Crossing an ocean

“There is nothing more enticing, disenchanting, and enslaving than the life at sea.” ― Joseph Conrad

Despite the fact that most people travel by air, the majority of trade still travels by sea— as much as 90% of it.  Most commodities have long shelf-lives to begin with, but cargo ships are also one of the most fuel efficient modes of transportation.  But how much energy is needed to cross the ocean?  This is a simple physics problem.  Most fuel will be used to work against the drag of the water against the ship.  The required energy, in horsepower-hours, is $$E \approx \frac{1}{10,000,000} \times \frac{\Delta x^3}{\Delta t^2} \times \text{WSA},$$
where Δx is the distance in nautical miles, Δt is the travel time in days, and WSA is the wetted surface area of the ship in square feet.

The real usefulness of this equation lies in it's ability to estimate the fuel requirement of these vessels, as one tonne of fuel carries 17,000 horsepower-hours of energy.  In general, the numbers will be enormous, especially over long distances and short travel times.  Pull out a calculator, and see how much energy certain trips might cost—use your imagination or use real shipping routes.

The moral of this story is that as our supply of fossil fuels dwindle and their costs rise, it is highly likely that the age of sail will return.  After all, wind was able to supply all the energy needed during the 19th century to transport goods and will supply our electricity during the 21st century.