This has nothing to do with stacking them on top of one-another, and everything to do with allowing the vessels to keep their balance inside the hull of a wooden cargo ship as it transported them from one port to another. The pointed bottom would be stuck vertically inside a wooden rack, usually packed with sand or straw, each vessel next to the other. This would prevent the cargo from shifting during sea transport (ships rock back and forth in the ocean, which tends to knock unsecured cargo around), keeping them from breaking open.

Source which also addresses flatbottomed vessels.