What is Propeller Drop?
Propeller Drop refers to the vertical movement or drop of the propeller shaft due to the combined effects of the propeller’s weight and the wear that occurs in the bearings that support the shaft. Over time, as the bearing wears down, a small clearance develops, causing the shaft to sink slightly. This sinking or drop is measured and referred to as propeller wear down or propeller drop.
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How is Propeller Drop Measured?
Propeller drop is typically measured using a poker gauge during a dry dock maintenance period. A dry dock is a period when the ship is taken out of water to inspect and perform maintenance on the hull and propeller, among other parts.
Detailed Explanation
The propeller shaft is a long cylindrical piece of metal that connects the ship’s main engine to the propeller. One end is fixed to the engine, and the other end is attached to the propeller. Because the propeller is heavy and rotates in water, it tends to exert a downward force on the shaft over time, especially as the bearings supporting the shaft wear down.
To prevent sea water from entering the engine room, the propeller shaft passes through seals when it exits the hull. These seals are usually located in the aft peak tank and are called lip seals. They are made from materials like nitrile rubber or viton, which press tightly against the shaft’s bronze liner to create a waterproof seal.
Over time, grooves may form on the shaft’s liner surface due to the wear of these seals, allowing water to seep through. This reduces the lubrication between the liner and the seals, causing wear on the propeller shaft.
As the wear increases, the propeller shaft begins to sink under its weight, creating a clearance between the shaft and its bearings. This clearance, or drop, is the propeller drop.
Precautions Before Measurement:
- Rope Guard: Before measuring the propeller drop, the rope guard (a protective cover) around the propeller needs to be removed.
- Poker Gauge Availability: Ensure that the vessel has a poker gauge, the specialized tool used to measure the drop.
Procedure for Measuring Propeller Drop:
- Position the Gauge: The poker gauge is inserted between the last and second-to-last seals in the stern tube (near the propeller).
- Plug Removal: A plug on top of the seal is removed to allow the poker gauge to be inserted.
- Taking the Measurement: The poker gauge measures how much the shaft has dropped due to wear in the bearing.
- Comparison: The measurement is taken at each dry dock, and readings are compared to previous records to monitor the progression of wear.
Instruments Used to Measure Propeller Drop:
- Poker Gauge: The most commonly used tool to measure propeller drop.
- Other tools like filler gauges or vernier calipers may also be used, but the poker gauge is specialized for this measurement.
Common Procedures for Stern Tube Wear-Down Measurement:
- Use of Poker Gauge: Insert the poker gauge or wear down gauge into the stern tube, after removing the rope guard and plugs.
- Mark Alignment: Align the tail shaft’s zero marks with the Simplex seal and stern tube.
- Reading: Take the top and bottom measurements and compare them with previous dry dock records to determine the wear progression.
How to Read a Poker Gauge?
The poker gauge provides a direct reading of the shaft drop. By comparing this reading with previous values, you can assess how much the shaft has worn down and whether maintenance is required.
This explanation covers the basics of propeller drop, its causes, and how it is measured. Let me know if you need further clarification on any part!