MTC Transmission & Drivetrain Systems 3 — Questions and Answers
Question 1: What causes a propeller shaft to vibrate excessively at higher RPM on an inboard vessel?
- Low transmission oil pressure
- Shaft misalignment or a bent/damaged propeller (Correct answer)
- Incorrect throttle cable adjustment
- Clogged raw-water strainer
Correct answer: Shaft misalignment or a bent/damaged propeller
A bent shaft or damaged propeller creates an unbalanced rotating mass that produces vibration that worsens as RPM increases.
Question 2: What is the purpose of a cutlass bearing on an inboard propeller shaft?
- Prevents axial movement of the shaft under thrust load
- Supports and lubricates the rotating shaft using water (Correct answer)
- Seals the shaft log against water intrusion
- Connects the shaft to the transmission output flange
Correct answer: Supports and lubricates the rotating shaft using water
The cutlass bearing is a water-lubricated rubber bearing in the strut or stern tube that supports the propeller shaft's outboard end.
Question 3: When bleeding air from a hydraulic marine transmission control system, the technician should:
- Run the engine at full throttle and cycle gear lever rapidly
- Start at the highest point in the system and work downward to the actuator (Correct answer)
- Replace the hydraulic fluid completely before bleeding
- Leave the engine off and manually stroke the actuator 50 times
Correct answer: Start at the highest point in the system and work downward to the actuator
Air rises, so bleeding from the highest point first ensures all trapped air is purged progressively down to the actuator.
Question 4: A stern drive unit that steers but does not trim is MOST likely caused by:
- Failed trim sender unit only
- Hydraulic leak in the trim cylinder or low hydraulic fluid (Correct answer)
- Seized tilt lock pin
- Worn gimbal ring bearings
Correct answer: Hydraulic leak in the trim cylinder or low hydraulic fluid
The trim function relies on hydraulic pressure; a cylinder leak or low fluid level will disable trim while steering (mechanical) remains functional.
Question 5: What is the correct method for measuring propeller shaft runout?
- Use a torque wrench while rotating the shaft by hand
- Mount a dial indicator against the shaft and rotate it one full revolution (Correct answer)
- Measure shaft diameter at three points with a micrometer
- Install the shaft and check vibration with a vibration meter at idle
Correct answer: Mount a dial indicator against the shaft and rotate it one full revolution
A dial indicator measures total indicated runout (TIR) as the shaft rotates, revealing bends or eccentricity that cause vibration.
Question 6: Which condition would cause a marine transmission to overheat?
- Operating at idle speed for extended periods
- Blocked transmission oil cooler passages or low fluid level (Correct answer)
- Using ATF Dexron instead of Type F fluid
- Propeller pitch set two inches lower than specification
Correct answer: Blocked transmission oil cooler passages or low fluid level
A blocked oil cooler prevents heat dissipation, and low fluid reduces lubrication and cooling capacity, both leading to overheating.
Question 7: The angle between the engine crankshaft centerline and the propeller shaft centerline in an inboard installation is called the:
- Shaft offset angle
- Down-angle or shaft inclination angle (Correct answer)
- Trim angle
- Rake angle
Correct answer: Down-angle or shaft inclination angle
The down-angle (shaft inclination) is the angle at which the shaft slopes downward from the transmission to the propeller; excessive angles reduce efficiency.
MTC Transmission & Drivetrain Systems 3
What causes a propeller shaft to vibrate excessively at higher RPM on an inboard vessel?