Propulsion basics: how a boat is pushed

The lesson in plain words

1. Cavitation
   • Cavitation forms when pressure on a blade falls low enough for water to become vapour.
   • The bubbles collapse, causing noise, lost thrust and pitting, tiny holes eroded in the metal.
   • Excessive propeller speed, damaged edges or too little blade area for the load can contribute.
   • The bubbles form from water itself, not air drawn from above.
2. Ventilation
   • Ventilation draws surface air, or sometimes exhaust gas, onto the blades.
   • They lose grip, engine rpm rises and thrust falls.
   • It can happen in tight turns, waves or when an outboard leg is trimmed too high.
   • Trim is the leg’s running angle.
   • Ease power, straighten the turn if safe and lower the leg’s trim as appropriate.
   • The anti-ventilation plate above the propeller helps keep surface air away; it is often misleadingly called an anti-cavitation plate.
3. Fouling
   • Fouling is weed, barnacles and other growth on the hull or propeller.
   • It increases resistance and spoils blade shape, costing speed and fuel.
   • A fouled hull, heavy load or damaged blade can make the engine struggle, like too much propeller pitch.
   • Clean and inspect before deciding that the propeller size is wrong.
4. Displacement and hull speed
   • A displacement hull pushes water aside.
   • Near hull speed, wave-making resistance rises steeply, so extra power buys little speed.
   • Hull speed in knots ≈ 2.43 × √LWL with LWL in metres, or 1.34 × √LWL with LWL in feet.
   • LWL is waterline length; √ means square root.
   • At LWL 9 m: 2.43 × √9 = 2.43 × 3 ≈ 7.3 knots.
   • This is a guide, not a hard limit; light, slender hulls can exceed it.
5. Planing
   • A planing hull gains lift from water flowing beneath it.
   • With suitable shape and enough power, it rises and skims over the water, allowing much higher speeds than an ordinary displacement hull.
   • Adding a bigger engine to a displacement boat does not turn it into a planing boat.
6. Matching engine and propeller
   • Check the maker’s full-throttle rpm range with a clean hull, sound propeller and normal load.
   • Too much pitch or diameter can prevent reaching the range; too little can let the engine over-rev.
   • First check fouling, loading and damage.
   • A sudden rpm surge during a turn suggests ventilation, while steadily excessive rpm in normal straight running can suggest too little propeller load.
   • Symptoms guide checks; they do not prove the cause.

Key data

Key points

• Cavitation forms vapour in low-pressure water; collapsing bubbles can pit blades.
• Ventilation brings air or exhaust gas onto the blades, causing lost grip and an rpm surge.
• Fouling increases resistance and can mimic too much propeller load. Clean before changing size.
• Hull speed ≈ 2.43 × √LWL (m) knots. It is a guide for displacement hulls, not a hard limit.
• Match the propeller to the maker’s full-throttle rpm range with a clean hull and normal load.

Check what you have read

True or false?

Practice case & exam keys

Exam keys

1. 1Cavitation: low-pressure vapour bubbles, noise, lost thrust and possible pitting. Ventilation: air or exhaust gas on blades.
2. 2Fouling costs speed and fuel. Establish clean, undamaged conditions before judging propeller load.
3. 3Hull speed ≈ 2.43 × √LWL (metres), or 1.34 × √LWL (feet), in knots; suitable planing hulls can go much faster.
4. 4Full-throttle rpm should fall within the maker’s range under normal load; low rpm and high rpm need different checks.