Operational Information

Holding Down and Chocking


The engine is mounted on resin or cast iron chocks and bolted to the hull using holding down bolts.

The engine must be securely fixed into the ship. As the engine turns the propeller, the propeller tries to push or thrust the propeller shaft and engine crankshaft forward into the ship. The thrust bearing which is situated at the aft end of the engine transmits this thrust from the crankshaft to the bedplate.

The bedplate is mounted on chocks and is securely bolted to the engine foundation plate on which it sits and which forms part of the structure of the hull.

The Engine must also be lined up with the propeller shaft. If the engine output driving flange was higher or lower, or  to port or stbd of the propeller shaft, then it is easy to visualise that trying to connect them would cause bending stresses to be set up.

The engine must also be bolted to a flat surface. If  the surface was uneven, then when the bolts were tightened the bedplate would be distorted, which in turn would distort the crankshaft, causing unacceptable stresses to be set up when the engine was running.

Before the engine is bolted down it is supported on jacks whilst it is aligned with the tailshaft bearing. This can be done by stretching a wire above the tailshaft and crankshsft, and measuring the distance from the wire to the crankshaft bearing centres. Modern methods use a laser.

When the bedplate is in perfect alignment, cast iron chocks are hand fitted between the machined underside of the bedplate and machined spots on the foundation plate. This is a skilled task and 80% contact is the aim.

Once the engine is supported by the chocks the jacks are removed and the holding down bolts are tightened using a hydraulic jack to stretch the bolts.

Holding down bolts should be checked regularly for tightness. If they are allowed to come loose, then the mating surfaces will rub against each other and wear away in a process known as fretting. If this continues and the bolts are subsequently tightened down, the bedplate (and main bearings) will be pulled out of alignment.

Side chocks are fitted to prevent the engine from moving sideways due to the movement of the vessel or because of the sideways component of thrust from the reciprocating and rotating parts. The chock is welded to the foundation plate as shown, a liner is hand fitted on a 100:1 taper and then driven home.


This is a side chocking arrangement, where after driving the liner home, locking screws are hardened down as shown.


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