Angle of Loll: Hidden Dangers to Stability

1.0 Introduction

PANIC! Well, at least severe concern. That should be your reaction if your ship developed a permanent list, and you eliminated the possibility of any extra weights. The only other possibility is an angle of loll. Angle of loll shines like a bright red warning sign, indicating serious stability problems. You need to track down this cause, because the next warning may be when your ship capsizes. Today we discuss an angle of loll (AOL): what it is, how to find it, and what to do about it.

2.0 What Is an Angle of Loll?

An angle of loll (AOL) is a symptom of serious stability problems. Basic stability theory imagines a point called the metacenter, which represents the stability of the ship. (Point M on Figure 2‑1) For a stable ship, we want the ship’s center of gravity (point G) to be vertically below the metacenter.

When a ship has poor stability, the points M and point G are very close together. With an AOL, when the ship is upright, the point G is actually above M. End result: the ship is unstable within a small range of heel angles. That is very bad.

We can also see this instability in the righting arm curve. (Figure 2‑2) Initially, the curve drops negative, and moves back to positive after some angle of heel. That crossing point is the angle of loll.

Sure, an AOL is annoying. But the real concern is that an AOL is a symptom of a borderline unstable ship. And unstable ships capsize.

3.0 Signs of an Angle of Loll

Don’t jump to find zebras when you only hear hoofbeats. A permanent heel may only be a list, which is far less concerning than an angle of loll (AOL). These are some classic symptoms of an AOL.

Vessel rolling never fully dampens out.
1. In calm seas, the vessel exhibits some minimum amplitude of roll back and forth.
In harbor, vessel list switches between port and starboard side. Similar list angles on both sides.
1. This may be masked by your mooring situation. The mooring lines can easily force the vessel to list on one side when tied to a pier. You may still have an AOL if the ship always heels towards the pier.
2. An easy check: try mooring on the opposite side and see if your list flips sides.
Adding counter weights does not correct the heel angle.
1. It either requires more weight or less weight than initially planned.

3.1 Free Surface Moment

AOL’s typically result from some change in your vessel’s tanks. This can drastically affect the free surface moment and create an AOL. Check for any of these changes to your tanks.

Excessively high engine room bilge level. High enough that water stretches across the entire engine room.
New cross connects in any tanks. Or cross connects that were recently opened.
Water leaking into ballast tanks.
Water leaking into void spaces.

4.0 How To Test for an Angle of Loll?

Angle of loll (AOL) is hard to detect. The best solution is to bring in an expert: call a naval architect. But naval architects are expensive. Maybe you want to check things yourself before opening the wallet. The following procedure helps indicate if your ship has an angle of loll. Just read all the warnings and if you have any safety concerns, a naval architect is the best route to go.

4.1 Equipment

To test this, you will need the following:

One or more water tanks, positioned on deck. We use water as heeling weights. You need enough weight to correct the current list.
A water pump and hose to fill the tanks on deck.
Something for accurate measurement of ship heel. Either a fine gauge bubble level, or even a pendulum setup.
A calm day in sheltered water, with almost no wind (less than 10 knots) and no waves (small boat wakes only).

4.2 Procedure

The goal here is to fill the heeling tanks on deck until the ship changes from the list angle back to zero heel, and slightly past. If the ship has a true list, it will remain at the new angle. If the ship has an AOL, once you pass zero heel, it should very quickly continue to heel further. For an example, assume our problem ship has an initial heel of 2.5 deg to port.

Place the heeling tanks on the starboard side of the deck. We are using these to create a correcting heel moment.
Start filling the water tanks on deck. It will be useful to periodically record the levels in each tank and corresponding ship heel angle. If you do have an AOL, this information can help a naval architect to understand how bad it is.
Fill the water tanks until the ship reaches exactly zero heel.
Continue filling until the ship heels slightly onto the starboard side. (Opposite of your initial heel angle.) You should only need 0.5 deg starboard heel or less.
Wait 10 minutes. One of two things will happen.
1. Angle of Loll: Without any further changes to the water tanks, the ship will continue to heel further and should rest at about 5.0 deg starboard. This new heel angle on starboard is the list angle created from the water tanks on deck (2.5 deg list to starboard), plus the AOL that you started with (2.5 deg).
2. True List: The ship will remain at the 0.5 deg of starboard heel, with no further changes.

This test gives you a clear and very noticeable distinction between AOL and genuine list.

4.3 Dangers of Test

This test does have some risks for the ship. For safety planning, assume you have an angle of loll (AOL). This means, when you create the counter moment, you created a list angle. The list angle will compound with the AOL and heel farther on the opposite side than you started with. This potentially leads to very large heel angles and possible deck edge immersion. This following video shows an excellent example of how that can happen.

Figure 4‑1: Scary Compounding AOL with List

Check your load cases before hand to ensure the vessel can tolerate the larger heel angle. If you are unsure about the safety aspects, consult with a naval architect.

5.0 Fixing The Angle of Loll

Fix two problems for the price of one! Address the underlying stability issues first, and the angle of loll will resolve itself. If this was a recent change to your vessel stability it probably stemmed from free surface moment problems. The most likely culprit is cross connects between port / starboard tank pairs. Cross connects should normally be closed. Check all your cross connects and repair any bad valves.

If this problem slowly developed over time, then it is time to call in the experts. Hire a naval architect to do a stability test, stability assessment, recommend corrective actions, and update the trim and stability booklet. These are not quick and easy solutions, but they are necessary. The angle of loll is a major warning about stability. Ask yourself which is easier: preventative maintenance, or dealing with a capsized ship.

5.1 Finding Hidden Cross Connects

An open cross connect valve often results in an angle of loll. Cross connects are normally associated with the port/starboard tank pairs and often sealed with a valve. These valves lie deep in the double bottom and can be devils to find.

How to find the hidden cross connect? Start by filling only the starboard tanks of your port/starboard tank pairs. Leave the port side empty. Then take continuous soundings of the full tanks. Any tanks with open cross connects, and you will quickly notice a change in the sounding. It should take no more than ten minutes to detect a definite change in tank level.

Repeat this process with the port tanks full and starboard tanks empty. You want to check it both sides to ensure you only have one unknown opening.

6.0 Conclusion

Angle of loll is actually a blessing. It provides a clear warning about underlying stability problems with your vessel. Not all ships show this warning before bad things happen. If you have an angle of loll, take advantage of the warning. Speak with a naval architect to address the stability issues and get your vessel floating straight and tall.

7.0 References

[1]	K. J. Rawson and E. C. Tupper, Basic Ship Theory, Oxford: Butterworth-Heinemann, Fifth edition, 2001.

Angle of Loll: Hidden Dangers to Stability

1.0 Introduction

2.0 What Is an Angle of Loll?

3.0 Signs of an Angle of Loll

3.1 Free Surface Moment

4.0 How To Test for an Angle of Loll?

4.1 Equipment

4.2 Procedure

4.3 Dangers of Test

5.0 Fixing The Angle of Loll

5.1 Finding Hidden Cross Connects

6.0 Conclusion

7.0 References

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