Does your boat vibrate or “whistle” from the bulb? All the fault of a Hungarian physicist, here’s why

Have you ever felt, at certain speeds, a seemingly inexplicable and undetectable vibration coming from the “belly” of the boat? This is something that can happen especially to trapezoidal-shaped keels, but not only to those, with a more abundant exit profile. Probably some of you, hearing this vibration, rejoiced, thinking that the boat was sailing so fast that it began to “whistle.”

Nothing could be more wrong. Theorizing this phenomenon was, in the early 1900s, Hungarian physicist and mathematician Theodore Von Kármán,1881-1963. Was he an expert on sailboats? Not exactly.

Von Kármán studied fluid dynamics and in particular one branch of it, aerodynamics; in fact, he also collaborated in his career with NASA and the U.S. military. He theorized the so-called “Von Kármán vortex wake,” or that phenomenon characterized by the alternating detachment of vortices from the trailing edge in some squat bodies.

To simplify and make the concept understandable: the trapezoidal bulb of a sailboat has two faces, the water outlet vortices, when the “whistle” comes into action, alternately detach from one or the other side discontinuously, creating a change in the distribution of pressures around the body and the resulting vibration. Turbulence that will then affect the propeller and the rudder, causing further “damage” to the progress of our boat, not to mention the fact that if the vibration is particularly pronounced it will even be transmitted to the rigging, the mast and then the sails, disturbing the boat’s propulsion.

Basically you think you are going fast but just then you start to slow down and most importantly you are not doing the boat any good. Why does this happen? Because very often, especially in boats not designed for competition (but not only in those) the drift outline is quite thick and stubby.

What is the solution to limit the problem? Let us preface this by saying that it is one of the classic “Don’t try it yourself at home,” but you need to get help from an expert or advice from the boatyard that built the boat. It is necessary to file, “cut off,” the bulb outlet, at the extreme tail and only for very few millimeters, to create an angle between 30 and 45 degrees .

A small fiberglass template that follows the 45-degree cut and will have the function of further thinning the trailing edge of the drift to clean its flows and eliminate vibration.

Mauro Giuffrè