Boats are like the cars of the water. Some motorboats are designed in ways that very much resemble an automobile. They run on gas, and the more gas you give it, the faster they go. Many are even designed so that the rudders are controlled by a steering wheel, which means you would steer the boat in the same way you steer a car. But do the similarities extend to stopping?
Do boats have brakes? No, boats do not have brakes. Stopping the propeller of a motorboat or lowering the sails of a sailboat stops any further propulsion, but the boat will continue in a forward motion until the friction of the hull against the water brings it to a stop. To stop faster, you can run the propeller in reverse, drop anchor, or steer the boat into the wind or current.
Cars are propelled by a motor spinning the wheels, which pushes the car against the ground. Automobile brakes work by slowing and stopping the spinning of the wheels, which in turn causes the car to stop. Boats, on the other hand, are propelled by a spinning motor or wind pushing against a sail. Because brakes cannot be applied to stop the boat, other methods are used to stop boats quickly.
Increasing the Wetted Surface Area
Because there are no brakes on a boat, what actually slows them down is called viscous drag, which is the amount of friction between the boat and the water. Viscous drag is equal to the density of the water (p) multiplied by the surface area (S) that comes in contact with the water, multiplied by the velocity (V) squared.
The Density of the Water
The density of the water affects the viscous drag. A boat traveling in jello will stop much quicker than a boat traveling in water because the jello is denser. In space, on the other hand, there is no density because space is a vacuum, and therefore spaceships will never be slowed by viscous drag.
We cannot, however, increase the density of the water, so we are not able to stop a boat in this way.
Velocity
Theoretically, according to the equation, increasing the velocity would also increase the viscous drag, and as the velocity approaches infinity, so too does the viscous drag, stopping the ship. In actual practice, however, this is not possible because it would require an infinite surge velocity.
On any practical scale, increasing the boat’s velocity will only increase the boat’s speed, not stop it.
Wetted Surface Area
One method that can be used to stop a boat more quickly is to increase the wetted surface area. This Increases the amount of friction created between the boat and the water, increasing the viscous drag and slowing the ship faster.
Cruise ships often deploy stabilizer fins. Not only do these projections help stabilize the ship against rolling, but the increased wet surface area also helps slow the ship faster.
Running the Propeller in Reverse
A motorboat works by a motor spinning a propeller in the water. The fins of the propeller are angled such that when the propeller spins in one direction, it catches and pushes water away from the boat’s stern, pushing the boat forward. Stopping the propeller from spinning will significantly slow down the boat, but it will continue gliding forward.
You can stop a boat faster by running the propeller in the reverse direction. Because of the angle of the fins, this will push water towards the ship’s bow, the same direction that the boat is traveling. This will cause the boat to slow down much faster because you are creating force in the opposite direction as the current momentum. Eventually, the boat will come to a full stop and then travel in reverse.
This process is slower than brakes on a car and can take even longer because you need to wait for the propeller to stop spinning in one direction before engaging it in the opposite direction. Just like suddenly shifting into reverse when driving at full speed would damage your car’s transmission, you could cause similar damage to your boat’s motor with the sudden shift.
In order to make the reversing action even quicker, some large ships have reversing buckets that are dropped in the path of the propeller’s force, causing it to push in the opposite direction. Whether using a reversing bucket or simply reversing the direction the propeller spins, the basic concept is that creating a force in the opposite direction will slow the boat faster.
Dropping Anchor
You could also try to stop more quickly by dropping the boat’s anchor. The anchor is designed to dig into the sea bed, and that, combined with the weight of the length of anchor chain dropped will help slow the boat. While it may offer some stopping power in an emergency, dropping the anchor isn’t really an effective method of stopping.
Anchors are designed to hold with horizontal force. It requires 5 to 7 times as much chain length lying along the seafloor in relation to the depth of the water in order to have enough horizontal force to hold the ship in place.
While in theory dropping the anchor would work similarly to dragging your foot on the sidewalk if you were skateboarding too fast down a hill, there are some technical issues that make this difficult. While anchors are often dropped using the weight of the anchor and chain as the driving force, doing so must be done carefully to avoid the chain dropping faster than the brakes on the windlass can stop it.
Turning into the Wind or Current
Instead of, or in addition to, using the force of reversing the propeller direction, you can also use the natural force of the wind or water current to stop more quickly.
Sailboats use sails to catch the wind in order to push the boat. Dropping the sails makes the sails no longer catch the wind. Just like cutting the motor, however, this only slows the boat, and it will continue to drift. If you can turn the sails so that the wind catches them in a way that opposes the direction the boat is traveling, it will stop the boat faster.
Currents in the water can also be used to slow a boat faster. An unanchored, stopped boat will not stay in one place, even on a windless day, because water is not stationary. Currents in the water will push the boat in the direction of the current. If you turn the boat so that it is traveling against the direction of the current, the force of the current will help stop the boat faster.
The Crash Stop Test
Because there are no brakes on a boat, captains need to know how far the boat will travel after they’ve stopped (and reversed) the propellers. To do this, they conduct crash stop tests during sea trials.
To conduct the test, the ship is driven in a straight path at a constant speed. The propeller is then reversed, stopping the ship as quickly as possible. Using the ship’s GPS, the captain measures the distance the boat travels forward, as well as how far it deviates laterally from the set course before it finally comes to a dead stop.
By running crash stop tests at various speeds, captains can gain an understanding of how quickly the boat is capable of stopping. Keeping this distance in mind helps them operate safely near obstacles.