Propeller diameter is the circle swept by the blade tips. Pitch is the theoretical forward distance travelled in one revolution through a solid, before allowing for slip. Blade area, blade count, section, cup, rotation and material also affect load and performance.
A propeller is part of the engine installation. Too much pitch or diameter can prevent the engine reaching its rated full-throttle rpm under normal full-load conditions. This is over-propping. It can increase smoke, exhaust temperature and engine loading. Too little propeller load may let the engine exceed its permitted speed and waste available thrust.
Fouling changes the equation quickly. A thin layer of growth can reduce speed and increase fuel use. A rope or net can damage the shaft seal, saildrive seals, strut or gearbox. After any strike or entanglement, inspect vibration, oil leakage and underwater hardware.
Sailing yachts may use fixed, folding or feathering propellers. Folding designs reduce drag under sail. Feathering designs rotate blades to a low-drag angle and may offer strong reverse performance. Both need correct hub maintenance and a compatible anode arrangement.
Record a clean-hull sea trial in safe conditions: loading, wind, sea state, maximum rpm, rpm versus speed, exhaust appearance and engine temperature. That baseline makes later diagnosis far easier. Propeller selection should be performed using the engine curve, reduction ratio, hull data and propeller specialist calculations.