Page 289 - Practical Ship Design

Design of Lines 247

The author has no practical experience of using lines developed by any of the
many mathematical hull generation programs that are available today but a reader
of this chapter who had this experience made the comment that most of these
programs do not take into account all the factors which have been considered in
this chapter, each of which contributes in some way to the overall performance of a
design. This can, of course, be overcome by manual intervention at the preliminary
body plan stage with mathematical refairing of the resulting sections.

8.5 TWIN-SCREW LINES AND APPENDAGES

The lines of a fine lined twin-screw ship can be designed almost entirely with the
objective of minimising resistance, without the need to consider flow to the prop-
eller which necessarily plays a large part in the design of single-screw lines. As a
result, twin-screw forms can have a slightly bigger block coefficient (and as
already noted a further aft LCB position) for a given Froude number andor should
have better specific resistance of the “naked’ hull.

8.5. I Bossing or shaft brackets
The naked resistance advantage of a twin-screw lines is, however, reversed when
the appendage resistance of bossings is added. In the past fully enclosed bossings
were the normal fit on twin-screw ships. These had the advantage of protecting the
shafts and allowing these to be supported at intervals that avoided problems with
whirling vibrations. The additional resistance was, however, high - of the order
of 10% of the naked resistance - so more recently the shafts have been left
exposed to the sea, supported by one or more “A” brackets. The resistance of this
combination can generally, with good detail design, be kept to about 6% of the
naked resistance.
These figures are very broad generalisations as appendage resistance can vary
widely (see also Chapter 7, 57.5).

8.5.2 Twin skeg forms

A novel approach adopted on several recent twin-screw passenger liners and
ferries is the twin skeg form shown in Fig. 8.11. With these forms the bossing in
effect becomes part of the main hull. This type of lines appears to increase the
wake and certainly provides good support to the propellers, and ought to minimise
“bossing vibration”, although this is not borne out by experience in more than one
ship of this type.