MST Marine Propulsion & Propeller Systems 1 — Questions and Answers
Question 1: What is cavitation in a marine propeller?
- The formation of air pockets when the propeller spins too slowly
- The formation and collapse of vapor bubbles on propeller blades due to low pressure (Correct answer)
- The buildup of barnacles on propeller blades reducing efficiency
- The slipping of the propeller hub under high torque load
Correct answer: The formation and collapse of vapor bubbles on propeller blades due to low pressure
Cavitation occurs when low pressure on the blade face causes water to vaporize, forming bubbles that collapse violently and erode the blade surface.
Question 2: What does propeller pitch refer to in marine applications?
- The angle of the propeller shaft relative to the keel
- The diameter of the circle swept by the propeller blades
- The theoretical distance a propeller would advance in one revolution with no slip (Correct answer)
- The number of blades on the propeller
Correct answer: The theoretical distance a propeller would advance in one revolution with no slip
Pitch is the theoretical forward distance the propeller would move in one complete revolution if there were no slippage through the water, similar to a screw thread.
Question 3: Which material is most commonly used for recreational boat propellers?
- Cast iron
- Stainless steel
- Aluminum alloy (Correct answer)
- Titanium
Correct answer: Aluminum alloy
Aluminum alloy is the most common propeller material for recreational boats due to its low cost, light weight, and ease of repair after minor strikes.
Question 4: What is the correct procedure for removing a stuck aluminum propeller from a splined shaft?
- Use a torch to heat the hub and tap the sides with a hammer
- Use a propeller puller or soak with penetrating oil and tap gently (Correct answer)
- Apply grease and run the engine briefly in reverse
- Use an impact wrench directly on the propeller blades
Correct answer: Use a propeller puller or soak with penetrating oil and tap gently
A propeller puller or penetrating oil with gentle tapping is the correct approach to avoid damaging the splined shaft or distorting the propeller hub.
Question 5: What does adding 'cup' to a propeller blade accomplish?
- Increases the overall weight of the propeller for better inertia
- Reduces the effective diameter without changing pitch
- Adds a curved hook to the trailing edge to improve grip and reduce ventilation (Correct answer)
- Dynamically balances the propeller at high RPM
Correct answer: Adds a curved hook to the trailing edge to improve grip and reduce ventilation
Cup is a curved trailing edge on propeller blades that improves water grip, reduces ventilation and cavitation, and helps the engine maintain rated RPM.
Question 6: What is propeller slip?
- The difference between theoretical speed based on pitch and actual boat speed (Correct answer)
- The lateral movement of the propeller shaft under load
- Hub failure allowing the propeller to spin freely under high torque
- Loss of pitch when the propeller strikes a submerged object
Correct answer: The difference between theoretical speed based on pitch and actual boat speed
Propeller slip is the difference between theoretical speed (pitch × RPM) and actual boat speed, expressed as a percentage, representing inefficiency in the propeller's grip on water.
Question 7: How does a 4-blade propeller generally differ from a 3-blade of the same pitch and diameter?
- 4-blade achieves higher top speed due to reduced drag
- 4-blade provides better hole shot and reduced vibration but slightly lower top speed (Correct answer)
- 4-blade is only suitable for inboard engines
- 4-blade reduces fuel consumption at wide-open throttle
Correct answer: 4-blade provides better hole shot and reduced vibration but slightly lower top speed
A 4-blade propeller offers better acceleration and less vibration due to increased blade area, but the added drag typically results in a slightly lower maximum top speed.
MST Marine Propulsion & Propeller Systems 1
What is cavitation in a marine propeller?