The devices, such as mountings, in an engine that serve to secure the engine in place are not an actual part of the engine. However, a discussion of these devices is included in this chapter since they are obviously essential for installation purposes and play an important part in reducing the possibility of damage to the engine and to the machinery driven by the engine.

Different terms are used in identification of the devices that secure an engine to a ship. Terms such as base, subbase, bed, frame, rails, mountings, and securing devices appear in various engine technical manuals. To avoid confusion with our discussion on the engine base, we will use the word subbase to refer to the supporting and connecting pedestal between an engine and the hull structure of a ship. We will also describe two devices used to attach the subbase for a diesel engine to the hull of a ship.

SUBBASE

The size and design of the subbase depends on the engine involved and its use. In many installations, the engine and the mechanism that it drives are mounted on a common subbase. One advantage of mounting both units on a common subbase is that misalignment is less likely to occur than when the units are mounted separately. Diesel-driven, electrical generator sets are usually secured to a common subbase.

A different type of mounting or subbase involves the use of handfitted chocks, or blocks, between the engine and the supporting structure of the ship. Bolts are the devices used to secure the engine rigidly in place and to maintain alignment.

SECURING DEVICES

The securing devices that are used to fasten a subbase to the structure of a ship may be classified, in general, as rigid or flexible. Propulsion engines are secured rigidly so that misalignment will not occur between the engine, reduction gear (or other driven mechanisms), and the propeller shaft. Engines that drive auxiliary equipment such as fire pumps or generators may be secured by either rigid or flexible devices.

In installations where rigidity is of prime importance, bolts are used as the securing devices. Flexible securing devices are generally used between the subbase of a generator set and the structure of the ship. Flexible devices may also be placed between the engine and the subbase. Although flexible devices are not necessary for every type of generator set, they are desirable for generator sets that are mounted near the side of the hull. In this case, flexible devices will serve to reduce vibration. Flexible devices also aid in the prevention of damage from shock loads imposed by external forces.

Flexible securing devices are of two general types: vibration isolators and shock absorbers. Both types may be incorporated in a single securing device.