What factors influence the maximum speed of an airship? I'm interested in learning about the physics of airships; in particular, how you determine an airships maximum speed. 

For example, the Hindenburg topped out at about 80 mph. The ship itself weighed about 130 tons, not including cargo, passengers, etc. 

If it weighed less could it have achieved a higher top speed, or would 80 mph have been about the fast it could have gone regardless, for other reasons such as drag/air resistance? 

If it had twice as many engines, could it have gone twice as fast?

These are oddly specific, noodly questions, and I'm not looking for the answers themselves, but rather I'd like to be pointed in the direction of the math/physics fundamentals for getting the answers. I'm working on designing some fantasy airships for an RPG, and of course while they're not real and very fantastical, I do want them to have some semblance of verisimilitude, and imposing real world physics (with some wiggle-room for the fantasy part) I think helps a lot.

Thanks so much for your time and answers!
I'll do this without most of the math since your target audience won't want to read equations in your story.

In the simplest terms, the maximum speed of an airship occurs when the maximum thrust generated by its engines is equal to the drag it experiences while being pushed through the air at that speed. That drag depends on the diameter and length of the airship (more specifically, the projected frontal area and the total "wetted area" of the entire vehicle's outer skin). 

This is a complicated business because the relationships between all of those variables are strongly nonlinear- for example, the drag varies with the speed squared and the power required at a given speed varies with the cube of the speed. 

So in your story: doubling the power of the engines will increase the airship speed only slightly, at the cost of doubling the fuel burn rate and therefore the fuel tank capacity- and thereby reducing the useful payload. 

Similarly, doubling the length of the airship will roughly double the wetted-area drag, which means you have to double the horsepower of the engines or their number in order to achieve the same maximum airspeed as the shorter airship.