C11-3.FAST2011.Cusanelli

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11th

International Conference on Fast Sea TransportationFAST 2011, Honolulu, Hawaii, USA, September 2011

Hydrodynamic and Supportive Structure for Gated Ship Sterns - Amphibious Ship

Stern Flap

Dominic S. Cusanelli

Naval Surface Warfare Center, Carderock Division (NSWCCD), Code 5800, West Bethesda, Maryland, USA

ABSTRACT

Stern flaps have now been at sea for more than two decades,on a variety of U.S. Navy (USN) and U.S. Coast Guard(USCG) classes, including destroyers, cruisers, frigates,cutters, and patrol craft. Application of flaps to large-deckamphibious-type ships is a fairly recent extension of thetechnology. Ship performance improvements such asdelivered power reduction and fuel savings, and maximumspeed increases, have been proven during at-sea trials and

are well documented.U.S. Navy amphibious ships contain well decks, which areaccessed through large folding stern gates. When open, thegates are supported by sizable structures, which are partiallysubmerged and affixed to the transom. A new concept, thehydrodynamic and supportive structure for gated ship sterns,i.e. the amphibious stern flap, was developed and patentedby the author. This design combines a stern flapshydrodynamic performance surface with the stern gatesupport structure.

The Naval Surface Warfare Center, Carderock Division(NSWCCD), has now designed and implemented this newtype of integrated stern flap and gate support structure on

several U.S. Navy amphibious ship classes. U.S. Navy shipdesign and R&D programs have resulted in amphibiousstern flaps being implemented as new construction items ontwo new Navy amphibious ship classes and one new sub-class. Fleet Readiness R&D programs have fundedamphibious stern flap design, retrofit installation, andevaluation trials on two existing amphibious ships. Eachamphibious stern flap design spiral, model-test series,optimization, full-scale implementation, as well asperformance benefits and fuel savings will be discussed.

KEY WORDS

Stern flap, amphibious ship, stern gate support structure,performance trial, fuel consumption.

1.0 INTRODUCTION

A stern flap is an extension of the hull bottom surfacecreated by a relatively small appendage welded to thetransom of the ship. Stern flaps have been at sea for morethan two decades (Cusanelli 2002), beginning with their useas back-fit devices, and now extending to their inclusion innew-design hullforms. An extensive body of knowledgeand experience has now been amassed in the use of flaps forperformance improvements.

Stern flaps have been proven, at sea, to reduce propulsivepower and exhaust emissions, and to foster significant fuelcost savings, while increasing both ship range (endurance)and top speed. They have been used to provide for a betterbalance between the ships power requirements and engineoperating envelope, increasing the interval between engineoverhauls, and extending the service life of the propulsionmachinery. Flaps also reduce propeller loading, cavitation,vibration, and noise tendencies. On a new ship application,stern flap performance could also foster greater speed,

range, or payload, or reduce propulsion system requiredpower, size, and initial acquisition costs.

Stern flaps have now been installed as new constructionitems or as retrofits on 173 U.S. Navy and Coast Guardships, comprising 13 ship classes. Full-scale stern flapevaluation trials on combatants, cutters, and patrol shipshave all indicated that ship performances were significantlyimproved by the installation of a stern flap. The recent trialon USS KEARSARGE (LHD 3) was the first stern flapevaluation trial on an amphibious ship. The installation ofthis amphibious stern flap greatly improved the shipperformance.

For use on amphibious ship applications, a new stern flapdesign concept was developed, which combines a sternflaps hydrodynamic performance surface with theamphibious ships existing stern gate support structure.Programs to design and implement this new type of sternflap and gate support structure were undertaken on severalU.S. Navy amphibious ship classes. Hull dimensions, sterngate size and support structure requirements, ship speed andmission capabilities, as well as widely divergent stern flapdesign criteria, resulted in dramatically different stern flapdesigns for each of these applications.

U.S. Navy ship design programs have resulted inamphibious stern flaps being implemented as new

construction items on the SAN ANTONIO (LPD 17) Class(seven of the planned twelve ships of this class are nowactive), as well as on the AMERICA (LHA 6) Class. As aresult of a ship energy enhancement R&D program, a newconstruction amphibious stern flap was installed on MAKINISLAND (LHD 8). Fleet Readiness R&D programs havefunded amphibious stern flap design, retrofit installation,and evaluation trials on WHIDBEY ISLAND (LSD 41) andKEARSARGE (LHD 3).

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2.0 CONCEPT

A stern flap is an extension of the hull bottom surface whichextends aft of the transom. It is a relatively smallappendage, built of plate, welded to the transom. Criticalflap geometries are chord length (fore-aft), span acrosstransom, and angle, referenced to the local buttock slope(run), positive with trailing edge down (TED), as depicted inFig. 1. Stern flaps have been successfully applied to a wide

variety of ships, 104 to 945 ft in length, 150 to 65,000 tonsdisplacement, and speeds of 8 to over 40 knots,necessitating a diverse array of geometric variations(Cusanelli and Karafiath 2001; Cusanelli 2003).

