With the rudder, I new that when we bought the boat, the rudder had moisture issues. I waited and waited until I though I couldn't wait much longer. Since I knew there was water in there, I drilled a 3/8" hole at the bottom of the rudder thinking that "what harm could it do, it was already saturated. When we hauled the boat each year, the water simply drained out and that way I didn't worry about expansion due to freezing.
Two years ago, I bit the bullet, dropped the rudder, cut her open, discovered all sorts of nasty, including mussles, worms and general stink...but no corrosion on the stainless steel structure...I was OK. The re-build process was straight forward: clean out all of the old foam, re-glass weak areas, epoxy resin areas of water ingress, re-foam, re-glass and correct anything on the pintle, paint, launch, drink a beer!
As far as detecting what's going on? You could use a moisture meter but if you tap the rudder with the butt end of a screw driver and if the sound is dull, you may have water. If the sound is sharp and solid sounding, maybe OK. If you drill a hole at the bottom of the rudder and water comes out, you have a saturated rudder. I kept my hole open knowing that water would make it's way back in even if I filled the hole with epoxy and that I would need to drill the hole back open when out of the water. As far as the stainless is concerned, I am afraid the only way to know what's going on is to expose it........cut it open.
This past winter, I re-built my cousins T-30 rudder, but this time, I had to re-construct the entire stainless structure, rudder post, tabbing et al. This was a 1976 boat, so 30 to 36 years is a good run for these rudders before a re-build.
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