(Published IndRastra Global 06 May 2020.)
Next-generation Propeller Design
The development of warships has been guided by the need for speed, range, firepower, and survivability at sea. However, to achieve speeds over 30 knots, it became apparent that the propeller material and design needed a change to ensure that cavitation effects are minimized. The transit of propeller blades through water results in positive pressure on the blade face and negative pressure on its reverse side, the negative pressure causes the absorbed gases in the medium to bubble and later collapse on the reverse side of the propeller. The hammer-like impact of the collapsing bubbles causes damage to propeller blade surfaces and results in a drastic drop in its efficiency. The cavitation of propellers is a very complex phenomenon, and the pitting becomes visible on the reverse of the blade in a clear radial pattern. It may also happen to the driving face of the propeller in case there is an incorrect pitch distribution along the length. There are various types of patterns of propeller cavitation like face cavitation, tip vortex cavitation, bubble cavitation, sheet cavitation, cloud cavitation, root cavitation, and boss vortex cavitation. The loss of speed and damage to the propeller blades caused due to cavitation can be mitigated to an extent by correctly designing the propeller blade area relative to the area of the circle described by the propeller blade tips.
Since modeling of high-speed propeller designs did not result in predicting cavitation damages accurately, recourse was taken to actual testing of prototype propellers on ships in the late 1930s. Triple bottom hulls and hull blisters were developed to counter the torpedo and mine damage effects; however these affected the speed of the ships, and so a process of iteration continued in the interwar years. With the advent of oil-fired boilers, additional fuel could be carried in the blisters and hull spaces leading to higher endurance. The length to speed ratio in respect of capital ships was arrived at as Capital ship speed = 1.19 √length at the waterline. The Iowa class ships were designed accordingly but with a limitation on their beam size at 108-1/6 feet to enable clear passage through the Panama Canal. An empirical relation between the waterline length and maximum beam was arrived at as 8.85 for a cruiser and 7.96 for a battleship. The Iowa class carried triple turrets of 16-inch 50 caliber guns along with an armor plating which tapered towards its waterline as protection. The propulsion was designed to provide 50,000 shaft horsepower to each of its four turbines and achieved the speed of >33 knots for a loaded displacement of 57,000 tons.
A sea change in the approach for the expansion of the US Navy took place after Germany occupied France in 1940, which saw the enactment of the Two Ocean Navy Act 1940 or the Vinson-Walsh Act. This act was brought about with the argument that the British Navy may not be able to maintain security in the Atlantic Ocean and therefore the US Navy would have to prepare itself for providing security both on the Atlantic as well as the Pacific Oceans. This act authorized US Navy for an expenditure of $8.55 billion towards an expansion program that placed emphasis on aircraft. Rep Vinson stated that “The modern development of aircraft has demonstrated conclusively that the backbone of the Navy today is the aircraft carrier. The carrier, with destroyers, cruisers, and submarines grouped around it is the spearhead of all modern naval task forces.” The increase of nearly 70% in the Naval strength envisaged procurement of 18 aircraft carriers, 2 Iowa-class battleships,5 Montana-class battleships, 6 Alaska-class cruisers, 27 cruisers, 115 destroyers, 43 submarines, 15,000 aircraft, the conversion of 100,000 tons of auxiliary ships, $50 million for patrol, escort and other vessels, $150 million for essential equipment and facilities, $65 million for the manufacture of ordnance material or munitions, and $35 million for the expansion of facilities. This impetus led to the development of warships and carriers with no restriction on their beams to cater for the passage through the Panama Canal. The length to beam ratio was changed to 7.35 for the Montana and the Midway class of aircraft carriers.
In the late 1990s, the British Type 23 frigate exemplified the benefits of an electric architecture in tandem with the hybrid power distribution and propulsion system in the Combined Diesel Electric and Gas (CODLAG).
