Propellers

Question 1

What types of damage can be found on a prop?

Answer 1

• Cavitation
• Stress corrosion
• Erosion
• Cracks
• Fractures
• Pitting

Question 2

Cavitation why it happens? Damage it causes? How to solve it?

Answer 2

Due to low pressure caused by the propeller, there is then small vapor bubble. these then collapse under the local high pressure and damage the surface causing corrosion and erosion has it removes small fragments and the protective layer.

Question 3

Explain galvanic erosion?

Answer 3

When two dissimilar metals are in contact with each other via a stray current. This causing one material to become a cathodic and the other, an anode. This depends on their rankings on a galvanic chart. The anode will deteriorate quicker than normal.

Question 4

How would you measure cutlass bearing wear down? Normal clearance/maximum clearance?

Answer 4

While in the water at the deepest draft you would send a diver down with a measuring device known as a Go - No go gauge. Maximum would be 5% of the shaft diameter.

Question 5

How would you measure oil bath shaft bearing wear down?

Answer 5

• Visual inspect the oil bath shaft bearing
• Collect an oil sample
• Measure the internal diameter with a micrometer
• Measure the outer diameter
• Compare to manufactures manual
• Measure the clearance between the shaft journal and bearing surface with feeler gauges

Question 6

What is the propeller cavitation limit?

Answer 6

20%

Question 7

Explain different propeller removal and fitting techniques

Answer 7

• Hydraulic pullers
• Hydraulic press
• Cranes and hoist

Question 8

How to repair propellers?

Answer 8

• Welding
• Brazing

Question 9

How would you check the taper fit on a propeller? What would be the normal %?

Answer 9

Engineers blue for the fit and between 75-85% for a reasonable value. Line of thought being that shipyards were sometimes not to be trusted and reports of them putting blue on prop and the shaft to get false 100%.

Question 10

Shaft run out test before reinstalling prop?

Answer 10

• Ensure the shaft is clean
• Place a dial indicator on a bearing housing and place the dial on the shaft
• Rotate the shaft slowly by hand
• Record the readings, especially and deviations
• Compare with manufactures spec

Question 11

Tell me about propeller dye test?

Answer 11

Dye Penetration Test

• Clean the surface
• Apply fluorescent dye and allow to penetrate
• Remove excess
• Apply developer and allow to develop
• Conduct visual inspection