What do I need to consider when choosing waterjet propulsion?

Jet skiers, ambulance boats, military boats, and ferries all love it – the waterjet. But how does it work, what are the waterjet’s pros and cons, and what do you need to know before choosing the right waterjet for your application and needs? Keep reading and I hope I will answer all your questions about the waterjet.  

Waterjet – a marine system that produces a jet of water for propulsion

A waterjet is a propulsion system that uses a pump to drive the boat forward. An intake, usually located at the bottom of the hull, guides the water into a pump powered by an engine. The rotating part of the pump is called an impeller. Unlike a propeller that is placed freely in the water, an impeller is enclosed in a pump chamber. Since the water rotates inside the pump, it must align before leaving the pump system. Therefore, the system includes a stator with guide vanes, turning the water beam in the right direction. The water beam then passes through an outlet nozzle at the stern of the boat. The difference between the speed of the water flow at the inlet of the waterjet and the speed of the water flow at the outlet nozzle will create a reaction force (thrust), pushing the boat forward.

An axial flow pump vs a mixed flow pump

There are waterjets adjusted for high-speed boats as well as low-speed boats. Waterjets differ depending on the type of pump. The main difference between the pumps is the ratio between pressure and flow. There are mainly two types of pumps suitable for waterjet propulsion systems: an axial flow pump and a mixed flow pump. An axial flow pump’s pressure increases by diffusing the flow as it passes through the impeller blades and stator vanes. The pump nozzle then converts the pressure energy into velocity, thus producing thrust. Axial-flow pumps produce large flows at lower velocity, making them well suited for low to medium speed boats.

A mixed flow pump incorporates aspects of both axial flow and centrifugal flow pumps. As a centrifugal-flow pump creates water pressure through a radial flow, the pressure from a mixed flow pump develops through both diffusion and radial outflow. Mixed flow pumps produce lower flows of water at high velocity making them suited for high-speed boats.

Waterjet principle

Designer’s Guide, Marine Jet Power

Operates in shallow waters, offers good comfort and high uptime

A waterjet is a versatile propulsion system as it is suitable for both small jet skis and larger boats. One of the great advantages of the solution is that there is nothing protruding beneath the bottom of the boat which results in zero added hydrodynamic resistance. The lack of protrusions under water also provides a great advantage when operating in shallow waters. This is useful for boats such as ambulance boats and military boats as they must be accessible in all situations and in all marine environments. Also, since a waterjet does not have a protruding propeller it increases the safety for the people in the water during, for example, a rescue mission.

The propulsion system has high maneuverability and offers good comfort as it creates low vibrations in the hull. This is highly appreciated, not least in the ferry industry. It is also a good propulsion solution when driving boats in waters with floating objects such as branches and timber. As there is nothing protruding underneath the boat the objects do not get stuck in, for example, a propeller. Instead, the boat runs over the objects and if something would enter the pumps it often disintegrates inside the pumps without affecting the function. In general, boats with waterjets require a limited amount of service and have higher uptime compared to boats with propellers.              

Monohedron – a hull shape adjusted to keep a straight course 

One of the challenges with waterjets is to keep a straight course. Since a waterjet doesn’t have a propeller and rudder protruding underneath the boat, which normally help to keep the right course, other measures must be made instead. For example, the boat’s hull must be adjusted to fit a waterjet already in the design phase. A monohedron, consisting of a constant deadrise and two parallel sides of the hull, is preferable to a warped hull. This hull has a changing deadrise from the transom through the midship. Also, a boat powered by a waterjet is more sensitive to load, and has poor efficiency at low speeds. This must taken into consideration, when designing the hull for different purposes.

The boat’s load and speed determine which waterjet to choose

To choose the right waterjet, aspects such as the boat’s load and speed must be taken into consideration. A waterjet is one of the most effective propulsion systems at around 40 knots. However, a waterjet does not recognise the speed of the boat but operates with the same capacity all the time. Thus, it does not increase its absorption of engine power if the speed of the boat decreases due to, say, high loads. This means the fuel consumption (litre per hour) will be the same regardless if the boat operates at 20 or 40 knots. Therefore, it is important to define the load and speed of the boat before choosing which waterjet to use.

To get optimal efficiency, it is also important to match your waterjet with the right engine. In a way, the waterjet is more gentle towards the engine, compared to a propulsion system with a propeller. This since the engine powering a waterjet does not need to handle varying loads. However, it often operates at a higher speed. A propeller, on the other hand, creates variations in engine torque and engine load. This means that different parts run down faster on an engine powering a waterjet compared to an engine powering a propulsion system that includes a propeller. In both cases, it is important to design the propulsion system for the highest displacement planned for the boat, to achieve best efficiency and longest time between overhaul.

Waterjet on workboat

Sima Charters, SC Elite Workboat

Active maneuverability results in high steering force 

Waterjets offer active maneuverability, compared to boats with propellers and rudders that have passive maneuverability. Active maneuverability means that the waterjet maneuvers in the same direction as a rudder would point, which results in a high steering force. There is also high maneuverability when driving in reverse. After the outlet nozzle, a reversing bucket is attached to the waterjet which produces a reverse thrust that can be achieved quickly and without the need to change gear or adjust engine thrust.    

Volvo Penta offers engines specifically designed to power waterjets

We, at Volvo Penta, have many years of experience working with waterjets and are well aware of what the propulsion system needs to function at its best. We design engines specifically adjusted to waterjets as we know they perform at their best in the power range where the waterjets are the most effective. Volvo Penta offers reliable marine propulsion systems, allowing our customers to completely trust their equipment and focus on their core business.    

Would you like to know more about our marine products? Please visit our website or contact us on social media. And don’t forget to visit our Professional Power Blog where we share more interesting blog articles.


Head image: Maritime Partner, Alusafe 1500 MPV

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