Materials in Blender can be reused. That keeps files smaller and lets you easily modify materials throughout a scene:
Select the Materials button on the Properties window header. It's the ninth button from the left, with the chrome ball on it.
In the Material panel, notice that Hull Material is already there. One clue to why that is can be found next to the box where you input the material's name. Notice that it has the number 2. That number represents how many objects use that material. Both the Sloop and Cabin were made from the default cube, and the Hull Material started out as the default material. So Blender has already been reusing the materials.
With the cursor over the 3D View window, press A to deselect all the faces.
Select the Object Data button on the Properties window header.
In the Vertex Groups subpanel, highlight the Cabin Roof group in the menu and press the Select button with theLMB.
Select the Materials button on...