DRAFT SURVEY vs. INCLINING EXPERIMENT

Here is a good question from one of my Bunker and Draft Survey Course students:

Draft Survey is cheaper to do than an Inclining Experiment. Can we use a Draft Survey to determine a change in Light Ship Displacement over time?

Sadly, the most Maritime Academies do not include Draft Surveying in their curriculums because it is not included on the License Exams.

I say the Answer is YES! Overtime you can determine over time with a Draft Survey changes in the Light Ship Displacement and its Constant. You can even calculate the weight of marine growth on the bottom.

 A draft survey will do many things:

1. Determine the vessel’s total Displacement at each observation which can be resolved to find the vessel’s Light Ship Weight and Constant or Crew, Stores and Effects.

2. Determine the cargo weight loaded or discharged taking the difference between two draft Surveys.

3. Determine the consumable liquids on board: FO, DO, FW, Ballast Water.

4. Total Displacement - FO - DO - FW - Ballast Water - Cargo - Ship's Constant = Light Ship Weight.

 5. By observing the vessel's drafts and confirming its Displacement and LCG, you are confirming your vessel's Stability Calculation!

 You cannot do an Inclining Experiment without a Draft Survey because you need a very accurate Displacement. A Draft Survey can yield a vessel’s Displacement within 1/2%.

 The Inclining Experiment Formula is Tan (angle of heel) = w d/ GM Displacement

Solving for GM = w d tan(angle of heel)/Displacement

where: d = distance inclining weight moves; and w = inclining weight

KG at the time of the Inclining Experiment = KM – GM

where: Free Surface Effects should be minimized or accounted for and KM is a function of draft observed. Keeping in mind the true draft is located at the LCF. The Light Ship KG is finally determined by removing all know extraneous weight by use of the weighted average method vertically.

There is one more thing that can easily be done during an Inclining Experiment knowing the vessel’s accurate Displacement. The Light Ship LCG can also be obtained by working Trim backwards during inclining experiment.

 Trim in inches x MT1 = Trim Moment

Trim Moment divided by Displacement = Trim Arm which is distance between LCB and LCG

Trim Arm - LCB = LCG at the time of the Inclining Experiment. The Light Ship LCG is finally determined by removing all know extraneous weight by use of the weighted average method longitudinally.

If you find this interesting you will like my remote Bunker and Draft Survey Course.

My Bunker and Draft Survey Course is done on your schedule via Podcasts and emails with attachments. I do expect to review and comment on you work as well as answer your questions on the subject.

Normally, my course can be completed in about one lesson per week at an affordable cost savings compared to remote courses. Your enrollment will be good for one year. 

What does this remote training cover?

The first five lessons are with the UN ECE Draft Survey Code:

Introduction to the Draft Survey Code

Surveyor's Qualifications

Vessel Documents

Draft Survey Instruments

Draft Survey Procedures

 My Workbook Lessons:

Correcting Apparent Drafts to True Drafts

Calculating M/M/M

1st Correction to Displacement for Trim

2nd Correction to Displacement for Trim

Correcting Displacement for Harbor Water Density

Ballast and Freshwater Tanks

Oil Tanks

 Complete Draft Survey Problems:

Start and Finish Loading Survey

Start and Finish Discharging Survey

Deadweight Survey

Bunker Survey

Barge Draft Surveys without Hydrostatic Data

 My Workbook Lessons are step by step. Each lesson that follows has the answers for the preceding lesson to help keep you moving forward.

If you have any questions, please email me at: [email protected]