Waterjets made a big splash, and now we see them on many high-performance craft. This prevalence of active installations prompts us to search for other applications. Can we put jets on a slow river boat? Or a fishing vessel at 20 knots? This article focuses on the merits of waterjets, with focus on the most important factor: efficiency.
Waterjets work by pumping a LOT of water through a closed duct. The acceleration of this water creates a forward thrust for propulsion. Figure 2‑1 shows a typical arrangement for a waterjet. Suck the water in at the bottom, the impeller adds power, and the nozzle accelerates the water out the back.
Waterjets operate completely different from propellers. A propeller tries to minimize the velocity change and focus on pure pressure differences. But a waterjet intentionally increases the velocity change between inlet and outlet. That change in momentum creates thrust. Multiply the thrust with a huge flow rate, and you get propulsion. This different paradigm leads to several differentiating features:
A waterjet is essentially a pump inside a very short pipe. Pumps work differently than propellers; they don’t show the same limits. Pump efficiencies around 90% or more are regularly attainable. In contrast, conventional propellers stop at 60%-72% efficiency. But waterjet efficiency involves more than just the pump. A host of factors reduce the efficiency:
The largest factor in waterjet efficiency is the ratio of velocity at the jet outlet to the inlet. This velocity ratio strongly influences the total efficiency because waterjets depend on high flowrates for efficient operation. Without any water acceleration, the jet fails to produce thrust. But if we force too much acceleration, the efficiency drops off fast. DMS contacted a waterjet manufacturer to obtain typical values for the velocity ratio. They rightly refused to disclose any information about this critical trade secret. Instead, DMS tested several velocity ratios to see how they impacted theoretical waterjet efficiency. (Figure 3‑1) These ratios were not based on any waterjet manufacturer.
A higher velocity ratio meant more acceleration and more thrust from a single jet, but at the cost of lower efficiency. The figure showed that changing the velocity ratio also impacted the range of efficient operation. A higher velocity ratio allowed the jet to maintain its efficiency across a wider range of ship speeds, but at the cost of lower peak efficiency. This variation in capabilities drove waterjet manufacturers to supply different models, targeted towards different speed ranges and power requirements.
In fairness to waterjets, we also need to remember the other drag elements required for propeller propulsion:
The waterjet sits flush inside the hull, with just one outlet. At speed, the exit nozzles completely clear the water. This reduces the total resistance on the vessel. We have no propeller shaft or shaft brackets to drag through the water. Another element removed: the rudder. Waterjets don’t need a rudder, because they direct the thrust through changing the direction of the outlet stream. Removing these elements already gives the waterjet an edge over propellers. But velocity ratios clearly show that efficient operation depends on picking the right jet for the right speed.
It all comes down to speed. At higher speeds, waterjets show greater efficiency. But at lower speeds, they struggle to create enough momentum change with limited water flow rates. DMS created a simple comparison, using the same waterjet for six different peak speeds. (Figure 4‑1)
In practical terms, this would be the same waterjet used on six different types of vessels. The peak speed of 60 knots represents a sleek, light patrol vessel designed to cut through the waves. The peak of 10 knots applied to a slow lumbering barge, pushing a heavy load. It was more instructive to assume a constant power and vary the peak vessel speed, because waterjet efficiency hinged on speed.
The graph showed how waterjet efficiency dropped drastically at lower speeds. Above 30 knots, waterjets offered a better option to open water propellers. Below 20 knots, propellers were the clear winner. The range of 20 – 30 knots remained uncertain. Propellers have a few tricks to remain competitive in this range, and waterjet efficiency hinges on selecting the right model. Either option has merits, depending on your circumstances.
The speed range of 20 – 30 knots is also a very popular set of design speeds for many vessels. Picking the wrong propulsor may drastically alter your fuel consumption for these applications. DMS offers careful review and comparison of propulsion options in this range.
Remember to consider your operational profile when selecting the propulsor. Notice that the waterjet efficiency drops off with speed. Don’t select a waterjet due to its efficiency at high speed if you spend 90% of the time trawling. DMS can work with you to balance all the operational needs and maximize your propulsive efficiency.
Sometimes we accept the lower efficiency of waterjets due to their massive benefits in maneuvering. Unlike conventional propellers, maneuvering works by controlling the direction of your waterjet thrust. Your steering force links to engine RPMs, not ship speed. Imagine slowly drifting up to a dock with full steering control, something a rudder will not achieve.
Reverse thrust on waterjets also promises greater control. On a conventional propeller, we reverse thrust by slowing the engine RPM, switching to reverse gear, and revving up again. On a waterjet, just drop the bucket. (see Figure 5‑1(b)) The bucket swings over the outlet of the waterjet and redirects the thrust into reverse. No need to reverse gear or change engine RPM. This offers extremely quick reaction time for crash stops.
Some waterjets include a neutral thrust position. The reversing bucket partially covers the outlet, sending half the stream in reverse and half in forward. Engine RPM’s are no longer tied to thrust. You gain the option to keep your engine at full speed for strong thrust control and only use a fraction of that when approaching the dock. Just leave the bucket in neutral. Then adjust the bucket slightly to nudge the ship. These extra options allow very fine vessel maneuvering in a range of situations.
Waterjets are fun. They give you great maneuvering control and promise much higher efficiency at high speeds. But that flexibility comes with the price of more subtle limits on performance. Efficiency rapidly drops off if you install the wrong waterjet or use it at the wrong ship speed. Used incorrectly, waterjets perform worse than propellers. The critical speeds of 20 – 30 knots are the transition from propellers to waterjets. DMS can help you decide on the right option. Achieve all the promises of waterjets without sacrificing efficiency.
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