Ships Draft and Tonnage
Overview
• Definitions and measurement of ships draft.
• Introduction and History of tonnage measurement.
• Tonnage terms and measurement.
• Basic displacement calculations.
Ships Draft
- Draft measured at six point on hull Forward, Midships, and aft, both port and starboard.
- Mean draft is the average of the forward and after draft
Hog and Sag
• Ships loading large deadweight cargoes will often hog when in ballast and sag when loaded. This can be calculated by comparing the mean draft to the midships draft.
Hogging
Sagging
Computation
- Displacement calculated by formula :
Dispacement (LT) = Length x Width x Draft x Block Coefficient
35 Cubic Ft / Ton
Determination Of Displacement from draft
- known as a draft or deadweight survey, Used to determine the amount of cargo loaded or discharged.
- Calculated displacement from draft information with corrections for :
1. Salinity
2. Hog or Sag
3. Trim
Hydrostatic Properties
Tonnage Definitions
• Measurement ton: 100 cubic feet of volume
• Gross tonnage: Ships internal volume in measurement tons
• Net tonnage: Internal volume of revenue earning space
• Displacement ton: Weight in long tons or metric tons of water displaced by the vessel.
• Deadweight tonnage: Amount of weight vessel can carry at Summer load marks.
Difference in displacement of vessel at lightship and fully loaded conditions.
Use of Tonnage Measurements
• Port Fees
• Canal Fees. Special Tonnages: Panama/Suez
• Drydocking
• Comparison
• Taxation
• Licensing
