Propulsion Disablers – A Game-Changing Opportunity and Possible Mortal Threat


Propulsion disablers (PDs) are small torpedo-like devices that destroy a ship’s screws and rudders. The purpose of this essay is to describe: 1) the strategic, operational and tactical opportunities that PDs offer the US Navy; and, 2) the threat that PDs will likely pose to Navy surface ships. Submarines are not addressed.

Origins of the PD Idea

The PD concept has arisen purely to meet a strategic need. PDs appear to be an ideal weapon to implement a blockade strategy, sketched out below under “Opportunities for Use of PDs by the US and Allies” and are detailed in the posts Blockade: Military-Economic Warfare vs. Russia and Blockade: Military-Economic Warfare vs. China. The PD is a child of strategic thinking, not technological development. The propulsion disablement idea has been around in principle for a long time. I first encountered it during the Cold War. The crisis intermingling between Sixth Fleet and the Fifth Eskadra led many to wonder: why don’t we have a way to put our opponent’s ships out of action short of sinking them? What’s new today is the technical possibility of actually doing that. The idea that PDs might also threaten US surface ships is born of a simple maxim: if I can do it to him, he can probably do it to me, so I should think hard about my defenses.


It is assumed that production of PDs is technically feasible today, or in the foreseeable future, by the US and by its adversaries. Because the pace of innovation in the current era is so rapid, no attempt is made to estimate how quickly effective PDs might arrive (nor is the writer remotely capable of offering an opinion on that subject). However, their eventual appearance seems nearly certain and because of their attractiveness, the interval is likely to be short. The most likely form falls under the USN’s category of an underwater unmanned vehicle (UUV).

Naval ships depend on their mobility to employ their reason for being—their lethality. Depriving a ship of its mobility has essentially the same result as sinking it: the ship loses its lethality against targets beyond the range of onboard weapons. It also makes the ship a stationary target, vulnerable to seizure.

If a PD can deprive a ship of its mobility with minimal—ideally zero—damage to the platform itself or its crew, there would be military and unprecedented political consequences:

Capabilities and Employment Concepts

  • Unlike sinking, there would be no irreversible loss of an asset that expresses national sovereignty. Thus, there might be a clouded, ambiguous casus belli. In a situation where a PD was delivered by stealthy means, it might not be possible to identify with certainty the state or even non-state actor that “fired” it.
  • Although PDs are inherently less violent than torpedoes, they are not benign. A ship, naval or civil, helpless before an unforgiving sea would present its owner (and the disabling party) with complex choices that need careful analysis. The owner would first be concerned with rescuing the crew and then with recovering the ship and cargo, if any, but might lack the means to do either or may simply choose not to. In that case, those tasks would fall to the disabler who, as a matter of moral, political, and possibly legal obligation, could not be indifferent to the fate of those he has put at risk. In any case, disposing of, say, many tens or even hundreds of disabled or seized ships would have to be part of a blockader’s plans. The adversary’s ships (perhaps not all) would have to be towed to safe harbor, their crews interned, or repatriated, and cargoes seized or returned to their rightful owners (assuming the ship’s or other documentation permits the owner(s) to be identified). This would be likely be a lengthy and resource consuming process. (I am indebted to K.J. Moore for raising the problem posed by disabled ships. He is of course not responsible for my treatment of the matter.)
  • In a severe crisis or in war the propulsion disabler might be viewed as a qualitatively new form of warfare at sea not yet seen in the modern era. The world at large is rapidly being transformed by robotics and ever-growing computational power. It is obvious that navies are caught up in this vast transformation of human activity. One of its plausible long-term manifestations may well be many thousands of small, distributed, unmanned drone PDs challenging the dominance of the relatively few hundreds of manned warships that comprise established power on the surface of the sea today.

