Re: What is the best way to get an airship to have optimum lift capacity?

You've asked some very intriguing questions.

First, airship lift depends on the difference in the weight of the air 
displaced by the airship and the weight of the airship itself. (It is the 
same for a submarine in water.  The buoyancy depends on the amount of water 
displaced versus the weight of the sub.)

In the case of a hot air balloon, you heat the air to make it less dense.  
The weight of the balloon is less than the weight of the air it displaces.

In the case of helium or hydrogen, the density of these two gases are less 
than the density of air.  The difference causes the lift.

It is true that a pure vacuum would give a great amount of lift, but the 
structure to contain the vacuum adds a lot of weight also.  The added 
weight of the structure would offset the lack of weight of the gas.  
Airships work because they are made of lightweight fabric and the payload 
is typically small.  Note that they have to have a tremendous volume to get 
a lot of lift.  I suppose if you could capture a vacuum in a foam like 
substance, you might be able to create a lift, but the overall ratio of 
structure to air pockets would still likely be less than the air displaced.

A mixture of hydrogen and helium would work better for lift, but hydrogen 
has two problems.  It is such a small gas, that it easily escapes through 
most substances, so it would tend to leak and create a hazard where it 
leaked.  It would also tend to separate from the helium and create a 
layering inside the balloon.  The lighter hydrogen would float to the top, 
which would still be a potential hazard if a spark was created.  I suppose 
you could stir the gas with fans, but that would add more weight.

Interesting questions.  Hope I have helped.