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How does a jet ski work? – This is one of the first questions that pops in the head of any beginner while purchasing a new jet ski.
The answer to this question can be simple and complicated at the same time depending on how deeply you want to study the particular topic.
Perhaps the first thing you need to know about a jet ski is that its propulsion is quite different from a regular boat that you might come across. While you might want to know everything about your jet ski engine and propulsion, not everybody has the patience to dive into the details of how a jet ski works with regards to the physics behind how these systems work.
Therefore, in order to make it all easier for you to understand we at JetDrift have compiled a short and long answer for you. Read on to get a clearer understanding of how a jet ski works!
How does a jet ski work?
Before you go into the details, we recommend starting off with this short educational video on the topic:
A jet ski is one of the fastest and the most easily handled vessels that you can use on the water.
It is because of their ease or riding and their high speed that lifeguards and marines use them. It is important to understand that a jet ski is very different from a normal boat which uses an outboard/inboard engine and a propeller to power it. It is also not in any way similar to a motorbike where the engine turns the wheel.
A jet ski is completely different from these two types of vehicles because it moves along the water by squirting a high powered jet of water behind it. It has a great deal of science behind it, which we will explain further!
A step-by-step guide
If you’ve ever seen a rocket take off, you can see that it leaves behind a huge cloud of what looks like fire.
Actually, that is the high pressure gas which the rocket uses to generate thrust. Similarly, a jet ski uses water to generate that thrust for itself.
This is done with the use of a jet drive. The jet drive contains an impeller that propels large amounts of water from under the jet ski, through a nozzle and towards the rear of the vessel; generating enough thrust for the watercraft to move forward.
Let’s take a detailed look at the entire process step-by-step!
There is a grate (called: intake grate) at the bottom side of the jet ski that sucks in water from the waterbody below. Or in the worst case, it sucks your tow rope or debris with the water.
The engine of the jet ski provides power to the impeller. This engine can be of varying size depending on the category. The most common are 4-stroke-engines around 900-1800 cc displacements that use gasoline to run.
While the engine of a car or motorbike is responsible for providing power to the wheels to rotate, the engine of a personal watercraft provides power to the water pump and the impeller. An impeller is essentially a propeller that is fitted inside a pipe in order to suck water from one end and blow it out the other with maximum speed. A jet ski impeller is made up of 3 stainless steel blades that work at top speeds to help the vessel move along. Some of the water sucked in is also used to cool the engine down in order to prevent over-heating.
Once the water is sucked in from the pipe, it exists from the other end which is actually a steerable nozzle. This is located at the back of the watercraft and is smaller than the end that sucks in the water. The difference in size is what helps build up the water speed.
This water being squirted out is what moves the watercraft. This holds true because of Newton’s third law of motion; every action has an equal and opposite reaction. In the case of a jet ski, the action would be the expulsion of water from the nozzle and reaction would be the movement of the watercraft in the opposite direction at the same speed.
As far as the steering of a jet ski is concerned, it is almost as easy as steering a motorbike. While on a motorbike, you use the handlebars to turn the front wheels in a certain direction, for a jet ski you turn the handlebars which pull a cable attached to the water jet. This pulling action swivels the water jet in the required direction and hence turns the whole jet ski. Because of this function, a jet ski is best maneuvered at top speeds whereas it can be a little difficult to steer when it’s running slowly.
Besides similarities don’t forget that there as differences as well. Unlike vehicles, most of the jet ski models require acceleration to turn.
If you take your hands off the throttle while turning, the jet ski loses steering power and floats straight ahead. This can be surprising for beginners and leads to many accidents and injuries.
Jet Ski Engine
A jet ski engine is almost exactly like a motorbike’s or a car’s. It can be either a two-stroke and a four-stroke engine.
A two-stroke engine has up to three cylinders while a four-stroke engine has 3-4 cylinders. The starting procedure of the engine is the same; with the ignition button on the handlebars sending a signal to the motor. The motor then turns the engine and that’s what gets it started. The next step is the engine powering the craft.
