nf 4-15 - 14_09 US Navy Carrier 9/4/15 2:07 PM Seite 54 NAVAL TECHNOLOGY NAVAL©2015 Mönch Verlagsgesellschaft mbH FORC ES INTER NA TION AL FORUM FOR MARI TIME POWER STEVE MAYNARD THE COST OF SPEED Today’s sophisticated weapons systems, complete with fire-and-forget missile technology and proliferation of both manned and unmanned aircraft at sea, call into question the value of speed. IRON TRIANGLE NAVSEA example of design space exploration displacement), or from smooth-water perfor- To help gain insight into the influence that using a dynamic contour profiler. mance predictions and other hull test data. Naval speed has upon a 3,500 tons frigate, three con- (All photos: Courtesy of GE Marine) architects develop these predictions using ship ceptual mechanical drive configurations are models of similar hull form, appendage, and compared. Results reveal a 40-knot ship requires propulsor configurations to estimate the neces- almost three times more power than 30-knot PAYLOAD sary shaft power to propel across a range of ship, and that a more lightweight, power dense Navies are also payload-centric. In fact, a desired speed. The profiles are not perfect, but propulsion system can enhance mission payload surface combatant’s primary focus is defined by can provide a reasonable starting point for engine by over 200 tons. While a high-speed vessel may the ship’s suite of weapons, sensors and matching and machinery plant options. be justified based on mission requirements, it unmanned systems. Payload is important to Using the projected speed-power relationship, can lead to inefficient ship design, which has less modern navies given that operational require- design engineers can consider various machin- combat lethality. These comparisons highlight ments dictate many vessels be capable of inde- ery arrangements for a proposed surface com- the benefits of having a family of gas turbines pendently conducting multiple missions such as batant. Depending upon the ship’s primary mis- with wide power ranges, to allow naval archi- Anti-Submarine Warfare (ASW), Anti-Air War- sion and its associated operating profile (i.e. how tects to best match the iron triangle of speed, fare (AAW), and Surface Warfare (ASuW). much time the vessel is projected to operate at endurance, and mission payload. As opposed to Naval architects focus on the available space and different speeds), a navy may further stipulate a single power class, GE Marine’s gas turbine weight – or design space – for fitting the mission general propulsion plant configurations. These solutions range from 6,000-70,275 shaft horse- payload aboard the ship. Available space is mechanical drive, hybrid or integrated electric power (shp)/4.5-52MW. These varying engine largely a function of vessel size, hull form, cost systems can include arrangements such as: sizes offer design flexibility in diesel engine constraints, maximum speed, and endurance – Combined Gas Turbine and Gas Turbine selection and smaller power increments to enable range. Reduced payloads limit self-defence and (COGAG); more efficient loading of prime movers. striking capabilities. Upcoming changes to the – Combined Diesel and Diesel (CODAD); Littoral Combat Ship (LCS) programme attest, – Combined Diesel or Gas Turbine (CODOG); ENDURANCE as the US Navy has hinted that it may be willing – Combined Diesel and Gas Turbine For the US Navy, endurance cannot be to sacrifice ship speed in the interest of enhanc- (CODAG); sacrificed given the transoceanic nature of ing weapons payload and lethality. In the end, – Combined Diesel Electric and Gas Turbine geopolitical interests, thus dictating a required the true purpose of a warship is to deliver ord- (CODLAG). minimum cruising range of 3,500nm (6,480km). nance on target. Yet many Navies set cruising range requirements Fuel efficiency and power density can signi - at 6,000nm (11,110km) for endurance purposes 40 KNOTS REQUIRES NEARLY ficantly influence design choices. Yet, in all of which oftentimes indirectly yields longer on- 270% INCREASE IN POWER these configurations, the amount of installed station time for independent operations. There The speed-power relationship for a 3,500 tons engine power will largely be influenced by is little advantage to ‘getting to the fight’ quickly surface combatant with a conventional clean maximum speed for short term operation, and to if on-station time truncates the mission. In most monohull design can be defined by a nominal some degree cruising (or economical) speed, for Navies, endurance for self-deploy- cubic load curve. The fundamentals longer transit operations. ing or independently operating ves- of the cubic load curve dictate that Prime movers should be selected to best match sels compels designers to conceive doubling speed requires eight times the demands of the iron triangle. As always, the larger ships. These must accommo- more power. Speed-power profiles ship design team must also account for the date the fuel and supplies necessary can be derived from past