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in ballast condition (all ships except data (Ref 8) and model data (Ref 9) as shown
containerships): in Fig 5.
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It follows that equations (1) will become
For C =0.55, 0.60, 0.65, and 0.70, vessel in progressively more suspect as the Beaufort
B
normal condition (containership): number increases. Equally, detailed calculation
methods become inaccurate at higher wind
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(Eq 1 c) and sea states. In practice, propeller racing
Fig 2 C
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= 0.60 (normal) F
n
= 0.30 may occur at about Beaufort 7 and above
s = 204,700m
3
. as Aertssen has shown. Voluntary speed
BN is Beaufort Number and s is volume of reductions also may be expected at higher
displacement in m
3
. Beaufort numbers depending upon fineness,
size, and operating conditions. For all the
Weather direction reduction above reasons equations (1), and the detailed
factors calculations which they represent, are unlikely
Weather direction has a marked effect to be accurate for Beaufort numbers above 6
upon speed reduction even to the point of or thereabouts.
increasing propulsion in following weather However, for most sea routes Beaufort
at low Beaufort numbers when the motions numbers above 6 occur infrequently. NA
are small. At present, calculations of added
Fig 3 C
B
= 0.70 (normal) F
n
= 0.20
wave resistance in oblique seas inspire less References
s = 273,000m
3
.
confidence than head sea calculations, the 1. Townsin, R L and Kwon, Y J: ‘Approximate
latter of which show some agreement with Formulae for the Speed Loss Due to Added
model test result. The view has been taken Resistance in Wind and Waves’, Tran RINA,
therefore that weather direction corrections Vol 125, 1983.
to the head sea results are best derived from 2. Van Berlekom, W B: ‘Wind Forces on Modern
full scale data. Ship Forms - Effects on Performance’, Trans
Advantage has been taken of Professor NECIES, Vol 97, 1981.
Aertssen’s formula (Ref 7) for calculating 3. Maruo, H: ‘On the Increase of the Resistance
the effect of weather direction. The ratio of of a ship in Rough Seas’, JSNAJ, Vol 108,
speed loss in oblique weather to speed loss 1960.
Fig 4 C
B
= 0.80 (normal) F
n
= 0.10 in head weather was calculated from the 4. Kwon, Y J: ‘The Effect of Weather, Particularly
s = 484,200m
3.
Aertssen’s equation for a range of ship length. Short Sea Waves, on Ship Speed Performance’,
For each direction the ratio was found to be PhD Thesis, University of Newcastle upon
little dependent on length but to vary with Tyne, 1982.
Beaufort number. The ratios, or weather 5. ISO 15016: ‘Guidelines for the Assessment of
direction reduction factor, can be closely Speed and Power Performance by Analysis of
represented and expressed conveniently as Speed Trial Data’, International Organisation
follows: for Standardisation, ISO/DIS 15016, 2002.
6. Townsin, R L and Kwon, Y J: ‘Estimating the
2m = 1.7 - 0.03 (BN - 4 )
2
30
o
- 60
o
bow
Influence of Weather on Ship Performance’,
Tran, RINA, Vol 135, 1993.
2m = 0.9 - 0.06 (BN - 6 )
2
60
o
-150
o
beam
7. Aertssen, G: ‘Service Performance and Trials
Fig 5 Comparison of the approximate formula
at Sea’, App V Perf Committee, 12th ITTC,
w
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ulation, model tests, and full
2m = 0.4 - 0.03 (BN - 8 )
2
150
o
-180
o
following
1969.
scale datVa.
8. Aertssen, G and van Sluys, M E: ‘Service
Justification of the formulae for Performance and Seakeeping Trials on a
head weather speed loss Large Containership’, Tran. RINA, Vol 114,
Head weather percentage
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The first step in justifying the approximate 1972.
speed lossV formulae is to see how well they represent 9. Takahashi, T and Tsukamoto, O: ‘Effects
For C = 0.75, 0.80, and 0.85, vessel in laden the results of the detailed calculations of of Waves on Speed Trial of Large Full
B
condition (all ships except containerships): percentage speed loss in head weather at Ships’, ISNAWJ, No 54, 1977.
'V
6.5 various Beaufort numbers. The accuracy of
10.Meyers, W G, Sheridian, D J and
100%
B
0.5BN
N
(Eq 1 a) fit may be seen in Figs 2 to 4. Salvessen, N: ‘NSRDC Ship-motion and
V 2.7 �
2/3
The result of the approximate formula is Sea-load Computer Program’, NSRDC
For C = 0.75, 0.80, and 0.85, vessel also compared with some published full-scale Report 3376, 1975.
B
16
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