What is a Stability Test

Stability tests do not arrive instantly after you order one. It takes time and planning, and you should expect extensive coordination with several different organizations. But the ultimate benefit justifies the expense: a ship with reliable stability performance. This article unravels some of the mystery behind a stability test and why...

Why You Deserve One

1.0 Introduction

Black clouds threaten the sky and waves roll across your bow.  Apprehension grows in your gut, and that stability book doesn’t seem very reassuring anymore.  The one that specified safe operating limits.  After all, just how precise is that stability booklet?  What is the margin for error?  What makes that book so smart?

As a naval architect, I worry far less about the stability book.  I rely on the extreme precision and reliability that forms the foundation of any stability booklet.  It all starts with a stability test.  Today I explain what we achieve in a stability test, and why you deserve one.

2.0 Goal of a Stability Test

Before going to sea, the Master of the ship checked the stability book against their current loadout.  (This presents as many different levels of complexity depending on the size of the ship.)  Imagine that you review that loadout, checking each of the weight items on-board.  When examining the weights, we naturally focus on the big items.  They have the most impact.  And what is the largest weight item in any load out?  The light ship:  the weight of the vessel itself, freshly minted from the shipyard.  The properties of the light ship can make or break vessel stability.

But how do you determine the light ship?  The first thought would be to track the weight of every item that goes on and off the ship during construction.  That simply isn’t practical.  Imagine the flurry of activity in a shipyard, with dozens of workers on the vessel. 

Instead, we wait until construction finishes and conduct a stability test.  Using the data from the stability test, naval architects calculate the light ship.  The light ship becomes the most important weight item in any stability calculation.

With something so important, accuracy becomes paramount.  We need to know the exact weight and center of gravity for the light ship.  Vertical center of gravity (VCG) is the critical number.  Shifting the VCG even 150 mm (6 in) can drastically change the vessel stability.  For the stability test to have any value, we need the accuracy and discipline of a finely controlled scientific experiment.  But ships are not the best place for scientific accuracy. 

2.1 Challenge of a Stability Test

The difficult part behind stability tests lies in the extreme accuracy and precision required when extrapolating a small measurement range to a much larger impact.  Imagine I allowed you to weigh a single pin, and then asked you to accurately calculate the weight for an entire crate full of pins.  In theory, simple enough.  Just multiply the weight of one pin by the number of pins in the crate.  But anyone with practical sense will probably ask, “What if the pins have slightly different weights?” 

Stability tests face many of the same challenges.  The theory behind a stability test requires us to measure heel angles as we intentionally apply a heeling moment.  But we only heel the ship 1 – 3 deg typically.  With such small angles, accurate measurements become paramount.  That challenge of accuracy is the driving force behind many of the procedures in a stability test.

3.0 General Test Process

What is the first step to a stability test?  It requires extensive planning over several weeks.  Other articles may focus on the day of the test.  But performance on test day reflects the preparation in the weeks leading up to the test.  To understand the test process, we need to include all the preparation.  The process follows these general steps:

1. Hire a naval architect to perform the test. They also handle much of the coordination.
2. The naval architect creates a test procedure.
   1. The procedure is sent to USCG for approval.
3. Coordinate to obtain all the required resources for the test.
4. Prepare the ship for the stability test.
5. Naval architect arrives and performs deadweight survey
6. Test day:
   1. Must wait for a day with good weather
   2. USCG inspector arrives
   3. Complete the rest of the stability test
7. Naval architect prepares formal test report with results
8. Send test report to USCG for approval
9. Use test results for stability analysis.

The last step often takes people by surprise.  A stability test only tells you the current state of the light ship.  You also need a stability analysis to determine if that state is good or bad for vessel safety.  If you need a stability test, also expect a stability analysis directly following the test.

4.0 Nomenclature

We don’t make things easy with the nomenclature.  Each naval architect uses a slightly different variation on a similar set of words to describe the various steps in a stability test.  So the best place to start is with some vocabulary.

Light ship:                     A vessel in the light ship condition is a vessel complete in all respects, but without any deadweight items or liquids onboard except machinery fluids.

Deadweight survey:          Taking an audit of all items to be added, removed, or relocated on the vessel at the time of the stability test so that observed condition of the vessel can be adjusted to the light ship condition.

Tank survey:                  Taking soundings of every liquid tank onboard the vessel to determine the exact amount of liquid in each tank.

Freeboard readings:         Measuring the freeboard at several points along the vessel length (port and starboard sides) to precisely determine the vessel’s draft and trim at the time of the test.

Lightweight survey:          A combination of a deadweight survey, tank survey, and freeboard readings to determine the weight, longitudinal center of gravity (LCG), and transverse center of gravity (TCG) for the vessel light ship.

Incline experiment:          Moving a series of known weights in the transverse direction and then measuring the resulting change in equilibrium heel angle to determine the vertical center of gravity (VCG).

Stability test:                  A generic term to refer any combination of the above tasks.  Most commonly used to describe a combination of a deadweight survey, tank survey, freeboard readings, and incline experiment to determine the weight, LCG, TCG, and VCG of the vessel in the light ship condition.

5.0 Heritage of a Stability Test

Aside from the science, stability tests also carry a large cultural heritage with them.  In the past, some naval architects were less than honest in reporting the results of stability tests.  They adjusted the test results to make the vessel appear more stable than it actually was.  This resulted in the coast guard approving the vessel as safe for sea . . . until the vessel sank.  That history and heritage still influence modern stability tests.

The modern stability test is a combination of science and tax audit.  At least in the U.S.A., the U.S. Coast Guard sends inspectors to monitor the stability test.  Their sole job is to observe the test and ensure that everything written into the test report matches reality.  The naval architect is still responsible for creating a test that ends with a reliable report.  The Coast Guard ensures no one can fudge the numbers.

5.1 Standard for Stability Tests

Reliability is the other challenge of stability tests.  As I said before, stability tests require extreme accuracy.  Without careful attention to detail, the stability test fails to achieve sufficient accuracy.  Standardization became key for the reliability of stability tests.  The golden standard for anyone involved in stability tests is ASTM F1321-92. [1]  This provides a step by step procedure of what to expect from the stability test.

6.0 Conclusion

Ordering a stability test seems similar to building a custom house.  It takes time, planning, and coordination.  Stability tests also provide the same benefits as a house.  A house protects you from the weather and keeps you safe at night.  Stability tests are just as important.  They form the foundation that goes into one of the major ship safety systems.  When night blackens the sky and bad weather rocks your ship, you want that reliable foundation.

7.0 References