Thanks John,
I work on the basis that if I make enough posts, (and they're long enough), some will make sense!!!!
Another aspect of this subject occurred to me today (I was driving at the time). It is that the (few ) modern airships currently in existence tend to have ducts on their (multi-bladed) props. I suspect that this is because the extra parasite drag caused by the duts at very low speeds is outweighed by the benficial effects of improved prop eficiency. This is of course pure conjecture on my part.
Any airship designers out there?