Draft Survey: Barge Overhang Method

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I found this alternative method from draft survey course book, hope it usable. Frankly, I never use this method yet, as we required the barge owners to provide the hydrostatic table for draft survey.

The overhang method use the same principles with the method of calculating the volume of a prism. The weight of the barge will be getting by multiplying the volume with the water density where the barge is afloat. Measure the dimension of the barge, before and after the loading/discharging is required for the length and width of the barge, barge freeboard, overhang aft and fore.

The method is applied when :
– The barge is of cut-up type.
– There is no table or data.
– The concerned parties agree on the use of overhang method for weight determination of barges.


Procedures :
1. Calculate: the length (L) and width (W) of the barge with inelastic measuring tape.
For example : during loading, measure the barge’s L and W before the barge goes down to the water line.
2. Look at the ABC and OPQ triangles.
3. Calculate BC and PQ freeboard.
4. Determine point B and P; measure AB and OP.
5. CQ (the part of barge immersed in the water), say LOA1, can be calculated as follow = (AO – AB – OP).
After loading, the barge’s part goes down to water line W1L1.
6. Using the same method, measure the freeboard DE and RS.
7. Measure DR = ES (LOA2) the same way as we measure CQ (no. 5).

Now we have :
– Lb (barge’s width),
– BC – PQ (the height of prism, CESQ),
– CQ (barge’s part which goes down under the water before loading)
– ES (barge’s part which goes down under the water after loading)

From those four dimensions, calculate the volume of the barge which goes down under the water, before and after loading, as follows :
Volume = (CQ + SE) x ½ (BC – PQ) x Lb.

The Volume is multiplied by the water density to get the quantity of cargo loaded.

If there is trim, use the mean figures of the freeboards, parts of the barges that go down under the water (left/right) before and after loading, and water density figures before and after loading.

Use the same method for calculating the discharged cargo quantity. Drop you comments regarding the post.


Author: Faisal Yusuf

Hi, my name is Faisal Yusuf, but you can call me Surveyor. I live in Jakarta, Indonesia and work as a surveyor.

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