Page 277 - Practical Ship Design

Design of Lines 235

both pitching and the amount of water shipped on the fore deck. Appearance and
the minimisation of damage caused to the other vessel in a head on collision are
further advantages of bow rake.
Care should be taken not to exaggerate flare too much as waves hitting one side
of a heavily flared bow can give rise to torsional vibrations and stresses. This was
first noted on some early container ships and Classification Society rules now
require additional strength to be provided if the flare is thought excessive.
The severity with which the forces generated by the sea can impact on flare has
been shown in a number of accidents, mainly off the South African coast, in which
complete bows have broken off, and of course in the recent tragic event in which
the bow visor was ripped off the Ro-Ro ferry Estonia.
See also $8.8.3 on the use of knuckles.

8.2.4 Sterns
Stems have to be considered in relation to the following roles:
(i) the accommodation of the propeller(s) with good clearances that will
avoid propeller excited vibration problems;
(ii) the provision of good flow to the rudder(s) to ensure both good steering
and good course stability;
(iii) the termination of the ships waterlines in a way that minimises separation
and therefore resistance;
(iv) the termination of the ships structure in a way that provides the required
supports for the propeller(s) and rudder(s) plus the necessary space for
steering gear, stern mooring and towage equipment etc. and is economical
to construct.

8.2.5 Flow to the propeller

Where the propeller diameter (D) on a single-screw ship is of normal size in
relation to the draft, Le. DIT is approx. 0.75, the main consideration is ensuring
good flow to the propeller, with a figure of between 28 and 30" being about the
maximum acceptable slope of a waterline within the propeller disc area.
Keeping to such a figure tends, of itself, to force the LCB forward on a full
bodied ship.
Lloyds' recommended minimum clearances as a fraction of the propeller
diameter for a four-bladed propeller are:
Tip to sternframe arch = 1.00 K
Stemframe to leading edge at 0.7 R = 1.50 K
Trailing edge to rudder at 0.7 R = 0.12
Tip to top of sole piece = 0.03