Displacement is extracted from hydrostatic table based on ‘quarter mean draft’ and ‘quarter mean draft’ is derived from the formula: (Forward corrected draft + Aft corrected draft + 6 X Midship corrected draft)/8. If vessel’s MSD is controlled by Dynamic Under keel clearance (DUKC) applicable for the vessel, maximum cargo lift can be achieved through loading the vessel to her even keel draft and without any hog or sag. Maximum cargo lift reduces when vessel suffers hog or sag and in case of hog, this loss is more significant. If MSD is controlled by load line mark and if there is no DUKC issue, only sag will reduce maximum cargo lift. Contrary to that hog will increase and this can be confirmed through analysis of aforementioned formula for ‘quarter mean draft’.
Looking at the intermediate draft as shown in FIGURE 1, Master requested me to change the trimming hold and instead of cargo hold no.6, to load in cargo hold no.4. According to him, if we use cargo hold no.6 & 2 for trimming, vessel’s hog will increase and consequently, cargo lift will be further reduced; which means loss of freight. Though I did not acquiesce with Master, but I had no documented evidence to defy his claim. In my opinion, considering huge size cargo hold of ore carriers, the possibility of increased hog could be eliminated through loading in after part of cargo hold no.2 and forward part of cargo hold no.6. However, it is always the ship’s call and I had no other choice but to accept the Master’s proposal. Now the question is how much cargo should be loaded in cargo hold no.4 & 2 to reach MSD of 18.33m. Here, Master refrained from affording any decision and was relying on my experience to decide on trimming quantity. According to calculation shown in FIGURE 3, distributing 6300 MT between hold no.4 & 2 is likely to bring the vessel’s draft to her MSD of 18.33m.
TRIMMING CALCULATION: Max Draft 18.330
Summer/mid F & A Draft 1/4 Mean
18.330 18.330 18.330
Balance to go 7705 6298 7354
Cargo Requirement 6300 Fwd Mid # Aft # Fwd # 0 No.2 4558 No.4 1742 No.2 Aft 0.000 Mid
17.871 18.037 17.870 0.000
0.000
17.871 18.037 17.870 0.182
0.365 -0.070 0.272
18.053 18.402 18.142 0.279
0.104 18.332 18.332 18.246 FIGURE 3
But can we rely on this calculation? My answer to this question is “NO”. I have been doing draft survey since 1995 and since 2014, I am collecting data to identify the change of hog or sag while using different pair of cargo holds for trimming of a cape size bulk carrier. In fact, I am proactively doing a research and the research question at this stage is “What is the effect on hog or sag of a cape size bulk carrier for loading cargo in different pair of cargo holds during trimming of a vessel” If we load 6300 MT and trim the vessel with cargo hold no. 4 & 2, my experience suggest that vessel would have never reached to MSD of 18.33m. It is relevant to mention here that minimum quantity that can be ordered is 500 MT. Any short load quantity which is less than 500 MT on completion of trimming pour also cannot be ordered later and subject to dead freight claim by Master.
At that stage the insight which I have developed through data collection for my research rescued me from the dilemma. I used the hindsight to determine cargo quantity to be loaded so that she can reach to her MSD and at the same time minimize the loss of freight resulting from hog. Figure 4 indicates that if we distribute 6800 MT among hold no.4 & 2, vessels fore and aft draft will exceed MSD applicable for the vessel. However, on completion of loading, we found that actual draft is less than calculated draft and also meeting the MSD requirement. Hog of the vessel was also significantly reduced and so the loss of cargo i.e. loss of freight, which is clearly evident from Figure 5.
0.000 0.349 0.166 Trim 0.166
LBP 318.000 LBM 288.000
The Report • March 2017 • Issue 79 | 51
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