Do Jet Skis Have a Transmission? [Explained]
Surprisingly, jet skis lack a regular transmission since they feature a direct drive system. This means that jet skis act like automatic vehicles, ensuring smooth easy handling.
If you want to find out more about the jet ski drivetrain, you’ve come to the right place.
We at JetDrift have compiled all you need to know under one roof!
Do Jet Skis Have a Transmission?
Unlike cars or motorcycles, jet skis don’t have a transmission or a clutch.
Instead, a jet ski engine is connected to an impeller via a direct driveshaft. On some models, the crankshaft is directly connected to this shaft, while other models feature a gear reduction system to decrease high engine RPM.
Due to this design, a jet ski continuously drives its impeller when its engine is on. The impeller does the same job as the propeller on a boat, which is to move the water backward to propel the vessel forward.
Despite this, most jet skis utilize two levers on the handlebars.
But while on a motorcycle the right lever is the brake and the left is the clutch, on a jet ski the lever on the right is the throttle while the brake/reverse lever is on the left.
However, you can find some budget jet ski models that lack brake and reverse system. These machines feature only one throttle lever, which is on the right handlebar.
It can be confusing especially for riders who have switched from a motorcycle to jet ski!
What is the Gear Reduction System on a Jet Ski?
There’s a lesser-known part in the engine of many jet skis, which is known as the gear reduction system or driveline reduction gearbox.
What does this unit do in a jet ski?
In a nutshell, the gear reduction system is intended to reduce the RPM before power is transferred toward the impeller. Thanks to this feature, the engine can rev at a much higher RPM while it still delivers an optimal RPM to the jet pump.
As a rule of thumb, Yamaha and Kawasaki jet ski engines house reduction gears, while Sea-Doo engines lack this feature. Why?
This is because Sea-Doo engines are purpose-built PWC engines that ensure an optimal RPM range. Therefore, their driveshaft can be directly connected to the crankshaft. (On a Sea-Doo the general formula is: Engine RPM = Impeller RPM.)
In contrast, Yamaha and Kawasaki’s jet skis borrow their engines from motorcycles.
For example, Yamaha’s MR1 WaveRunner engine is basically a YZF-R1 sportbike engine modified for marine environments. Kawasaki jet ski engines are derived from the Ninja sport bike series.
But since these power sources are primarily designed for sport motorcycles, they rev much higher than purpose-built jet ski engines. And running at these high RPMs the impeller would start to cavitate.
This is where the reduction gear comes into play. This gearing system is mounted in a reduction gear housing on the PTO side of the engine, located between the engine crankshaft and driveshaft.
As reported by Boats.com, the reduction gear ratio of Yamaha’s MR1 engine is 1.47:1, meaning that the impeller is spinning 1.47 times slower than the engine crankshaft. (Formula on WaveRunners: Engine RPM / 1.47 = Impeller RPM.)
In other words, at the engine RPM of 8,000, the impeller is still running at only 5,442 RPM.
This simple system allows the engine to reach its peak rev range and produce its maximum power while keeping the impeller speed in the optimal range. This results in the best performance available without any impeller cavitation.
Here’s an example of how the reduction gear drive looks in a WaveRunner:
Do Jet Skis Have Gears?
Since jet skis are manufactured without a transmission system they don’t feature traditional gears either. As we’ve discussed, some manufacturers release their jet skis with driveline reduction gears. But these gears connect the driveshaft with the crankshaft and don’t act like the gears in cars or motorcycles.
In these vehicles, the transmission provides a selection of gears for different speeds and driving conditions. In a manual vehicle, you can shift the gears via a clutch and a shift lever, while an automatic transmission vehicle does the job for you.
Unlike street vehicles, jet skis lack a transmission system as their impeller is connected to the crankshaft directly or with a gear reduction system.
This means that the jet ski starts to spin the impeller when you fire up its engine. If you hit the throttle, the impeller starts spinning faster, which accelerates the jet ski.
The speed of the machine strictly depends on the engine speed; the more throttle you apply the faster the jet ski goes.
Do Jet Skis Have Neutral?
Do jet skis have neutral? – we get this question all the time and the answer is yes, but it’s not a traditional neutral. Instead, the minimal thrust produced in “neutral” is split into forward and reverse thrusts by the reverse bucket. This balanced thrust keeps the jet ski still in off-throttle situations.
Since jet skis feature a direct drive system, the impeller is driven by the engine when it’s on.
At idle the impeller spins very slowly, but it still generates enough thrust to move the jet ski forward.
To avoid this, the manufacturer uses the reverse bucket to block these thrusts. This foldable bucket, also known as a reverse gate, is intended to slow down and brake the machine when it’s in motion.
This is the part of the brake and the reverse system that’s mounted behind the jet pump and controlled by a small electric motor.
When the jet ski is in “neutral” its engine runs at idle speed, which causes the impeller to spin slowly, resulting in a minimal thrust. The bucket partially reverses this thrust, balancing between reverse and forward.
Eventually, the reverse and forward thrusts balance each other out, which prevents the ski from moving.
What’s more, turning the handlebar directs this thrust to the side, ensuring a slow precise control around docks and in tight spaces.
But unfortunately, not all jet skis have “neutral.” Many vintage 2-stroke and some modern budget jet skis lack brake and reverse systems. And if a jet ski doesn’t feature this system it means that it doesn’t have “neutral” either.
In other words, the small thrust in neutral starts to move these skis slowly forward, whether you want to or not!
The only way to stop these jet skis is to shut down the engine.
Conclusion – Are Jet Skis Manual or Automatic?
It’s safe to say that jet skis act like automatic transmission vehicles. Although they don’t feature a regular transmission, jet skis still have neutral, forward, and reverse.
Jet skis come with a direct drive system, meaning that their impeller is connected to the engine via a direct driveshaft.
On purpose-built jet ski engines (like Sea-Doos) the driveshaft rotates at the speed of the crankshaft.
Other manufacturers like Yamaha or Kawasaki use modified motorcycle engines in their jet skis. These power sources can hit very high RPMs, which is reduced by a driveline reduction gearing system.
These gears are mounted between the driveshaft and the crankshaft. Unlike regular transmission gears, the only purpose of these gears is to reduce engine RPMs before power is transmitted to the driveshaft.
If the engine is on, this durable shaft continuously spins the impeller, producing some thrust.
To prevent the jet ski from moving in “neutral” its reverse bucket is used to partially reverse the thrust.
Only vintage and some budget skis lack reverse systems, so their impeller slowly propels the machine forward, even in ”neutral.” This can be surprising, especially for beginner riders.
More advanced models with a brake and reverse system will stay in one place when they are in “natural” and can be maneuvered more precisely around docks.
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