Span Across Transom34 ft (10.36 m)

Chord Length: 5.3 ft (1.62 m)

Angle: 8.10

Fig. 1. Stern flap critical geometry, depicted on A. W.RADFORD (DD 968)

2.1 Hydrodynamic Mechanisms

Stern flaps modify the pressure field under the hullafterbody, causing the flow to slow down over an areaextending from its position to generally forward of thepropellers. Decreased flow velocity causes an increase inpressure, which in turn, reduces resistance due to reduced

after-body suction force (form drag).Wave heights in the near field stern wave system, and farfield wave energy, are both reduced by the flap, inspiringthe credo less show - more go! Localized flow around thetransom, which represents lost energy through eddy-making,wave breaking, and turbulence, is significantly modified.The flow exit velocity from the flap trailing edge isincreased relative to the transom knuckle, leading to a lowerspeed for clean flow separation, and further reducedresistance.

Vertical forces developed by the flap can affect the trimangle on planing or semi-planing craft, however, trimeffects are somewhat negligible on displacement hulls.Additional secondary effects are due to lengthening of thehull, and to improved propeller performance.

2.2 Amphibious Ship Application

U.S. Navy amphibious ships contain well decks that areaccessed through large folding stern gates. When open, thegates are supported by sizable structures, which are partiallysubmerged and affixed to the transom, as depicted in Fig. 2.Previously, stern flap installation on these ships wasprecluded by the existence of the gate support structures.

A new amphibious stern flap concept was developed andpatented by the author (Cusanelli 2004), which combines astern flaps hydrodynamic performance surface with thestern gate support structure, as depicted in Fig. 3. Stern flapdesign flexibility is extremely limited in these types ofamphibious ship applications, in that, the hydrodynamicsurface as well as the entire structure of the flap must beintegrated through, and around, the large stern gate supports.

The amphibious ship stern flap derives its performance froma combination of hydrodynamic mechanisms and the partialor entire masking of the stern gate support structure (i.e.,deflection of flow around the structure).

Fig. 2. Stern gate support structure, depicted on model ofWASP (LHD 1) Class

Fig. 3. Amphibious stern flap concept, integrating flap andgate supports, depicted on model of WASP (LHD 1) Class

3.0 DESIGN AND OPTIMIZATION

All of the amphibious stern flaps presented in this paperwere designed, tested, and optimized through model-scaleexperiments. Stern flaps are designed as several series, tosystematically investigate variations in flap dimensions of

chord length, span, angle, and planform area distribution.Through computational fluid dynamics (CFD), designparameters can be refined, reducing the number ofcombinations necessary for testing. However, the cost,difficulty, and limited level of effectiveness of these CFDcalculations, precludes their use as a sole design tool.Model-scale experiments are performed to ultimately selectthe optimal stern flap design, and to assess its impact oneach particular ship class.

A stereolithography apparatus (SLA) is now used for themanufacture of the model-scale amphibious stern flaps

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made from a UV laser-curable photopolymer resin, such asthat depicted in Fig. 3. This rapid manufacturing -prototyping technology has allowed for the design andtesting of the complex, integrated hydrodynamic andsupportive structures necessary.

Stern flap target speed or target speed range has beenestablished as the principal design criteria for determininggeometry and performance. Target speed varies with the

flap design goal, such as to decrease power at cruise speed,to increase or achieve a top speed, or to foster fuel savings(speed range where the ship annually consumes the mostfuel is targeted). For an energy savings application, a classannual speed-time profile, based upon current reported shipoperations, is critical to establish and optimize the stern flapdesign. While flap power reduction is generally optimizednear the target speed or range, it is generally designed toexhibit acceptable performance over the entire operationalrange, necessitating some degree of compromise. Flapperformance could be exceptional in a point designscenario (operation at a single constant speed), when otherspeeds are truly of little concern to the operator.

Stern flaps have always worked superbly on relatively high-speed naval combatants, however, amphibious ships operateat much lower speeds. Beneficial performance at low speed,once elusive, has been achieved with increased designknowledge. Two recent flap applications on U.S. CoastGuard cutters, despite low speed optimization, achieved theexemplary fuel savings that they were designed for, whilestill increasing top speed. All six at-sea flap performancetrials have shown flaps to be effective at all speeds, reducingpower at speeds as low as 8 knots.

3.1 Fuel Savings

An annual speed-time profile, based upon current reported

ship operations, is required not only to establish andoptimize the stern flap design, but also to estimate total fuelconsumption and fuel savings. Model powering is evaluatedat the speeds depicted in the class operating speed-timeprofile. Annual time-weighted delivered power and annualfuel consumption are determined using delivered power,corresponding engine fuel consumption flow rates, speed-time profile and propulsion plant operations profile, andclass average annual underway hours and fuel consumption.The amphibious stern flap configuration is evaluated basedon annual fuel consumption when compared to the baselinehull with the stern gate support structure and no flap.Effects on main propulsion engine power is determinedonly, secondary hotel loads generators, etc., have not beenconsidered. Likewise, engine exhaust emissions can becalculated from reported exhaust output rates at specificpower levels.

