Originally Posted by menon.ramesh

Could you suggest me how can i overcome this issue ?

and pls tell me what you mean by international recommendations.

R/m

Dear menon.ramesh,

Please follow https://forum.bulk-online.com/showthread.php?t=17241

There is a link to United Nations website. Document ECE/ENERGY/19 Draft Survey Code.

Try to avoid misunderstanding.

Draft surveyor must do all measurings and calculations twice: before and after loading.

Before loading he calculate Net Displacement

After loading he calculate Net Load Displacement

Weight of cargo is difference between Net Load and Net.

Surveyor must documentally fix all datum: draft readings, ballast soundings etc. The Shipmaster must check and sign all datum and calculations.

This is normal process of draft survey.

Unfortunately your information is insufficient to understand are you losing cargo or not.

If acceptable you may send me a copy of Draft Survey Report for examination.

Best regards,

Anatoly ■

Dear Teus,

On-board software for displacement calculations now installed on each ship. Presence of this software is required by SOLAS convention. However it is necessary to remember that on-board software is originally designated for safety navigation but not for commercial calculations of cargo quantity. In base of all modern on-board software are vessel drawings. Real dimentions of ship are different from drawings.

Plenty of useless formulas and corrections for draft survey are developed. Criterion of theirs using is opinion of surveyor. Sometimes these formulas are comical. For example Lloyd recommends to use two formulas for calculation of mean draft. Surveyor must choice one formula visually subject to shape of hull. Surveyor performs functions of some wizard.

Actual accuracy of any sensors and other gauges is very easy checked with step-by-step ballasting of ship.

Best regards,

Anatoly ■

Dear Teus,

Possibility of manipulations while draft survey exists always.

After developing the international recommendations ECE/ENERGY/19 Draft Survey Code in 1992 the situation was improved. Unfortunately the most essential requirements of Code still ignored.

It was expected that IMO will supervise performance of Code. Unfortunately this was not executed.

Required modifications were realized only on oil fleet. Probably due to absence of necessary technologies. For example it is possible to calibrate oil cargo tanks with 3D laser scanner or theodolite. And It is practically impossible to do the same in ballast tanks of dry bulk ship.

Best regards,

Anatoly ■

Dear menon.ramesh,

My e-mail: info@shipsurvey.ru

Best regards,

Anatoly ■

Dependant on how the ship shape can be related to a rectangular block of the same extreme dimensions, i.e. the Block Coefficient, the following are alternative methods of calculating correction for hog or sag:

1) For finer lined vessels (block coeff. < 0.75):

Mean Adjusted Draft = [(6 x DM) + D FP + D AP] / 8

2) For fuller form vessels (block coeff. > 0.75):

Mean Adjusted Draft = [(4 x DM) + D FP + D AP] / 6

However, most surveyors use the first formula, which is based on Simpson's Second Rule, regardless of whether the vessel is a fuller form or not. My questions are the following:

a) Is correct the use the first formula in all cases, regardless the form of the vessel?

b) When applying these formulas, one assumes that the deformed shape of the vessel follows a regular mathematical curve (parabola). What happens when the bending moment diagram indicates multiple deflections or the deformed shape is different than a parabola. Regardless this issue, most surveyors use the formula, anyway. Is that correct?

Thank you! ■

Dear captain,

Plenty of useless formulas and corrections were developed for draft survey till 1992.

Then by request of the United Nations the international group of experts has developed the Draft Survey Code.

a) Using of the formula Mean Adjusted Draft = [(4 x DM) + D FP + D AP] / 6 is not provided by the Code. So qualified surveyors don't use this formula because it is not present in a Code.

According to repeatedly effected theoretical and practical researches the Block Coefficient criterion 0,75 is doubtful.

b) Presence on a vessel of special documents is required for the correction of non-parabolic deformations. Such documents are very seldom. Surveyors are compelled to use this primitive formula because absence of ship documents.

According to modern researches each vessel should be supplied by personal formulas to ensure accuracy of draft survey 0,5 %

Now real accuracy of draft survey is much worse than 0,5 %. It is very easy to prove it on any vessel.

You have mentioned very important question. Many experts until now do not see a difference between calculations connected with commercial account of cargo (draft survey) and calculations connected with safety of navigation (stability etc.). It is already recognized clearness on tankers but misunderstanding on bulkers. ■

Dear Draftmaster,

A commonly adopted method of marine surveyors for assessment of the effects of small global hull longitudinal deflection on the ship displacement is to perform standard hydrostatic calculations supposing parabolic deflection shape with an equivalent value of draft amid-ships, denoted as "quarter mean draft" or “mean of means draft”, defined as:

QM or Mean Adjusted Draft = [(6 x DM) + D FP + D AP] / 8

The above formula is obtained by applying the 2nd. Simpson's rule, using four ordinates along the length of the ship. However, if instead of using only four ordinates, we use more, for example ten, always considering a parabolic shaped flexion, then the result should be more accurate.

This is possible using the formula of the parabola instead of 2nd. Simpson's rule:

y = a.x2 + b.x + c

Formerly, when calculations were done manually, it was very annoying the use complicated formulas. Nowadays, however, the work is done by computers, so it's easy to make calculations using an Excel spreadsheet or maybe an even more sophisticated software.

Finally, the formula applicable to any number of stations along the length of the ship is as follows:

y = ((2DFP+2DAP-4DM)/LBP2).x2 + ((4DM-3DAP-DFP)/LBP).x + DAP

where:

x = segment or distance from aft perpendicular at each station;

x2 = x squared

y = draft corresponding to x;

DFP = draft observed at forward perpendicular;

DAP = draft observed at aft perpendicular;

DM = draft observed at midship;

LBP = lenght between perpendiculars;

LBP2 = LBP squared.

Worked example (using an Excel spreadsheet):

LBP 148.00 m

DFP 3.193 m

DM 4.770 m

DAP 6.295 m

We will take 10 stations along the length between perpendiulares, separated by 1 / 10 LBP.

STATION X

0 DAP (LBP*0) 0.00

1 (LBP*0,1) 14.80

2 (LBP*0,2) 29.60

3 (LBP*0.3) 44.40

4 (LBP*0,4) 59.20

5 DM (LBP*0.5) 74.00

6 (LBP*0.6) 88.80

7 (LBP*07) 103.60

8 (LBP*0.8) 118.40

9 (LBP*0.9) 133.20

10 DFP (LBP) 148.00

Draft calculations at each station:

Station Draft

DAP 6.295

D1 5.994

D2 5.691

D3 5,386

D4 5.079

DM 4.770

D6 4.459

D7 4.145

D8 3.830

D9 3.513

DFP 3.193

Draft average 4.760

My proposal is to use this formula instead of "quarter mean" or "mean of means", using at least 10 stations along the lenght, considering a parabolic shaped flexion. What do you think about?

Thank you! ■

Dear captain,

Your formula is more accurate indeed. However it is suitable not for all vessels.

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