Since a propeller provides the necessary thrust for the movement of the ship, it can be intuitively imagined that bigger the size of a ship, the heavier will be the propeller. Keeping the size of various types of propellers in mind, you would realize that it is a daunting task to mount such propellers onto the propeller shaft during the ship building process. Moreover, there are various other attachments that are to be fitted with and around the propeller for safety and efficiency. In this article we will learn about ship propellers, shafts, and the way these are secured together.
Methods of Shaft Mounting
If you think in the simplest of terms, basically the problem of shaft and propeller is just about having the right fit as you would face regarding any small shaft and a mounting on it, in a routine workshop procedure. It is only the scale (size) of the objects which makes a difference here, and makes the process all the more difficult. A propeller can be mounted on the taper, located at the tail shaft, in two ways :
- With-key arrangement
- Keyless arrangement.
With Key Arrangement
In a with-key arrangement, a key is inserted through the propeller and the taper on the tail shaft. A large nut is used to fasten and lock this key in the place, at the end of the tail shaft. Once done, a cone is mounted over the end of the tail shaft to provide a smooth flow of water from the propeller.
The keyless arrangement is most commonly done by fitting in the method of oil injection system.
In this arrangement , the propeller bore has a series of axial and circumferential grooves machined into it. The propeller is mounted on the tapered section of the tail shaft and high – pressurized oil is pumped through these groves in between the tapered section of tail-shaft and the propeller. The oil is injected mainly to reduce the friction and for the absorption of heat. Moreover, this is also done at the time of mounting the propeller on the shaft. First, high pressure is built between the two parts and the propeller is pushed up the shaft taper by a hydraulic jacking ring. When the propeller is properly aligned on the shaft, the oil pressure is released and the oil runs back. The release of oil pressure leaves the shaft and propeller fastened together.
Pilgrim Nut Operation
Pilgrim nut is used to ensure a solid frictional drip between the propeller and shaft. The use of pilgrim nut facilitates the transmission of the engine torque without using the key.
Pilgrim nut is basically a hydraulic jack with threads, which is screwed into the slot provided on the tail shaft . The propeller is forced onto the tapered tail shaft region by using a steel ring, which receives the thrust from a hydraulically pressurized nitrile rubber tyre. The same process can be revered, by using an additional withdrawal plate attached to the propeller boss by studs , in order to take the propeller off off the shaft. In simple, when the tyre is pressurized the propeller will come off the tapered region.
Introduction of Marine Engineering by D. A. Taylor