An Evaluation of Tanker Resistance
Prediction Accuracy in NavCad
A HydroComp Technical Report
Product link: NavCad
This report evaluates
the use of NavCad for the resistance prediction of tankers. A
discussion of the unique prediction challenges for modern tankers
- and their solution - is presented.
Due to a modern
tanker's full form and its operation at relatively slow speeds,
a large portion of its resistance is viscous. Viscous resistance
(made up of a frictional and a "form" component) does not scale
from model to ship in the same way as the wavemaking resistance,
which is directly proportional at both model and ship scale. Therefore,
hulls with large viscous resistance must separate the "form" component
out of the traditional "residuary" resistance, and isolate this
from the wavemaking resistance. This means that tankers should
utilize the three-dimensional ITTC-78 Cw-based (wave-making) model-ship
extrapolation method. Values of the "1+k" form factor frequently
in excess of 1.3 underscore the need to use this approach. Experience
has shown that the use of a two-dimensional Cr-based (residuary)
extrapolation can overpredict resistance by as much as 10%.
The value of the
form factor generally varies with the overall hull parameters
(L/B, B/T, Cb), where finer ships have lower values. The wavemaking
resistance, on the other hand, has been shown to be most influenced
by the bow shape - notably the half angle of entrance and the
bulb characteristics [Tagano, 1974].
NavCad has two algorithms
that can be successfully used with tankers - the BSHC method
and the Holtrop-1984 method. Both methods include recent
revisions by HydroComp to improve the specific evaluation of bulbous
bows. A more analytical "complete" algorithm for the wave-making
coefficient replaces the previous "simple" design-oriented algorithm
in the BSHC method [Kostov, 1991]. A slightly revised bulb
resistance algorithm replaces the previous formula in the Holtrop-1984
method [Holtrop, 1988]. The previous Holtrop algorithm significantly
overpredicted the added pressure resistance with large-area, high-centered
bulbs often found on modern tankers.
method provides good quality, consistent prediction of resistance.
While applicable to a broad range of hull parameters, it does
seem to slightly underpredict (2% to 5%).
The BSHC method
shows excellent correlation to tests when all parameters are within
the range of data set values. However, the accuracy of applying
this method to hulls with parameters out of range is generally
poor. It appears that the method is most sensitive to the half
angle of entrance and block coefficient hull parameters.
With either method,
extrapolation to full scale should always be made with the ITTC-78
form factor approach.
As a final note,
it is important to remember that water density and viscosity change
with water temperature (as might be found in tankers operating
in the Middle East). An inaccuracy in viscosity could significantly
affect the viscous resistance, and therefore the total resistance.