Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
Line putting lateral force the peak of the gaff
Schooner with gaff vang at yellow arrow
A gaff vang is a line on a gaff rigsailboat used to exert lateral force on the gaff and thus control the shape of the sail. Rarely used now they are commonly shown on old pictures and drawings. Typically separate port and starboard vangs were fitted. The primary purpose of the gaff vang is to reduce the twist in the sail caused by the gaff "sagging away to leeward". This sag is the main cause of the gaff rig's poor performance to windward relative to the Bermuda rig.
The gaff vang "works well on a ketch or schooner, but is often found to be unleadable on a cutter"
Another use of the gaff vangs was to steady the gaff on a boomless gaff rig like a bawley when the sail is brailed up.
References
^ Cunliffe, Tom (1992). Hand, Reef and Steer, Adlard Coles Nautical, London, page 26.
