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However, if you can find an article or source book that explains the exact function of the balloons with regard to magical propulsion, please feel free to share. My first guess would be the Shining South book from 1993.
I will double check Shining South but I dont recall seeing that information when I was writing about Halruaa. Oh and please respond using the forum format shown on the top of this page, thank you!
I'll certainly take a look there, but at the risk of sounding defensive, its not just a depiction in Tomb of Annihilation, and yes while it can be boiled down different interpritations, or just a lack of understanding of the magic behind them, the exact description is "It flew like a blimp, with the aid of an amazing lighter-than-air gasbag."
I'll leave it at this unless I find anything else
It's hard to say that "some" skyships were propelled by gas bags, because we only have it written down that one of them did so. So at this point, I think the best we can do given the references we have is to say that "there is one" Halruaan skyship that used a gas bag to stay aloft (plus, since it crashed, as far as we know there are no longer any). At least until we find text evidence that there were others.
I hope this helps.