Vessel constant can be calculated when vessel is unloaded by using the following method:
Vessel displacement - deductables (ballast, freshwater & fuel) - light ship = constant.
Hi to all,
would like to ask if theres really a difference conducting draft survey in deep water level areas than surveying in low level areas closer to ground.Is there such implication to this matter in terms of draft survey accuracy?
I cant imagine im getting short of invty. when the draft survey calculation resulting a certain 5000 mt. but when the material was disposed im just getting 4800 mt. out of it. I was short of 200 mt.
Pls. reply thanks....
Effect of suction to ground in low level area really exists at presence of heavy current.
This can be reason of mistake in draft readings about 2 cm. Therefore mistake in definition of quantity of cargo will be about 2 TPC.
Vessel 5000 DWT has TPC about 20 MT. Mistake may be up to 40 MT but never 200 MT.
Mistake of 200 MT may be due to vessel hog/sag.
Dear allanis77DM, Draftmaster,
Indeed, draft readings are influenced by strong currents.
However, this effect is also related to the water depth in combination with the ship’s draft (keel clearance) and is called squat.
A draft survey consists of 2 readings, 1 loaded and 1 unloaded.
When unloading in shallow water:
The loaded draft reading will result in a higher cargo mass, due to the fact that the ship is deeper in the water, because the current sucks the ship down.
The unloaded draft reading, however, is almost correct, because the keel clearance is then much more.
This would result in a higher unloaded cargo mass and allanis77DM is 200 tons short.
When loading in shallow water, the effect is opposite and when the draft survey at the unloading site is then correct, a calculated shortage will occur.
A 5000 dwt ship has a length of approx. 100m and a width of approx 15m, resulting in a waterline area of 1500 m2
With a loaded waterline coefficient of 0.8, this gives an area of 1200 m2
The resulting TPC is then 1200/100 = 12 tons/cm immersion.
Assuming a current under the keel of 3 knots (3 * 1852 / 3600 = 1.54 m/sec), the following pressure decrease is estimated:
dp = ½ * 1000 * 1.54^2 = 1187 Pa (N/m2)
The assumed bottom area coefficient is approx 0.6, resulting in a bottom area of approx 900 m2.
The pressure at the keel reduces then with 1187 * 900 = 1068300 n # 106.8 tons, compensated by an increased draft.
If the current under the ship is 6 knots (2*3 knots), then the squat causes 1.41*106.8= 150 tons cargo error.
The mentioned discrepancy of 200 tons at the unloading site, due to shallow water and current (if that is the cause), can only be caused by a shallow water (resulting in a very small keel clearance) and high current situation at the loading site.
The keel clearance must be very small to generate enough squat.
The clearance between the ship and the quay wall is also a key parameter that can influence the current underneath the ship.
Have a nice day
Teus and Draft Master,
Thanks for the info. ,this is a great help in understanding draft survey inaccuracies. I APPRECIATE MUCH OF YOUR BIG HELP.
At the moment of loading, the master just notices a certain draft.
If the master is qualified and experienced, he should be aware of the effect of shallow waters and high currents.
However, when he sails from the loading port, he must also notice that the ship’s draft is suddenly less.
Arriving at the loading port, the draft survey is repeated again and when there is no shallow water and high current situation, the out-loaded cargo mass is correct, but not corresponding with the draft survey at the loading site.
Comparing the 2 draft surveys will show where it went wrong.
A squat at very small keel clearance and high velocities of 15 cm (½ ft) does not seem impossible to me and actually, that is what we discussed here.
(I remember an old article that squat measurements on large bulk carriers entering the port of Rotterdam showed a squat up to 1 foot)
The squat effect exists and I tried to quantify that effect.
Whether it is the cause of Allanis’s problem, we do not know.
Dear Teus and Draftmaster
Your comments on the ship squad and underkeel current influence are great
Beside, some point need to be considered - as most of inaccuracy are due to ship condition, i.e:
- Blocking on the sounding pipe: likely a sounding bob parted and left in the sounding pipe that can cause to an error = bob length in cm * TPC of tanks - may be large if tank is DB and large tanks
- Parted to the bob touch plate of sounding pipe, caused to sounding bob passed over touch point and got into tank bottom: big sounding error
- Air void in the TST, specially in case of high trim - while TST overflow
- Sometime, crew delibrately block sounding pipes to cheat shore rep.
In our country, most of inaccuracy to draft survey are due to defective of vessels.