Page 102 - Practical Ship Design

The Design Equations 71

30

w
W
I
I-
I - General Cargo
(3
n

0 10 m 30 40
DEPTH - METRES

Fig. 3.10. Draft-depth relationship.

This formula is now historic with the unlamented demise of the open shelter
deck ship with its undesirable features. For a depth of 15 m the ratio TID given by
this formula for a closed shelter deck ship would be 0.746 which is quite close to
the value which applies to a cargo ship today.
In the 1975 plot, reproduced as Fig. 3.10, two lines for TID = 0.7 and 0.8 were
shown together with a mass of data spots which mainly lay somewhere between
these lines, with tankers with a class “A” freeboard tending towards a higher value
of about 0.78, whilst cargo ships with a class “B” freeboard were nearer the lower
line and a TID value of 0.73.
The scale of this plot is not suitable for reading off an accurate value emphasis-
ing the advantage of the ratio presentation in Fig. 3.8. This shows bulk carriers and
refrigerated ships averaging at a TID ratio of 0.7 1. The reason for this being a little
less than the 1975 figure for cargo ships may be partly due to the fact that the ships
are larger and partly because many of the 1975 ships had sheer, whereas most ships
in 1991 do not. The T/D value for tankers now appears to average at 0.67,
contrasting with earlier practice when an “A” class freeboard was general and
confirming the statement that tankers are now “volume design” ships.
The TID value of container ships varies quite a lot but the average value is about
0.62.
With designers now having access to computer programmes which makes the
calculation of an accurate rule freeboard easy and quick, the use of a TID ratio
value is confined to preliminary estimates.