(especially Marius and tarheel to answer your questlions)
@Marius: The magnets I have used for the magnetic rudder couplers were samples I had got from a dealer on who had been on a model exhibition selling various electronic components, tools and so on... This had been more than 10 years ago (you can see it by the coin on the photograph for size comparison, a former DM, since 2002 we have the EURO).
Meanwhile you can buy powerful magnets in various sizes for example from ebay (look for "Neodym magnets") for almost reasonable prices. But note: for applications where they are exposed to water you shall protect them from water contact by some moulding (e.g. epoxy glue) because if their nickel plating is injured (fine cracks in the surface) the magnet material beneath the plating will start to corrode rapidly.
@tarheel: Efficiency is a question of running speed, whether the friction of the seal or the "water friction" of a wet running rotor is more lossy. For higher rpm speed a conventional shaft seal will have less losses in comparison to a wet running motor. A wet running rotor is stirring the water around and those water whirls (that do not help for propulsion)cause losses as well.
But the real advantage of the wet running motor is the tightness, there is no leakage at all even at extreme diving depths.
Kind regards
Hermann
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