3.2 Scaling Issues

As a consequence of the smaller scale, the flow conditionsaround the model stern flap are somewhat different fromthose on the ship. Actual performance of all full-scaleprototype stern flaps have exceeded their model testpredictions. A beneficial stern flap scaling effect has beenfirmly identified, through geosim model testing, CFD

analysis, and correlation with full-scale trials on combatants.A non-traditional ship/model scaling procedure, by whichfull-scale flap performance can be projected from model-scale data, is now generally utilized at NSWCCD.

Model stern flap scaling effects on combatants are mostprevalent at low speeds, where the high flap angles producedetrimental effects at model scale. No such effects havebeen measured at full-scale. Amphibious stern flaps,

optimized for lower speeds, consequently are designed withlower angles. These low-angle flaps do not exhibit thedetrimental low-speed powering trends at model-scale asseen in their combatant counterparts. Due to this model-scale experience, and because a full-scale stern flapperformance trial had not been conducted until recently onan amphibious ship, a conservative approach was preferred.The model-scale stern flap performance predictionspresented herein do not include stern flap scalingadjustments as applied to smaller, faster combatants.

4.0 AMPHIBIOUS STERN FLAPS AS NEWCONSTRUCTION ITEMS

U.S. Navy programs have resulted in the design andimplementation of amphibious stern flaps as newconstruction items on three ship classes.

4.1 SAN ANTONIO (LPD 17) Class

An amphibious stern flap is featured on the SANANTONIO (LPD 17) Class, the U.S. Navys newest class ofamphibious transport docks. The LPD 17 is 200 m (656 ft)long, 25,254 tonnes (24,855 Ltons), twin-screw, amphibiouswell-deck vessel, with a folding stern gate. First-of-classSAN ANTONIO (LPD 17) was commissioned January2006. Seven of the planned twelve ships of this class arenow active.

The target speed for the LPD 17 Class amphibious stern flapwas the ships 21.5 knot sustained threshold speed. Asecond performance criteria for the stern flap was to enablethe ship to attain its objective speed. As this flap was not adesign optimized for fuel efficiency, only a minimalreduction in annual fuel consumption was expected.

Fig. 4. Amphibious stern flap installed on San Antonio(LPD 17)



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The stern flap hydrodynamic surface was affixed to thelower surface of the stern gate support structures, restrictingthe flap angle to 12.5 only. Model-scale experiments, withvarying chord lengths and spans, indicated that a stern flapcould be designed to be effective at this 12.5 angle. 1 Theupper surface of the flap is integrated through the threesupport fixtures, and depicted in Fig. 4.

4.1.1 Stern Flap Performance on SAN

ANTONIO (LPD17) Class

Model tests indicated a stern flap delivered power reductionof 3.4 percent at the LPD 17 Class 21.5 knot sustainedthreshold speed.1 More importantly, the 0.25 knot speedincrease provided by the stern flap enabled the ship to attainits objective speed.

Annual fuel consumption for the LPD 17 was estimatedfrom the ships envisioned propulsion power expenditure,fuel consumption rates, and expected speed-time operationsprofile. Based on model-scale results, the stern flap isestimated to effect an annual fuel reduction of 900 barrels(2.1%), corresponding to an annual fuel cost savings of

$135K-$270K ($150-$300/barrel), per ship of this Class.Owing to the fact that the stern flap is a new constructionitem on this class, there does not exist the opportunity toconduct pre- and post-flap installation trials to explicitlydetermine flap performance. NSWCCD has not yetconducted adequate full-scale powering trials on a ship ofthe SAN ANTONIO (LPD 17) Class from which thedelivered power for the ship, with the amphibious stern flapinstalled, can be determined. A series of trials on a ship ofthis class is pending. A ship/model correlation of poweringperformance, between trials data from a ship of the LPD 17Class and the geosim model, would verify the accuracy ofthe model-scale performance data and instil a higher degree

of confidence in the model-scale amphibious stern flapperformance prediction.

4.2 MAKEN ISLAND (LHD 8) sub-Class

The MAKEN ISLAND (LHD 8) is a sub-class of the WASP(LHD 1) Class amphibious assault ships, 257 m (844 ft) inlength. As the eighth ship of the class, LHD 8 represents amodified design, in which the former steam propulsion plantis replaced by marine gas turbines as the main propulsionengines. Five-bladed controllable-pitch (CP) propellers,along with associated appendage variations, also replace theformer six-bladed fixed-pitch (FP) propellers. In addition,electric motors will be used for low-speed auxiliarypropulsion. Nominal draft for the LHD 8 was 8.38 m (27.5

ft), with a corresponding displacement of 43,093 tonnes(42,412 Ltons).

