The Two Stroke
The Bedplate is the foundation on which
the 2 stroke engine is built. It must be rigid enough to support the
weight of the rest of the engine, and maintain the crankshaft, which
sits in the bearing housings in the transverse girders, in alignment. At
the same time it must be flexible enough to hog and sag with the
foundation plate to which it is attached and which forms part of the
If the bedplate was too rigid, then as
the hull flexed, the holding down bolts, which secure the engine into
the ship would be likely to break, and there would be a danger of the
Basically the bedplate consists of two longitudinal
girders which run the length of the engine. Connecting these
longitudinal girders are the transverse girders which are
positioned between each crankshaft throw, and either side of the
thrust collar. Built into the transverse girders are the main bearing
pockets for the crankshaft to run in.
On the small bore engines, the bedplate can be made from
cast iron as a single casting. Larger engines have a fabricated
bedplate. This means it is welded together from steel sections, steel
castings and plate. The steel is to Classification Society
specifications and is a low carbon steel with a maximum carbon content
Earlier fabricated bedplates had box section
longitudinal girders and box section fabricated transverse girders.
Problems were encountered with cracking of the transverse girders, which
increased as engine powers and crankshaft throws got larger.
modern bedplate is constructed from fabricated longitudinal
girders with cast steel transverse sections containing the bearing
pockets and tie bolt holes welded into place. After manufacture,
the bedplate is stress relieved, the bearing pockets are line
bored and landing surfaces machined.
The bedplate should be inspected regularly for
evidence of cracking. This can occur in the welds joining the transverse
girders to the longitudinals, and under the bearing pockets, where the
cracks can be radial or follow the line of the pocket. As well as faulty
manufacturing techniques, causes can be uneven loading/ overloading of
engine units, loose tie bolts and loose holding down bolts.