Why some warship propeller shafts are hollow

Are propeller shafts hollow, and what advantage does that provide?

Some are hollow to reduce weight and, with controllable-pitch propellers, to accommodate the control mechanism.

Older shafts were often solid, while modern arrangements may use a hollow form to save weight without surrendering the required strength. A controllable-pitch propeller can also require space through the shaft for its operating mechanism. This is a simplified introduction because the structural explanation needs a diagram to show how the load passes through the shaft.