The MAKEN ISLAND (LHD 8) amphibious well deck isaccessed through a large folding stern gate. A sizable, sterngate support structure is affixed to the transom to supportthe stern gate in its opened position. This structure ispartially submerged, and consists of four large structuralsupports that extend forward, aft, and below the transomknuckle, and a large diameter perimeter protection pipe.

The primary design goal of the MAKEN ISLAND (LHD 8)stern flap was fuel efficiency, therefore, it was a speed-timeweighted compromise design, tailored to produce theoptimal reduction in annual fuel consumption. An increasein the ships endurance range and attainable top speed werealso desired. A principal design limitation of the stern flapwas that the four large gate structural supports could not berelocated or modified in any manner.

The design selected for the MAKEN ISLAND (LHD 8) wasan amphibious stern flap with chord length 6.2 ft, span 26.0ft between stern gate supports, and 0 degrees centerlineangle (parallel to centerline buttock), as depicted in Fig. 5.The four structural supports project well below thehydrodynamic (bottom) surface of the stern flap, as seen inFig. 5. Therefore, the masking of these supports by thestern flap is minimal.

Fig. 5. Amphibious stern flap installed on MAKENISLAND (LHD 8)

4.2.1 Stern Flap Performance on MAKENISLAND (LHD 8)

For an estimation of the amphibious stern flaps annual fuelsavings on the MAKEN ISLAND (LHD 8), the speed-timeprofile available for the LHD 1 Class was utilized. Thetime-averaged flap performance was applied to the annualunderway hours and fuel consumption for the LHD 8 asreported for fiscal year 2011 (FY11) in the Navy EnergyUsage Reporting System (NEURS). For the MAKENISLAND (LHD 8), the amphibious stern flap performance,projected from model-scale teats, will provide for an annualfuel reduction of 4814 barrels (3.3%), resulting in an annualfuel cost savings of $722K-$1.4Mil ($150-$300/barrel) forthis ship. This analysis utilized the existing WASP (LHD 1)Class speed-time profile and projected annual fuelconsumption for this modified ship with gas-turbinepropulsion.

Secondary goals of the amphibious stern flap were alsoachieved. In the range of the ships transit speed, the flapprovided a 5 percent powering reduction. An increase in topspeed of 0.4 knots was also predicted.

MAKEN ISLAND (LHD 8) was commissioned in October2009. Again, as a new construction item, there does notexist the opportunity to conduct pre- and post-flapinstallation trials. NSWCCD has not yet conductedadequate full-scale powering trials from which to determine



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delivered power for this ship. A ship/model correlation ofpowering performance has therefore not yet been completedfor the verification of the stern flap performance.

4.3 AMERICA (LHA 6) Class

The AMERICA (LHA 6) is the lead ship of the Navy'snewest class of large-deck assault ships. At this time, twoships are envisioned for this class. It is an all-aviation

modification of the class it is scheduled to replace, theTARAWA (LHA 1) Class amphibious assault ships.Although the ships hull classification and numberingdesignation associates it with the TARAWA Class, the newAMERICA will have more in common with the amphibiousassault ship MAKIN ISLAND (LHD 8). The LHA 6 sharesthe same basic hull form, 257 m (844 ft) in length,controllable-pitch propellers, and gas-turbine mainpropulsion with the LHD 8. However, the LHA 6 designdraft was increased to 8.8 m (29 ft), and displacement wasincreased to 45,835 tonnes (45,114 Ltons), representing asubstantial increase over that of the LHD 8.

As an all-aviation platform, the well-deck and stern gate,

necessary for amphibious operations, has been eliminatedfrom the LHA 6 design. Therefore, the stern gate supportstructure has also been eliminated from the transom. Forthis reason, the stern flap design criteria specified for theLHA 6 is somewhat more relaxed than that of the LHD 8.

The design goal of the AMERICA (LHA 6) stern flap wasto reduce delivered power at ship speeds of 20 knots andabove, with an emphasis at the ships 21 knot sustainedspeed, while incurring minimum powering penalties at lowspeeds. This was a flap designed for high-speed powerreduction, and therefore, not a speed-time weightedcompromise design optimized for fuel efficiency. However,a reduction in annual fuel consumption, though minimal,

was expected. Increased endurance range and reducedpropeller blade loading at higher speeds were also desired.

Fig. 6. Amphibious stern flap installed on a model ofAMERICA (LHA 6)

Full-scale dimensions of the LHA 6 stern flap are chordlength 7.78 ft (2.4 m), span 80.0 ft (24.4 m), and 0centerline angle (parallel to centerline buttock). Full-scalephotographs of the LHA 6 were not available as of thiswriting. Fig. 6 depicts the amphibious stern flap installedon a model of the LHA 6. Note that the hydrodynamic(bottom) surface of the LHA 6 stern flap continues the

contour of stern propeller tunnel sections. This geometrypresents a much fairer (cleaner) and more efficienthydrodynamic surface when compared with the previousamphibious application where the stern flap was integratedwith a stern gate support structure.

