MST Marine Propulsion & Propeller Systems 3

Question 1

How do you verify that a replacement propeller has the correct pitch for a given engine?

Accepted Answer

Confirm the engine reaches the manufacturer's rated wide-open throttle RPM range under full load

Answer

Select the highest pitch propeller that physically fits the shaft

Answer

Match propeller diameter in inches to hull length in feet

Answer

Use the same pitch as the original propeller that came with the boat

Question 2

What is a counter-rotating propeller system?

Accepted Answer

Twin propellers on concentric shafts rotating in opposite directions to cancel torque effects

Answer

A variable-pitch propeller that reverses blade angle to go astern

Answer

A single propeller designed with asymmetric blades to prevent ventilation

Answer

A system where the propeller freewheels when the engine is in neutral gear

Question 3

What physical process causes the characteristic pitting (erosion) seen on cavitating propeller blades?

Accepted Answer

Micro-shockwaves from the violent collapse of vapor bubbles on the blade surface

Answer

Chemical corrosion from saltwater dissolving the blade surface

Answer

Sand and grit abrasion from running in shallow coastal waters

Answer

Electrolytic action between dissimilar metals in the propeller assembly

Question 4

What is propeller rake?

Accepted Answer

The angle at which propeller blades are swept forward or backward relative to a perpendicular plane through the shaft

Answer

The degree of trailing-edge cup added to each blade

Answer

The angle of the propeller shaft measured below horizontal

Answer

The ratio of blade area to total propeller disc area

Question 5

How does a folding propeller primarily benefit a sailboat with an auxiliary engine?

Accepted Answer

Its blades fold parallel to the shaft when not in use, greatly reducing drag under sail

Answer

It provides significantly more thrust when motoring in reverse

Answer

It automatically adjusts pitch based on vessel speed through the water

Answer

It eliminates the need for a thrust bearing in the shaft system

Question 6

What is the primary purpose of a sacrificial zinc anode installed on a propeller shaft or strut?

Accepted Answer

To corrode preferentially and protect the propeller and shaft from galvanic corrosion

Answer

To lubricate the shaft cutlass bearing and reduce wear

Answer

To dynamically balance the propeller at operating speeds

Answer

To dampen shaft vibration transmitted to the hull

Question 7

Why is anti-seize compound applied to the splined shaft before installing a propeller?

Accepted Answer

To prevent galvanic corrosion between dissimilar metals and ease future removal

Answer

To increase the torque capacity transmitted through the spline connection

Answer

To seal the spline against water intrusion into the shaft bearing

Answer

To increase friction and prevent the propeller from loosening at high RPM