Zhang Zhuo Lu Fei Tang Jin-jing Gao Bin

（Shanghai Merchant Ship Design and Research Institute，Shanghai201203，China）

0 Background

With more and more importance attached to environment protection,fuel saving and emission reduction has become an important concept of ship design.Due to steep price of oil and market downturn,reducing speed and saving fuel hasturned toa leadingtrend.The application of fuel saving measures on container vessels with high speed and oil consumption is favored by owners in particular.At present,popular ways of saving fuelsincludeoptimization of hull form,main engine derating,propeller redesign and other fuel saving devices.In this research,as for converted vessels,emphasis was put on maximizing of benefits by means of minimizing conversion,cutting cost and shortening payback period.

1 Fuelsavingfor3600TEUcontainer vessel

3 600 CV,well designed by SDARI,was taken as the research target in this paper.The original design speed of the vessel is 23 kn and daily oil consumption of main engine is 116.71 t during operation.Speed reduction is the trend and most effective way for fuel saving.If theservicespeed islowered to19 kn,the ship will save about 50%power and oil consumption will decline toaslow as58.27 t/d.See Table1.

Table 1 Benefit of reduce the service speed from 23 kn to 19 kn

Apart from speed reduction,following measures aretaken tomaximizefuel saving effects.

1.1 Main Engine De-rating

Theconcept of engine de-ratingisto fit the engine to run on a lower CMCR than its nominal maximum continuousrating.A more optimized layout rating of the engineisselected by reducingthe enginepower and the appurtenant engine speed,leading to a reduction in the fuel oil consumption and possible increased mechanical efficiency.See Figure1.

When the service speed of the vessel is lowered from23 kn to 19 kn,the main engine is operated in low load,whichisnogood for maintenance.Therefore,proper conversion of the main engine is highly recommended.The turbochargers,some accessories such as fuel cams,exhaust cams,crosshead gaskets may be adjusted or replaced.After adjusting,withefficiencyof turbochargers andmeaneffective pressure of engineincrease,coefficient of mechanical efficiency of engine improves and SFOC decreasesthen,while theengineisrunning on low load.The following table shows the contrast between original design and improved one.See Table2.

In this research,CMCR of the retrofitted engine is compatible with theoriginal propeller.Tosuit newspeed of 19 kn,changing propeller is also studied.The power of main engine will be de-rated further,resulting in reduction of CMCR which is 21 910 kw×89 r/min.Comparing the latter power and SFOC with the former ones at the same speed,we can only expect close to 1%saving.That is to say,changing propeller is not economical.See Table 3.

Figure 1 principle of de-rating

1.2 Install energy saving devices

There is a variety of energy saving devices with different principles and effects, and some even are compatible with others.The following three devices are introduced toimprovesavingeffects,oneor moreof which could be installed on 3 600 CV.

Table 2 Contrast between original design and improved design

Table 3 Benefit of main engine de-rating

1.2.1 HVAF

After accurate evaluation of hull and propeller forms of aft part,HVAF(hub vortex absorbed fin)is recommended to install.Becauseflowline of aft part is relatively smooth and the resistance is declining with the reduced speed,thus leading to an increase of light running margin of propeller.HVAF consists of small fixed fins attached to the propeller hub.These fins can reduce or eliminate the propeller hub vortex,hence improve overall propulsion efficiency.The propeller hub vortex can be examined at model scale(open-water propeller test,preferably in a cavitation tunnel or by meansof CFD).See Table4.

1.2.2 Optimized rudder

Optimized rudders are often called “Reaction”,“Asymmetric” or “Twisted” rudders.Their working principle isto reducetherotational losses.Any conventional rudder recovers some of the rotational losses behind the propeller,yet more rotational energy can be recovered with a slightly twisted rudder designed to be better aligned with the flow after the propeller.A thinner,moreprofiled rudder can alsoreducethefriction resistance of the rudder,and thus reduce the thrust needed to achievea certain speed.See Table5.

1.2.3 Rudder fins

Rudder fins can consist in four fins in X-shape f itted ontheleadingedgeof thepropeller,or twohorizontal fins.Thefinsaredesignedtogeneratethrustintherotating propeller slipstream,meaning that they can recover part of the rotational loss and reduce the need for thrust to achieve a desired speed.The design of the rudder fins requires CFDsimulations and vast experience.During model tests,the generated thrust can be recognized in a reduced thrust deduction fraction.Thisresults in higher hull efficiency and thus better propulsive efficiency.See Table 6～7.

Table 4 Benefit of install HVAF

Table 5 Benefit of install optimized rudder

Table 6 Benefit of install rudder fins

Table 7 Compatibility of three fuel saving devices(F-fully,P-partly)

1.3 Optimization of hull form of bow part

Considering speed is reduced,original hull form of bow part is not suitable any longer and even may increaseswave making resistance.So designing a whole new bowpart tosuit 19 kn istaken asresearch.Contours of pressure and working sketch of bow part before

Figure 2 contoursof pressureof hull surface before and after optimization

We can see from the following figure that the Crspeed curve of original hull form is climbing fast especially when the speed is higher than 19 kn and is relatively smooth at lower speed.Modifying hull form of bow part affects wave making resistance,reducing residuary resistance then.Because the resistance of the original hull form is quite low at 19 kn,only about 1%savingeffectcan beexpected bymeansof CFDoptimization of bow part.Although optimizing hull form of bow partisnotthateffectivefor thisvessel,it’sstill apopular way for fuel saving and may serve asreference for other ships.See Figure 4 and Table 8.and after optimization at 19 kn by means of CFD are displayed asfollowing.See Figure2～3.

Figure 3 result of optimization of bow part(the upper part is optimized wave pattern)

Figure 4 Cr－speed data sheet of original hull form

Table 8 Benefit of optimize hull form of bow part

1.4 Low Frictional antifouling coatings

The working principle of fuel saving with low frictional antifouling coating is to reduce the friction resistancecomponentbymaintainingahull surfaceassmooth aspossible.In addition to preventing hull fouling,some coatings are designed to gradually smooth down the paint roughness by the action of sea water.And accordingto the data provided by suppliers,2.5%-4%saving isassumed.See Table 9.

Table 9 Benefit of antifouling coating

2 Conclusion

All the measures(Takeany one of the three energy saving devices,2%saving effect is assumed)applied to 3 600 CV can reduce fuel consumption by 11.4%,from 58.27 t/d to 51.63 t/d.Taking into account the saving effect resulting fromreduced speed,oil consumption is lowered from 116.71t/d to 51.63 t/d,saving fuels by 55.8%.See Table10.

Table 10 Effects of conversion items

As for 3 600 CV,reducing speed properly and doingcertain conversion make a fuel saving vessel with a short return-on-investment period.All the measures taken could beapplied to such relatively new shipswith characteristics of high speed and oil consumption,bringing a prosperous future for owners,shipyards and suppliersaswell.

［Referance］

［1］DNVFuel Saving Guideline ［R］.