The only testing I had seen earlier was on air injection through small holes in a hull of conventional shape.  I think the Japanese did a lot of development and got drag reduction of the order of 10 to 15% but a good deal of that was offset by the air pump power input.  The best overall result was a fuel saving around 7%.

DK Group have done tests with large cavities in the bottom of bulk carriers and claim fuel savings up to 15%:

SES have taken the concept further and applied it to a panning hull.  The air cavity is somewhat larger by proportion and the makers of the so called ASV have certain patents covering their technology:

If you go down to the comparisons they claim fuel usage for vessels of similar weight and speed is halved.  One of the factors that would come into play there is the improved propeller efficiency due to lighter loading.

SES state they have patents covering the technology:

I am not certain what part of it can be patented.

I started looking at the idea of a large cavity as a means to reduce the significance of viscous drag for a scale model.  The viscous component of drag tends to increase as a proportion of overall drag as scale is reduced.  That is when I found that it had been applied to full scale.

So taking the idea to a slender light displacement hull there are factors that suggest a favourable result - at least for calm water drag.  The wave drag is already low by virtue of the slenderness.   The volume can be achieved by increasing length but reducing draft while keeping beam the same such that wave drag does not increase but the ratio of bottom area to side area increases.  That means the bottom area is an increased portion of the overall wetted surface.  So that is getting closer to the planing hull idea where there is very little wetted surface hence drag is dramatically reduced because a planing hull does not have significant wave drag.  

Rick

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