Page 327 - Practical Ship Design

Machinery Selection 285

Table 9.2
Contributors to fuel economy
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The transportdtion need, ship size, speed, routing Shipowner dnd trdnsportdtion economi\t
Design optimisation lines and propeller Naval architect
Shell finish and minimising fouling Shipbuilder dnd paint manufacturer
Fuel etficient engine, use of wdqte heat Marine engineer
Reduction in hotel lodd dnd other electricdl demand\ Ndvdl architect
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Table 9.3
Savings in fuel coats per cargo tonne-mile 1975-1995
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Chmge Reduction in fuel co\ts
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I. Speed reduction of 15% to a more economical speed (e g 12 knots in lieu of 15 25
knots or 23 knots in lieu ot 27 knots)
2. Increaw in deddweight by use of a fuller C, together with weight saving due to 2
improved structural design
3. Improved line\ and proportions of ship main dimenqions reducing resistance 8
3. Improved shell finish reducing trictiondl resistance and fouling 6
5. Slower revving propeller increasing propulsive efficiency 12
6. AIymmetric line\, Grimm wheel or reaction fairings reducing or recovering 10
rotational energy
7. Improved \pecific consumption ot main engine (170 g/kWh v 210 g/kWh) 19
8. Shaft-driven alterndtor reducing cost of fuel wed for electricity generation 2
9. Reduction\ in electric load due to insulation etc 1
IO. Better utilisation ot waste heat 2
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Cumulative \wing 56

help to increase another, as happens when a reduction in EHP makes it possible to
lower the propeller revs and thereby gain in propulsive efficiency.
The extent to which the economy of a typical modern ship built in the last five
years has improved compared with a similar ship built about twenty years ago is
shown in Table 9.3. On this basis, the fuel consumption of a good modern ship per
tonne-mile would be about 44% of that of a ship built twenty years ago. The
cumulative saving is, of course, obtained by multiplying the percentage savings
and not by addition.
This table is open to criticism and the author would not attempt to defend it in
detail, but believes it gives a feel for the savings that have been made. A naval
architect should always be on the look-out for new ideas that will provide improve-
ments but should assess these critically before accepting the claims made!