Page 214 - Introduction to Naval Architecture

200 RESISTANCE

The Schoenherr and ITXC resistance formulations were intended to
apply to a perfectly smooth surface. This will not be true even for a
newly completed ship. The usual allowance for roughness is to increase
the frictional coefficient by 0.0004 for a new ship. The actual value will
depend upon the coatings used. In the Lucy Ashton trials two different
coalings gave a difference of 5 per cent in frictional resistance. The
standard allowance for roughness represents a significant increase in
frictional resistance. To this must be added an allowance for time out of
dock.

FORM PARAMETERS AND RESISTANCE

There can be no absolutes in terms of optimum form. The designer
must make many compromises. Even in terms of resistance one form
may be better than another at one speed but inferior at another speed.
Another complication is the interdependence of many form factors,
including those chosen for discussion below. In that discussion only
generalized comments are possible.
Frictional resistance is directly related to the wetted surface area and
any reduction in this will reduce skin friction resistance. This is not,
however, a parameter that can be changed in isolation from others.
Other form changes are likely to have most affect on wave-making
resistance but may also affect frictional resistance because of con-
sequential changes in surface area and flow velocities around the
hull.

Length
An increase in length will increase frictional resistance but usually
reduce wave-making resistance but this is complicated by the inter-
action of the bow and stern wave systems. Thus while fast ships will
benefit overall from being longer than slow ships, there will be bands of
length in which the benefits will be greater or less.

Prismatic coefficient
The main effect is on wave-making resistance and choice of prismatic
coefficient is not therefore so important for slow ships where it is likely
to be chosen to give better cargo carrying capacity. For fast ships the
desirable prismatic coefficient will increase with the speed to length
ratio.

Fullness of form
Fullness may be represented by the block or prismatic coefficient For
most ships resistance will increase as either coefficient increases. This is