Originally Posted by aronhk_md
Although...the tanks are aluminum held in place by aluminum framing which is attached to the aluminum ribs of the boat which is attached to the skin.......are they not grounded this way?
Let me offer a qualified yes.
If the tank is installed and secured by metallic straps that are bolted to structural components then it is fairly safe to assume the tank is electrically bonded. Use a meter and find out.
In the case of installations like yours, corrosion is most likely the result of local electrolytic "cells" created by contamination between things like mounting straps and the tank wall, or galvanic corrosion where a dissimilar metallic fitting is attached. Internal electrolytic corrosion can occur beneath sludge or deposits from minerals or silt that precipitated from the water over the past 44 years.
Bonding is not a cure for any of those issues, but a bonding wire from the tank itself (where you attach it may be problematic without welding on a tab or stud) to a nearby frame or structural hull fitting can't hurt and may reduce the severity of localized corrosion where the tank contacts structural members. That is where contamination collects and if you can "short circuit" the cell you may reduce the rate of corrosion in that spot.
A large aluminum tank subjected to sloshing and flexing for 44 years might be reasonably expected to suffer cracked welds that have nothing to do with galvanic or electrolytic corrosion.
The type of anode used on the exterior of the hull is so irrelevant as to be unworthy of further comment.