-pressure side.
2. Diameter and pitch
   • Diameter is the width of the circle swept by the blade tips.
   • Pitch is the theoretical forward advance in one revolution, with no slip.
   • A 15 × 11 propeller has a 15-inch diameter and an 11-inch pitch; diameter comes first. One inch = 2.54 cm.
   • rpm means revolutions per minute.
   • Propeller rpm = engine rpm ÷ gear ratio.
   • At 2,600 engine rpm with 2:1 reduction, the propeller turns at 1,300 rpm.
3. Speed and apparent slip
   • Theoretical knots = pitch in inches × propeller rpm ÷ 1,215.
   • With 10-inch pitch at 1,300 rpm: 10 × 1,300 ÷ 1,215 ≈ 10.7 knots.
   • Apparent slip compares this with ship speed through the water: apparent slip (%) = (theoretical − through-water speed) ÷ theoretical × 100. Typical values are 10–40 %, depending on hull and conditions.
   • At 20 %, 10.7 knots becomes about 8.6 knots.
   • Hull wake changes the water speed reaching the propeller, so real slip differs.
4. Right or left hand
   • Judge hand from astern: stand behind the boat, looking forwards, with the propeller driving ahead.
   • Clockwise means right-handed; anticlockwise means left-handed.
   • Looking from the other end reverses what you see.
   • Check the marking or handbook rather than assuming the hand.
5. Matching the load
   • An overpropped boat has too much pitch or diameter for its engine: the load prevents it reaching the maker’s full-throttle rpm range.
   • An underpropped boat has too little: the engine may exceed that range because the load is too light.
   • Check with a clean hull and propeller and normal loading.
   • More pitch only helps if the engine can turn it properly.
6. Blades and materials
   • More blades, commonly 3, 4 or 5, generally give smoother running and better low-speed grip; fewer can be more efficient.
   • Aluminium is inexpensive but soft; stainless steel and bronze are stronger.
   • Sailing yachts use folding blades that fold back, or feathering blades that turn edge-on, to reduce drag under sail.
   • Folding propellers often give weaker astern thrust and may open late; test astern response before a tight manoeuvre.

Key data

Key points

• Read size as diameter × pitch. Pitch is advance per turn with no slip.
• Use propeller rpm, not engine rpm, in the speed formula.
• Judge hand from astern, driving ahead: clockwise means right-handed.
• Overpropped: too much load, low rpm. Underpropped: too little load, possible over-revving.
• Folding and feathering reduce sailing drag; folding propellers often give weaker astern thrust.

Check what you have read

True or false?

Practice case & exam keys

Exam keys

1. 1Diameter × pitch, in inches. Pitch is theoretical advance per revolution.
2. 2Theoretical knots = pitch (in) × propeller rpm ÷ 1,215; apparent slip compares theoretical and actual through-water speed.
3. 3Right-handed is clockwise from astern, driving ahead. Match propeller load to the engine’s specified rpm range.
4. 4Blade count and material affect performance; folding and feathering reduce drag under sail.