The British Type 45 destroyer has the integrated power system (IPS)-derived advanced induction motor (AIM) development from the United States and the WR21 ICR gas turbine. The WR-21 is an advanced cycle (intercooled and recuperated) gas turbine system developed to provide power to the new generation of warships. Its design allows higher thermal efficiency, decreased signature, higher reliability, and ease of maintenance. Its development has been funded by the US, British and French Governments. The main contractor in the design and development program is Northrop Grumman Marine Systems (NGMS) along with Rolls-Royce as the major subcontractor responsible for the design of the gas turbine. Further, Honeywell is subcontracted for the design and supply of the intercooler, Ingersoll Rand Energy Systems (IRES) for the recuperator, and CAE Inc. for the Electronic Engine Controller (EEC).
British warship HMS Defender (Pennant number: D36) visited Goa in the second week of November 2019. It allowed the Indian Navy to study this type 45 destroyer’s integrated electric propulsion system. It is reported in the media that a team of Indian naval engineers visited the warship and got a first-hand view of the propulsion system, combat systems, and crew berthing for women.
The commanding officer of HMS Defender Commander Richard Hewitt stated that “HMS defender is propelled by an integrated electric propulsion system. I understand that the Indian Navy is also looking at technology. So today we have marine engineers from the Indian Navy and the Royal Navy working collaboratively to understand how this system works,”
The Indian Navy is said to be considering the induction of electric propulsion in its future warships and aircraft carriers. Electric propulsion provides savings in cost, better efficiency, survivability, and flexibility. The reduction in numbers of prime movers, integrated systems, flexibility in layout, and proven commercial designs make it a viable option. Electric propulsion, EP, systems are of three types, that is a hybrid, integrated (IEP) and integrated full (IFEP). The terms electric ship and electric warship are also used. In the hybrid system, mechanical drive and electric drive systems are combined. In the IEP, a common power source is utilized for both the propulsion system, and ship services, the propulsion is electric. The IFEP incorporates energy storage and power electronics and into the system to achieve operational and cost advantages. The Electric ship incorporates into the IFEP architecture, advanced prime movers, and electrification of auxiliaries. The Electric warship has high-power weapons and sensors to take benefit of the available system power.
Modular Warships and Common Hull Forms
Navies today operate various types of warships with different hull designs. The hulls last over three decades; however, due to the advancement of technology, the propulsion systems, weapon and sensors packages, and other major systems undergo upgrades or complete changeovers during prolonged refit schedules. There is a thought to standardize as much equipment and systems as feasible to achieve modularity on ever-increasing scales. The ideal would be to achieve a plug and play design interfaced with a single basic system design. For instance, there is an increasing thought that the Navies should move towards decoupling the payload from the platform by going in for two types of hull designs namely a Destroyer (D) hull and a Cruiser (C) hull. The modular systems fitted in D class could also be plugged into the C class. The same D hull shape can meet the different types of mission requirements by retrofitting with the least amount of time in the dockyard. These hull shapes would standardize the Navy-wide maintenance, training and manning requirements with the required scalability for the bigger hull type. The C type would be larger ship with a flight deck but with standard D hull control systems, interfaces, and basic armament modules. The standardization of hull shapes is in tune with the merchant marines 20-foot equivalent container unit concept that has led to the modern container ships with universal and interchangeable platforms and payloads. As technology upgrades arrive, the payload kits can be designed to fit into the existing interfaces, spaces, power buses, and other systems. The D hull type could accommodate electric drive propulsion, a medium caliber gun, vertical launch system cells, a point defense system, a double helicopter capability, hull sonar, an advance 3D radar, an EW suite, a station for boats and unmanned craft, and excess power, bays, and spaces for multiple utilities and so on. The C hull type could accommodate a flight deck.
Navy has already been the spearhead to imbibe advanced technologies in its platforms, sensors, and weapon systems. The interplay of technologies like autonomous systems, cyber and electronic warfare, energy management, big data analytics, smart materials, and advanced manufacturing are going to play a leading role in the upgrades of current warships as well as the design of future ships.
