TECHnic Your cruising boat already has foils. Here’s how to use them well

Class with Helium / 1. In this lecture, Italian-Brazilian “prof” Elio Somaschini explains in his own way that the rudder blade, the drift, the bulb, the hull And the sails are foil. Just like those Of the America’s Cup flying boats! Here’s how to use them well to navigate better, such as moving the rudder gently

Vi had already stopped to think that not only Luna Rossa, but also your boat has foils? In fact, the rudder blade, centreboard, bulb, hull, and sails are foils, just like those on boats that fly. Now we explain how your appendages (primarily the rudder) should be used gently, to make your boat go better. You will realize, in practice, the benefits you will gain by conducting yourself more consciously, knowing that for every action there is a reaction.

WHAT IS A FOIL?

First, what is a foil? It is the cross section of a wing. Imagine that a wing is like a salami. If you cut it into thin slices, these slices are called foil. So, the foil is not a wing. The wing is three-dimensional; the foil is two-dimensional. It is very important to study wings, though, because the reasoning is like this: we study what happens on a slice of the salami and then, assuming the slices are all the same, we just combine all the slices and we know what happens on the salami, oops-I mean, the wing!

Figure 1 is a sketch of a foil of your boat, which is actually like a slice of the rudder blade. When the water comes exactly front to the blade, some of it goes to the top (extrados) and some to the bottom (intrados). Since the two parts have the same shape, what happens in the upper half happens in the lower half. This is said to be a symmetrical foil.

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However, if we tilt it in relation to the flow of the water, a force arises that we can decompose into two: one perpendicular to the flow, which is called Lift (I think it is called sustenance in Italian) and the other called Drag (I think it is resistance). See fig. 2.

RESISTANCE.

Let us dwell a little on resistance. Clearly, the less resistance, the faster and without much effort our boat moves. So it is essential to decrease drag in order to sail better.

What does drag produce? Two forces: one is the difficulty for the water to slide over the foil surface and is parallel to the foil surface, the other is caused by pressure and is perpendicular to the foil surface. The other types of drag we will not consider.

Imagine taking a rectangular piece of plywood and placing it perpendicular to the flow of a fluid(fig. 3). In this case, there is practically no slip force, but the pressure difference between the front where the fluid comes from, and the back where the fluid comes from is huge. Then a great force pushes back this plywood of ours.

This is an example of drag caused by pressure. Note that the force is perpendicular to the surface (important!).

Now try putting our sheet of plywood horizontal, so that the fluid can slide over its surface. As you can guess (correctly), there is practically no difference in pressure, however, the fluid will have to slide over the entire surface of our plywood, and the larger the surface area, the greater the resistance force(Fig. 4). Important: in this case the drag is parallel to the surface!

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NON-LAMINAR FLOWS

Let’s look at another example where pressure drag is important: imagine a ball receiving wind. In the front part of the ball, the pressure becomes great, then, in the back part, the pressure will be less. There you have it: if there is a pressure difference, there is a force going from the side where the pressure is high to where the pressure is low. Then, because the air moves in a nonlaminar fashion, here is where the resistance force becomes very large(Fig. 5). To sail well, we must avoid non-laminar flows at all costs!

What does it mean to “not laminate?” It means that the air (or fluid) acquires chaotic motion, with cavitations, flows with turbulence. And why does this happen?

When the fluid moves up the ball, the pressure decreases and the fluid velocity increases (this is called favorable gradient), however, when the fluid moves down the ball, the pressure increases and the velocity decreases Is called an unfavorable gradient. See fig. 6. And here you have to look well at what’s going on to understand.

The fluid moving in contact with the surface has a lower velocity than that of the fluid a little farther from it, due to resistance. Because the fluid’s velocity increases but the part that is in contact with the surface brakes, there comes a certain point where this part “lags behind,” and there it begins to break away and create vortices and turbulence. In a ball this happens around 80º(fig. 7).

And here I ask you: could you tell me why we need to do the twist in the main sail (foiling), especially if the wind gets stronger and you are sailing upwind or sideways?

Think about it, we will talk more about it in another installment, but the answer is written above.

THE SECRET? IS IN THE GOLF BALLS (AND IN YOUR HANDS)

Just to rest your brain a little bit, a nice detail: to make the air not come off a ball at 80º, just make small hollows in its surface! With that when the air slides over the surface, small cavitations are created and the flow comes off toward 120º, which considerably reduces the resistance force(Fig. 8).

Now you know why golf balls have that surface that allows them to go much farther than if they were smooth, and when you take the next plane (covid permitting!) look at the wings and (especially if it’s a Boeing) you can see little aluminum triangles on the wing. They are made to create vortices that help the air stay out of the way, reducing drag, reducing fuel consumption and increasing the speed of the plane.

To understand better, it would be, more or less, like when you rub your hands together. Feel the resistance (hands get hot!), however, if you put pencils resting on one hand and rub the other on top of it, the pencils work as rollers, and the hands slide by heating less (less resistance force). This phenomenon is important in meteorology when there are currents of air moving in the opposite direction. In the region of separation, these eddies are created that bring storms and very rapid changes in weather. Very important if you sail in the famous Roaring 40 or near the south of Iceland.

FROM THEORY TO PRACTICE

We return to our boats. To try to make sure that the fluid does not come off the profile, airfoils (for air) or hydrodynamic profiles (liquids) are created. As an example we have the sails, rudder, bulb or hull of the boat.

When a profile moves laminarly, the most important resistance becomes that of the fluid moving over the surface, and the greater the surface area, the greater the “rubbing” of the fluid, that is, the greater the resistance. A conflict remains to be resolved: to prevent pressure resistance from existing, the wing must be long, wide and thin, however, this increases the surface area and as a consequence increases the resistance(fig. 9).

How to reduce this resistance force created by the fluid moving over the surface? There is no single answer. For our boats, it is important to keep the submerged surfaces well cleaned. Anti-vegetative paint offers a lot of resistance, which is why racing boats do not use it, however, they must be withdrawn and washed every fin day.

Finally an important tip: If you use the rudder blade at an angle greater than 5º or 6º you are braking the boat because it creates turbulence(fig. 10)! When you have to jibe or make a turn, don’t push the rudder! You are braking the boat and she will have difficulty turning. Have you ever seen someone who always sails monohull take a Hobie Cat 14 for the first time? He can’t change edges because he pushes the rudder! The boat brakes, faces into the wind, stalls, and then goes into reverse….

Think about it, use the rudder gently! You will have more speed, more control and more safety, faster turns, as well as not forcing and smashing anything in your boat. Hi, in the next installment we will talk about lift, the lift force. And why it’s important on your cruising boat, too!

Elio Somaschini

WHO IS THE WEB’S MOST BELOVED SAILING “PROF”

Physicist, sailor, sailor (he sailed around the world for 20 years without instruments, only with a watch). Elio Somaschini, discovered by the Journal of Sailing, has quickly become one of the most followed “popularizers” on the web. His secret? He has a gift for making complex concepts simple and understandable to everyone by applying physical principles to practice. Elio makes you understand why you are doing something. In this lecture series, he explains how to make the most of appendages and sails to sail happily, you and your boat.

Would you like to personally contact Elio Somaschini to learn more about boating physics? Do you want to a ver it on board, or for a transfer? Write to him at cr****@***il.com

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