The Gold-Rush Clippers
Tom Cunliffe Aboard Jolie Brise

Remembering Arvel Gentry and John Letcher, Pioneers of Sailing Science

It was nearly 30 years ago that Sailing World published "Fluid Dynamics: How Modern Science and Sailing Discovered Each Other." (You can read it here in PDF form.) The main players covered in it, Arvel Gentry and John Letcher, have gone to Fiddler's Green, Gentry in 2015 and Letcher in 2018. Both were geniuses and gentlemen of the sport.

Almost a quarter-century before that article appeared, Gentry had written a series of lessons in Sail magazine that debunked some of the popular concepts of how sails worked. "Gentry and company were weighing in with talk of stagnation streamlines, separation bubbles, starting vortices, Kutta Conditions. They were saying outright that much of what modern sailors had been taught about lift and drag on an airfoil was just rot. And they weren't appreciated."

They weren't appreciated because, as sailors and human beings, we tend to simplify concepts until our brains are comfortable with them. Well, as Einstein said, "Everything should be made as simple as possible, but not simpler." To this day, despite the prodigious advances in both the hardware and software that have made computational fluid dynamics (CFD) commonplace in the design of vessels, sails, and underwater appendages, there's still plenty of mystery in how these structures operate in the real world, and it's not for want of either brain power or computer power.

CFD_Shuttle
A CFD image of turbulence behind the space shuttle on re-entry. Source: Wikimedia Commons.

What aero-hydrodynamicists and computer engineers have been able to do with CFD is create an animation of dynamic events, and they can make pretty accurate predictions of how things will work given a particular set of imagined conditions. But of course fifteen degrees of heel angle are not always fifteen degrees. And real waves don't observe a constant height or frequency. Windspeed varies. Wind direction varies. The same goes for the set and drift of water current. Then add in the human factor: People steer differently, and trim sails differently, and move around the boat, changing everything all the time.

As the Fluid Dynamics article says, "The essential challenge... is in trying to make a vessel move nicely through two fluids simultaneously, part of it stuck down into thick, slow-moving water, and the other part stuck up into thin, fast-moving air. Add peripheral challenges like waves, local wind dynamics, geographical effects on both fluids, and the limitations of  boat design, and you've got an excellent puzzle to solve." Which is why people like Gentry and Letcher have always been drawn to the sport, and why sailmakers and Formula One engineers and aeronautical engineers enjoy talking to each other.

There's still mystery in sailing because the elements we sail in are always chaotic, slightly or greatly. It seems likely that there are some people who, by nature or practice or both, are better attuned to the what's coming through the chaos pipeline than the most powerful and sensitive computer program. Will that always be the case?

- DL