Download Formula Draft Survey
Rikke Reifel
With draft survey we measures the initial weight (displacement) of the ship and it measures the final weight (displacement) of the ship after loading. The difference plus all the weights that were taken out (like ballast) will be the cargo loaded.
This cargo has air in it and thus the density of this cargo is not uniform. Calculating the way we calculate the weights for other liquids will give wrong quantity. Draft survey is the answer in this case too.
So the correction to the forward draft would be 9.95 x 2.10 / 155. This will be equal to 0.135 meters. As the forward perpendiculr is forward of the draft marks and we have stern trim, this correction will be negative.
As per Archimedes principle, a floating ship displaces the amount of water equal to its own weight. And the ship floats at the center of floatation. So the correct displacement is the displacement corresponding to the draft at the center of floatation and not at the mean draft.
Why ? As per archimedes principle the ship displaces water equal to its own weight. If the density of the water is more, ship would displace less volume of water (less draft). And if density of the water is lesser, it would need to displace more volume of water (more draft) to have same weight as of the ship itself.
For draft survey, we need to measure the density of dock water just before or after reading the visual drafts. The density is measured by draft survey hydrometer with a sample of dock water drawn from around the vessel.
The density of dock water changes with depth. So it is important to know at what depth the sample is drawn. Many take the sample from a depth that is half the draft of the vessel. Others prefer to take the sample by running the emergency fire pump.
Lightweight of the ship and constants will remain same and so these does not matter. We would know the arrival and departure displacement from draft survey. All other weights we know by sounding and measurement. The only known is the cargo loaded. It is simple mathematics then to know the cargo quantity.
We can sometimes get negative constants. This would mean that we have done some mistake and we must find it and correct it. It may be that we have wrong drafts, wrong soundings etc. Whatever it is, we must correct it before loading starts.
I just went thru your article on draft survey. Well depicted and explained. Good guide for people actually doing it specially Surveyor appointed to do so, as many a times they do not have perfect knowledge and may get carried away ...thanks for sharing
Dear Capt. Rajeev Jassal! Thank You for this post! I just want to ask one more question. What to do, if we have thickness of keel plate in our calculations. For example, thickness of keel plate is 22mm. Base line is line above keel plate. Should we substact this value when calculating true draft at perprndiculars or when calculating Mean of Means? Or we shoud add this value? Thank You!
Look on www.draftsurvey.org download software and create new ship, there is a nice help where is perfectly explained how is it with keel thickness correction. Generally this correction is used to match two drafts: read drafts from draft marks and hydrostatic table drafts column. Usually draft marks has datum welded on the bottom line of keel plate. So once you calculate MMM draft you have a draft from 'bottom edge' of keel. Now you open your hydros. table booklet and you like to read displacement. There is a column with drafts from e.g. 2.50 to 15.00 m, these drafts could be from bottom or top edge of keel plate. This information is always included in hydrost. table. (here in Mr. Rajeev article hydros. table fragment has drafts from top of keel plate - see picture). In case that read draft is from bottom of keel plate and drafts in HT column from top of keel plate you have to subtract the keel thickness from MMM draft. In case that you will have hydros. table from bottom of keel plate you are omitting this correction.
Sir, my question is: is the formula for getting apparent trim is constant: trim = aft draft - forward draft , Is that formula constant ??? Or no constant formula just apparent trim is derived from always subtracting the smaller draft into the bigger draft.... for example: aft draft = 7 m fwd draft= 5 so trim= aft - fwd = 2m or aft draft = 5 fwd draft= 7 m so trim= fwd - aft = 2m i just want to clarify this, sorry for stupid question .. thanks.
First trim correction: TRUE TRIM always minus to stern, everything running left with minus, everything running right is plus. E.g. LCF is longitudinal center of flotation, it mean distance between AP (aft perpendicular) and center of flotation (f), the value is always positive because running right from AP. The distance from midships to flotation (Xf) = (LBP / 2) - LCF, in this case when running left from midships (LCF < LBP/2) it will be minus, when right it will be plus (LCF > LBP/2). The first trim correction will be positive when, Xf is positive and Trim is positive or Xf is negative and Trim is negative, otherwise the first trim correction is positive. So remember left and right hand rule and you cannot make mistake. Same in Astronavigation what we are using is 'right' like (E) as + and on the other side 'left' (W) as minus. Do not implementing this crazy rules bigger draft and trim to bow or stern. Follow the maritime standards.
Nice article, very comprehensive. I have small remark, your logic around first trim correction is little off (results are ok, no mistakes), but the logic is off. Exactly this approach to first trim correction is making mess in students heads. Nobody properly follow Cartesian coordinate system and this is the main problem in determination of the positive and negative value of trim, LCF, Xf and finally first trim correction. If your are looking for best software, try www.draftsurvey.org
I just wonder why so many site talk about draft survey but few or almost none mention about Heeling correction and deflect ( hogging and sagging) since we accept condition of ship for draft survey should not has list more than 0.5 degree. In real situation it's can easily found 1-2 degree listing sometime.
quatermeans does not take care off hull deflection, Quatermean is only calculated to find the draft at the point of the Xf. heel correction should not be neglected but is subject to discussion, specially then a claim or LOP is imminent it should be calculated as wel as an exact deflection calculation.
Question: What will be the draft when the barge is empty? Barge details are 8000DWT(YXH- 2000), Length 122.25M, Breadth MLD 20M, Depth MLD 7.2M, Design Draft 4.8M, Scantling Draft 5.2M, Dead weight 8000DWT, Gross tonnage 6000T.
Capt. I am an avid fan of you, but for this topic I just want to know how do you come up with the figures for interpolation of LCF and TPC because it doesn't coincide with my computation, e.g. the given figures for LCF of 7.45 and 7.5 drafts were 1.0465 and 0.9555 respectively. My computation goes like this, I would subtract 1.0465 and 0.9555 so I would have 0.091 then divide this for 5000 units, therefore giving me 0.0000182 for each unit. I will multiply the later to 675 so I would have 0.012285. Subtract this from 1.0465 so I came up with the answer for interpolation of 7.45675 to 1.034215, which is quite too far from your given answer (0.9851).
The 2nd trim correction must be demonstrated for peaple using draft survey . Draft survey is accepted within only small trim about LBP/100 max . This means the Change in weight from evenkeel to very small trim . Ex : trim 3 m gives 2nd correction 9 times as much compared to trim : 1m in the same condition .
This is a masterpiece and a well articulated writeup. I've read two of your great work on cargo survey- Cargo calculations with ASTM Tables and this one. How I wish you were my classroom teacher. I indeed appreciate you for sharing your knowledge with us. Please do you have more articles on cargo survey for us? Could you let us know the topics of the articles? Once again, thanks a million times.
I would like to know the correct procedure to apply first trim correction....First trim correction can be applied to displacement obtained from quarter mean draft by formula (LBP/2)-LCFXTRIMX100XTPC/LBP OR by applying first trim correction by formula (LBP/2)-LCFXTRIM/LBP to get true mean draft TMD nd therefter get corresponding displacement from True mean draft TMD. Kindly clarify this doubt.
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