Page 350 - Practical Ship Design

Freeboard and Subdivision 307

(4) Timber freeboards
These freeboards which are also less than type B, are assigned to ships designed to
carry timber on deck, in recognition of the fact that this type of buoyant cargo
provides a major contribution to the ship’s survivability. There are specific require-
ments for the stowage of the deck cargo.
(5) Dredger freeboards
Dredgers which have open hoppers without hatch covers and also have bottom
dump valves, have in the first of these a facility that results in cargo spillage if the
ship heels beyond a certain angle and in the second an ability to dump cargo rapidly
in the event of an emergency. They are also usually employed in waters close to land.
The increased survivability which these features give is recognised by giving
these ships the possibility of a freeboard less than the statutory minimum and relief
from the statutory bow height.
Under different conditions the freeboard may be one of the following:

5/8 B; or 1/2 (B-60); or 1/2 (B-100)

subject in all cases to a minimum of 150 mm. The conditions attached to this
dispensation are given in the Department of Transport Instructions to Surveyors.
They include requirements:
- that the longitudinal strength is adequate for the corresponding draft,
- that the ship has operational limits which normally do not exceed 15 miles
from land,
- that the ship complies with special stability and flooding requirements, which
are discussed in $12.3,
- that draft indicators are fitted.
The last requirement may not appear to be of quite the same importance as the others,
but it is in fact an important contribution to the safety of a ship which loads at sea.
Hopper dredges/barges with a less than statutory minimum freeboard are also
marked with statutory marks. (The non-statutory marks are red in colour and their
use is limited to very specific (named) local applications for which they are
assigned.)

11.2.2 Seasonal freeboards
The freeboards mentioned so far are in each case the basic or summer freeboard (S)
which applies in salt water in summer. Ships are in general also given a number of
subsidiary freeboards which apply in different conditions.
These are respectively tropical (T), winter (W), winter North Atlantic (WNA),
freshwater (F) and tropical freshwater (TF) and are derived from the basic summer
freeboard using the following relationships: