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
