Practical Stability Test: Naval Architect's Guide

Every naval architect learns the theory of how to perform a stability test. But a well executed stability test employs very little theory, and a great deal of practical experience. This guide imparts some of that hard earned experience to make your next stability test go well.

1.0 Introduction

The boss just assigned you to perform a stability test.  Awesome!  I think most naval architects aspire to conduct a flawless stability test.  Every naval architect learns the theory in school, and now you get to apply it.  Except, a well executed stability test employs very little theory, and a great deal of practical experience.  This guide imparts some of that hard-earned experience to help your next stability test finish without problems.

2.0 Clear Division of Labor

A large part of your job is to act as the Test Coordinator.  A stability test requires coordination with several companies.  A typical test may include the following parties, all working together:

• Naval Architect (Test Coordinator)
• US Coast Guard
• Vessel owner
• Vessel crew
• Shipyard
• Crane company

With all the people involved, you need to keep clear lines of communication and clear division of labor.  Take the lead here and everyone will be grateful.  Clearly identify which tasks the vessel owner must handle, and which you will handle.  Stability tests hinge on clear communication.

This is actually a great chance to help out your vessel owner.  Stability tests are fairly rare in a vessel’s life; so this may be the first time your vessel owner has worked on a stability test.  If you just tell them to rent some test weights, they will be lost.  Instead, ask which tasks the owner would like to outsource to you.  Take the load off them and help out with coordination. 

3.0 Follow the ASTM to the Letter

ASTM standard F1321-92 [1] gives step by step instructions on how to conduct a stability test.  This is the gold standard.  Get a copy, read it.  Read it again, word by word.  There are several key elements that the ASTM lays out, but a warning:  the standard is very succinct.  Critical information will only get a single sentence.  Some highlights that need emphasis:

1. Total weight from your deadweight survey must be less than 2% of the new light ship weight. (This is not in the standard, only a common practice enforced by USCG inspectors.)
2. The vessel trim must be less than 1% of length between perpendiculars at time of test.
3. The vessel heel must be less than 1/2 deg at time of test.

Use the ASTM to form your test procedure and plan the stability test.

3.1 Anticipate Change

No test procedure survives the first hour with the ship.  Expect to make changes on the fly.  Come to the stability test prepared to adapt your plan.  Every stability test I ever worked on required modification of the test procedure once we actually saw the vessel.  Bring backup equipment, extra drawings, extra marking utensils, etc.  Anticipate change, and you will easily adapt to it. 

4.0 Careful with Tank Configuration

When you plan the stability test, one critical decision will be the tank configuration.  You need to decide which tanks will be full, empty, and partially full.  The ASTM limits the number of partially full tanks that are permitted during the test.  In simple description, the goal is to limit the free surface moment from all tanks.  Ideally, all tanks should be either completely empty, or pressed full.

Easier said than done.  Most working ships still hold some liquid in them during the test.  As the naval architect, your job is to find a combination of tank loads that limits the number of slack tanks, but it should also achieve a fairly level trim.  Pay special attention to fuel tanks.  These are the most difficult, because fuel presents environmental risks if spilled.

Any requirements to press a fuel tank full presents great risks on a stability test.  Any tank listed as full must be demonstrated as such to the USCG Inspector.  You demonstrate a full tank by pumping extra liquid in until it overflows out the vent pipe.  No chief engineer wants to risk spilling fuel out their vent pipes; it presents a danger of major environmental fines.  Fuel tanks will require creative solutions.

5.0 Incline Weights

After deciding on the tank configuration, you next need to pick the incline weights and their locations.  The goal here is to balance several competing goals:

1. Generate sufficient heeling moment
2. Stay under the capacity limits for the crane
3. Minimize the number of weights to move

Based on those goals, you select the size of your incline weights and the type.  Table 5‑1 compares the common different options for incline weights.

5.1 NO WATER!!

One option is almost always forbidden:  pumping water as an incline weight.  Partially full water tanks create free surface moments.  And water flow meters perform poorly at accurately tracking the water movement.  Put simply, pumping water introduces far too much error.  USCG only allows water pumped as an incline weight in extremely rare circumstances.

This does not stop you from using the weight of water, contained in individual containers.  The water needs to completely fill the container so that you eliminate any free surface.  The key difference is that you move the entire container, not just the water inside the container.

5.2 Check Deck Loads

When planning the location of the incline weights, check the strength of the deck.  Most of these weights get placed on superstructure decks, which are built with lighter scantlings.  If you are not careful, a heavy weight can easily damage the deck. 

Even with a strong deck, it’s wise to lay down wood cribbing to ensure the incline weight gets properly distributed to the deck beams.  This also helps to protect the paint job on the deck.  These little details make the Owner and crew happy.

5.3 Certified Scale

Several of the incline weights will require you to weigh them just before the test.  Round estimates fail to impress for the stability test.  We need to know the exact measured weight for each incline weight.  This is usually measured with a crane dynamometer / load cell.   

6.0 Perform an Advisory Survey

I worked on plenty of stability tests.  In most cases, the owner felt very confident that the vessel was ready for the test.  And in every case, the vessel was not actually ready.  If at all possible, visit the vessel a few days before the deadweight survey starts.  Sit down with the Master and Chief Engineer to answer any questions.  And then walk through the vessel with them.  Mark out any areas where deadweight needs to be removed from the ship.  This advanced survey gives the crew extra time to prepare.

7.0 Conclusion

I promise this:  despite your best efforts, the stability test will not go perfectly.  Every test comes with its own challenges and obstacles.  If it were easy, anyone could do it.  But hopefully these practical tips removed a few obstacles.  I wish you a smooth and successful stability test.

8.0 References