Page 278 - Practical Ship Design

236 Chapter 8

where

2.56Cb .P
+ 0.3)

where P = power in kW.
The recommended clearance for a four-bladed propeller on a twin-screw ship, is
1 .00 K. Other values are given in the rules for three, five and six-bladed propellers.

8.2.6 Large propellers

Where the propeller is large in relation to the draft of the ship, a number of options
exist:
(i) The propeller can be fitted in such a position that the lower tip is below the line
of the keel. This is common practice on warships, but merchant ship owners have
been reluctant to allow this because of possible damage to the propeller in shallow
water and possible additional dry docking problems and costs. With a better
understanding of the gains that can be obtained by the use of large diameter prop-
ellers it is possible shipowners may be more willing to consider this in the future,
although even when a clear water stern is used, most owners demand a substantial
rise of both the propeller tip and the bottom of the rudder above the base line.
(ii) The ship can have a designed trim or a raked keel. This is commonly used,
and for precisely this purpose, in small ships, notably tugs and fishing vessels. It is
also used for the same reason on warships, even large twin-screw vessels. So far it
has not been adopted on large merchant ships, partly due to a wish to limit the
extreme draft of these ships, and partly because of the increase in structural
complexity which is an unfortunate corollary. Winters 1997 R.I.N.A. paper “Applic-
ation of a large propeller to a container ship with keel drag” merits study and may
lead to a greater adoption of this simple and effective way of improving propeller
efficiency on large ships.
(iii) A Mariner type rudder, supported by a skeg, can be fitted eliminating the
sternframe solepiece and thus permitting a small increase in propeller diameter,
but see comments under (i).
(iv) A tunnel type form can be used. The design shown in Fig. 8.3 was used very
successfully on shallow draft river craft. It may be noted that the propeller tip can
come right up to the static waterline with the tunnel configuration ensuring that it is
kept fully immersed. The use of much the same technique on single-screw vessels
was introduced by Burmeister and Wain on their fuel economy vessel, and a body
plan showing two body plans with lines of this sort used by Port Weller is
illustrated in Fig. 8.4. This type of stem brings an incidental gain in displacement
and deadweight.