When You Need a Stability Test

Certain as the sea, ships will require stability tests at some point in their life. Or maybe not. This article discusses the different types of stability tests and when regulations require them.

Regulatory Requirements

1.0 Introduction

Just like death and taxes, stability tests are certain, but rarely specific.  And the government always seems to have a say in the matter.  Following that idea of taxes, stability tests go better when you plan for them and anticipate government regulations.  So today, I want to highlight the schedule for stability tests, when you need them, and explain your options.

1.1 Regulatory Jurisdiction

Stability tests heavily involve regulatory bodies; they resemble a tax audit more than a scientific experiment.  You need to know the politics, history, and attitudes of each nation’s regulator to run a smooth stability test.  For this article, I limit my experience solely to working with the United States Coast Guard (USCG).

2.0 Two Important Ideas

Before we talk about regulations, there are two important ideas that underpin much of stability test requirements.  Once you understand the logic behind this, all the other regulations seem more sensible.

2.1 Aggregate Weight Change / Risk

The word aggregate weight change appears in many regulations surrounding stability tests.  When you hear aggregate weight change, the importance is not the physical change in light ship weight.  We use aggregate weight change as a method to keep score of the risk behind the light ship weight.  (Which is why I italicize it in this article, to remind you that this represents a risk, not a physical weight.)

Every time you added or removed a permanent item to the ship, there was always a risk that we don’t really know the weight of that item.  You may know the shipping weight, but how much did the welding add?  What about the paint job?  Each change to the ship comes with a little uncertainty.  Over time, those tiny variabilities add up to the point where the total effect could potentially shift your light ship by a large amount.  This is the risk that we focus on when we calculate aggregate weight change.

So how to calculate aggregate weight change?  In normal weight calculations, we count additions and subtractions to the vessel.  But remember that aggregate weight change is a risk, not a weight.  We can only increase the risk, not decrease it.  Follow these simple rules to calculate aggregate weight change:

• For new weights added to the vessel, add them as a positive aggregate weight change.
• For existing weights removed from the vessel, also add them as a positive aggregate weight change.
• Existing weights relocated to a new location on the vessel need to get counted twice, because there are two opportunities for uncertainty.
  • First, count the weight as removed from its existing location: add this as a positive aggregate weight change.
  • Second, count the weight as added to its new location: add this as a positive aggregate weight change.

There is one way to reduce the risk.  Your aggregate weight change resets to zero after every stability test or lightweight survey.  These tests remove the uncertainty because we physically measure the vessel weight.  That is why regulations periodically require stability tests.  Because ships do change over time.

2.2 Ships ALWAYS Change Weight

Ships ALWAYS change their weight over the lifespan of the vessel.  The ship always has maintenance, equipment changes, paint touchups, etc.  These little changes alter the weight of the vessel and shift the center of gravity slightly.  It happens to be bad luck that most changes increase the vertical center of gravity, which reduces the ship stability.  Over many years, a ship can age from healthy and stable into a wreck ready to capsize.

It sounds odd to say that small changes lead to trouble. But consider a few examples.  First, imagine that you own a vessel for 10 years, and the Master keeps detailed records of any weight changes.  So it should be impossible to have any unknown weight changes.  Ok, but who owned the vessel before you?  Did they keep detailed records?

Another classic example is paint.  From an operations perspective, you may discount paint as a weightless object, something applied in small patches to maintain the hull and prevent corrosion.  But look at a shipyard crew as they paint the entire hull of a ship. (Figure 2‑1)  Imagine the pallets full of paint feeding the paint sprayers.  Can you lift all that weight by hand?  Paint adds several tonnes worth of weight.  Some ship owners add that weight to their vessel every year!

As you can see, the small changes are deceptive, and nearly impossible to pinpoint.  This is why regulations simply assume that a ship will ALWAYS increase in weight over time.

3.0 Different Types of Tests

Given these assumptions, regulations look for ways to reduce the risk and ensure vessels remain stable throughout their working life.  Thankfully, we get a little leniency in the area of required tests.  There are three different types of tests associated with stability, each with their own limitations.

3.1 Lightweight Survey

The lightweight survey consists of a deadweight survey, plus a freeboard reading.  Naval architects take an audit of the vessel to identify everything that is not part of light ship.  Then they carefully measure the current freeboard of the vessel.  The freeboards report the current weight of the vessel, on the day of the test.  Subtract off everything that doesn’t belong, and you have the light ship weight.

Table 3‑1 summarizes the output from a lightweight survey.  This test leaves out one critical element, the vertical center of gravity (VCG).  The main appeal to this test is cost savings.  Without the VCG, we don’t need an incline experiment.  No requirements for expensive crane rentals, or incline weights, etc.  Lightweight surveys often present a low budget solution.

3.2 Stability Test

Most ships will require a full stability test at least once in their lifetime.  This includes everything in the lightweight survey, plus an incline experiment.  With the incline experiment, we use weights to generate a known heeling moment and measure the resulting vessel heel.  Based on the response, we can calculate the vertical center of gravity (VCG). 

Table 3‑2 summarizes the results available from a stability test, which is basically everything.  Stability tests provide all the information the naval architect requires for the light ship.  The trade-off is cost.  Stability tests are large complicated affairs, requiring coordination of several parties, including shipyards, crane operators, and ship personnel. 

