U.S. Navy ships have used screw propellers almost exclusively for nearly two centuries, but in recent years, the Navy has shifted to waterjet propulsion for a handful of surface ships.1 The submarine force may not be ready to embrace an all-waterjet primary propulsion system (à la The Hunt for Red October’s “caterpillar drive”), but as a graduate student in mechanical engineering at the Naval Postgraduate School (NPS), I developed the outline for a novel method of auxiliary waterjet propulsion for submarines. Using it, the Navy could modify current submarines inexpensively to quickly create a nearly silent auxiliary system.
Propulsion Pros and Cons
A propeller moves a large volume of water at a slow speed, while a waterjet moves a small volume of water at a high speed. Waterjet propulsion is therefore able to propel a ship faster than a conventional screw propeller can.
In a waterjet, a pump pushes water through a converging nozzle, increasing or decreasing the speed of the water by varying the nozzle’s aperture (in much the same way a garden hose nozzle can eject a thin, high-speed stream or a wider, slower one).2 Waterjet pumps typically have the ability to change speed almost instantaneously, so thrust can change at the push of a button. Changing the speed of a conventional propeller, on the other hand, requires spinning up or braking the rotational speed of the massive main shaft, which takes time. To change a propeller’s direction from forward to reverse takes even longer; the shaft must stop completely, then start again in the opposite direction. A waterjet can change thrust’s direction quickly by positioning a flow diverter, also known as a “bucket,” in the jet flow.3
Not only do waterjets save considerable time reversing thrust, but they also have few to no external moving parts (other than the thrust-directing buckets), making them ideal for operating in places where quiet is the highest virtue—that is, wherever the U.S. submarine force typically operates.4
Applications requiring rapid changes between forward and reverse thrust are perfect for waterjet propulsion. For example, marine dynamic positioning systems use computer controllers receiving precise navigational input and outputting commands to waterjets to maintain a ship’s position. These systems are popular in the oil and offshore wind industries.5
Despite the waterjet’s advantages, a stigma exists among some ship designers, because the jets are efficient at high speeds, but inefficient at low speeds—the opposite of propeller efficiency.6 Submarines routinely operate at slow speeds, making waterjets appear to be a bad idea, because they will be inefficient. Efficiency, however, is just one design parameter, and it may not be the driving factor in a design choice, considering the nearly limitless energy source (a nuclear reactor) on U.S. submarines.
Outside of fiction, little research has been done into submarine waterjet propulsion. The Navy has taken a small step toward it by using ducted propellers, or “propulsors,” for the Virginia and Columbia submarine classes.7 A propulsor surrounds a propeller with a converging shroud, both to block cavitation noise and increase the speed of the water, but the propeller still otherwise operates just as a conventional one does.
To abandon propellers and make submarines move entirely by waterjets would take an expensive and Herculean effort, involving not only Naval Sea Systems Command, but also Naval Reactors as well as designers from shipbuilders around the country. The merits of such an effort are outside the scope of this article. But what if an existing system could be adapted to create an auxiliary waterjet propulsion system, at low cost and low risk?
Stand by for Thermo
The second law of thermodynamics dictates, among other things, that a machine cannot convert 100 percent of its heat generation into work; some heat must be rejected to the environment—the reason “perpetual motion” machines are impossible.
Submarine nuclear reactors reject waste heat by pumping cold seawater through a heat exchanger and then back out into the ocean. This flow exits perpendicular to both sides of the boat hull, producing no net thrust. Thus, the energy used to pump the seawater is as wasted as the heat it exchanges.
But a converging nozzle could be placed at the outlet to redirect the flow to produce thrust. If only forward thrust is desired, all that is required is one stationary component—an elbow joint, in effect—that could be added to any existing U.S. submarine. For directable thrust, a control system would need to be added as well, to adjust each bucket’s position. An enormous operational advantage could be gained at low cost, with minimal effort, and taking up almost none of a submarine’s limited space.
Fewer moving parts means quieter submarines, but only to a point. Bernoulli’s principle states that as the speed of a fluid increases, its pressure drops. If it drops low enough, the water can actually vaporize, forming bubbles that collapse—loudly. This is called cavitation, a generally undesirable phenomenon in the submarine world.
The higher the speed of the outflow, therefore, the lower its pressure, and the greater the probability of cavitation.
Higher water speed is achieved with a smaller nozzle outlet area. Theoretically, an infinitesimally small outlet surface area would make the water velocity nearly infinite. Enter friction: As the outlet area decreases, the friction in the nozzle increases, in turn reducing the volumetric flow rate and thrust. For any given fluid, there is a “sweet spot”—a nozzle outlet surface area that maximizes thrust at a given flow speed.8 Since there is no reason to increase the water speed beyond the “sweet spot,” cavitation is intrinsically limited. In addition, because cooling the nuclear reactor is the main purpose of the seawater system and there is a minimum design seawater flow rate, the nozzle area cannot be reduced to a size that would go below this minimum.