The Navy has long had in place a wide variety of UUV programs guided by Master Plans dating from the 2000’s. However, as publicly described, these plans do not give priority to PDs nor to defense against them. Existing technologies (e.g., miniaturization, computing power, extended battery storage, exotic propulsion means, etc.) and, critically, a warhead a small fraction of the size of torpedoes designed to sink ships—all suggest that a PD UUV would be small. (Many might be carried in the space occupied by a 3,000-pound torpedo.) It would also be passive, difficult to detect and capable of considerable range in both mobility and target detection, especially of large surface ships. PDs would also be smart. Based on a library of the sonic signatures of the adversary’s naval and civil ships, collected in peacetime, PDs could distinguish between an enemy ship and those of third parties, and between categories of enemy shipping, allowing excluded targets like ferries, passenger ships, and the like to be avoided.

Emerging technologies are likely to enhance such capabilities, while efforts to reduce or mask the detectable signatures of traditional big ships are less likely to keep pace. PDs would mainly be delivered by air or submarine, though surface ships could also be armed with them for use in offensive blockade. A highly likely use would be as the warhead for stationary mines. Aircraft might deliver PDs against many enemy civil ships fairly rapidly over a wide area. Because of their size, it cannot be excluded that small numbers of PDs could be delivered by cruise or ballistic missiles. In the latter case, the missile would not have to hit its target—the golfer’s hole-in-one—it would only have to hit the green or just the frog’s hair. Submarines might deliver many tens of PDs from modules already under development for other uses, or of new specialized types.

PDs might be employed singly against civil ships, e.g., container ships, tankers, LNG carriers, etc. Against warships they might be employed singly, depending on their stealth, or perhaps in swarms. Swarms would seek to saturate defenses, overwhelm countermeasures, and increase the probability that multi-screw ships could be completely disabled.

Opportunities for Use of PDs by the US and Allies

PD devices have potential for offensive use versus China, Russia, and lesser adversaries. PD capability would provide useful payoff at all levels of planning: at the tactical level in acute crisis, where threats might be ambiguous, the US NCA would not face a binary choice between sinking the adversary’s ship or letting it sail on. Deprived of its mobility, a PD-ed ship becomes a helpless, floating hulk to be seized or left for the adversary to deal with.

PDs would also be ideally suited for blockade enforcement. (see the posts Blockade: Military-Economic Warfare vs. China and Blockade: Military-Economic Warfare vs. Russia). Runners could be disabled, and blockade-breaking defeated, with an as-yet-unspecified level of violation of the owning state’s sovereignty.

This is no minor matter. Enforcing blockades has been fraught with moral, legal, and political problems. The propulsion disabler would likely transform blockade operations. Consider the historical example of Lusitania. An artist’s rendering of her sinking is below. This picture would have looked very different if she’d been hit not by a German torpedo but by a German PD. Lusitania was a British-flagged ship but had aboard several hundred US citizens, many of whom were among the 1200 who lost their lives when she went down. As a result of those unprecedented losses, American public opinion turned against Germany and stayed decisively so until the US entered World War I two years later. If she had been PD-ed, Lusitania would have gone dead in the water, then likely be towed to Liverpool, and the war might have taken a different direction.

Sinking of the Lusitania. Engraving by Norman Wilkinson
The Illustrated London News, May 15, 1915

PDs likely will reshape naval warfare. No one needs reminding that throughout history new weapons have changed the ways navies have been employed. The propulsion disabler may prove to be such a weapon, both to use in blockade enforcement on offense, and—of equal importance—for the navies of the United States and its allies to defend against.

These observations are obviously hypothetical, used here to illustrate a point: a propulsion disabler would have given a radically new dimension to the submarine war against the SLOC a hundred years ago, just as it would change blockade enforcement today. There is little reason to expect that the PD will remain hypothetical. Whether the US fields one or not, adversaries almost certainly will.

At the operational level in war, PDs might prove almost as effective as torpedoes in defeating the enemy because they would render target ships essentially useless and burden the enemy with retrieving ships and crews. At the strategic level mass use of PDs could yield considerable leverage. Consider the case of a hypothetical war with China: if a half-dozen Chinese warships and several dozen civil ships were disabled, the rest might then be kept in port, producing the effects of a successful blockade.

China’s vulnerability to blockade has not yet been thoroughly analyzed, but the nation is already highly dependent on seaborne importation of hydrocarbons, raw materials, and even foodstuffs, production for exports dominate its economy, and it has made great investments in ship-building and other industries that depend on use of the sea. (see the post Blockade: Military-Economic Warfare vs. China. PDs are not a sine qua non of a design for strategic blockade, but their low cost, widespread deployability—air, surface, and subsurface—and likely efficiency would make them a highly desirable component.

