MST Marine Propulsion & Propeller Systems 2 — Questions and Answers
Question 1: What causes propeller ventilation on an outboard motor?
- Running in water that is too deep for the anti-ventilation plate
- Surface air or exhaust gases being drawn into the spinning propeller (Correct answer)
- Using a propeller with too much pitch for the engine
- Running at idle speed for extended periods in warm water
Correct answer: Surface air or exhaust gases being drawn into the spinning propeller
Ventilation occurs when surface air or exhaust gases are drawn into the spinning propeller, causing it to over-rev as it loses its grip on water.
Question 2: What is the primary function of the splined connection on a propeller shaft?
- To provide a watertight seal against water ingress into the shaft log
- To transmit rotational torque from the shaft to the propeller hub (Correct answer)
- To allow the propeller to freewheel when the engine is in neutral
- To automatically adjust propeller pitch based on engine load
Correct answer: To transmit rotational torque from the shaft to the propeller hub
The splined connection between the propeller shaft and hub transmits rotational torque from the engine to the propeller without slipping under load.
Question 3: When should a damaged propeller be replaced rather than repaired?
- When it has minor pitch distortion from a light grounding strike
- When blade erosion pitting from cavitation is superficial
- When a blade is missing or there is a crack extending through the hub (Correct answer)
- When there is surface oxidation on the aluminum blades
Correct answer: When a blade is missing or there is a crack extending through the hub
A missing blade or hub crack compromises structural integrity and dynamic balance to the point where repair is unsafe — replacement is required.
Question 4: What effect does increasing propeller diameter have on engine performance?
- Increases RPM while decreasing torque load on the engine
- Decreases thrust while allowing higher top speed
- Increases thrust and torque load but typically reduces maximum RPM (Correct answer)
- Has no measurable effect if pitch remains constant
Correct answer: Increases thrust and torque load but typically reduces maximum RPM
A larger diameter propeller displaces more water per revolution, increasing thrust and the torque load on the engine, which typically prevents the engine from reaching maximum RPM.
Question 5: What alloy is a 'nibral' marine propeller made from?
- Nickel, bronze, and aluminum (Correct answer)
- Nickel, iron, and brass
- Nib rubber composite and aluminum
- Non-corrosive iron-based alloy
Correct answer: Nickel, bronze, and aluminum
Nibral is an alloy of nickel, bronze (copper-tin alloy), and aluminum, prized for its high strength, excellent corrosion resistance, and use in demanding marine applications.
Question 6: What causes a boat to 'prop walk' sideways when shifted into reverse?
- The pitch angle of the blades reversing to a negative angle
- The asymmetric thrust pattern created by the propeller's rotation in reverse (Correct answer)
- Ventilation drawing exhaust gases under the hull
- Cavitation on the pressure face of the blades in reverse
Correct answer: The asymmetric thrust pattern created by the propeller's rotation in reverse
In reverse, a single-rotation propeller creates an asymmetric thrust pattern (similar to P-factor) because blade loading differs between the top and bottom arcs of rotation, pushing the stern sideways.
Question 7: What is the purpose of the thrust bearing in a propeller shaft system?
- To reduce friction between the rotating propeller and surrounding water
- To absorb and transfer the propeller's axial thrust force to the engine mounts and hull (Correct answer)
- To prevent the propeller shaft from spinning when the engine is in neutral
- To align the propeller shaft angle with the engine output flange
Correct answer: To absorb and transfer the propeller's axial thrust force to the engine mounts and hull
The thrust bearing transmits the propeller's forward or reverse axial thrust force from the rotating shaft to the engine mounts and ultimately to the hull, propelling the boat.
MST Marine Propulsion & Propeller Systems 2
What causes propeller ventilation on an outboard motor?