In general, a gudgeon is a circular fitting, often made of metal, which is fixed onto some surface. It allows for the pivoting of another fixture. It is generally used with a pintle, which is a male type of hinge pin which pivots in the hole in the gudgeon. As such, a gudgeon is a simple bearing.
In sailing, pintles insert into gudgeons that are normally affixed to the transom of a boat. Normally, the corresponding pintle fitting on the rudder slides, lowers or clips into the gudgeon. There are variations where gudgeons are mounted to the rudder and boat, and a pivot clevis pin is inserted into these gudgeons, or the pintles are fastened to the boat, and gudgeons are attached to the rudder. In any case, the fitting with the hole is referred to as a gudgeon. They are used to attach the rudder to the boat so that it can swing freely. The rudder can then be turned with the tiller. There must be at least two gudgeon/pintle sets for stability in the rudder's attachment to the transom. The pintles must face the same direction for insertion into the gudgeons and usually one is a bit longer so it can be the first into its gudgeon, giving some stability for the insertion of the other pintle. To prevent the rudder from rising out of the gudgeons there is often some preventer such as rudder weight or a locking device slid across the path of the pintle's removal from the upper gudgeon.