Page 266 - Practical Ship Design

224 Chapter 7

7.7.2 Weather and fouling - historical treatment

In the past, naval architects used to lump allowances for weather and fouling
together as an addition to be made to the power required under trial conditions to
enable the trial speed to be maintained in service.
Typically, the service power allowed for an increase of 20% over trial power,
although this addition was almost certainly much lower than the actual increase
required as was demonstrated by a number of investigations. However under the
commercial pressure of competitive tenders 20% remained the general guide line
for shipyard naval architects.
The allowance for service should take into account the weather conditions that
may be expected on the vessels trade route. Guldhammer and Harvald in the paper
quoted in $6.8 suggested average percentage service allowances for different
routes as follows:
Summer Winter
North-Atlantic Eastwards 15 20
North-Atlantic Westwards 20 30
Pacific 15 30
South-Atlantic and Australian routes 12 28
East-Asiatic route 15 20
These figures predate the improved anti-fouling discussed below but it remains the
case that the percentage which must be added to the trial power for weather can
depend significantly on the shipping routes for which the ship is intended and on
the importance attached to the maintenance of the schedule.

7.7.3 Fouling with modern anti-jiouling paints

The 7% power reduction that can be gained by achieving an 80 pm finish on a new
ship compared with the 165 pm standard has been referred to in $6.2.6. Even more
important, is the fact that, with an advanced constant emission system, the finish
will actually improve with service. This is because this type of paint self-polishes
in service often actually improving the finish, in marked contrast to conventional
paints whose roughness increases, not only with service but also with further paint
applications.
With conventional paints the poison leaches out from the interior and the rate of
release decreases with time in an exponential way. The use of deeper layers of
paint results in the release being slower. With these paints, fouling started to affect
the performance of a ship after as little as 20 days out of dock - a fact acknowl-
edged when this was made one criterion of a “best trial”.
With the SPCs the paint ablates or dissolves steadily (6 pdmonth is a typical
figure) and the new surface exposed by this process is as poisonous and active as