Your basic fantasy-style airship
is typically a balloon of some kind with a regular wooden boat (anything from a canoe to a galleon) slung underneath it and some kind of rotary propellers at least on the rear of the ship, to provide thrust, and often some along side the sides aligned vertically to - ostensibly - provide some extra lift but mostly there to just look nifty. The balloon is usually the length of the ship, and occasionally slightly - slightly! - larger, overhanging by less than 10% of the length of the ship.
Last time I looked at this what I came up with was that, even using hydrogen, at that kind of ratio - of a balloon of roughly the same dimensions as the ship - and using modern light materials and a moderate ship size (mostly-fiberglass yacht of around 60 feet length and 20 feet wide) that the amount of lift provided by the balloon would be minimal, enough that you might as well ignore it entirely and rely entirely on propellers/rotors.
1) Does that sound about right? I understand that modern blimps use helium for safety reasons, but the balloons are VASTLY larger than the cabins.
2) If a ~60'x20' elliptical balloon of hydrogen is not enough to lift a similarly-sized fiberglass yacht, what kind of gas WOULD you need?