A problem we hear from time to time with jet ski & jet boat watercraft and boat owners is the congestion of debris in the direct drive water jet propulsion system but this is something that can be minimized with some understanding of what is going on underneath your vessel.
Remove the Suck!
Lets first look at what a Direct Drive Water Jet Propulsion system consists of.
A Direct Drive Water Jet Propulsion system starts at the intake, this is a tunnel or track that enters the bottom of the hull to feed the encased impeller. Now hold this vision for just a few sentences… In front of the intake inside the vessels engine compartment rest the engine, mounted directly onto the bottom of the hull with the in-line drive shaft angled straight back (towards the transom) traveling through the hull and connected to the front, center of the impeller. No transmission here, it is a “direct-drive” system.
Okay back to the water flow. The impeller is a bladed or veined component that when turned transfers water from front to back, this is what creates the water propulsion. But there is more to it than that, behind the impeller blade are directional veins angled the opposite as the impeller blades to straighten the twirling water flow to force it straight back to use that energy as efficiently as possible to create maximum forward thrust. Behind the jet pump is the Venturi. The venturi is shaped similar to a cone, with the inlet being much larger in opening diameter than the smaller, tighter exiting outlet. This compresses the water and provides greater thrust when forced out. Imagine holding a garden hose with water running freely out of the end. now squeeze your thumb over the end and wow, it shoots a stream of water a lot harder, faster and further, this is what the venturi does.
Now to provide steering (and trim on some models) there is a pivoting nozzle on the rear of the venturi, this redirects the water flow from side to side to push the transom to side to side to change direction – or up and down to change/trim planning angles.