Final answer:
Engine speed is the input condition that may not have an effect on the control of the steering gear when steering on autopilot. Environmental factors such as sea state, wind speed, and current direction directly impact steering control, whereas engine speed controls the vessel's overall speed.
Step-by-step explanation:
When an autopilot system is engaged on a vessel, the control of the steering gear can be influenced by various environmental factors. The autopilot must constantly adjust to maintain the set course. Factors such as sea state, wind speed, and current direction have a direct impact on the vessel's ability to maintain its course, thus affecting the autopilot's steering control. However, the engine speed refers to the rate at which the vessel's engine is running, which typically controls the speed of the vessel rather than its steering capabilities.
In the context of the question asked, it's important to note that the function of the autopilot is to maintain a set course despite variations in environmental conditions. While environmental factors like sea state, wind, and current can push the vessel off course, the engine speed does not have a direct impact on the control of the steering gear. As such, engine speed is the input condition that may not have an effect on the control of the steering gear when steering on autopilot.