Mastering Draft Surveys - How to Derive the Perfect Draft for Hydrostatic Tables

To perform a Draft Survey, it’s essential first to determine the correct draft to enter into the Hydrostatic Tables. But what exactly is this draft, and how do we derive it with precision?

Imagine we are pioneers in conducting a Draft Survey, with no established procedures at hand.

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.

Trim Adjustments and Draft Marks

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.

The sign of each correction is critical. A systematic sketch of the vessel with its trim, including a diagram showing draft mark locations relative to the perpendiculars, is useful here:

Forward Perpendicular: Correction usually involves a minus sign since the forward draft mark reads higher than the actual forward perpendicular draft.
Midship Perpendicular: This correction is also typically negative.
Aft Perpendicular: Here, a positive correction is applied, as the aft draft mark reads lower than the actual draft at the aft perpendicular.

Once these adjustments are made, averaging the corrected drafts from the forward, midship, and aft perpendiculars gives us a mean draft to enter the Hydrostatic Tables. However, there’s an additional consideration—longitudinal deflection.

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.

Future discussions will cover additional corrections, such as First and Second Trim Corrections.

About Author

Artur Sokolov holds a Master’s Degree in Navigation and is a licensed Master Mariner with extensive experience at sea. A prominent figure in maritime education, Artur has authored numerous training programs tailored to the needs of today’s mariners. As a marine educational speaker and coach, he brings a wealth of knowledge and practical insights to his audience, helping to shape the next generation of seafarers. In addition to his teaching, Artur serves as a marine operations consultant, providing expert guidance on complex maritime challenges.

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