They're 80 gallon fiberglass(in hull above the lead keel) with bolted steel plate tops. I have pretty good records and I don't think the tanks have been opened in the last 30 years, so there's a good chance they haven't been opened since built, 52 years ago.

Removing the original mechanical fuel sender gives an 1 1/2" diameter hole. I stuck a piece of garden hose to the bottom and pretty sure, the lowest part of the tank, screwed the hose onto a drill pump, put the outlet hose in a 2 litter coke bottle.

The first 2 litter came out nearly black, but I expected that as I use this pump for draining the engine oil(and it was full of dirty oil). The second 2 litter looks like new fuel. No sediment in the bottom of either bottle. My worry was the steel plates could corrode and the rust scale could fall inside(I've seen a photo of a similar boat that happened to).

I'm pleased. I have no fuel polisher, add a standard diesel additive. I have a Racor 500 and stock Bosch filter on the engine.

So why do some tanks go very bad in a shortish(compared to these) time? Are these mostly southern boats(many northern boats spend a good portion of their lives in warm climates)? Are they tanks left empty for years promoting condensation?

I wonder if with the vast amount of info available on the internet today, tank problems are a focus because of photos and forums. Problems like these(with photos) go viral. I've seen them, a few here have had problems. But overall, fairly common or rare?

I believe these tanks were always left full over the winter in the past. Makes sense to me, besides, with oil cost history, 80 gallons is an annuity.

Unrelated but for those that followed my compass problems, the compass today, sitting in the galley...has no bubble. I picked it up, tilted it both ways, no bubble. I swear it was there, big around as a quarter, right under the top of the glass. What's going on it there?

I screwed the compass back in the binnacle and will cut a new gasket for the fuel sender, and bolt that back down. Two dogs are put back to sleep.