Hi Nerys, I found that the salt tends to get into places you can't wash out, and even oil won't stop the corrosion happening. When I rebuilt my Graupner Optimist yacht recently, I had to destroy the rudder shaft and tube to remove them from the boat, as it had blended with its tube. I used to sail it in the salt mainly, and religiously washed it and oiled the rudder shaft, but it didn't stop it corroding. The head of the 12mm keel bolt was also seriously rusted, due to a small amount of un noticed salt leakage sitting in the bottom of the hull under the floor. Now rust proofed and seriously glassed in.
Don't think it would be such a problem if you used stainless tube etc, but the problem is sticking the bits together, (need to be TIG welded),- not something most people can do at home. With non stainless mini shaft ball bearings there is also the same problem.
Probably need to have rudder shafts etc easily removable as a precaution, for long term storage if used in salt.