# Stopping a Ship: Why Ships Cannot Have Brakes?

A very common question regarding ships has been – if they can brake to stop like any other automobile or aeroplane on ground. The answer, as we all know, is “NO”, and that is not what this article focuses on. The article delves into the reasons behind the answer- Why cannot a ship brake itself to stop? And if it cannot, how do ships stop at all?

To understand the answer behind this question we first need to know how a brake works? The working principle of a brake is that it creates friction between the wheel and a stationary body (generally the brake shoe) to stop the rotatory motion of the wheel, with respect to the surface with which the wheel is in contact.

Here’s a video on how car brakes work:

Extrapolate this concept to a moving ship â€“ there are no wheels in this case, given the fact that a ship propels itself in water by a propeller that is located at its stern. Now when a propeller rotates, it generates a thrust which propels the ship in the forward direction. The most vital thing to note here is that the hull of the ship moves in a fluid medium and hence it is not possible to instantly bring a ship to stop, given the shearing tendency of water medium.

Hence, a ship can be braked, but not by using a conventional braking system. It uses various methods, sometimes applied individually, and sometimes combined with each other, to brake and maintain station (geographic position)

When a ship moves in water, there is a viscous drag generated between the submerged hull surface and water particles in contact with the hull. This drag is generated due to the viscosity of water, and is directly proportional to the wetted surface area of the ship, and varies to the square of velocity of the ship (expression below):

The primary aim of braking any moving object, is to increase the drag force experienced by the body. Therefore in case of a ship, according to the above formula, the drag force should be increased (in other words, the ship should brake) by:

– Increasing wetted surface area of the ship: Though this method would increase the resistance, it wouldnâ€™t contribute to complete braking of the ship. Hence, in some cases, increase in wetted surface area is used to just decrease the speed of a ship to some desired level. For example, cruise ships often deploy stabiliser fins to increase the wetted surface area, and take advantage of decreasing the speed along with roll stabilisation.

– Increasing the velocity of the ship: Though theoretically, this would increase the resistance, and with a velocity tending to infinity, the resistance should tend to an infinite value. In other words, theoretically, the ship should come to rest at an infinite surge velocity, which is practically not possible, and is hence a paradox. The secret lies in looking at the problem from the correct angle.

What we want to do in order to brake a ship to a stop is to decrease the magnitude or change the direction of thrust on the ship, or sometimes both one after the other. If the thrust on the ship is reduced in magnitude (that is achieved by decreasing the RPM of the propeller), then the resistance of the ship instantaneously exceeds the magnitude of thrust, thereby slowing the ship down. This effect can be attained rapidly by changing the direction of thrust, that is, by changing the direction of rotation of the propeller (in case of fixed-pitch propellers) or by reversing the pitch angle of the propeller blades (in case of controllable pitch propellers). So when the ship is surging with some velocity, a reversal in direction of thrust would result in a braking condition.

The important parameters of breaking to be considered by a ship designer are measured during a crash stop test, which is carried out for every ship during sea trials.

## Crash Stop Test

The purpose of the crash stop test is to estimate the braking parameters of the ship, and record them so as to provide a database to the ship crew.

The test is carried out at the design speed, and the ship is surged on a straight path and allowed to surge with negligible acceleration for some time until it attains a steady motion in the desired path. The path of the ship is tracked by using the GPS system installed on the vessel. Once the engine is powered to the rated RPM, the direction of rotation of the engine is reversed in one go. The distance covered bt the ship before coming to complete stop is calculated by the GPS system, and is recorded as the track length of the ship.

The track length of a ship is very important as in, it helps the captain to estimate the time and distance required to bring the ship to a complete stop in case of emergencies.

Hence, what can be concluded from the above explained theories is that a ship is a sluggish body, and therefore it has a certain lag of response to the action carried out to bring about any change in its motion. Ships are not braked conventionally, rather, cannot be braked conventionally primarily because of the medium in which it moves.

Having understood the physics behind the stopping of a ship, itâ€™s also useful to understand how a ship maintains station. When at heavy seas, it uses Dynamic Positioning System, which is a networked system of sensors, computers, thrusters and propellers to help the ship maintain a steady position depending on the disturbance forces from the sea. When at harbour or port, station is maintained by mooring lines that tether the ship to the berths.

Some ships in certain cases can also use anchors as a braking mechanism, but it would only help in reduction of speed, and is not as abrupt as that of crash stop method.

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The information contained in this website is for general information purposes only. While we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the website or the information, products, services, or related graphics contained on the website for any purpose. Any reliance you place on such information is therefore strictly at your own risk.

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## Types of Bow Designs Used For Ships

Soumya is pursuing Naval Architecture and Ocean Engineering at IMU, Visakhapatnam, India. Passionate about marine design, he believes in the importance of sharing maritime technical knowhow among industry personnel and students. He is also the Co-Founder and Editor-in-Chief of Learn Ship Design- A Student Initiative.

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1. alex bacaltos says:

Very educational appreciate it very much. Many thanks

2. rilwan olanrewaju odediran says:

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4. Capt.Salim A Chagla says:

you stop a ship by stop the engine (in fuel~change Gas oil)then ! use a 8 knot in IN & OUT
If say a crew/fell or stopping the Engine !

best/regards

Very educational, learnt something today.. thanks..

6. Karim says:

More education on ships breaks.

7. Raunek says:

We have several articles on ship navigation and handling. Kindly use the search box on top right.

8. Sunil mittal says:

Generally ships are sailing in a certain speed because we’re not able to stop them all of sudden like a train if we are able to stop them all of sudden much lesser time as on date then it will be a golden era

9. capt.s.chagla says:

except latest AHTS, PSV, DPI,11,111, DCV Offshore’s Vessels can stop a.s.a.p…… action to stop in these tugs

10. Capt. Alejandro Malayas says:

Nothing mentioned here about ship’s with azimuth thruster/propellers in which can be actually stopped in a very short distance even running full speed compared to ships with conventional propellers.

11. Capr.Murali M .K says:

Thats a lot of information for those learning Ship handling. Informative.

12. Anish says:

@Capt. Murli Glad the information is useful ????????