What must cranes and derricks with variable angle booms have?

Cranes and derricks with variable angle booms are required to have a boom angle indicator visible to the operator for several important reasons. This tool is essential for the safe operation of the equipment, as it allows the operator to monitor the angle of the boom accurately throughout the lifting operation. Understanding the angle is critical because it directly affects the crane's lifting capabilities and stability.

The operator needs to maintain the boom within safe operational limits to prevent tipping or losing control of the load being lifted. A visible indicator ensures that the operator can quickly reference the angle, allowing for adjustments to be made as necessary. This contributes to overall safety by minimizing the risk of accidents related to boom misalignment or exceeding the crane's operational specifications.

While the other options mentioned (such as an emergency stop button, weight sensors, and sound alarms) serve important safety functions, they do not specifically address the unique requirement of monitoring the boom's angle, which is crucial for cranes with variable angle booms.