Mike, I agree. I have seen some "hydro-formed" Cessna parts. A compromise in the structure would allow the sinking process to happen faster.
I am in total agreement about the positive buoyancy effect of the AVGas however I am sure that empty tanks must be even more positive.
Greg, I bet that adding some floating foam wouldn't add much weight. Perhaps this stuff?
I wouldn't put it into my project because I don't plan on going swimming!
OTOH, if I had a little ultralight or microlight floatplane, I certainly would consider it!
Neal, I totally missed that story and am glad that you brought it to my attention! There is LOTS of great information there!
Once again, there is nothing new under the sun. I knew that there must have been some experience in this arena.
I am glad that it all turned out well for the pilot. I think that a reasonable approach to a buoyancy solution is the inflatable Class V PFD's that I have.
As a floatplane pilot, I know that capsiztic entrapment is a genuine concern. As a SCUBA diver, I think that I would be level-headed in such a situation. As a pilot, I know that some real training in that area would be a solid chioce in the right direction.
I know that Thermos (RV-7 builder here) has had precisely that training and he told me how invaluable it is. I totally agree.