HYDROSTATIC DATA & ASSUMED TRIM

Dealing with Hydrostatic Data for Different Assumed Trimmed Conditions

In the days before all the computing power that Naval Architects now have the construction of Hydrostatic Data, the assumed and understood or unmentioned trimmed condition was zero trim.

Back then, and you can still do it today, the basic Hydrostatic Curves were drawn and from those curves data was tabulated by simply scaling it off the Curves. This information was so standard that no one seemed to ever mention that the vessel’s trim was assumed to always be on an “even keel” or “zero trim” condition.

To load a ship with maximum cargo down to its marks you would like to also have the minimum draft, so an “even keel” or “zero trim” condition is desired due to restricted harbor drafts. If you did understand that simple assumed condition and you knew how to calculate trim, it should be obvious the LCB, “Longitudinal Center of Buoyancy” listed for a zero trim condition was also the required LCG, “Longitudinal Center of Gravity” for zero trim with no other calculations needed. Sadly, most Mariners did not get this detail because this was not stressed in their education. I do stress it in my book, STABILITY AND TRIM FOR THE SHIP'S OFFICER. 4th Edition, at the end of the first chapter.

With today’s computer programs Naval Architects and Ship Builders and generate numerous Hydrostatic Tables for multiple assumed trimmed conditions. They could also generate more Hydrostatic Tables for Assumed List or Heel angles as well as Trim. Fortunately, the Loading Manuals that now can be more than 500 pages still present Hydrostatic Data with and assumed List or Heel Condition of zero or upright.

In taking a close look at one Loading Manual the traditional Hydrostatic Table for a traditional assumed condition of zero trim and list covers 19 pages. The same Loading Manual also had Hydrostatic Tables for an assumed condition of zero list and trimmed conditions of 2 M by bow, 1 M by bow, zero trim, 1 M by stern, 2 M by stern and 3 M by stern which cover 153 pages.

I have also inspected different Loading Manuals and there seems to be no standard method of presenting this additional Hydrostatic Data. For example, I believe the result will be confusion and errors because you really need the time to study each ship’s Loading Manual format to extract the information that you would need to do a proper draft survey. I do not consider doing a Draft Survey by just attending the vessel, observing the drafts sounding the tanks and walking off with the Chief Officer’s Computer Printout of the Ship’s Draft Survey. While the Draft Surveyor can use their own computer, it is their job to extract the correct data on their own.

The following is a couple of examples of how Hydrostatic Data with assumed trimmed condition can be used.

In this first example, I used the same LCG and the different assumed Hydrostatic Data for 5 different assumed trim condition listed in the Loading Manual’s tables. Please note the Displacement, LCB, and MTC are different for each assumed condition, however the answers for True Trim vary no more than 9 centimeters. My conclusion, you can get the correct answer even if you do not know which assumed trim table you used for all practical purposes. You still need to know how to do a trim calculation.

In my second example we want to calculate the Displacement given the True Trim, Zero Trim Hydrostatic Data and a water density of 1.025 in the traditional Draft Survey manner as described in the UN ECE DRAFT SURVEY CODE and compare the results to simply interpolating the Displacement from the Assumed Trimmed Hydrostatic Data. (Depending on how the data falls it may be necessary to do a double interpolation instead of a single interpolation.) My conclusion is the results are within less than 3 Metric Tons. While this method is not recognized by the UN ECE Draft Survey Code, it is a good way to verify the first method.?

Final thought if the Surveyor or Ship's Officer enters a Hydrostatic Table not realizing what they are working with (ie. an assumed trim other than zero) calculated the error could be that of the combined 1st and 2nd Trim corrections to Displacement. Hopefully a comparison of the ship's constant will make this error stand out.

DO YOU LOVE TO WORK ON SHIPS, BUT YOU NOW NEED TO LIVE AT HOME?

My?Bunker and Draft Survey Course?is a correspondence course done via email. Sadly, the Maritime Academies do not include Draft and Bunker Surveying in their curriculums because it is not included on the License Exams.

Did you know that a Draft Survey properly done can yield the Total Displacement within 0.5% of the true cargo figure or Displacement? Just for practice you can determine your vessel's total cargo weight by reading the drafts instead of the cargo manifest which may or may not be correct. This method works on all vessels as long as they are afloat. You can also use it to independently confirm a vessel's Stability Calculations.

Why not consider becoming a Cargo Surveyor. Most of us would like to take our knowledge and experience from going to sea and use it ashore. Knowledge from this course could help you start a new shoreside career. Years ago, I got a great job and a 38 year career, because I knew about SHIPS, STABILITY & TRIM and DRAFT SURVEYING calculations.

To apply for enrollment to become one of my private Remote Students please respond to me directly by email me at:

WilliamEGeorge@gmail.com

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Looking for a copy of my book, STABILITY AND TRIM FOR THE SHIP'S OFFICER, 4th Edition? You can find it on AMAZON.COM there are new and used copies available.

https://www.amazon.com/dp/0870335642/ref=tsm_1_fb_lk

If you like my book, you will like my Podcasts about the book.

STABILITY AND TRIM FOR THE SHIP'S OFFICER, 4th Edition

https://podcasts.apple.com/us/podcast/stability-trim-for-the-ships-officer/id1493267414

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