The hull is the base component module used in building minecarts with Steve's Carts. The hull determines which components and how many components can be built into the minecart.
A Wooden Hull's complexity cap is 15, so it can only hold modules that have modular cost of 15 or less, such as the Tiny Coal Engine, which has a modular cost of 2. Additional modules can then be added up to a maximum of 50.