Drydock Maintenance


    When ships need hull inspection and maintenance, or propeller and/or shaft work, they must be drydocked. A drydock is simply an open-ended floating structure that has large tanks on the sides and bottom that can be flooded or pumped out to sink or float itself. The buoyancy of a pumped-out drydock is sufficient to lift a ship.

    Before sinking the dock to enable the vessel to float in over it, adjustable blocks are carefully positioned by a skilled dockmaster, who consults blueprints to ensure that the block locations conform to the hull shape of the ship to be lifted. A final check of the block positioning is made as the vessel starts to lift out of the water.

    The drydock is then pumped out enough to raise the ship completely out of the water. On the particular occasion these photos were taken, one of the propeller shaft flanges had some loose bolts, causing noise and vibration when giving astern thrust. The flange holes were reamed, with new fitted bolts assuring a tight connection.

    Here is Circle Line XI out of the water. The vessel is in the May Ship Repair facility in Staten Island, New York.
    A bow view shows the shape of the hull. The drydock is supporting a weight of about 340 tons.
    Most of that weight is on the keel blocks along the centerline of the dock.
    In order to keep the vessel upright on the keel blocks, adjustable side blocks are positioned against the hull before the drydock is fully floated.
    When placing the blocks, it is very important to check the ship's blueprints, so that the blocks are not placed where they would damage protrusions like the roll period retarding fins.
    Whenever the vessel is drydocked, the sea suction strainers are checked and cleaned. During USCG inspections, the sea valves are taken apart and checked internally as well.
    Another routine (but vital) maintenance item is replacing worn zinc plates. These sacrificial anodes protect the steel hull and shafts from wasting away due to electrolytic corrosion from dissimilar metals such as the bronze propellers.
    Zincs are placed throughout the hull, but mostly around the rudder and propeller areas.

    The propeller shafts are very well supported, extending through the propeller into a water-lubricated bearing in the rudder post. The large size of the rudders relative to the vessel size gives Circle Line XI remarkably good handling characteristics, ideal for the busy New York waterways. 

    On each side, the tailshaft exits the stern tube and is connected by a tapered coupling to a shaft section that passes through a strut-mounted  bearing, which gives additional support, and prevents bending of the shaft assembly along its length as it rotates.
    This shaft section has a flange at the aft end (the tapered coupling is used at the forward end so that this piece can be withdrawn through the strut) and connects to another section of shafting which is flanged at both ends. This one is designed to be removed so as to make space for pulling either the previous section or the propeller stub shaft.

    The last shaft section is this stub shaft, which connects to the last flange and goes through the propeller (which is keyed to it) and into the rudder post bearing.

    Here is the problem;  the bolts on the flange just aft of the strut had loosened (probably after being struck by large debris in the water). I had noticed abnormal vibration underway, especially when backing down, and a diver had confirmed something was wrong. A reamer was used to ensure exact hole size for the new interference-fitted bolts.

    There have been no further problems in the several years since this was done. 


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Copyright 2002, Brian Bailey, W4OLF
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