Page 247 - statbility for masters and mates

True mean draft 235
The actual correction itself can again be found by using the above formula, but in this case the correction is to be subtracted from the mean of the drafts forward and aft. Similarly, by substituting forward for aft and aft for forward in this ®gure, it can be seen that the correction is again to be subtracted from the average of the drafts forward and aft when the ship is trimmed by the head and the centre of ¯otation is aft of amidships.
A general rule may now be derived for the application of the correction to the draft amidships in order to ®nd the true mean draft.
Rule
When the centre of ¯otation is in the same direction from amidships as the maximum draft, the correction is to be added to the mean of the drafts. When the centre of ¯otation is in the opposite direction from amidships to the maximum draft, the correction is to be subtracted.
Example 1
A ship's minimum permissible freeboard is at a true mean draft of 8.5 m. The ship's length is 120 m, centre of ¯otation being 3 m aft of amidships. TPC 50 tonnes. The present drafts are 7.36 m F and 9.00 m A. Find how much more cargo can be loaded.
Draft forward 7:36 m Draft aft 9:00 m
Trim 1:64 m by the stern Correction t FY 1:64 3
Correction Draft forward Draft aft Sum Average Draft amidships Correction True mean draft Load mean draft Increase in draft Cargo to load
Ans. Cargo to load 1400 tonnes
L 120 0:04 m
7:36 m
9:00 m
16:36 m
8:18 m
0:04 m
8:22 m
8:50 m
0:28 m or 28 cm
Increase in draft required TPC 28 50