The most common draft survey errors are reading draughts on one side only or in a swell, taking density samples at the wrong depth, missing or misreading tank soundings, accepting the constant without checking it and applying correction signs wrongly. Most of them can be reduced substantially through good preparation and by carrying out the survey jointly with the ship's officers.
Which mistakes occur when reading draughts?
The most basic error is reading only the quay side. If the vessel has a list, port and starboard draughts differ, and an average taken without the offshore marks does not reflect the true immersion. Skipping the midship marks means deflection is not accounted for at all.
In a swell, a single snapshot reading is misleading. The waterline should be observed over several oscillations and the mean of the highs and lows recorded. Where marks are obscured by rust, paint or marine growth, keeping the eye as close to the water as possible reduces parallax error.
What goes wrong with density measurement?
Taking samples only from the surface is a frequent mistake. Near river mouths or after heavy rain, a lighter layer of fresh water may sit on top, and a surface sample will not represent the water the vessel is actually floating in.
A dirty hydrometer, air bubbles in the sampling can or reading the wrong part of the meniscus all introduce errors. It is also worth checking that the type and calibration of the hydrometer match the density basis of the hydrostatic tables.
- Sample at around half the vessel's draught
- Take samples at more than one point along the hull
- Rinse the hydrometer and sampling can before each use
- Read at eye level with the sample at rest
Which tank sounding errors affect the result?
Slack ballast tanks make soundings highly sensitive to trim and list, so tanks should ideally be pressed up or completely empty. Tanks reported full should be verified as genuinely pressed up, and tanks reported empty checked for unpumpable residues.
Blocked sounding pipes, reading the tape against the wrong reference point and failing to apply trim corrections to tank tables are all common. Ballast pumps left running during the survey mean the readings no longer match the tank condition.
Why can the constant cause trouble?
The constant represents unrecorded weights on board, such as sediment in tanks, stores and spares not listed elsewhere, and accumulated mud. If the constant found at the initial survey is accepted without comparing it with values from previous voyages, survey errors can pass unnoticed into the cargo figure.
An unexpectedly large or negative constant usually points to an overlooked quantity in the tanks, a wrong density or a reading error. In that case it is sensible to repeat the readings or re-sound the doubtful tanks before cargo operations start.
Which errors happen at the calculation stage?
Typical calculation errors include reversing the sign of the first trim correction, omitting perpendicular corrections or applying them twice, and interpolating the tables in the wrong draught range. Some vessels carry more than one set of tables, so confirm that the correct one is in use.
Inconsistent deductibles also distort the result. Including sewage or bilge tanks at one survey but not the other creates an artificial difference that ends up in the cargo quantity.
Which errors come from documents and tables?
A light ship weight that has not been updated after a conversion or a significant change in equipment is a common reason for an unexpected constant. Similarly, tank tables prepared for a different trim, or tanks missing from the tables altogether, will find their way into the result.
On older vessels, tables may use a different system of units. Conversions between draught marks in feet and inches and metric tables can create errors that are hard to spot unless they are checked carefully. Before the survey, it is worth confirming that the tables actually reflect the vessel's current condition and that the version on board is the approved one.
How can these errors be reduced on site?
Carrying out the survey jointly with the ship's officers, cross-checking readings and tank figures on the spot and re-checking doubtful values before leaving the vessel are the most effective safeguards. Recording weather and sea conditions in the report gives context when the results are reviewed later.
Karan Gözetim provides draft survey services from its base in Gemlik; survey conditions and scope are agreed with the parties at quotation stage.
Frequently asked questions
Are draught readings taken at night reliable?
They can be, provided there is adequate lighting. Where the marks cannot be seen clearly, reliability drops and the condition should be noted in the report.
Is it a problem if different surveyors do the initial and final surveys?
Not in principle, as long as the method is the same. Make sure both surveys use the same tables, the same list of deductibles and the same calculation approach.
Can a report be corrected if an error is found later?
A calculation error can be corrected and the revised figures shared with the parties. Readings and soundings, however, cannot be repeated once the vessel has sailed, which is why checks on site matter most.
Topics
- draft survey errors
- draught reading errors
- draft survey discrepancy causes
- hydrometer reading error
- ship constant problems
- tank sounding errors