Page 103 - Practical Ship Design

72 Chapter 3

Although the drafts of warships are kept carefully concealed by the world’s
navies (or are quoted as drafts to the propeller tip, which are of little value for
design purposes), a line for frigates has been added to Fig. 3.10 and shows a mean
TID value of 0.46. The high freeboard that this low ratio indicates shows the
concern for seaworthiness that is so necessary a feature of the design of these ships.

3.3.5 DeptWlength relationship D = f(L)
In the discussion about the BID ratio it was noted that deadweight carriers have a
higher value of this ratio than capacity carriers.
In deadweight carriers, stability is generally in excess (sometimes greatly) of
rule requirements and depth and breadth are therefore independent variables. For
these ships, control of the value of D is exercised more by the ratio LID which is
significant in relation to the structural strength of the ship and in particular to the
deflection of the hull girder under the bending moment imposed by waves and
cargo distribution. The largest LID ratios were formerly used on tankers whose
“A’ type freeboard needed a comparatively small depth for the required draft and
whose favourable structural arrangements with longitudinal framing on bottom,
deck, ship sides and longitudinal bulkheads together with the fact that this type of
ship has minimum hatch openings meant that the steel-weight penalty for an
unfavourable LID value was minimised.
When higher tensile steel is used to save weight, it is generally desirable to use a
smaller LID value in order to limit the deflection of the hull girder.
LID values as presented in the 1975 paper are shown in Fig. 3.11, whilst the
values in use in 1991 are shown in Fig. 3.8. A comparison of these figures shows
little change in bulk carriers, with the LID ratio averaging at about 11.8 in both
cases. Tankers, however, show a striking change from a value of about 12.5 in
1975 to one of 10.5 in 1991, a change brought about by the need to provide separate
clean ballast capacity.
The line for frigates which has been added shows a mean value for these ships of
LID = 13.3. This comparatively high value would appear to be the consequence of
the need for length which has already been the subject of comment together with
the need to limit depth which is the corollary of the limited beam. The fact that
warships do not have large hatch openings reduces the adverse structural effect of a
high LID ratio.

3.3.6 DrafAength relationship T = f(L)
This is essentially a secondary relationship resulting from either of the following
combinations of relationships:
T =f(D> or T=f(D)
and D =f(L) and D =f(B) and B =f(L)