While preparing to cut holes out of the cabin, you must first create the shape of those holes. Follow the listed steps:
With the cursor over the 3D View header, press A to deselect the cabin.
In the Layers button of the 3D View header, select Layer 1 with the LMB, then select Layer 2 using Shift + LMB. So, you can see the hull of the sloop as well as the cabin and Layer 2 is the active layer.
Press 3 on the NumPad for the Right view.
Press Shift + S. Select Cursor to Center from the menu.
Put the 3D Cursor in the center of the rear panel of the cabin, as shown in the following screenshot:
Press Shift + A, and make Cylinder from the Mesh menu.
In Tool Shelf, set the number of Vertices to 18. Set the Cap Fill Type to Triangle Fan.
Check the Align to View checkbox so that it is properly oriented.
Press S, 0.8, and Enter to scale the cylinder down in size.
Press S, X, 7, and Enter to make the cylinder longer than the cabin's width.
In the...