Even in the event cavitation occurs, a sound suppressing device—an ejector—could be placed around the nozzle exit to reduce the noise transmitted to the open ocean.9 Ejectors have been used for decades to reduce jet engine noise, and a huge benefit is that they provide thrust augmentation thanks to a phenomenon known as entrainment.10 For a nozzle with no ejector, the high-speed water exiting the nozzle collides with relatively stationary seawater (also called entrained flow), imparting momentum to it. With no surface for this entrained flow to push off of, no extra thrust is produced. But the ejector’s surface allows this entrained flow to push off it, thereby increasing the thrust.
Ejectors are not commonly used in waterjets because their surface areas must increase gradually to have the proper effect. This yields long—sometimes obnoxiously long—ejectors that protrude beyond the stern, where most propulsion systems are located.11 But submarine seawater system outlets are not at the stern; ample distance exists along the hull to attach an ejector without altering hull length.
Thunder (and Quiet) Below
Seabed and deep ocean warfare are topics of significant interest for the Navy. Deep ocean warfare should be to the Navy what air warfare was to the Army in the 1980s: the means to win cross-domain warfare. But the service at present is ill-prepared to gain and maintain dominance there.12
The U.S. Navy must be able to operate routinely on or near the seabed, in open ocean or the littorals, with either manned or unmanned vehicles. The precision required to maneuver a submersible along the ocean floor must be significantly higher than away from the floor, so as not to damage the submersible and manipulate devices such as mines or cable taps. In some applications, submersibles must have the ability to remain completely stationary while fighting ocean currents, similar to surface ships’ dynamic positioning systems.
Most transatlantic communication occurs through cables laid on the seabed between North America and Europe.13 It is possible for adversaries to tap into, or even sever, these lines of communication. Russia has small deep-diving submarines made for this specific purpose.14 The United States also has assets with similar capabilities. Waterjets could improve their stealth and handling on and near the seabed.
An auxiliary waterjet system would be useful elsewhere in the deep, away from the ocean floor, as well. A submerged submarine whose propeller is incapacitated, whether by a near-miss torpedo detonation or an engineering casualty, is at grave risk. In the case of enemy action, damage to the main shaft or propeller may well be accompanied by damage to the secondary propulsion unit (positioned near the stern), leaving the sub without motive power. Waterjets could provide emergency propulsion.
My research suggests these auxiliary waterjets could propel a sub between 5 and 10 knots. The surprising implication is that the most useful and cost-saving application would be to add waterjets to ballistic-missile submarines (SSBNs), which routinely patrol at slow speeds to minimize risk of detection. Adding seawater outflow nozzles and ejectors to SSBNs would allow them to routinely patrol without using the main shaft at all, transmitting less noise to the water and burning less nuclear fuel. This could increase the life of the boat, since the hull can be strengthened and recertified as needed, without refueling or replacing the reactor. The life of aging Ohio-class boats could be extended modestly, while new Columbia-class submarines could expect service lives increased by many years—all for the cost of some ducting and a simple control system.
1. Nicholas Newman, “Pump It Up! Bigger and Better Uses for Water-Jet Technology,” Engineering & Technology, 2 October 2019.
2. Morton Alperin and Jiunn J. Wu, Underwater Ejector Propulsion Theory and Applications (Washington: Office of Naval Research, 1976).
3. Newman, “Pump It Up!”
6. Camarc LTD, “Waterjets vs. Propellers,” camarc.com (2020).
7. Globe Composite Solutions, “The ‘Silent Service’ Becomes Even Quieter,” www.globecomposite.com.
8. Chang Tao Wang, et al., “Research on Optimization of Water Jet Propulsion,” Applied Mechanics and Materials vol. 380–84 (August 2013): 205–8.
9. Alperin and Wu, Underwater Ejector Propulsion Theory.
10. M. J. Werle and W. M. Presz, “Shroud and Ejector Augmenters for Subsonic Propulsion and Power Systems,” Journal of Propulsion and Power 25, no. 1 (2009): 228–36.
11. Alperin and Wu, Underwater Ejector Propulsion Theory.
12. Bill Glenney, “The Deep Ocean: Seabed Warfare and the Defense of Undersea Infrastructure, Pt. 1,” Center for International Maritime Security, cimsec.org, 4 February 2019.
13. Pete Barker, “Undersea Cables and the Challenges of Protecting Seabed Lines of Communication,” Center for International Maritime Security, cimsec.org, 15 March 2018.
14. Thomas Milsen, “From This Secret Base, Russian Spy Ships Increase Activity Around Global Data Cables,” The Independent Barents Observer, 12 January 2018.