A ship or submarine moves because engine power is carried through a rotating shaft line to a propeller in the water. The propeller shaft must pass through the hull without letting seawater flood the vessel, which makes the stern tube a critical piece of marine engineering. This system matters because it connects propulsion, watertight safety, alignment, lubrication, and vibration control in one compact region of the stern.
A well designed shaft and stern tube allow high power transfer while protecting the hull and machinery.
Key Facts
- Shaft power is P = Tω, where P is power, T is torque, and ω is angular speed in rad/s.
- Torque can be found from T = P/ω when power and rotational speed are known.
- Rotational speed conversion is ω = 2πN/60, where N is revolutions per minute.
- Propeller thrust pushes water backward, and the equal and opposite reaction pushes the ship forward.
- The stern tube supports the propeller shaft as it passes through the hull and helps keep the hull watertight.
- Bearings reduce friction and support the shaft, while seals prevent seawater from entering and lubricant from escaping.
Vocabulary
- Propeller shaft
- A long rotating shaft that carries torque from the engine or gearbox to the propeller.
- Stern tube
- A watertight tube built into the stern that guides and protects the shaft as it passes through the hull.
- Bearing
- A support surface that holds the shaft in position while allowing it to rotate with reduced friction.
- Shaft seal
- A sealing device around the shaft that helps stop seawater from entering the ship and lubricant from leaking out.
- Torque
- A twisting effect that causes rotation and transfers mechanical power through the shaft.
Common Mistakes to Avoid
- Confusing thrust with torque, which is wrong because torque twists the shaft while thrust is the forward force produced by the propeller pushing water backward.
- Ignoring units when using P = Tω, which is wrong because angular speed must be in rad/s, not directly in rpm.
- Thinking the stern tube is just an empty hole in the hull, which is wrong because it contains supports, lubrication paths, seals, and alignment features.
- Assuming bearings remove all friction, which is wrong because bearings reduce friction but still need lubrication, cooling, and maintenance.
Practice Questions
- 1 A propeller shaft delivers 900 kW at 120 rpm. Calculate the angular speed in rad/s and the torque in N m.
- 2 A ship shaft has a torque of 45,000 N m and rotates at 180 rpm. Calculate the shaft power in kW.
- 3 Explain why a stern tube must combine shaft support with sealing, and describe what could happen if the shaft is misaligned.