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New Paradigms in Boat Design: Exploring Unmanned Surface Vehicle

Wavelengths: An Employee's Digest of Events and Issues (NAVSEA Carderock)

June 2002

Synopsis by Leslie Spaulding

The following article is a synopsis of a paper written by Seth Cooper (2420) and Matthew Norton (7430) titled, "New Paradigms in Boat Design: An Exploration into Unmanned Surface Vehicles." The paper will be presented at the Association for Unmanned Vehicles Systems International's annual symposium in July.

Unmanned vehicles are critical components of the future naval forces. Significant research and development has been performed on unmanned underwater vehicles (UUV) and unmanned aerial vehicles (UAV), yet little effort has gone into examining unmanned surface vehicles (USV).

With future conflicts taking place primarily in the littoral regions around the globe against adversaries who possess increasingly more effective weapon systems, placing people in harm's way may no longer be a viable option. Unmanned systems-air, ground, underwater, and surface-present an effective and low cost alternative to risking the life of a highly trained Soldier or Sailor.

A USV offers many benefits to the Fleet. The first, and most obvious, is no risk to Sailors' lives. A USV can be deployed in waters where it's unacceptable to send a manned vessel, including high threat environments or areas contaminated by nuclear, biological, or chemical agents. A USV could also remain on station for extremely long periods of time (up to several weeks) without resupply or human intervention. Such a capability could allow for long-term anti-submarine warfare (ASW) or mine countermeasure (MCM) operations in areas of the world where future conflicts are expected. Additionally, a USV squadron could be deployed in advance of a carrier battle group or amphibious ready group to sanitize the area of potential threats and assure access for troops. Furthermore, USVs have large payload capacity, allowing for a multi-function mission package to be deployed with each USV. A single USV could simultaneously conduct many operations.

By operating on the water's surface, a USV could operate on conventional power sources, such as diesel, gas turbine, etc., rather than relying on more exotic and limiting power supplies, such as batteries or fuel cells. Additionally, a USV could communicate in all three mediums of interest-undersea, air, and space, relaying information from submerged assets (submarines, UUVs, etc.) to any combination of surface vessels, aircraft, or satellites and vice versa. No other unmanned system has this capability.

USVs could range from small, "floating log" intelligence missions to a large (10,000+ ton) UAV/USV mobile base. In consultations with the Navy's future strategists-the Navy Warfare Development Command (NWDC), the Naval War College (NWC), and the Strategic Studies Group (SSG)-and based on studies published by NWDC, Office of Naval Research, and the SSG, the missions deemed most critical to the future Fleet were ASW and MCM.

A Paradigm Shift in Design

Changes to the design of Navy boats and ships happen at a very slow pace. In the last 50 years, the main hullform change to naval ships has been the addition of bulbous bows to displacement hulls. Although many innovative hulls have been designed and tested for small boats, none have proven more suitable than a standard planing hull. A boat designed to operate as an unmanned vehicle, however, has a completely different set of requirements than a manned one. This new set of requirements leads to a paradigm shift in hull design over a traditional manned vessel; the old requirements need to be reworked from the ground up to arrive at the optimal design for an unmanned surface vehicle. USVs may call for either completely new hullforms or modified versions of existing hulls.

Since they are unmanned, the USV design need not be limited by human factors. It can be completely mission-driven-allowing for a more efficient design. Certain characteristics common to all USVs could form a "base model" to which mission-specific modules could be attached or mission-specific technologies could be designed into the system, ensuring a finished product optimized for its intended purpose.

For example, an unmanned vehicle designed for mine hunting would incorporate the latest acoustic sensing technologies, a hullform optimized for low speeds high endurance, an onboard power generation capability designed to support a wide array of sensors and propulsion, and an intelligent mine-recognition capability to detect, classify, and neutralize any mines. A vehicle designed for surveillance and sensor delivery would utilize stealth technology both in its physical and electronic design to evade detection by hostile sensors. It would utilize the latest imaging and electronic eavesdropping devices to gather a wide array of intelligence data and remain on station for extended periods of time.

Hullform Comparison

Certain hullforms have specific characteristics that make them more suitable to certain operations. The requirements for a manned vehicle center around human limitations, such as motion, temperature, habitation space, etc. The requirements for an unmanned system are based solely on what the machinery can handle. Due to this shift in requirements, new and innovative hullforms, which may be unsuitable for manned operations, become prime candidates for USVs.

