consumption but no carbon emissions, that would certainly be a useful
technology. And, as a person-transporter, the Electra delivers a respectable
11 kWh per 100 p-km, similar to the electric car in our transport diagram
on p128. But in this book the bottom line is always: “where is the energy
to come from?”

Many boats are birds too

Some time after writing this cartoon of flight, I realized that it applies to
more than just the birds of the air – it applies to hydrofoils, and to other
high-speed watercraft too – all those that ride higher in the water when

Figure C.13 shows the principle of the hydrofoil. The weight of the
craft is supported by a tilted underwater wing, which may be quite tiny
compared with the craft. The wing generates lift by throwing fluid down,
just like the plane of figure C.2. If we assume that the drag is dominated by
the drag on the wing, and that the wing dimensions and vessel speed have
been optimized to minimize the energy expended per unit distance, then
the best possible transport cost, in the sense of energy per ton-kilometre,
will be just the same as in equation (C.26):


where cd is the drag coefficient of the underwater wing, fA is the dimen-
sionless area ratio defined before, ε is the engine efficiency, and g is the
acceleration due to gravity.

Figure C.13. Hydrofoil. Photograph by Georgios Pazios.