If land vehicles are excluded from the GvK diagram, then a large triangular "gap" appears, spanned by merchant ship, destroyer, and commercial airplane, with airship being the lone inhabitant of the gap. After the GvK diagram became more widely known to marine engineers, a large number of designs were promoted to fill the gap, such as planing boats, hydrofoils, hovercraft and ground-effect vehicles, without success.[5]
Extract from Wikipedia's entry on the Karman-Gabrielli graph that compares efficiency of different types of transport versus their speed. The triangle described is bounded by jet airliners (very fast but relatively inefficient) through merchant ships (very slow but very efficient) to destroyers (altogether faster than the previous at considerable costs in terms of efficiency.
The 'sweet spot' central to the triangle is inhabited by the airship, which shares the efficiency of the merchant ship whilst encroaching upon the higher speeds. As noted in the entry, the only other means of travel that combines this speed and efficiency is those maritime craft that largely get out of the water.
Hydrofoils meet this criteria, but have largely fallen from favour because of damage to which they are prone by flotsam and jetsam. The tide is turning though with electrical powered hydrofoils, whose re-amplified efficiencies are making the damage issue the more sustainable.
