Page 132 - Practical Ship Design

Weight-Based Designs 99

4.3 Bollards and fairleads, chain pipes
4.4 Seats for windlasses, capstans, winches, cranes, steering gear
4.5 Masts and derrick posts
5. Shell fittings
5.1 Bilge keel
5.2 Bulbous bow
5.3 Hawsepipes, thruster tunnels
6. Structure below upper deck
6.1 W.T. bulkheads
6.2 Tank bulkheads
6.3 Casings below upper deck
6.4 Decks and flats not in weight per metre
6.5 Pillars and girders
6.6 Sparring, ceiling
7. Machinery space structure
7.1 Main engine seats
7.2 Auxiliary seats, engineers tanks, sewage tanks
The net weight calculated in this way must be corrected for weld metal and rolling
margin before it is used when making up the lightship, whilst for cost purposes a
suitable scrap percentage must be added.

4.3.3 Detailed structural weight calculations - warships
The midship section weight per metre method outlined above can be used for
warships in the preliminary stages of design but in later stages it is usual to draw a
number of structural sections to investigate the structure required at different
stations along the length of the ship. When such sections are available their use
provides the possibility of a more accurate structural weight estimate.
Warship structural weights are generally estimated and recorded in the categories
shown in Group 1 on Fig. 4.14, although warship designers use more detailed
three-digit weight groups for their calculations.

4.4 OUTFIT WEIGHT CALCULATIONS

4.4.1 Approximate methods - all ship types

The traditional method of estimating the outfit weight for a new merchant ship was
by proportioning the outfit weight of a similar ship on the basis of the relative
“square numbers”, i.e., L x B, and then making corrections for any known differ-
ences in the specifications of the “basis” and “new” ships.