The shape of an aluminium anode can vary depending on the specific application and the design of the cathodic protection system in which it is used. However, some common shapes for aluminium anodes include:

1. Rod-shaped: These anodes are cylindrical in shape and are typically used in smaller-scale applications, such as boat hulls or small pipelines.
2. Plate-shaped: These anodes are flat and rectangular or square in shape, and are often used in larger-scale applications, such as offshore platforms or large pipelines.
3. Tubular-shaped: These anodes are cylindrical with a hollow center, and are often used in applications where the anode needs to fit around a structure, such as a pipeline or a piling.

The specific shape and size of the anode will depend on a number of factors, including the size and shape of the structure being protected, the environmental conditions present, and the design of the cathodic protection system.