Christopher Columbus out-and-out said the earth was not flat. Then, Copernicus and Galileo declared that it was the earth that revolved around the sun. Then, Arvel Gentry completely "debunked the popular explanations of what made the sailboats go". Doug Logan writes "the foundations of sailing have been demolished, quietly and completely. No one saw it coming, and very few understood it when it happened." (Sailing World, May 1995.)

Arvel Gentry's original sail theory work was conducted in the 1969-1971 time frame. Gentry, a Technical Fellow of the Boeing Company, got his masters degree in Aeronautical Engineering from USC in 1958. At the time Gentry launched on his quest to figure out how sails work, he was working at the Douglas Aircraft Company. In 1977, after 19 years at Douglas the last ten of which were in the Aeronautical Research Group, Gentry moved from Douglas to Boeing in Seattle. He mostly worked in the Aerodynamics Research Department, and retired recently in 1995. In contrast to what we inadvertently claimed in an earlier version of the commentary on the WB-Sails "Telling Tales" article, Gentry never worked for NASA (although in 1953-54 he did work for NACA before the name was changed to NASA).

Since then, Gentry's material has been included in a number of books including those by Marchaj, Wallace Ross, and Whidden/Levitt. His paper "The Application of Computational Fluid Dynamics (CFD) to Sails" (presented at The Symposium on Hydrodynamic Performance Enhancements for Marine Applications, Newport, Rhode Island, October 31, 1988) describes how he used CFD in understanding sail aerodynamics. Today, no serious naval architect or sailmaker isn't well aware of the scientific basics championed by Gentry and his colleagues - A.M.O. Smith and C.A. Marchaj.

As mentioned in the commentary, Arvel started out by using an "analog field plotter" because he could do all of the basic investigations using the equipment at his home. However, he then used a digital computer program (based on a panel method) to verify all of his streamline plots and to calculate the pressure distributions on the sail. The actual airflow streamline plots and pressure distributions shown in all of his publications were actually obtained from a digital program. The use of the digital program was not mentioned in any of his early publications because the program was a Douglas proprietary program and could not be mentioned in any external reference.

Gentry's articles in the book, "The Best of Sail Trim," (which form the basis of the comments here on the WB-Sails "Telling Tales" article) were first published in SAIL magazine in 1973. At that time, Arvel's sail theory ideas did not set very well with the established "experts." From their high and lofty positions, they had been telling us that it was important that sailors understand how their sails worked (the wrong "venturi effect" and "slot effect" theories, etc.), and here was Arvel, telling them that they were all wrong, that (As Logan puts it) "much of what modern sailors had been taught about lift and drag on an airfoil was just rot". So, the stir he created was not appreciated, and his ideas faded quickly.

Gentry experimented on his Cal 20. He would go out sailing, with a bubble-blower in front of his sail plan, top to bottom, and with 500 telltales on his sails, starting right at the luff: "People sailed by and thought I was crazy", he says. He was doing research on "laminar separation bubble".

Gentry frequently sees the same negative reaction of the 70s even today. And many "experts" still write and teach sail aerodynamics with either completely wrong or misleading ideas. As recently as 1990, we could see sailors and engineers denying (even on the pages of the performance-oriented Sailing World) what was and is obvious to Arvel.

For example, the WWW article by Mark Johnson ("Racing Basics, Chapter 2 -Rigging") noted in the "Instruction Hotline" of this home page, references the book "Art and Science of Sails," by Tom Whidden and Michael Levitt. Much of Chapter 5 of that book is based on Gentry's work. However, Johnson then proceeds to say that "Looking at the top of the foil...you can see an air particle will have further to travel than his little twin particle going over the bottom. These two want to reach the back of the sail at the same time, so what gives? The particle over the top needs to travel faster." This is wrong! A particle of air traveling over the top of an airfoil reaches the trailing edge long before its companion particle moving along the lower surface. Both wind tunnel and computational results prove this.

So, a quarter of a century after Arvel Gentry developed his theory of sails, it's still not uncommon to see erroneous descriptions of how a sail provides lift. In a recent correspondence, Arvel notes: "Aerodynamics is a difficult subject, and all attempts to simplify it for the average person leads to wrong interpretations. The facts are that lift comes about because air has viscosity, which leads to the starting vortex. This is followed by the formation of a circulation field about the airfoil necessary to meet one of Helmoltz's theorems of vortex motion. Then the Kutta condition is satisfied at the trailing edge, and bingo -- we have lift. These principles, together with knowledge of boundary layer theory, lead to a correct understanding of the interaction between the jib and the mainsail."

Easy for him to say, right?

Gentry puts is as "You can't just sit there and stare at your navel and come up with conclusions."