Page 246 - Practical Ship Design

208 Chapter 7

- Large unfaired hubs of C.P. propellers. These can have quite a large drag, but
are not fitted when the resistance is measured and the effect may appear as a
reduction in propulsive efficiency
- Bilge keels used to be assumed as included in the naked power, but as
mentioned in $6.2 the resistance of these is now added as an increase in the
wetted surface area. The importance of taking great care to align these
appendages with the stream line flow in the region in which they are fitted
must be emphasised.
- Fin stabilisers
- Bow rudder andor bow propeller. Bow or stern thruster openings. These
must be carefully married into the lines, and indeed the lines may need to be
modified to accommodate them. It is worth noting that the additional resist-
ance of a bow propeller can be significantly less behind a bulbous bow than
behind a conventional bow.
- Sonar domes on warships and research vessels
- Cathodic protection anodes
- Moon pools on oil exploration and offshore production vessels
- Hopper doors and the ship side slides for the overboard suction pipe trunnions
on dredgers. All these can cause a considerable increase in resistance which
can, however, be minimised by careful design.

7.3.2 Some approximate figures for appendage resistance

Some approximate figures (regretfully as factors based on the naked resistance)
which should be used with care are:
Twin rudders 0.0 15
Twin screw bossing 0.10
Shaft brackets and open shafts 0.06
Fin stabilisers 0.02
Sonar dome 0.01
If there is more than one appendage the appropriate factors should be added.
An alternative treatment of appendage resistance is suggested by Holtrop and
Mennen in their papers to which reference has already been made. This calculates
appendage resistance based on their wetted surface area using the C, value for the
ship according to the 1957 ITTC formula.

.
.
Rap, = 112 . c, . r. sapp v2 (1 + K*) (7.19)
Approximate K2 values are quoted as follows:
Rudder behind skeg 1.5-2.0
Rudder behind stern 1.3-1.5