Explain a Wrap Casing Rudder
There are many rudder designs, although all rudders are hinged. Any rudder includes the rudder or flap that affects the direction of the boat, a hinge of some kind and a mechanism to actuate or turn the rudder, such as a rudder arm. Some rudders hang over the stern of the boat and mount to exterior hardware; other rudders pass through the hull at which point they are sealed. Wrap casing rudders refer to rudder designs that are cased and sealed where they pass through the hull or designs that also employ an extended casing as the hinge mechanism -- and perhaps both design elements.
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Rod
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Simple rudders have three sections of rod -- the rudder axle, an angled armature to provide leverage for turning the axle and a tiller arm to rotate the rudder. The axle must connect to the rudder, so much of the rod or axle must live in the water. Designs that pass through the hull must have some form of seal to maintain a mechanical guide, allow the rudder to turn while keeping water from penetrating the hull. The axle operates partially in and partially out of the water.
Casing
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The casing has at least one and sometimes two roles: it may function as a hinge, but it must function as a seal. The casing isn't literally wrapped around the rod as though it is made of flexible material such as tape; it must mechanically wrap around the rudder axle, encasing it. This is often an extended encasement or tube, rather than a single hinge-point.
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Seal
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The long encasement has benefits as a seal, its greater surface area makes it easier to withstand water pressure, which will try to penetrate the seal. The seal will likely be augmented by bearings to help the smooth operation of the rudder; the bearings themselves can also act as seals within the casing.
Hinge
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Rudders do not have to use extended seal/hinges or encasements; many employ multiple hinge-points in the same way a door might have two or three hinges functioning as hinge-points along a single axis. A wrap casing rudder may use discrete hinge-points inside the casing in the form of bearings between the case and the axle, but the entire length of such an axle is typically in contact with the inside of the case with sealed bearings at either end; it is one long hinge-pin. The entire assembly is greased for motion and sealing.
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References
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