Sec. 69.10 



GENERAL DESIGN OF PROPULSION DEVICES 



577 



0.95 of Moximum Designed 5hoft Power =■ P fv^q,, less Mochirier>) 



Ends of 

 Curves 

 Not 

 Shown 



No Covilotion Allowances Shown Here 



Curve of Shaft Power on Speed for 

 FULL OVERLOAD Conditions, Involvinc) Storrrnj 

 Weather, Heavu Foulinc),and Unexpected 

 Adverse Effects~^ 



Curve of Shaft Power on Spaed for^ 



AVERA&E OVERLOAD, with Averoe^e 



Adverse Wind and Sea and Averaoe 



Foul Bottom. Points on This Curve. 



ore Often Selected for 



Propeller-Desiqn Points, 



Represent(nq Averaqe Conditions ^-r , ^, ;. 



'^^ > _^ ^^^Curve of Shaft 



Lower ^^ ^^^wer on Speed with 



NO OVERLOAD but Includinej 



Still-Air Resistance and Normal 



Rouqhness Allowance for o Clean, New 



Vessel, Correspondinq to Model Basin 



Prediction for These Conditions 



All Curves Are for Desicjned Displacement and 



Trial 5 peed Vy 

 ot Power Pg5 

 Under Perfect 

 Trial Conditions 



Ship Speed, kt 



"^Sustained 



^Triol 



Fig. 69. a Explanatory Diagram for Powering Allowances in a Large Vessel 



95 per cent of maximum power. Under what 

 might be termed "FULL OVERLOAD" con- 

 ditions, corresponding to the curve FGaE, and 

 utilizing only 95 per cent of the designed maximum 

 power, the ship is able to maintain a sustained 

 speed Fsu.t , at the speed-power point Gj . 



If the ship is really to sustain this speed for 

 long periods it must be capable of running faster 

 than Fsust part of the time. Assuming full-over- 

 load conditions all the time this is not possible 

 by Fig. 69. A unless the power is increased above 

 P95 (and above Pm^i as well). Rather than to 

 do this it is assumed, and logically so, that the 

 sustained speed is to be achieved under average- 

 overload conditions, along the curve DG2C in the 

 figure. This means that with a sort of average 

 power, represented by the ordinate of the point H, 

 the ship has a reserve of power represented by 

 the ordinate HG2 . With this reserve, extending up 

 to P95 at the point Gi , it is capable of achieving 

 the speed Fa • This speed is somewhat less than 

 FTriai , but as long as the speed margin (F, — 

 Fsu.t) is greater than the possible speed reduction 

 (F3 — F4) to be anticipated over any lengthy 

 period, the ship is assured of maintaining the 



sustained speed and of keeping up with its 

 schedule. 



Unfortunately in practice the designer rarely 

 if ever knows the precise location of the average- 

 overload or the full-overload speed-power curves 

 on his plot, despite the availability of procedures 

 such as those in Sees. 45.22 and 60. 15. He is reason- 

 ably certain in the design stage of the no-overload, 

 speed-power curve AGB, with its values of 

 Fjuns and FTrioi • He knows, furthermore, that 

 the speed-power values along the curve AGB can 

 be checked by carefully conducted ship trials. 

 He also knows the required sustained speed. He 

 may therefore select a speed Fi such that the 

 speed margin (Fi — Fsust) is sufficient in his 

 opinion, and in the judgment of the owner and 

 operator, to make good the sustained speed. 

 From the no-overload ctu've of the figure, this 

 speed Fi then corresponds to the point A, also 

 easily determined in advance with reasonable 

 accuracy and subject to confirmation on trial. 

 The corresponding power is PNom, , which with 

 an average overload should give a speed F3 , 

 slightly in excess of the sustained speed or at 

 least not less than that speed. For most ship 



