Page 245 - Practical Ship Design

Powering I1 207

Using this data, C, is then established and to this is added (1 + K) C,, and
roughness, appendage and air resistance elements all as in eq. (6.7).

7.2.6 Calculation of P,
The calculation of P, from C, uses the formula:

P, = 0.0697 C,, . S . V3 (in kW) (7.18)

All the C, values must of course be corrected for the ship’s dimensions. If the
process outlined has been followed that for length will have already been made, but
corrections for beam and draft will still be necessary. These should be made using
the ratios of the ship’s beam and draft, proportioned to the length of the basis ship
divided by the beam and draft of the basis ship together with the appropriate
Mumford indices (use twin screw ship also for warships). Although these indices
were originally intended for use with 0, they apply to equally to C, as they are
multipliers.
Using LIB and TIL ratios for both the model (1) and the new design (2), the ratios
required for the Mumford indices are easily derived as follows:
L, IB, B, xL, IL, T, IL, - T, xL, IL,
-- - and ~ -
L, lB, B, TI ’4 TI

7.3 APPENDAGE RESISTANCE

To complete the resistance (or effective horsepower) calculation it is necessary to
add the resistance of any appendages that are fitted to the ship, but were not fitted
to the model.
In power estimates appendage resistance is generally added as a percentage of
the naked resistance, although there is little logic in this as this implies that an
appendage behind a resistful hull has more resistance than an identical appendage
behind a less resistful ship.

7.3. I Most common appendages
The most common appendages are:
- Twin rudders. A single centreline rudder is normally included in the “naked’
model, but twin rudders are treated as appendages.
- Twin screw bossing and/or shaft brackets. ‘A’ brackets, in general, appear to
have significantly less resistance than enclosed bossings on twin screw ships,
but both need careful alignment with the flow pattern.