I assume your neighbour's boat is an older J&J design, in which case if you google Bavaria rudder bearing replacement you will find a lot of information, in particular a youtube that shows what the problem is and how to replace the lower bearing. Seizure of the rudder is caused by one of two things, most common is the seizure of the lower composite bearing in the aluminium housing - as in the video, and the second, less common is the rudder stock seizing in the bearing because of corrosion of the stock itself.
Unfortunately there is a lot of misunderstanding of galvanic corrosion. It occurs when two dissimilar metals are in contact in an electrolyte (seawater). The most common examples are brass alloy props on stainless shafts such as fitted to most modern shaft drive boats. It is the zinc in the propeller that erodes leaving just copper, so the solution is to introduce an additional amount of zinc electrically bonded to the assembly, such as a shaft anode or an anode attached to the hull close to the assembly and bonded to the shaft with a wire, often by being attached to the engine/gearbox that the shaft is attached to. In the case of a saildrive, the two metals are aluminium for the housing and stainless for the shaft, bearings and fastenings. This is protected by isolating the whole drive from the rest of the boat (including the engine AND propeller) and attaching an anode to the housing. The video is wrong in suggesting the rudder should be bonded to the saildrive anode. If a folding or feathering prop is fitted, it is still isolated from the drive and may have its own anodes because mixed metals (usually brass alloys and stainless steel) are used in the mechanism. Most of the alloys are very corrosion resistant and anodes are small, or even non existent as in my older FlexoFold 2 blade, but others require larger anodes with short life.
Getting back to the bearing housing that is the root cause of most rudder seizures, you will see that it is not made of two dissimilar metals immersed in seawater, so bonding to an anode as the narrator suggested will not stop the corrosion. The cause is staring him in the face in the video - it is the copper based antifouling that coats the aluminium housing, and it is the copper that is the second metal. If you look at your saildrive manual it stresses the need to use a copper free antifoul such as Trilux on the aluminium housing. Exactly the same should be used to antifoul the area of the hull around the bearing housing - usually a 10cm circle.
No anode was fitted to earlier Bavarias because none was needed. The one fitted to the current boats is there to comply with RCD requirements for the electrical system. As you have discovered it is not connected to anything in the steering system, although I think that the lower bearing design is different from the earlier boats. There has never been any need to bond seacocks in a GRP boat because there are no dissimilar metals in contact with seawater. Corrosion of seacocks and fittings happens because some are plain brass (copper and zinc) and not corrosion resistant in seawater, although in reality even these can take many years to show any signs of dezincification. Attaching an anode will not make any difference to this type of corrosion.
The boat shown in the video is the same as my old 37 which I had for 14 years from new and no sign of stiff rudder when I sold it. The video does make the point that the boat was left for long periods in the water unused and this makes the build up of corrosion in the housing worse as regular use rubs the deposits away. As he said, take the boat out of the water and the shaft and bearing lose lubricating water and seize.
Hope you (and others) find this explanation useful.
