BOAT PROP
Boat propeller sizing, pitch, materials, flex shaft
BOAT PROP NOTATION
READING BOAT PROP SIZES
Boat props use Diameter × Pitch in inches (eg. 40mm × 1.4" or listed as X445 = 4.45" prop). Some use metric (mm diameter, mm pitch). Unlike aircraft, boat props usually have left-hand or right-hand rotation — twin-engine setups use counter-rotating props to cancel torque. Resin or stainless steel props — check shaft size (M3, M4, 3/16", 1/4").
BLADE COUNT
2-BLADE
Lower drag, higher top speed. More efficient at high RPM. Better for fast-running mono and catamaran hulls. More sensitive to cavitation. Standard for racing.
3-BLADE
Better low-speed thrust and grip. Smoother power delivery. Less top speed. Better for scale hulls, heavier boats, and rough water. Less susceptible to cavitation.
PROP MATERIALS
| MATERIAL | PROS | CONS | NOTES |
|---|---|---|---|
| Plastic / Nylon | Cheap, won't damage hull on capsize | Flexes, loses efficiency at high RPM | Beginner boats. Replace often. |
| Stainless Steel | Rigid, efficient, durable | Expensive, can damage hull if loose | Racing and performance boats. More precise pitch. |
| Bronze/Brass | Good compromise, machinable | Heavier than nylon | Scale boats and medium performers. |
FLEX SHAFT vs DIRECT DRIVE
FLEX SHAFT
Flexible cable connects motor to prop shaft. Motor sits in hull at any angle. Absorbs shock from hits. Water seal at strut exit. Common in brushless racing boats. Requires greasing. Can whip if too loose — prop exits water.
DIRECT DRIVE
Motor directly coupled to prop shaft. More efficient power transfer. Motor must align precisely. Common in scale boats and pod-and-boom racing designs. Less forgiving of shaft runout or misalignment.
CAVITATION AND AERATION
Cavitation: Low pressure on prop blade face causes water to vaporise and form bubbles — these collapse violently and erode the prop. Caused by damaged prop edges, too high pitch for RPM, or improper depth. Signs: sudden loss of thrust, high-pitched whine.
Aeration: Air sucked down from the water surface into the prop. Common in rough water or when the prop breaks the surface on turns. Causes sudden loss of thrust. Reduce with deeper prop setting or turn fin adjustment.
Aeration: Air sucked down from the water surface into the prop. Common in rough water or when the prop breaks the surface on turns. Causes sudden loss of thrust. Reduce with deeper prop setting or turn fin adjustment.