Are Jet Skis Inboard or Outboard?


Painted Bottom of Hull

If you have ever wondered about the science of how a jet ski goes forward at such high speed, this article will detail the science of Jet Ski propulsion for you. Not a boat and not like the engine on a motorbike, jet skis are a unique vehicle that has a simple function that we will detail. Also, we want to give you a comparison of what an outboard and an inboard motor are so you can understand the benefits and concerns of each type of engine. 

Are jet skis inboard or outboard? A jet ski is not like a boat with an outboard motor that spins a propeller. It can be called an inboard motor because the engine inside of the jet ski shoots a jet of water out of the back of the jet ski, which propels it forwards. 

What Are The Differences Between An Outboard And Inboard Motor? 

An inboard motor is a engine that is mounted inside the boat. It is usually below the waterline and has a pole or shaft with a propellor that extends into the water. You steer the boat with a standard rudder that is mounted directly behind the propeller. Large ships will use inboard motors for the most part. 

Outboard motors have the motor on the outside of the boat. These motors have a single-engine and steer by pivoting the whole motor or the propeller using the tiller fixed to the motor, or sometimes a hydraulic rod connected to the wheel for larger motors. The engine sits upright and has a 90-degree gearbox that is under the water. This gearbox powers the propellor and the boat forward. You use outboard motors on smaller vessels. Outboard motors are more visible and are easier to work on or repair and even replace. 

An Inboard/Outboard (IO) for short is a combination. The engine is in the hull of the boat while there is a prop and outdrive to the exterior. The outdrive turns to steer the craft and can be trimmed up and down.

A jet drive has the motor located within the hull of the jet ski. There is a jet pump mounted at the bottom of the hull. There is an impeller which is closely related to a propeller, however it is internal. The impeller sucks up water from the intake grate and spits it out the steering nozzle. The nozzle turns left and right to direct thrust. Jet drives are safer as you will not get hit from a propeller if you fall off your jet ski.

How Does A Jet Ski Inboard Motor Work? 

Newton’s third law states that every action has an equal and opposite reaction. If you think about this law, it is precisely the type of motion that propels vehicles like jet skis forward. A great example of this physics law is if you are skateboarding. You must push backward to go forwards, which is opposite, but equal in force. 

The Motor of a jetski is a marvel of modern technology. It does not use a propeller like an ordinary boat. Instead, a water intake valve with a grate over it at the bottom of the jet ski sucks water into the engine. The engine then uses power from the fuel to eject the water out the back. The water is discharged in a powerful jet from a nozzle at the end of the jet ski. The nozzle of the jet is slightly smaller than the intake valve on the bottom of the boat. A smaller nozzle than the intake valve increases the power of the jet stream. The jet stream power going backward is what propels the jet ski forward in the water. 

Why do jet skis use a jet pump verses a propeller?

Jet skis use a jet pump instead of a propeller for several reasons. The main reason is that they are safer. It would be very hard to fall off a jet ski and get your hand cut by an impeller which is guarded by a intake grate which blocks out large items. If you had a propeller on the other hand, the exposed blades could be unsafe.

Jet skis also use a jet drive as you can direct the thrust to give yourself sharper turns than if you had just a rudder directing thrust. If you could not spin out on a jet ski, they may not be as fun to ride.

Are jet drives harder to drive?

Jet drives take some getting used to to drive than a propeller drive. If you are coasting on a prop boat, you can turn the wheel and it will turn. If you try and turn a jet ski, you won’t. The only way to turn a jet ski is to apply power and direct thrust to turn the boat.

A major cause of accidents is that people forget you need thrust to turn a jet ski, so people will try and spray a boat or another jet skier and end up running straight into it due to forgetting to hit the gas.

How To Take Care Of Your Jet Drive

Caring for your jet ski jet pump can keep it running efficiently and even make the jet ski last longer. The following list is a few tips and maintenance tricks to keep the driveline of your jet ski maintained well each season. 

Oil Change, Like A Car

The oil in your pump needs to be changed out periodically. This can be done relatively quickly by a mechanic, or you can open up the jet cone and drain and replace the oil and filter yourself.

Visual Inspections

My intake grate is missing on of the sides. This was replaced due to lack of performance.

Before using the jet ski, you will want to do a visual inspection of the watercraft, especially the jet drive. Look for anything that could have gotten stuck in the impeller, anything which could have damaged your wear ring, or any damage to your intake grate or ride plate. You should be looking for any loose or missing components that could cause the jet ski fail at steering as well as your reverse or brake lever.

Pump Maintenance

Follow your owners manual to complete maintenance on your pump, driveshaft, and other items to make sure they are operating at 100%. The last thing you would want to happen is have a part fail and ruin your time on the water.

Conclusion

A jet ski is a single inboard motor. There are no duals, (that we know of yet) It does not have a visible propellor that pushes it forwards in the water. Instead, the power of the engine is used to pump water into the engine and out a nozzle at the back of the craft, creating a powerful jet of water that propels it forwards. The motor is potent, almost as much as a subcompact car or large motorbike. This means that care for the engine is essential to maintaining a jet ski that will last for years to come. Follow these tips and make sure to always check your jet ski before getting in the water. This is especially true at the beginning of the season. 

Eric

I have been an avid boater / jet ski fanatic since I was a kid.

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