How do we accurately determine the weight of a ship’s cargo? Traditionally, draft surveys have been used to calculate this. But how do we ensure that the draft measurement we use is precise? Let’s explore the process of conducting a draft survey from scratch, according to Marine Insight.
Precise Method
We know our goal: to determine the vessel’s Displacement, a parameter obtainable through the Hydrostatic Tables, using the draft. But the question remains—which draft should we use?
Initially, one might consider taking a single draft reading from midship. However, this approach would likely yield inaccuracies, as it doesn’t account for the vessel’s potential list or trim.
A more precise method involves taking six draft readings: two forward (port and starboard), two amidships, and two aft.
By calculating the average of these readings, we can determine a mean draft. However, trim and the exact location of draft marks relative to the perpendiculars can still influence accuracy.
If a vessel is trimmed by the stern, the draft readings at the forward and aft draft marks won’t align with the drafts at the perpendiculars. For accurate Hydrostatic Table entry, draft readings must be adjusted to a common “coordinate system”—that is, to the forward, midship, and aft perpendiculars.
To correct each draft to its perpendicular, calculate the draft mark correction by considering the vessel’s trim and the horizontal distance between each draft mark and its corresponding perpendicular. This information can typically be found in the Stability or Loading Booklet. The draft mark correction formula adjusts each draft reading to reflect the draft at the perpendicular.
Accounting for Longitudinal Hull Deflection
The vessel may experience longitudinal deflection, often in the form of hogging or sagging. To account for this, the middle draft is given a greater “mathematical weight” due to its higher displacement contribution.
In practice, this is done by using a multiplier of 6 for the middle draft when calculating the mean draft. This weighted draft calculation, known as the ¾ Mean Draft, reduces potential errors due to hull deflection, providing a more accurate Displacement reading. In summary, the calculated ¾ Mean Draft prioritizes the middle draft, minimizing the impact of longitudinal deflection and improving accuracy for Displacement determination.
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Source: Marine Insight
