Damn “Coanda Effect”!

Maurizio GaripoliBy Maurizio Garipoli11 Giugno 20217 Minuti

I have to tell the truth: before experiencing it in reality, I didn’t know it… I’m not even sure I studied it at school. It is also true that many physical laws or theorems that I learned at Nautical, if only without a brief review, I cannot expose on the spot. Still, I really don’t remember ever studying the Coanda effect! Yet such a singular name, I should remember …

Instead, I have experienced it!
To handle ships in the canal, it is necessary: ​​first of all to study the theory of hydrodynamic effects; it is then necessary to experience its existence in daily practice; finally, if the effects were insidious for that type of manoeuvre, we must implement certain measures such as – if the available manoeuvring space permits – the lengthening of the tow cable by enough to make the wake effect against the ship’s hull irrelevant. (as shown in the following illustration)

The most common effects – described in many ships handling texts – are those due to the interactions between ships and seabed, ships and ships, ships and docks; we, therefore, speak of attractions and rejections.

Bank Suction and Bank Cushion. They are determined by different pressures, positive or negative, acting on the ship’s hull.

Positive pressures repel, negative pressures attract.

Also, in our bookA bordo con il pilota”, we address the subject with detailed explanations and practical examples because they are part of the baggage that every Captain must have with him.

The Coanda effect is less common in the maritime field, and I am sure that not all sailors know what it is. I am sure of it because even some colleague, to whom I asked the question, did not know its existence, or rather, they found its effect but ignored its name and theory.

I guess someone reading will smile.

Pilot work is essentially practical work.

In the pouring of experience during the apprenticeship year, the student pilot is instructed and bombarded with knowledge by his future colleagues. The training continues beyond, in the following years. Each manoeuvre teaches something, and it is quite common to believe that “it takes at least 5 years to make a Pilot”.

Memories...

When I was a training pilot, I remember that it was common to moor ships 150 meters long by 25 meters wide, moving backwards in a channel only 50 meters wide.

Passing from Pilot to Pilot, I compared everyone's precautions and “tricks” and, based on them, forged my own personal style. Not that there is an infinite range of different techniques ... the same manoeuvre is usually done in a couple of ways, maybe three. However, the nuances are innumerable.

In that particularly delicate circumstance, some pilots used both tugs to maintain the correct attitude of the ship. In contrast, others gave the only initial order to the stern "tractor" always to maintain the centre of the channel, using only the forward tug to correct any yaws.

Obviously, the latter obtained a simpler and less "worked" manoeuvre.

What happened to the former was that when they moved the towing tug from the canal axis to correct a deviation, Its wake was wedged between the ship and the bank, creating a pressure zone that countered the action of the towline.
The engine and rudder often had to be used repeatedly to help the tugboat in its task.

Is this the Coanda effect? Not exactly.

More properly, we can blame the pressure generated between shore and ship and the repulsion that derives from it. However, if by removing the channel and positioning our ship in larger spaces, we notice the same phenomenon … well … yes, then we are experiencing the Coanda effect!

 

In the aeronautical field Henry Coanda, like Bernoulli and Venturi, is well known; those who have studied flying know who and what we are talking about. The theory describes the Coanda effect through the speed variation of a flow that strikes a convex surface: “the part in close contact with the surface slows down due to friction, the external part accelerates and a pressure reduction is generated, but since the fluid is molecularly cohesive, it remains adherent to the convex surface following its profile, generating, at the same time, a low-pressure zone “.

Low pressure = attraction.

Thus it happens that, under certain conditions and at a certain pulling angle, the tugboat wake can hit the side of the ship and part of its flow create, in the opposite side, a force of attraction capable of cancelling or even reversing the direction of rotation that we want to give to the ship itself.

Immagini estratte dal libro: A bordo con il pilota

If now, finished reading these few lines, we leave the page and search for the “Coanda effect” on Google, we can find enormous amounts of information. Among these, the example of the spoon under the tap is nice and instructive.

I did it! I took the spoon, opened the tap and, holding it with two fingers so that it was free to swing, I made the water flow on the convex surface, empirically experiencing the attraction that the fluid exerts on it.

After this experiment, what is written becomes more understandable and difficult to forget.

At the next opportunity, observe the ship’s behaviour, and among the effects, you will experience trying to recognize the one described in this article. With a little attention and refining our manoeuvring technique, you will see that it will become easy to prevent and compensate for it.