For the purposes of this paper, the authors grouped vessel characteristics and mission capabilities by the size of the vessels. A small USV would be about the size of a torpedo; a medium would be about the size of an 11-m rigid inflatable boat; a large USV would be the size of an LCAC, and an extra large USV would be the size of a Corvette.

Certain missions lend themselves to a specific size, whereas others can be accomplished by a USV of any size. Missions were categorized by size, with some missions fitting into more than one category. Attributes specific to certain sizes were also identified.

For the missions of ASW and MCM, simplified analytical models were created to determine the relative characteristics of a USV design, such as speed, range, payload capability, fuel load, squadron size, and other features. Conclusions were drawn for each scenario separately, as well as overall.

For ASW, a probabilistic model using search theory was created. In the model, squadrons of USVs were assigned to a pre-defined area, set at 5,000 and 10,000 square nautical miles and were to pre-search the area for enemy subs for 24 hours after which a LSD 41 Class ship would enter the area. The USV would continue searching to protect this ship. The model accounted for the ship characteristics, the threat characteristics, the operating environment, the detection ranges of the ship and the threat, and the transit, search, and attack speeds of all the vessels, as well as USV squadron size. The model then calculated the probabilities that the USVs would detect the threat in the 24-hour pre-search and that the LSD 41 would survive the 24 hours it was operating in the area.

For the mission of MCM, a model was developed using a simple ladder pattern to search out randomly placed mines. It was assumed that each mine found was also neutralized using a mine-missile deployed from the USV. A minefield of variable size was modeled using a density of 20 mines per nautical square mile. The fact that this is not a valid assumption for "real life" scenarios is not important toward the goal of obtaining maximum coverage with minimum resources. The number of USVs searching the field, the number of disposable mine neutralization devices, and the transit speed were all varied to determine their relative effects.

From these modeling efforts, several conclusions were drawn. It was determined that a relative squadron size of 16 USVs could accomplish not only the missions identified, but numerous others as well. In addition, it was highly desirable to have the USVs remain on station for up to two weeks at a time without intervention. The minimum range the USVs should have was 500 nautical miles with ranges of 1,000 to 2,000 nautical miles being more desirable. Finally, the faster the USVs could deploy to their designated operating areas, the more effective they could be in their mission. A sustained speed of 30 to 35 knots was determined to be the most advantageous, both from a naval architecture point of view, as well as from a tactical view.

With these design parameters determined, the next stage of the study focused on a point design for a USV to provide a potential solution for these mission needs.

Point Solutions

The main point solution identified in the paper is the Planing Hydrofoil Assisted SWATH (Small Waterplane Area Twin Hull) Transport or PHAST. The concept for this hullform is to behave as a SWATH at lower speeds-obtaining the efficiency, seakeeping, and stealth advantages of a SWATH, and at higher speeds operate in a dynamic planing mode to allow for the efficiency gains at higher speeds. Since the craft will perform in two modes it creates a difficult task for the designer of having to converge the design for optimal performance in both modes. At lower speeds, the hull was modeled as a SWATH with extra drag to account for the added surface area and the foil. At higher speeds, it was modeled as a planing catamaran with the addition of lift and drag from the foils, and an iterative solver was used to ensure that all the forces and moments were balanced.

Conclusions

USVs will provide the Fleet with additional military capabilities especially where loss of life is unacceptable. USVs can perform many missions better, more cost effectively, and for longer than other unmanned systems available today.

The success of a USV system is contingent on many factors. Among these are successful integration into both the current Navy and the Navy After Next, deployment to desired areas with a minimal impact to Fleet activities, the ability to obtain and deliver in real-time a wide array of sensor data, the ability to deliver command control information over large distances, and the development of technologies critical to long-duration unmanned operations.

Unmanned surface vehicle technologies can be grouped into six categories: hull and propulsion, machinery and electrical, autonomous replenishment and reconfiguration, communications and telemetry, command and control, and signatures. New and innovative hullforms may be required to take full advantage of the fact that the vehicle is unmanned. Near zero maintenance machinery will need to be reliable enough for long-duration unmanned operations. Since unmanned vehicles will be used as long-range sensor platforms, they will require the ability to stream large amounts of data back to the Fleet command for evaluation. Currently this capability is not readily available. A robust command and control system must also be developed to ensure fault-free operation of the USV over long distances for extended periods of time.

These are just a few examples of the research and development efforts to be undertaken to ensure successful unmanned surface vehicle operations. The technological and design challenges are not easy, but with up-front design of unmanned vehicles, host ships, and associated interfaces, clear military benefits can be gained.



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