Rudder stock

What it is

A rudder stock is the vertical shaft that links the steering to the blade, on bearings, with a seal where it leaves the hull or the skeg. The stock is the shaft; the blade is the foil. Extra play, scoring and leaks at the seals are survey points. A heavy helm does not prove that the stock is sound. Universal millimetres of play are not printed from the dock.

Steve D'Antonio writes in Cruising World: the part of the stock that is almost unseen is where the shaft passes through the log, and where the wet packing of the gland lies on the stainless steel. If the boat sits for a long time, the oxygen in the water in the tube is used up, and crevice corrosion almost always sits right under the packing or under the bearing. The joint between stainless steel and the FRP blade opens early; water in the gap is again without oxygen. If the armature inside the blade is mild steel, that is ordinary rust. Rust streaks on a blade that looks whole are a reason to look further, not a medal that stainless steel does not rust. Stainless steel in crevices and on welds fails anyway.

In Professional BoatBuilder he writes the same point: a stainless stock is prone to crevice corrosion in oxygen-poor water inside a composite blade and under flax packing. For welding, the alloys are low-carbon 316L or stronger shaft alloys. Mild-steel mesh on a stainless stock is a known recipe for failure once water has reached the welds. Bearings of nylon or Delrin can swell and seize; UHMWPE does not take up water that way. A rudder gland, unlike a propeller-shaft seal, is often kept nearly dry: the shaft turns slowly, and there is nothing to heat the packing.

Pacific Yachting, on a production blade: two shell halves, the stock and the webs, foam inside. Water eats the welds of the webs. The sign: turn the wheel or the tiller and the stock moves, while the blade lags or does not follow. Crevice pits where the shaft enters the seal or the bearing: in their text, more than a local light pit deeper than about 1/16 inch (about 1 mm) is for a surveyor or the yard to look at; lowering the rudder and looking about every five years is their habit, not a law. On the slip: take the bottom of the blade and rock it sideways. It should be stiff, with no noticeable play in the bearings. Some of the play is visible afloat; the bearings and the bottom are often seen only on the hard. One wiggle at the dock does not give this millimetre.

Ocean Marine Surveyors separate the blade, the stock, the bearings and the link to the wheel. They sound the blade; they take the bottom in the hands, fore and aft and side to side. A bent stock is often the mark of a grounding. ISO 12215-8, for small craft up to 24 m length of hull, sets rudder scantlings and the required diameter of a solid metal stock, and it speaks of the clearance between stock and bearing. That is a design calculation, not a table of one play figure for every yacht at the dock. Class-ship clearances and IACS clearances are not carried here in one line. A heavy helm does not prove a sound stock. Universal millimetres of play are not printed. Nearby on the site: rudder bearing, skeg, haul-out survey and survey checklist are other pages, not this millimetre.