Page 196 - Practical Ship Design

162 Chapter 6

the imperfections that give rise to the h value. A large number of essentially
streamline elements will have less effect than a smaller number of less streamlined
solidified droplet-type imperfections.
Values of AC for different roughness values for the 330 m vessel used in the
calculations in 56.2.4. for a speed of 15 knots are as follows:
MAA lo3 AC
75 0.052
100 0.079
125 0.102
150 0.122
Both (1 + x) and AC values make very clear the bonus to be gained from a good
paint finish and the penalty imposed by a poor one.

6.2.7 Steelwork roughness

Townsin and his co-authors, encouraged by the success of constant emission toxic
coatings in reducing fouling, saw roughness of steelwork and paint finish as a
major factor to be attacked in pursuit of fuel economy. Under laboratory conditions,
they obtained a roughness of 50 microns with a ship’s paint system applied to
shot-blasted steel and thought that 80 microns could be attained when applied to a
ship in drydock in very strictly controlled conditions.
Grigson, whose paper “The full-scale viscous drag of actual ship surfaces” was
published in 1987 in the Journal of Ship Research, states that it is not only the
height of the average roughness that matters but also the form that the roughnesses
take. He also makes the point that with good modern practice we ought not to be
speaking of surface roughness but of imperfect smoothness. With shop-blasted and
shop-primed smooth steel plate, welded by automatic machines producing smooth
rounded weldments and up to four coats of paint applied by airless spraying
machines under cover, a very high quality finish is possible. Unfortunately, unless
senior management insists on the best practice, airless spray can be incorrectly
applied resulting in “dry overspray” which can increase AC to as much as 20% of
cts

6.3 AN AIDE MEMOIRE ON THE COMPONENTS OF POWERING

(i) ESfective horsepower (P,)
This is the power required to tow a ship, overcoming its resistance. It can be
expressed either as