Because hot air has only a third of the lift of helium thermal airships are very large for a given payload. However they are very impressive vehicles to look at and are excellent advertising platforms in fair weather condition.
In 1970s the Lindstrand team was the first to build a Type Certified pressurised thermal airship. Since then over 100 thermal airships have been produced by the Lindstrand Team.
The most unique thermal airship was constructed for French botanists in 1993. The AS-300 is the world’s largest thermal airship (300,000 cubic foot) and has an underslung raft, which is positioned by the airship on top of the tree canopies in the rain forest. The raft is detached from the airship and the botanists can now carry the research without damage to the rainforest. When the research is finished at a given location the airship will return and pick up the raft and the scientist and go to a new location to investigate.