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Maritime Forum

TOPIC: Propeller Removal in Dry-Dock

Propeller Removal in Dry-Dock 21 Jul 2016 15:06 #4598

In removing a propeller in dry dock how it is ascertained that the nut is tighten back to its original position on reassembly and how it is locked? :unsure:
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Propeller Removal in Dry-Dock 22 Jul 2016 03:23 #4599

The System used for Mounting and Dismounting the Propeller is called the Pilgrim Nut System.

The pilgrim nut is a threaded hydraulic jack, which is screwed on to the tail shaft. A steel ring receives thrust from a hydraulically pressurized nitrile rubber tyre inside it. This thrust is applied to the propeller to force it onto the taper sleeve.


Mounting of Propeller with Pilgrim Nut

The Pilgrim nut employed for propeller mounting has an internal nitrile rubber tube which when inflated hydraulically, forces a steel loading ring
against the hub, as shown below.




First step in the Mounting of the propeller is to check the propeller's orientation while it is still hanging in the Air, and position the propeller correctly with the tail shaft and then lightly jacked on to the Tail shaft taper.


(Before the Pilgrim nut is used to apply an initial loading, a reference mark is made at this point about 25 mm from the forward end of the hub.)

The hydraulic pressure required by the Pilgrim Nut is provided by an air driven pump. When the pressure is applied, the piston of the Pilgrim Nut extends, pushing the propeller along the tapered shaft. The applied pressure and travel of the piston correspond to the required push-up distance of the application.

The nut is then turned until the loading ring is again flushed (Ozzing hydraulic fluid with it) before full pressure is applied.

During this stage, the dial gauge should show the movement.A second mark 25 mm from the forward face of the hub is then made. Push up, registered by the distance between the two reference marks, is
measured and noted.

The nut is again vented and turned to bring the loading ring to the flush position and finally nipped up with a tommy bar.

How it is ascertained that Nut is Tight

The thing to understand here, is that the Propeller boss is "interference fit" on the taper of the tail shaft. Thus it is not the Nut which holds the propeller on the tail shaft (as is the case in other mechanical system assemblies).

You can see in the Figure No.2, that to tight and loosen the Pilgrim Nut, a Tommy Bar (just a simple looking steel's cylindrical bar of about 6 inch length) is used to open/close that by hand.


Thus when the hydraulic pressure is given in the pilgrim nut to push the propeller up on the Taper of the tail shaft, the propeller get pushed by few centimeters.

Then once again the pressure inside the Pilgrim Nut system is relieved (i.e.the tire deflates and loading ring comes back to the level of Nut), the Nut is again tightened by the Hand with Tommy bar and same procedure to push more the propeller boss is carried out every time, till the propeller does not move any more on the Taper of the tail shaft.
(the pressure inside the Pilgrim Nut system and movement of the propeller is closely monitored by the Expert engineers,technicians and Fitters of the Dry-Dock and is done within the limits provided by the Manufactures of Pilgrim Nut system & Propeller System a)

As in the end of mounting the propeller, when it does not move and is in the "marked" position on the Tail Shaft, the pressure inside the Pilgrim nut is released and the Nut is tightened by Hand and Tommy bar. (A o-ring will also come in between Nut and Propeller boss,as shown in the figure No.1).

Now when the propeller would rotating under load in sea, the Nut may open as it is just hand tightened.

To prevent this a Bar of rectangular section is used to lock the Nut with the tail shaft (inserted in a groove of tail shaft, as shown in Fig 1). That is simply called Locking plate.

Now ,Out of the Answer for the Question asked .... :)

Even this Locking plate and tread of Tail shaft will get corroded in sea. To prevent that A Cone is mounted over the tread of the Tail shaft and is bolted to the propeller boss and is filled inside by Tallow sort of material.
Last Edit: 22 Jul 2016 03:52 by anandgardener.
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