4.3.1 Stern Flap Performance on AMERICA(LHA 6)

Based on model-scale experiments, the selected LHA 6amphibious stern flap reduces delivered power byapproximately 6.5% at 21 knots.1 Maximum ship speed ispredicted to increase by 0.5 knots when the stern flap isinstalled. Additionally, this stern flap design did not incurany low speed powering penalties, for speeds as slow as the9 knots tested.

For an estimation of the amphibious stern flaps annual fuelsavings on the LHA 6, the speed-time profile available forthe LHA 1 Class was utilized. However, since the LHA 6shares the same propulsion system as the LHD 8, the time-averaged flap performance was applied to the annualunderway fuel consumption reported in the FY11 NEURS

for the LHD 8. This estimation indicates that theamphibious stern flap will effect an annual fuel savings of5861 barrels (4%) on the LHA 6, resulting in an annual fuelcost savings of $879K-$1.75Mil ($150-$300/barrel).

AMERICA (LHA 6) is currently under construction and isdue to be delivered to the Navy in 2012.

5.0 AMPHIBIOUS STERN FLAPS AS RETROFITSTO EXISTING DESIGNS

Fleet Readiness Research & Development (R&D) programswere undertaken at NSWCCD to design and implementamphibious stern flap designs as retrofits on two existingamphibious ship classes, WASP (LHD 1) and WHIDBEYISLAND (LSD 41). A stern flap evaluation trial hasrecently been conducted on KEARSARGE (LHD 3), thethird ship of the WASP Class. A stern flap has beeninstalled on WHIDBEY ISLAND, however, a Stern FlapTrial has not yet been conducted.

5.1 WASP (LHD 1) Class

The U.S. Navys WASP (LHD 1) Class amphibious assaultships currently consists of seven ships, LHD 1-7, with theeighth ship, LHD 8, being a modified sub-class. Each shipis 257 m (844 ft) in length, powered by an oil-fired boiler,steam turbine propulsion plant, driving twin-shaftline six-bladed fixed-pitch (FP) propellers.

The primary objective of the WASP (LHD 1) stern flap

design was fuel efficiency. The design was tailored foroptimal performance at speeds where these ships have thegreatest annual fuel consumption. An increase in the shipsendurance range and attainable top speed were also desired.

The existing LHD 1 Class stern gate support structure, ofthe same design as described for the LHD 8 program,consists of four large structural supports that extendforward, aft, and below the transom knuckle, and a largediameter perimeter protection pipe. First generation sternflap designs were affixed to the lower surface of the sterngate supports, limiting the flap to excessively high angles of



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20 and above. These high angles produced unfavourablelow speed powering penalties and precluded thedevelopment of fuel efficient stern flap designs.

Second iteration designs were developed utilizing theamphibious stern flap technology, and integrated the flaphydrodynamic surface through the gate support structuresrather than installing it under the supports. In this fashion,longer chord lengths and shallower angles, necessary for

powering reduction at the lower speeds, could beaccommodated into the stern flap design.

The LHD 1 amphibious stern flap design has principaldimensions of chord length 6.2 ft, span 26.0 ft, and an angleof 0 degrees at the centerline.

5.1.1 Installation on KEARSARGE (LHD 3)

The third ship of the class, KEARSARGE (LHD 3), wasallocated for the prototype amphibious stern flap installationand performance trials. The flap was fabricated, Fig. 7, andthen installed on the KEARSARGE (LHD 3), Fig. 8, at theNorfolk Naval Shipyard (NNSY) during a dry-dockavailability period ending 10 Sept. 2009.

Fig. 7. Fabrication of amphibious stern flap forKEARSARGE (LHD 3)

Fig. 8. Amphibious stern flap installed on KEARSARGE(LHD 3)

Inspection and measurements of the KEARSARGE sternflap were made by NSWCCD personnel.2 Measurementswere taken at approximately 40 locations on the flaphydrodynamic surface. At the time of the inspection, thefinal production welding had been completed. Principaldimensions of the stern flap (determined with a weightedaverage based on surface area, 20% port side, 60% centersection, and 20% starboard side) were chord length 6.44 ft,

span 26.0 ft, and angle 1.7 degrees (trailing edge downrelative to centerline buttock).

5.1.2 Stern Flap Performance Trials onKEARSARGE (LHD 3)

A Baseline Speed/Power Trial (pre-flap) was conducted onthe KEARSARGE (LHD 3), 08-09 July, 2008. However,ship operations commitments did not allow for the

collection of adequate trials measurements in satisfactoryenvironmental conditions nor at Class representative shipconditions during the scheduled trials period, and precludedfurther attempts to conduct the trials on alternate dates.

