A rudder hangs on two bearings, one where the stock enters the hull and one lower down, and turns thousands of times a passage. The bearings wear. The blade, usually fibreglass over foam and a steel frame welded to the stock, takes on water. Both show up on the haul-out if you push on it.
How a surveyor checks it
Push the rudder. Pacific Yachting's routine: at each haul-out, grip the bottom of the rudder and apply sideways force back and forth. The rudder should feel stiff and not have noticeable play. A plain bush that's worn gives a solid thunk. Roller bearings, fitted on larger rudders, tend to fail by binding.
Hold the rudder and move the wheel. With someone holding the blade, turn the wheel or tiller. Any movement between the two is play in the quadrant and keyway, or between the blade and the stock. Jefa notes that tiller or quadrant movement can wear the keyway.
Sound the blade. Andy Cross, in Good Old Boat, says well-bonded glass gives a ping when tapped and delaminated areas a thud. Rust-coloured water from the blade can be a sign of corrosion inside. Movement of the blade on the shaft is an early sign the internal plate is breaking away.
See the stock. Drop the rudder a few inches to look at the stock where it leaves the hull. Pacific Yachting says pitting deeper than about 1/16 inch should be looked at by a surveyor or boatbuilder.
Fouling. Jefa says marine growth most often fouls the lower bearing, and that bearings should be inspected annually and at each haul-out.
What decides the grade
What the rudder is hanging on. A blade that moves on its stock, or a deeply pitted stock, is an A. Noticeable play, a thunk or a weeping blade is a B. Stiff steering from a fouled bearing is a C.