On our way to Belize from St Petersburg in September we had a problem. North of Cuba the engine stopped. Tim went below thinking it was clogged filters. Well, not so bad, got the engine reprimed and started. Good until we rounded the Western tip of Cuba and clobbered by a Tropic Wave we saw a week earlier but our weather router (that's another story) said leave and we did. Now we were without power in 25-35 knot winds and seas building past 6 feet...
Fast forward from 2 days in Cuba and 5 days in Isla Mujeres. Filter fuel before it goes in the tank if you have any questions about it's purity. Keep the biocide in your tank. And based on our experience the additive in marina diesel isn't enough. We found it was best to continue with the biocide additive. We use Star Brite based upon West Marine's recommendation. Problems gone in our fleet.
As Tim said:
"That pattern you see at the bottom is from my fingers scooping out a handful of that coffee ground consistency crud. It was a half inch deep in the center of the tank in this partition. There was also a wooden plug from a hole saw down in there!
All we can assume is that over the years as fuel was put in the tank, sediment from the fuel storage tanks on shore was transferred into the boat's tank. After 43 years we had our own tank crud sand bar."