I did some more investigation with the drill. Paul's description is right on for hull #62. It is balsa right up to the tube. Looks like solid glass but most certainly is not. Like most other places on the boat, if something needed to go through a cored section they just drilled through and put it in.
Mine happens to be bone dry right up to the tube, but I will fix this design flaw anyway, just like I'm fixing all the deck hardwear installation. Drilling through balsa core and putting bolts in without isolating the balsa is not the way to build a boat if you want it to last 50 years!