All the recent models of jet skis come with a key that needs to be inserted into the slot in order for the engine to work. This key is then attached with a lanyard to your jacket or your wrist so that you don’t lose it. It is important to note that the watercraft will not work without the key in place.
This feature is a recent addition to watercrafts and it is very helpful in preventing collisions or the jet ski from going too far in case you fall off it.
Jet ski cooling systems
Most jet skis come with either of the two types of cooling systems; the closed loop and the open loop.
The closed loop system is mostly like a car’s engine. For a closed loop cooling system, the water on the outside of the watercraft never enters it. There is an anti-freeze liquid available that is used on the inside of the jet ski engine.
The cooling system contains a ride plate that is also called the heat exchanger. This works to disperse the heat from the inside of the engine to the natural water outside. This is effective because the water bodies on the outside are naturally cooler.
The ride plate in the jet ski performs the function of the radiator in your car. The only difference is that the radiator is cooled by air while the ride plate is cooled by the surrounding water. Both of them work effectively to remove the heat from the inside of the engine to the elements on the outside like air and water.
The anti-freeze liquid heats up and then the heat is transferred to the ride plate. This cycle goes on and on. The ride plate works as a transmitter of heat from the anti-freeze to the water on the outside. The heat exchanger is located under the PWC so it’s in the raw-water continuously.
This reduces the temperature on the hot anti-freeze liquid that exits the engine and it is then pumped back to the engine to cool it. Thus, the engine does not meet the external raw-water at all.
The most commonly used cooling systems in the watercraft industry are the open-loop systems. As their name suggests, the open loop cooling systems use the water from their surrounding water body to cool their engines. The cool water is sucked into the engine and thoroughly circulated around it before being expelled through an exhaust back to the water body.
There are some models that use squirting water from the watercraft’s hull. Contrary to popular belief the rooster tail (squirting water from the stern) doesn’t came from the cooling system.
The physics of a PWC operation
The science behind the engines of watercrafts and how they work is based on the discoveries and theory of a brilliant Englishman Sir Isaac Newton.
One of his greatest discoveries was his third law of motion. As mentioned before, the law states that every action has an equal and opposite reaction. This holds true when you take the practical example of blowing up a balloon to the fullest. Once you let it go without tying a knot to its mouth, it whizzes around the room. While it is funny sound, it is also important to understand the science behind it.
Newton’s third law, the one that we’re most concerned with, is “action and reaction”. It is sometimes also written as, “every action has an equal and opposite reaction”. Examples of this can be taken from everyday actions that we perform in our lives. Let’s take the example of a skateboard. When you’re riding skateboard, in order to move forward you have to kick the floor backwards. This is where the 3rd law of motion is applied.
Another example could be swimming freestyle. In order to move forward you have to pull your arms under you towards the back. The pulling of your arms is the action and your whole body moving forward as a response to it is the reaction.
It is important to note, however, that both the action and reaction forces have to act on different things in order for you to move forward. For example, if both the forces acted on the same body, you just won’t go anywhere.
Therefore, we have to understand that while swimming, the pulling force of your arms is applied to the water and the reaction, push force is applied to your body. Thus, moving you forward.
Action and reaction is how a jet ski works. The key to a personal watercraft moving in the water is the engine of the vessel making an impeller move in the water. This impeller rotates because of the motor that provides the power. The rotation is what sucks in the water from one end and then expels it with high force through the other.
This forcing it from the nozzle pushes the jet ski in the opposite direction and hence, the jet ski moves in the water.
Conservation of momentum
Therefore, the law of conservation of momentum which is mass x velocity, tells us that the speed of the water firing outward from the impeller should be exactly equal to the momentum of the personal watercraft that is needed to move forward. Therefore, in order to make the huge jet ski move, the water needs to be pushed out at the highest possible speed.
This was our detailed answer answer to the question “How Does a jet ski work?”
If you would like know more about jet ski engines, don’t miss our other posts about cooling systems and superchargers. If you would like to discover and compare new PWCs head to head, you may make a good use of our jet ski comparison tool.
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