Big Data Analytics: The humongous amount of data that is going to be received from diverse sources on the warship is going to be incorporated in the design of its data handling capacity as well as into the analysis of this data in real-time to provide actionable inputs for decision making. The knowledge-based approach to warfare has already made inroads into the development of situational awareness systems and decision-making tools coupled with high bandwidth requirements and associated communication networks. Cybersecurity aspects and interfacing with legacy systems are the primary concern areas that are continually being addressed in the development of big data analytics systems.
“I have ships with a number of sensors on them, measuring things like reduction gears, shafting components, turbines, generators, water jets, air conditioning plants, high packs, a number of components, and we’re actually pulling data off those ships, in data acquisition systems,” the above was stated by Rear Adm. Lorin Selby while talking about the process which is called condition-based maintenance plus (CBM+), based upon bigdata analytics for enhancement in maintaining ships. He further said that a pilot program using enterprise remote monitoring will be run on an Arleigh Burke-class destroyer. The data collected will be analyzed, and operators will learn to use that data to understand how their systems are performing and if maintenance or repairs are needed.
Unmanned Systems: Semi-Autonomous systems are already finding their place in navies be it in survey vessels, minesweepers, or surface ships. However, it does not appear likely that a fully autonomous system with the decision to carry out a lethal strike would be operationalized in the next decade due to various legal and ethical issues tied up with its induction.
Unmanned systems would increasingly be inducted onboard warships for carrying out tasks like surveillance on the surface, in the air, and underwater. Interfacing with existing systems is a challenge. It is being resolved innovatively to utilize the tremendous potential of the Unmanned systems across the board in running, maintaining, and warfighting on board a warship. The Medium Unmanned Surface Vehicle (MUSV) and Large Unmanned Surface Vehicle (LUSV) are being developed by the US Navy as “anti-surface and strike warfare” vessels. The US Navy is developing a family of unmanned vehicles for undersea warfare which includes the Razorback and the Snakehead which can be launched from a manned submarine.
Advanced materials: Advanced materials with unique properties are making their appearance in all sort of hardware applications, for example, Buckypaper is 10 % the weight of steel but 500 times stronger than it, it is a thin sheet comprising of carbon nanotubes (CNT) whose hardness is twice that of diamonds. The buckypaper is also corrosion resistant and flame retardant. Such a material can be used in shipbuilding and would significantly reduce the weight which in turn would lead to higher speeds, reduced fuel consumption, and increased endurance. Design and development in using materials like Buckypaper, in shipbuilding are already being progressed.
Additive Manufacturing: Additive Manufacturing or 3D printing is a process of joining materials to make objects from 3D model data, usually layer upon layer of polymers, as opposed to subtractive manufacturing methodologies. It is based upon computer-aided design (CAD) software and used for designing complex geometries and features. 3D printing technology allows the construction of real objects from virtual 3D objects. In a first for the US Navy, the US Navy’s Naval Sea Systems Command (NAVSEA) had approved the first metal part created by 3-D printing for installation on a warship last year. The US Navy has been using additive manufacturing technology utilizing polymers for many years; however, using it to print metal parts for naval systems would provide a tremendous boost to 3D manufacturing in the shipbuilding industry. A ‘3-D printed-metal drain strainer orifice for a steam line’ was installed on the carrier USS Harry S. Truman for a one-year test and evaluation trial after the prototype passed all tests, like welding, shock, vibration, material, and steam tests. It may be of interest to note that US Navy’s USS George HW Bush (CVN 77) aircraft carrier would be upgraded with conventional and 3D printed parts during its ongoing refit.
From the foregoing, it can be appreciated that naval shipbuilding is keeping pace with the developments in emerging technologies. The warships in the coming decade would display a large absorption of technologies that have not yet been commercialized. The Warship would be lighter, faster, much more lethal, and would be able to survive better at sea.