Page 207 - Introduction to Naval Architecture

RESISTANCE 193

Figure 8,9 Extrapolation to ship

Notwithstanding this, the skin friction resistance should be calcu-
lated as accurately as possible so that an accurate wave-making
resistance is obtained for comparing results between different forms
and for comparing experimental results with theoretical calculations.

Wetted surface area
To obtain the frictional resistance it is necessary to calculate the wetted
surface area of the hull. The most direct way of doing this is to plot the
girths of the ship at various points along its length to a base of ship
length. The area under the curve so produced is approximately the
desired wetted surface area. This is the way Froude derived his circular
S values and the method should be used when using Froude data. For
a more accurate value of the actual wetted surface area some allowance
must be made for the inclination of the hull surface to the centreline
plane especially towards the ends of the ship. This can be done by
assessing a mean hull surface length in each section and applying this
as a correction factor to the girth readings. Alternatively an overall
mean surface length can be found by averaging the distances round the
waterline boundaries for a range of draughts.
A number of approximate formulae are available for estimating
wetted surface area from the principal hull parameters. With the usual
notation and taking Fas the draught, those proposed by various people
have been

where C is a constant depending upon the breadth/draught ratio and
the midship section coefficient. With A in tons and Lin feet C generally
lies in the range 15.2 to 16.5.