Q1. What is the meaning of “de-rating” of machinery? a) Explain the principles behind de-rating a ship propulsion engine as a retro fit. And the benefits. (8) b) Can a de-rated engine be run at full power? If yes, under what conditions? (8)
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Adi S.
A vessel is outfitted for propulsion with four (4) identical high-speed marine diesels. All four engines are connected to one propeller shaft through a reduction gearbox with fixed reduction ratio. When all engines are run at CMCR (Contract Maximum Continuous Rating) the brake power delivered by each one of them is 900 kW and 14.9 knots is the ship forward velocity (speed) attained. The weather and sea condition as well as ship’s loading (displacement and draught) and hull fouling conditions are assumed to remain the same, but the ship is now required to run at velocity of 11 knots. (a) Can 11 knots be attained if only three (3) of the engines are running? If so, calculate the brake power delivered by each one of the three engines running, assuming equal load sharing. On the other hand, if 11 knots not attainable, calculate the ship velocity with three engines running at CMCR. (b) Can 11 knots be attained if only two (2) of the engines are running? If so, calculate the brake power delivered by each one of the two engines running, assuming equal load sharing. On the other hand, if 11 knots not attainable, calculate the ship velocity with two engines running at CMCR. For the engine admissible envelope is known that at a given crankshaft rotational speed below the CMCR value, the upper boundary complies with constant-torque. In effect PB(NE) ≤ PB,max(NE) = (NE / NCMCR) · PCMCR In the above: NE is the rotational speed of an engine’s crankshaft (which in this case is a multiple of the propeller shaft rotational speed nP due to the use of the reduction gearbox), NCMCR is the rotational speed of an engine’s crankshaft at CMCR (in this case a multiple of the propeller shaft rotational speed nCMCR at the same point), PB(NE) is the brake power delivery of one engine which depends among others on NE, PB,max(NE) the maximum admissible brake power delivery of one engine depending exclusively on NE as dictated by the constant torque behavior, PCMCR = PB,max(NE=NCMCR) = 900 kW.
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Luke H.
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