KEARSARGE Baseline Trial delivered power was adjustedfor the sea state and mean sustained wind speedsencountered during the trial. Post-trial ship/modelcorrelation experiments were conducted from which theBaseline Trial delivered power could be normalized to theclass operational displacement and trim. The adjustedKEARSARGE Baseline Trial data was then combined withprevious trials data from WASP (LHD 1) andBONHOMME RICHARD (LHD 6), to form a concatenated

WASP Class powering baseline. After the completion ofthe KEARSARGE Stern Flap Trial, the WASP Classpowering baseline was normalized to the displacement ofthe stern flap trial, and is presented in Table 1.

The KEARSARGE Stern Flap Trial was conducted 17-20June, 2011. The trial location was in open water southeastof the mouth of the Delaware Bay, where minimum waterdepth was 1280m (4200 ft). Ship conditions were 42,166tonnes (41,500 Ltons) displacement, with fwd and aft draftsof 8.2m (27.0 ft) even keel. Ideal environmental conditionswere encountered, with low winds and sea state 1 or below.

Preliminary delivered power results of the KEARSARGEStern Flap Trial, and the stern flap powering performance,

are presented in Table 1.

Table 1. Amphibious stern flap performance,KEARSARGE (LHD 3) - preliminary

ShipSpeed

BaselineDelivered

Power

Stern FlapDelivered

Power

Stern FlapEffect

(knots) (MW) (MW) (%)

10 2.9 2.6 -9.8%12 4.8 4.3 -11.0%14 7.6 6.8 -10.3%16 11.3 10.4 -8.0%18 16.3 15.3 -5.9%20

23.0

21.9

-4.9%

21 27.1 25.8 -4.9%22 32.0 30.2 -5.5%23 37.6 35.2 -6.4%24 44.4 40.8 -8.1%

24.9 52.2 - -25 - 46.9 -

25.6 - 51.1 -Preliminary results of the recently-conducted KEARSARGE(LHD 3) Stern Flap Trial indicate that the installation of theamphibious stern flap reduces delivered power by as much



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as 11%, and reduces annual time-averaged delivered power(annual hP-hrs) by 7.2%. Also, the maximum attainableship speed was increased by 0.7 knots. The stern flapunloaded the propellers to the extent that full installedpower of 52.2 MW (70,000 hP) could not be attained priorto reaching the maximum allowable shaft speed limit of 180RPM. However, ship speed was nonetheless substantiallyincreased. As has been the result of all previously tested

stern flaps, the actual performance of the full-scaleamphibious stern flap exceeded the model-scale prediction,especially at low speeds, as illustrated in Fig. 9.

Fig. 9. Stern flap performance on KEARSARGE (LHD 3)compared to model-scale prediction

The LHD 1 Class speed-time profile as reported by

NAVSEA was utilized for an estimation of the amphibiousstern flaps annual fuel savings. The time-averaged flapperformance in terms of delivered power-hours, was appliedto the required boiler operating scenarios and specific fuelconsumption, and annual underway hours and fuelconsumption averaged for ships of the LHD 1 Class asreported in the FY11 NEURS. This preliminary analysisindicates that the installation of an amphibious stern flap ona ship of the LHD 1 Class has a potential for annual fuelreduction of 6141 bbls/yr/ship (3.4% underway fuel), with aconsequential annual fuel cost avoidance (savings) of$930K/yr/ship to $1.9 Million/yr/ship (fuel cost range $150-$300/bbl).

5.2 WHIDBEY ISLAND (LSD 41) ClassThe U.S. Navy WHIDBEY ISLAND (LSD 41) Classamphibious dock landing ships, currently consists of eightships. In addition, the HARPERS FERRY (LSD 49) Class,which shares the hydrodynamic-equivalent hullform andpropulsion, currently consists of four ships.

A program was undertaken at NSWCCD to design anamphibious stern flap for the LSD 41 Class. The primaryobjective of the flap was a reduction in annual propulsionfuel consumption. The design was again tailored for

optimal performance at speeds where the ships have thegreatest annual fuel consumption. An increase in the shipsendurance range and attainable top speed were also desired.

The stern gate support structure on the LSD 41 Classconsists of two primary structural gate supports that extendaft and above the transom knuckle, several smaller cross-connecting structural members, and a large diameterperimeter protection pipe.

The amphibious stern flap as designed for the LSD 41 Classhas principal dimensions of chord length 7.3 ft (projectionaft of transom knuckle), span 42.4 ft (transverse widthacross the transom), and angle 0 (parallel to centerlinebuttock). Because the existing stern gate supports for thisclass do not extend beneath the transom knuckle, they canbe masked nearly to their entirely by the stern flap. Onlythe uppermost surface of the supports, where the gate makescontact with them, remains outside of the stern flapenvelope. Therefore, the stern flap derives its performancenot only from hydrodynamic mechanisms, but also from thesignificant masking of the stern gate supports, and theremoval of the perimeter pipe. This allows the LSD 41stern flap to be more efficient than the previously describedLHD 1 design.

