What function does the trick wheel serve in a steering system?

The function of the trick wheel in a steering system is to provide local control of the rudder. This means that the trick wheel allows an operator to directly manipulate the position of the rudder from a location on the vessel, facilitating precise steering adjustments. This is particularly important in scenarios where immediate feedback and control are necessary for safe navigation.

By using the trick wheel, a machinist's mate or helmsman can make small, deliberate changes to the rudder's angle, which in turn affects the vessel's course. This local control is essential for responding to conditions such as changing tides, wind, or obstacles in the water.

In other options, adjusting the speed of the rudder relates more to the implementation of hydraulic or electric systems rather than a mechanical manipulation, operating the main engine is a separate function not directly tied to the steering system, and locking the steering system generally involves additional mechanisms that do not include the trick wheel. Thus, the emphasis on local control aligns closely with the essential duties of maintaining precise steering operations on a vessel.