Understanding the use of riding sail

 

From Practical sailor, the full article can be found here.

There are four common factors that can cause an anchor to drag, (assuming it is well designed and ideally matched to your boat): poor bottom, short scope, insufficient shock absorption, and yawing.

The effects of a boat yawing at anchor can often go unnoticed until it is too late. Constant yawing prevents an anchor from setting deeply, and because the rode tension of a boat sailing vigorously at anchor is typically 50-100 percent greater than one resting quietly, it may cause the anchor to slowly walk downwind.

Using a riding sail.
A riding sail functions both by pushing the transom back in line when the boat sails to one side, and by increasing windage at the stern, steadying the pull on the rode. Traditional one-dimensional riding sails work by encouraging the boat to rest at a slight angle to the wind. These triangular panels are rigged from the backstay and sheeted to a side cleat at a 15- to 20-degree angle to the boat's centerline.

A riding sail functions both by pushing the transom back in line when the boat sails to one side, and by increasing windage at the stern, steadying the pull on the rode. Traditional one-dimensional riding sails work by encouraging the boat to rest at a slight angle to the wind. These triangular panels are rigged from the backstay and sheeted to a side cleat at a 15- to 20-degree angle to the boat's centerline. This causes the boat to ride at an angle of about 10 degrees to the wind. The stabilizing force on one tack comes from the sail, and the stabilizing force on the other tack comes from the side of the boat. Because the boat generally lies to one side, the wind load is higher, even when the sail itself is aligned with the wind. Three-dimensional Y- and V-shaped riding sails, including the Fin-Delta and V-Delta, set in alignment with the boat's centerline, providing a correcting force on either side when the boat yaws, as well as some steadying drag force even when aligned with the wind.

The weakness of anti-yawing strategies at the bow, including chain and kellets, is that they fail when they are needed most when the wind really howls. A riding sail, on the other hand, works by correcting the aerodynamic balance of the boat, in both light winds and extreme weather.

A conventional riding sail will reduce mild yawing, but its effectiveness is limited, and wed not use it in storm force winds. after all, Its 19th-century technology.

photo from https://sailrite.wordpress.com/2013/09/12/how-to-use-an-anchor-riding-sail/