The identical amphibious stern flap design is applicable toall twelve ships of the combined WHIDBEY ISLAND(LSD 41) and HARPERS FERRY (LSD 49) classes.

5.2.1 Installation on WHIDBEY ISLAND(LSD 41)

The lead ship of the class, WHIDBEY ISLAND (LSD 41),was allocated for the prototype amphibious stern flapinstallation and performance trials. The flap was fabricatedat QED Technologies, Norfolk VA, Fig 10, and theninstalled on WHIDBEY ISLAND at Metro Machine Corp

(Norfolk, VA) during a dry-dock availability period endingSept 2009, Fig. 11.

Fig. 10. Fabrication of amphibious stern flap forWHIDBEY ISLAND (LSD 41)

Inspection and measurements of the WHIDBEY ISLANDflap were made by NSWCCD personnel.2 Measurementswere taken at approximately 16 locations on the flaphydrodynamic surface. Principal dimensions of the sternflap were determined with a weighted average based on



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surface area, 48% for center section and 26% for each portand starboard sides (outboard of the two primary structuralgate supports), were chord length 7.8 ft, span 43.2 ft, andangle -1.1 (trailing edge upward relative to centerlinebuttock).

Fig. 11. Amphibious stern flap installed on WHIDBEYISLAND (LSD 41)

5.2.2 Stern Flap Performance Trials on

WHIDBEY ISLAND (LSD 41)A Baseline Trial (pre-flap) was conducted on WHIDBEYISLAND, 16 Oct, 2008, off the Virginia Capes. Idealenvironmental conditions were encountered and theresultant data measurements are exemplary, as presented inTable 2.

The Stern Flap Trial on WHIDBEY ISLAND has twicepreviously been scheduled and then subsequently cancelled.A Stern Flap Trial on LSD 41 is anticipated for early 2012.Regrettably, full-scale stern flap performance data will notbe available prior to the submission of this final FAST 2011manuscript.

Until the Stern Flap Trial is conducted, model-scale tests1will be used to predict the stern flap effects throughout thespeed range, and to project the delivered power of theWHIDBEY ISLAND (LSD 41) with the stern flap installed,Table 2. The tests indicate that the installation of anamphibious stern flap on a ship of the LSD 41 Class reducesdelivered power by as much as 7.8%, and that the maximumattainable ship speed is predicted to increase by 0.3 knots.

The LSD 41 Class speed-time profile as reported byNAVSEA was utilized for an estimation of the amphibiousstern flaps annual fuel savings. The time-averaged flapperformance was applied to the annual underway hours andfuel consumption averaged for ships of the LHD 41 Class as

reported in the FY11 NEURS. This analysis indicates thatthe installation of an amphibious stern flap on a ship of theLSD 41 Class has a potential for annual reduction in fuelconsumption of 1320 bbls/yr/ship (3.4% underway fuelreduction), with a consequential annual fuel savings of$198K to $396K/yr/ship (fuel cost range of $150 to$300/bbl).

Table 2. Amphibious stern flap performance,WHIDBEY ISLAND (LSD 41)

ShipSpeed

BaselineDelivered

Power

Stern FlapDeliveredPower*

Stern FlapEffect*

(knots) (MW) (MW) (%)

10 1.60 1.53 -4.6%12 2.73 2.61 -4.6%14 4.27 4.03 -5.6%16 6.39 5.96 -6.8%18 9.16 8.45 -7.8%20 12.53 11.59 -7.5%22 16.76 15.66 -6.6%23 19.52 18.29 -6.3%

23.8 22.06 20.67 -6.3%24.3 24.61 23.05 -6.3%24.6 - 24.61 -

*Based on mode-scale predictions while awaiting trial

6.0 SUCCESS AT SEA

The stern flap is now a mature and proven technology that

has been transitioned to sea since 1989 on destroyers,cruisers, frigates, cutters, patrol boats, and most recently, onamphibious ships. A listing of the current status ofcompleted U.S. Navy and U.S. Coast Guard stern flapretrofits and new construction installations is presented inTable 3. Full-scale installations now total 173 stern flaps on13 ship classes. At-sea stern flap evaluation trials have beenconducted on seven of those classes (denoted withasterisks). These trials have proven the performancebenefits of decreased ship power, reduced fuel consumption,reduced emissions, and increased speed and range.

The benefit to the Navy in terms of fuel cost savings ismillions of dollars annually. Since their introduction in

1989, stern flaps have accumulated over 1300 active ship-years of service and $655 Million in fuel savings.