And directly following the stability test, expect an update to the stability analysis for your vessel, with an updated trim and stability booklet. 

3.3 Simplified Stability Test

The simplified stability test only applies to certain smaller vessels.  The previous two tests assumed you had extensive documentation and drawings for the vessel.  Those drawings are necessary to complete the test.  But smaller vessels (less than 65 ft) may not have that documentation. 

Instead of determining the light ship, the simplified stability test takes the criteria of the stability analysis and tests them directly on the vessel.  We calculate anticipated heeling moments for the vessel, from two major sources:

• Passengers crowding on one side
• Wind heeling

Taking the larger of the two, we reproduce that heeling moment directly on the vessel, using sandbags, buckets of water, or other convenient weights.  The vessel passes if it does not heel beyond certain limits.

Table 3‑3 highlights the limitations of a simplified stability test.  It works very well when you do not have documentation for the ship.  But if you plan to alter the vessel, this test provides none of the necessary information to predict the new light ship after your alterations.  Discuss the trade-off of a simplified stability test with your naval architect.

4.0 Weight Criteria Drive Stability Tests

Several times, I indicated that regulations stipulate when you need a stability test.  So what is the schedule?  Thankfully, USCG does not punish you with unnecessary tests.  Rather than a prescriptive schedule, test requirements were driven by changes in the vessel weight.  There are two criteria that can require a test. [2]

4.1 2% Aggregate Weight Change

Remember in Section 2.1 when I explained aggregate weight change.  If the aggregate weight change does not exceed 2% of the current light ship weight, then no tests are required.  Once you go over the 2% limit, one of two tests will be required:

1. Aggregate weight change in the range 2%-10%: only need a lightweight survey
2. Aggregate weight change over 10%: full stability test required

Most owners are tempted to go with a lightweight survey and save some money.  But a second requirement can trip you up.  If you perform a lightweight survey, and the results of that survey show that the actual light ship weight changed by more than 1% from the old light ship weight, then USCG will require a full stability test. [2]  Notice the difference.  The previous limits compared aggregate weight change, but results from the lightweight survey compare actual weight change.

The risk behind a lightweight survey is paying for the added cost of a full stability test.  You can’t use the results from the lightweight survey and tack on an incline experiment some other day.  The lightweight survey and the stability test need to be two separate procedures, each with their own approval and review.  Lightweight surveys were only intended for cases when the vessel showed no real changes and you just needed to confirm that.  If you suspect your vessel changed, and still decide to go with a deadweight survey, understand that you risk paying for a lightweight survey and a stability test.

4.2 1% LCG Change

In addition to weight changes, the regulations also worry about the LCG changing.  When reviewing the results of the lightweight survey, the new LCG needs to be within 1% of the old LCG.  Put a little more formally:  calculate the change in LCG and divide by the length between perpendiculars (LBP).  This change should be less than 1%.  If your LCG shifted by more than 1%, you need a full stability test.

4.3 The Dummy Clause

Occasionally, certain vessels are ridiculously simple, and we can demonstrate safe stability, plus a huge margin for error.  In these cases, exact location of the weight and center of gravity becomes less critical.  And the regulators may not require any form of test.  I call this the dummy clause. 

With a larger margin for error, USCG becomes more lax in their requirements for stability tests.  Instead, you may be able to satisfy them with simple weight / moment calculations.  No testing required.  Just some engineering calculations.  One classic example are barges, where stability is almost always guaranteed by virtue of the hull dimensions.  But, the USCG always has final say and can still require a stability test at any time during the review process.

5.0 Passenger Vessels – Special Rules

5.1 International Passenger Vessels

Some vessels are required to conduct a lightweight survey every five years, regardless of recorded weight changes. [3]  For this regulation to apply, your ship must meet the following criteria:

• Commercial vessel, carrying passengers for hire.
• Travel in international waters
• Be subject to IMO rules (specifically, SOLAS)

Generally, cruise ships are the only passenger ships that travel in international waters.  If you fall under the IMO, budget to conduct a lightweight survey every five years.  And if that survey shows excessive changes to the light ship weight, expect a stability test immediately after.

5.2 Subchapter T Vessels

Subchapter T of the federal regulations applies to small vessels, with a complex definition of what constitutes “small”.  But generally, any vessel less than 100 gross tons.  In this case, if the vessel is also less than 65 feet in length, USCG will probably allow the vessel to go with a simplified stability test, rather than the full complex stability test. 

The key benefit of a simplified stability test is the documentation.  Full stability tests require a detailed lines plan, general arrangement, and several other ship drawings.  Very frequently, these drawings never exist for smaller ships.  A simplified stability test avoids the expense of generating more engineering documentation just to support the test.

6.0 Conclusion

Certain as the sea, ships will require stability tests at some point in their life.  Or maybe not.  As we discovered, the regulations recognize that a stability test is a large expense.  They employ performance based criteria to only require tests when absolutely necessary.  Even then, you typically have options to decide on the type of test you want.  The rules surrounding stability tests try to balance safety against cost, aiming for a happy medium.

7.0 References

Relevant Ship Science Articles