Page 194 - Practical Ship Design

160 Chapter 6

prediction” questions the accuracy of the 1957 ITTC line. He is almost certainly
right in doing this if this line is to be used as a base for a form factor as the ITTC
line was never claimed to be a friction line having originally been introduced as a
ship-model correlation line. Grigson’ s paper shows that at Reynolds’ numbers in
the model area (4 x IO6 to 2 x lo7) C, ‘57 values are up to 6% higher than what he
suggests are the “correct” values, whilst in the ship area which is of the order of
(4x10’ to 2 x lo9), C, ‘57 values are about 5% below the “correct” values (see Fig.
7.18). This would mean that both C, and C,, are being underestimated, and as the
paper also revises the (1 + K) values upward the overall effect is to increase Pd by
about 7% and propeller RPM by about 1.5%.
When relating K new line to K ITTC’78, the physical quantity that remains the
same is the viscous drag, so:

with C, remaining unchanged.

6.2.5 Ship-model correlation
As well as (1 + K) Cfs the total viscous resistance for the ship includes the term AC
which is intended to allow for the influence on resistance of the roughness of the
paint. This is now seen as an addition to the frictional resistance and not, as in the
past, a factor (1 + x) applicable to the total resistance.
It is interesting to take a brief look at the history of ship-model correlation
which came to prominence with the change from all riveted construction to all
welded ship hulls in the early fifties, when it was found necessary to bring in
shipmodel correlation factors to reflect the differences in smoothness between an
all-riveted ship (1. lo), a ship with riveted seams and welded butts (1 .OO) and an
all-welded ship (0.95).
By the late fifties/early sixties, most shipyards were building all-welded ships
and ship-model correlation seemed to have reached such a satisfactory state that it
was possible to start taking a more sophisticated look at the effect of the smooth-
ness of the platework and its paint coatings.
Within a few years, however, trial results from significantly larger ships started
to be tabulated, and it was found that many of these vessels had performed much
better than had been predicted.
A new factor was accordingly introduced into ship-model correlation -
a
factor which scaled with ship size. This had a value of 1.00 for a ship length of
about 105 m reducing linearly to 0.80 at a length of 275 m.
The necessity for a size-dependent factor clearly indicated a fault in the extrapo-
lation from model to full size with a factor less than unity predicating full-size
ships with a finish superior to that of a wax model!