Limiting factors during DRAFT SURVEY | blog

luka turmanidze

A draught survey starts with a reading of the ship’s draught, on both sides; forward, amidships and aft. There are a number of limiting factors:

<> It is often difficult to accurately read the draught because of prevailing weather conditions and the presence of waves on the water surface. A vessel may have also developed a slight roll, leading to further inaccuracies 

<> the draught should be read from a position as close to the waterline as possible to avoid parallax, although this may not always be practicable 

<> a ship moored in a tidal stream or current will be affected by squat, particularly in shallow water, and this will have a further effect 

<> a draught can be affected when there is a large difference between the temperatures of the air and the water. This will cause a difference in the expansion of the submerged and emerged sections of the ship. There is currently no acceptable method of correcting for this 

<> when a ship is not on an even keel (as is always the case before loading and after discharge), the draught readings must be corrected for trim. It should be borne in mind that, at such times, the draught marks are not in line with the forward and after perpendiculars 

<> the draught must be corrected for the density of the water in which the vessel is floating. It is difficult to obtain a reliable average density because this will vary at different levels and locations around the ship 

<> the draught has to be corrected for hog and sag. This correction is generally calculated on the basis that a ship will bend parabolically, although this is not always the case.

source: UK P & I Club, Bulk carriers safety precautions

The Blog post is edited by luka turmanidze Mar 24