The Threat PDs May Pose to USN Surface Ships

The first obligation of the planner is to defend his own vulnerabilities. Our adversaries are surely as aware of the merits of PDs as are students of naval warfare in the West. They may be capable of producing large numbers of PDs from home-grown robotic and computer technologies, as well as strength in mass production of modern electronic devices. These factors suggest this potential should be taken seriously.

It remains to be seen whether PDs will prove to be just a new form of undersea threat to be answered with traditional ASW measures, or whether they may be transformative. It is hard to imagine an asymmetric capability more attractive to China or Russia: a fairly simple, inexpensive way, possibly difficult to defend against, to neutralize the surface ships of the world’s most powerful navy. Nor one where the disparity in costs were so great: many thousands of PDs produced at a fraction of the cost of a carrier strike group and whatever may be prove necessary for its defense against PDs.

Possible Scenarios for Use of PDs by Adversaries

Consider three cases involving China:
(1) In peace, China successfully uses a PD against a US warship on a Freedom of Navigation operation near a Chinese-claimed area of the South China Sea. China denies all responsibility. The US searches for an appropriate response and is preoccupied with retrieving the ship.
(2) In a crisis at the brink of war, the Chinese do not fire explosives at an approaching CVSG. Rather they use PDs against the carrier. A successful attack would be a US nightmare: 110,000 tons of useless steel, drifting helplessly and displaying US impotence on worldwide television screens—a scene repeated over weeks until the ship can be towed away for repair—assuming China does not PD the tug. On the grounds of prudence, the US withholds commitment of the rest of the carrier force.
(3) In war, PDs will likely find a place in a mix with kinetic and explosive weapons. They may be the weapon of choice because of their unprecedented military advantage: putting ships out of action and at the same time forcing the opponent to deal (or not) with his disabled ships.

Suggested Actions

  • Develop PD capabilities for offensive uses as outlined here. Just as important, give counter-PD a high priority in Navy planning for the defense of the carriers and the rest of the surface Navy.
  • Direct the Intelligence Community to search for signs of PD development in all of our adversaries’ actions, including in their open military writings.
  • Ensure that intra-Navy research and analysis addresses PD/counter-PD. (See the note below on some relevant Navy efforts.) The Navy should also request the Joint Non-Lethal Weapons Directorate to similarly adjust its focus. The JNLWD’s “Strategic Plan, 2016-2025” includes a category “Stop Large Vessels.” But the category had no content in 2018, and the Directorate’s general perspective is offensive with little corresponding concern about defending against an adversary’s non-lethal weapons.
  • Suggest to OSD national options to respond if a US Navy ship (or perhaps even a civil ship or ships of allies) were PD-ed. This should be done as a precautionary minimum to avoid being caught flatfooted by a surprise PD attack. Even today it may be possible that the Chinese could produce a primitive PD warhead for a stationary mine placed on the perimeter of claimed territorial waters. Should evidence come to light that an adversary has or may soon possess operational PD capabilities, policy decisions on the matter would be urgent and mandatory.
  • Assess the international and domestic (if any) legal implications of PDs and take legal and policy actions as necessary.


The US should develop and field PDs with as little fanfare as possible. It seems highly likely that our adversaries will do so independent of US action. Even if that likelihood is estimated to be small today, the probability of the eventual appearance of PDs is high, and the possible consequences of their introduction could prove revolutionary. This combination of probability and consequence dictates a serious need to think through immediate and long-term measures both to exploit PDs on offense and counter them on defense.

Note: Currently a wide variety of Navy UUV efforts are underway, including some recent and soon-to-be-deployed hardware. None is focused on PD/counter-PD, as far as I have seen in information publicly available. The Coast Guard has shown specific interest in using a small torpedo—the Surface Ship Torpedo Defense (SSTD) currently being evaluated—for what is a PD in all but name. Employment of swarms of small underwater devices is in early stages of technical evaluation of their feasibility independent of a conception for their tactical use.

Bradford Dismukes, San Francisco, January 15, 2021

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