profiles of propulsion plant, since an optimal prime mover to transit long distances while still similar surface combatants (of a given match can improve the ship’s cruising range having a useful mission payload. To that end, high speed requires high Steve Maynard, Captain (Ret.) US Navy, power and that means bringing and is Engineering Director, Military Applications, GE Marine, Cincinnati, Ohio, USA. He joined GE burning lots of fuel. Marine in 2008 as a marine applications engineer after retiring as a Captain in the US Navy. 54 NAVAL FORC ES IV/2015 nf 4-15 - 14_09 US Navy Carrier 9/4/15 2:07 PM Seite 55 NAVAL TECHNOLOGY Example of mechanical drive cubic load curve. Note the dramatic increase required in power to generate vessel speeds above 30 knots. This illustrates the profound influence that the selection of a propulsion plant configuration and prime movers has on the speed-power relationship: nearly a 270% increase in power will be needed to reach the 40 knot speed. and/or increase mission payload. This allows increases have a marked effect upon mission maximum space, weight, and cost allocation to payload in terms of affordability and capability. each critical systems. ENHANCED PAYLOAD WITH CONCEPTUAL PROPULSION COMPACT POWER PLANT PLANT CASE STUDIES Machinery weight is an important design con- To illustrate the effect speed has on propul- sideration because it has an intrinsic relationship sion plant design, three conceptual propulsion on payload and speed. By reducing machinery plant mechanical drive configurations were and installing the lightweight CODOG propul- developed that achieve the different maximum sion system (similar to that of Case 2 or 3), the capa city were derived using typical percentages speeds of 29, 32, and 40 knots. Four 800kW mission payload may be enhanced by more than for surface combatants. A good rule of thumb diesel generators supply electrical power for ship 200 tons. Machinery volume cannot be ignored, for fuel allocation is 15% of total weight distri- service requirements. To simplify matters, 4% especially in smaller surface combatants. How- bution, with mission payload nominally gear and shaft losses (i.e. power delivered to the ever, to simplify the discussion, weight was accounting for 11% of total weight distribution. propeller) and 7% shipyard design margin (i.e. addressed as the most critical factor. Estimates Assuming the baseline fuel load, cruising ranges effective power) were assumed. Although more for machinery weight were developed using were calculated using fuel consumption attrib- detailed transmission loss and design margins these three plant configurations, as well as utes of both the propulsion engines and ship ser- come into play for a thorough analysis of power similar propulsion plant arrangements from past vice generators at various ship speeds. Cruising requirements, this approach provides a snapshot studies. The weight summaries of main ranges fall off precipitously with higher speeds of the maximum and cruising speed capabilities machinery components clearly demonstrate that on the larger diesel engines. In fact, as illu strated of the three propulsion plant configurations. Case the combined diesel and gas turbine architec- for Case 1, at speeds above 19.5 knots, the 9MW comparisons are presented as follows: tures for all three cases significantly increase in diesel engine is unable to attain 6,000nm Case 1 represents a 40-knot option, featuring weight as installed power increases. This has an (11,110km) – the common cruising range for 90MW of installed power in a CODAG arrange- attendant effect upon the available weight small surface combatants. ment; main combining and splitter gears accom- dedicated to mission payload and fuel capacity. Diesel engines operating at reduced power modate four steerable/ reversible water jet To help understand this relationship, weight normally attain better cruising ranges, which propulsors; allocations for fuel and mission payload may allow for reduced fuel weight and a Case 2: 51MW CODOG with controllable pitch corresponding increase in the mission payload. propellers; it represents a 32-knot option with 90MW CODAG with water jets. However, fuel load reductions may not be 51MW of installed power in a CODOG arrange- ment. A conventional main gear allows for either the gas turbines or main propulsion diesel engines to drive the power train. The propulsors are controllable pitch propellers; Case 3 represents a 29-knot option with 33MW of installed power in a CODOG arrangement; a main gear allows the gas turbines or the propul- sion diesels to drive both controllable pitch pro- pellers simultaneously. While each conceptual configuration serves as a feasible solution for a 3,500 tons surface combatant, the variance in both maximum and cruising speed capabilities translates into sig- nificant increases in cost and weight. These NAVAL FORC ES IV/2015 55 nf 4-15 - 14_09 US Navy Carrier 9/4/15 2:07 PM Seite 56 NAVAL TECHNOLOGY Cruising range for various speed.