Table 3. Current status of U.S. Navy and U.S. Coast Guardstern flap installations

FlapsInstalledon Class

Annual, per ShipFuel Reduction

CumulatedFuel

SavingsShip Class

(Total) (Barrels) (%) (Millions)

DDG 51 I/II* 28 8880 9.1 $223DDG 51 IIA 31 7286 7.5 $165CG 47 24 9619 8.3 $183DD 963* 7 7329 7.0 $12

FFG 7* 30 1992 4.9 $51PC 1* 14 416 7.9 $6.7LHD 1* 1 6141 3.4 $2.0LHD 8 1 4814 3.3 $2.4LSD 41/49 1 1320 3.4 $0.5LHA 6 0 5861 4.0 $0LPD 17 7 900 2.1 $3.0WPB 110* 28 270 3.7 $5.5WHEC 378* 1 952 9.5 $0.9

TOTALS: 173 $655 Mil

*Stern Flap Evaluation trials conducted on these classes



9/9

7.0 CONCLUSIONS

While stern flaps represent a proven technology oncombatant-type ships, their application to large-deckamphibious-type ships is a fairly recent extension. U.S.Navy amphibious ships contain well decks, which areaccessed through large folding stern gates. When open, thegates are supported by sizable structures, which are partiallysubmerged and affixed to the transom. The presence of the

gate support structure formerly precluded the installation ofa stern flap.

A new concept, the hydrodynamic and supportive structurefor gated ship sterns, i.e. the amphibious stern flap, wasdeveloped. This design combines a stern flapshydrodynamic performance surface with the stern gatesupport structure. U.S. Navy programs have now resulted inamphibious stern flaps being implemented as newconstruction items on three ship classes, SAN ANTONIO(LPD 17), MAKEN ISLAND (LHD 8), and AMERICA(LHA 6). Between these three classes, a total of 15amphibious ships will be newly constructed with stern flaps.

Two U.S. Navy amphibious classes are receivingamphibious stern flaps as retrofit items, WASP (LHD 1) andWHIDBEY ISLAND (LSD 41).

A Baseline (pre-flap) Trial was conducted onKEARSARGE (LHD 3), and an amphibious stern flap wassubsequently installed. The recent KEARSARGE SternFlap Trial was the first stern flap evaluation on anamphibious ship class. Preliminary analysis shows that thisamphibious stern flap greatly improved the shipperformance, resulting in a potential for annual fuelreduction of 6141 bbls/yr/ship (3.4% underway fuel). Theconsequential annual fuel cost avoidance (savings) is$930K/yr/ship to $1.9 Million/yr/ship (fuel cost range $150-

$300/bbl). In addition, maximum attainable speed wasincreased by 0.7 knots.

A Baseline (pre-flap) Trial has also been conducted onWHIDBEY ISLAND (LSD 41), and an amphibious sternflap has subsequently been installed. A Stern Flap Trial onLSD 41 is anticipated for early 2012.

ENDNOTES1Reports containing model-scale experimental results areclassified as limited distribution, and therefore, cannot becited herein.2Ship Trials Agendas containing results of stern flapmeasurements are classified as limited distribution, and

therefore, cannot be cited herein.

REFERENCES

Cusanelli, D.S., and G. Karafiath, "Advances in Stern FlapDesign and Application, FAST 2001, SixthInternational Conference on Fast Sea Transportation,Southampton, U.K., (Sept. 2001).

Cusanelli, D.S., Stern Flaps - A Chronicle of Success atSea (1989-2002), SNAME Innovations in MarineTransportation, Pacific Grove CA, (May 2002).

Cusanelli, D.S., Stern Flap: An Economical Fuel-Saving,Go-Faster, Go-Farther Device for the CommercialVessel Market, FAST 2003, International Conferenceon Fast Sea Transportation, Ischia, Italy, (Oct. 2003).

Cusanelli, D.S., U.S. Patent 6,698,370 Hydrodynamic andSupportive Structure for Gated Ship Stern (March 2,2004).

ACKNOWLEDGEMENTS

SAN ANTONIO (LPD 17) Class design program wassponsored by Naval Sea Systems Command (NAVSEA)Office SEA O3D4, RDT&E funding.

AMERICA (LHA 6) Class design program was sponsored

by NAVSEA, Office PMS377.Model-scale stern flap design and testing program forMAKEN ISLAND (LHD 8), entitled LHD(8) stern flap,was sponsored by the Energy Plans and Policy Branch(OPNAV N420) through the Shipboard Energy R&D Office(NSWCCD Code 859).

The initial model-scale stern flap design and testingprograms for the WASP (LHD 1) Class and for theWHIDBEY ISLAND (LSD 41) Class, entitled Hydro-LSD/LHD Stern Flap, were sponsored by the Energy Plansand Policy Branch (OPNAV N420) through the ShipboardEnergy R&D Office (NSWCCD Code 859).

Funding for the full-scale installation, trials, and evaluationon both the KEARSARGE (LHD 3) and the WHIDBEYISLAND (LSD 41), is through the Fleet Readiness Research& Development Program (FRR&DP) administered by R.Griggel (NSWCCD Code 916).

All amphibious stern flaps described herein were designedby the author with the exception of that for the AMERICA(LHA 6), which was designed by K. Forgach (NSWCCD5800).


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