My opinion is that you should match alloys with shaft and shoe. Manganese and naval brass are close on the galvanic scale. But I really would argue NOW that they present a problem when matched with silicon bronze. That includes any fastenings you will put with the shaft or fitting. I think the can of worms applies to mixing alloys. Even bronzes. I want to hear exactly what any metalurgist has to say on this.
338's shoe was quite corroded, and it may have been that if the alloy was manganese, it was 40% zinc. Bristol Bronze said it was manganese bronze which is highly resistant to salt water corrosion. But obviously NOT in all cases. DFO has an entirely different rudder in place of the original, that had a stainless shaft. Believe it reacted with the rudder shoe, despite, or because of the zincs. Or forgetting about them. Manganese bronze cannot be considered an underwater bronze like silicon, because in an electrolyte like salt water it can self distruct by galvanic action.
I went with silicon because silicon fastenings and rod are common. I went with silicon because it is mostly copper. And I thought the active bearing surfaces of shaft and shoe should be the same metal. If the boat is going to be parked in a marina where strange electric currents abound it is something to consider. Under water, manganese bronze (brass) fittings can self destruct in modern marinas.
Reply With Quote