Good question! Airships do get more efficient as they get larger (more volume to surface area). But at the 650 lb payload size our vehicles fly more efficiency than a comparably-sized plane - small planes are also aerodynamically inefficient!

Our goal isn't to maximize efficiency, but rather to build something that fits a market need and that we can deploy quickly. We actually think that the quest to build huge, efficient airships has lead past airship projects astray. There are several projects that have struggled to build 10 ton or 50 ton vehicles, but no past attempts to build something in this size class.

Define what you mean by fly more efficiently. I'd like to see some numbers on efficiency.

The two most important efficiency metrics are Lift to drag ratio (L/D) and energy per payload mass per distance.

Lift to drag ratio describes the aerodynamic efficiency of the vehicle. Typical values, depending on flight conditions, are 4-5 for a helicopter, 8-13 for small planes, and 12-16 for our airships. (Large planes can get as high as 17, but a larger airship would be also be even more efficient). https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Examples_of...

From the L/D ratio and the propulsion system efficiency you can calculate the total energy used, and divide that by the payload mass to get energy/kg/km.

L/D ratio is not enough, you also have to take into account speed. A L/D of 6 at 120 mph has an equivalent per mile efficiency of an L/D of 12 at 60 mph (and a better von Karman efficiency).

The figure of merit is maximum (speed * L/D), not L/D alone, and this is where the efficiency of airships fall apart. Due to their greater parasite drag compared to aircraft (necessitated by the lifting gas volume), airships generally can’t match the same per mile efficiency, and even if they can, not at the same von Karman efficiency.

That's not the case. The energy per mile depends only on the L/D ratio and the mass of the aircraft.

L/D is a ratio of forces, and work = force times distance. Going faster at the same L/D decreases the travel time but increases the power used, for the same energy usage. Generally L/D depends on speed, but you can get an apples-to-apples comparison of the energy usage of two different aircraft by comparing only L/D.

I think we might be discussing two different figures of merit. The efficiency that I care about is "energy per lb of payload per mile." You haven't defined what you mean by "efficiency per mile," but from the reference to von Karman efficiency, I think you have a figure-of-merit in mind that includes speed - specific resistance or productivity or something like that.

But the economics of our vehicle and use case don't really change with speed. The middle mile legs that we're flying are not fully-utilized - the most important thing is to get the cost per flight down, so that we can dispatch the aircraft whenever there's a delivery ready.

But aren't these L/D numbers calculated for a theoretical case where there is no wind? What kind of L/D ratios would you see for helicopters, planes and blimps given a 20 knot head wind?

Sure, I don't doubt a blimp is more efficient when the wind is blowing it to where it wants to go, but what happens when the wind doesn't cooperate?

EDIT: yes, the Wikipedia page states that L/D numbers are usually presented as graphs, because they are dependent on speed. This means that without knowing the wind speed the blimp is facing, especially given it travels at a lowly 35 MPH, you can't really calculate an L/D number. For the helicopter and airplane, the wind matters less as they are going much faster.

Planes encounter a lot of air resistance because they're constantly creating lift, which means they're pushing a lot of air around -- which means drag. Planes also fly fast which means a lot of drag (drag forces are proportional to velocity squared). Blimps don't need to produce lift, and they move much slower.