There is another way to do it. I was invloved with a vessel here that needed ballast, but could not have a keel and did not have room for lead or anything extra. All the lower part of the hull
was taken up with fuel. But the Fuel was kept inside flexible fuel bag tanks, which was inside the main fuel tank
. So as Fuel was used, the bag squashed down and water filled the outer area. It was some years ago now, but if I remember rightly is was something like 5000ltrs or maybe more in fuel. The vessel had no engine
by the way, the fuel was for a 50ft support vessel. It was a Maori canoe of 100ft or more and it was people powered. They were going to paddle to Raratonga or some similar Island. But on a practice around NZ, they got into difficulty and it sank I think. Not sure, it was kinda kept a little hush hush so as not to embarrase some. I ftiied 100G's worth of Electronic gear
to the support vessel. It went back to commercial fishing
. All funded by us Kiwi taxpayers, but I won't go there.