Rudder repair, blister & more
Ok, I noticed two raised areas on the port side of the rudder (aluminum stock, foam core), one smaller one around 3" diameter near the bottom, and one much larger one around 8" diameter around 2/3 of the way down. Both were about 50-80% aft from the rudder''s leading edge.
After attacking both with a sander, I started chiseling open the smaller one. The very thin and obviously yellowed outer glass quickly yielded to a white waxy substance that smelled none too good. I dug out around 1/4" depth of this stuff until I got to good fiberglass and aluminum on all sides. No foam down this far on the rudder. The aluminum was quite pitted under this area as well... seems like standard blistering from what I''ve read. Planning to clean it out and fill it with epoxy, then sand & paint.
Now, the upper big area is a little more of a mystery. The fiberglass had a mottled red and green appearance to it. After sanding some of it off I realized that I was now past all the glass and into the light yellowish/green foam over a wide area of this raised bump. The foam seemed dry, but tapping this area brought out a definite hollow sound. I decided to dig through the foam and see what lay underneath. After digging out a small hole with a screwdriver perhaps 1/2" deep through the foam, I broke through a very thin hard backing layer of some sort and into a hollow area inside the rudder, with perhaps 1/4 or 3/8" of air space between the back of the foam and more aluminum inside. I dug a second exploratory hole in a different section of the raised area with the same results.
I didn''t note any water inside this air opening, and as I mentioned before the foam itself seems dry, although the smell of air coming out from the space had a very rotten chemical odor to it. So far nobody knows of any rudder construction where an intentional air pocket would be included, and given that this section was raised up it seems to me that some sort of blister reaction possibly caused internal gassing and pressure, separating the foam from the aluminum??
So, the question is what to do about this now. If my hunch is correct, then I''d have to chop out a fairly large section of the rudder and a lot of foam, and then come up with some means of redoing the foam (type?) and glassing over it. I don''t think that simply epoxying over such a large area would be the best idea, and leaving out a bunch of foam may change the buoyancy of the rudder?
Ok, I''m open to ideas.