And strangely, there was dampness underneath, despite the boat having been under cover for a couple of months.
I drilled a hole in the bottom, and 3 litres of water poured out, which tasted salty.
I drilled a couple of holes in the top, the same distance apart as the camera lens on my phone, and the flash, and took a snap through the hole:
There's a rivet at the bottom I'd drilled out previously, dropped onto what looks like yellow sludge. Not knowing how much corrosion this water might have caused, I had not choice but to cut out the beam:
Sludge in the ends too:
Anyone with a catamaran with a beam that is embedded into the hulls and which has any rivets in it would do well to drill a small hole in the bottom to check for water inside! Bad design! I think embedded beams can be OK - it is surely a strong connection, but if the beam is not entirely water-tight, there'll be trouble, and rivets can't be relied upon to remain water-tight.
The good news is the extent of the corrosion in the beam - it's not enough to require a new beam, but enough that it was essential that it wasn't allowed to progress further. A bonus is that I will now be able to tweak my plans for the new arrangement with a seagull striker, because I can now easily transport my beam to the welding shop and have the new fittings welded on instead of clamped on as I'd planned previously.
Here's a beam stump ready for laminating over:
A piece of ply was used to fill the space where I've removed the foam, and the whole lot glassed over with biax. The ply knees and other structure I built into the hull to support the beam can be used to give support to the aluminium pads I've have made to provide support to the new beam.
Here's the modified plan:
Off to the welders!