Ship Draft Survey Hull Deflection and Quarter Mean Draft 🌊
When cargo is unevenly distributed on a ship, it causes Hull Deflection, which affects draft readings at the bow, midship, and stern. Even when the ship appears level, this deflection can impact accuracy, making it essential to measure drafts at multiple points for reliable calculations.
Types of Hull Deflection:
Sagging 😮: The midship draft is greater than the average of the bow and stern drafts.
Hogging 😬: The midship draft is less than the average of the bow and stern drafts.
Why Use Quarter Mean Draft? 🤔
The Quarter Mean Draft (QMD) compensates for hull deflection, providing a more accurate draft value for hydrostatic table entries. This method prioritizes the midship draft, which has the greatest influence on displacement.
Formula for Quarter Mean Draft:
Quarter Mean Draft (QMD) = (6 × MID Draft + AFT Draft + FWD Draft) ÷ 8
Example Calculations: 📝
Initial Draft Survey: QMD = (6 × 6.0351 + 7.1191 + 4.9283) ÷ 8 = 6.0323 m
Final Draft Survey: QMD = (6 × 12.6605 + 13.1412 + 12.1484) ÷ 8 = 12.6556 m
Sagging vs. Hogging: What’s the Impact? ⚖️
Sagging: The Draft Survey Report may overestimate the ship's displacement.
Hogging: The Draft Survey Report may underestimate the ship's displacement.
💡 Key Insight:If loading begins with hogging and ends with sagging, the report may show more cargo than actual. Conversely, starting with sagging and ending with hogging could underreport cargo weight.
Practical Tips for Mariners 🚢
Prioritize Midship Draft: Accurate measurement of the midship draft is essential, as it has the highest mathematical weight in QMD calculations.
Check Hull Deflection: Use this rule of thumb to determine deflection direction:Sagging:FWD and AFT Mean < MID Draft Corrected (more displacement shown)Hogging:FWD and AFT Mean > MID Draft Corrected (less displacement shown)
Visualize Deflection: Drawing the ship's profile can help you understand whether it's in sagging or hogging.
Leave a Comment! 💬
Does your ship often experience sagging or hogging during loading or unloading? Have you encountered discrepancies in displacement due to deflection? Share your experiences and thoughts in the comments below about Ship Draft Survey Hull Deflection and Quarter Mean Draft! 😊
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