Common Sails	March 1, 2011

This type of sail or sweep is considered common because it does not have mechanical louvers or other contrivances to catch the wind. Instead, the common sail relies on a panel of canvas that is affixed to the framework.

The parts of the common sail (shown in the figure below) are very utilitarian and descriptive of their purpose. They are:

• Whip: The whip is the center shaft of the sail that is bound to the windmill's stock. The whip has through mortices that are angled from 15 degrees nearest the windshaft to 0 degrees at the tip. These mortises receive the sail bars and give the sail a shape that is responsive to the wind.
• Sail Bars: 11 Sail bars are morticed into the whip.
• Hemlath: Hemlaths are the outter, wooden strips that tie the sail bars together and create a smooth, fair surface along the length of the sail.
• Uplong: The uplong is a correlary to the hemlath. It runs along the sail bars at intervals between the hemlaths and the whip and provides additional rigidity and strength. Typically, the uplongs are installed on alternating sides of the sail bars.

Click here to watch a time progression of a windmill sail being assembled.

The bulk of the time in constructing the sails was spent fabricating the whips. The first step in the process was to combine laminations of white oak with epoxy and wood screws.

The rough whips are then belted sanded to remove high spots and then run across the jointer to true-up one face.

With one side of the whip square and true, the body of the whip is run through the planer to smooth the opposite edge and produce a consistent thickness.

Next, any voids, cracks or knots in the whip are filled with catalyzed wood filler.

Through mortises are then created in the whip using the drill press. Because each of the mortises must be created at a unique angle, angled support boards are created and are used to position the whip for drilling.

Once the whips are completed, fabricating the sail is a fairly straight forward process involving isntalling the bars, hemlaths and uplongs. Each element is connected using adhesives and galvanized fasteners.

Finally, the sails are filled with an elastomeric putty that will remain flexible as the temperature and humidity changes. Once sanded, the sails are painted with a high grade paint and they're ready to be installed.

Note: During the construction and finishing of the sails, there were a significant number of cracks that emerged in the paint. In an attempt to mitigate this problem, we used a mixture of 1 pint of elastomeric putty to 1 gallon of paint. The finish was thicker, provided a more durabble coating and was resistent to scuffing and environmental changes.

Thanks to everyone who came out to help with this part of the project...