List, Loll and Heel

Read first: flotation, centres and metacentric height and good and poor stability. This topic builds on G, M and GM.

Three ways to lean

In short: a ship can lean for three different reasons. List = weight off the centreline. Loll = no stability upright. Heel = an outside force. Same look, very different cure.

• All three are a transverse lean (port or starboard). A fore-and-aft lean is trim, not heel.
• On a GZ curve every inclination on the bottom axis is called “angle of heel”, whatever the cause.
• The MCA stability syllabus asks you to distinguish between list and loll (MCA Master (Yachts) Stability syllabus 2.4.1.6).

Angle of list

In short: a list comes from weight off the centreline. G moves sideways, GM stays positive, and the ship is stable at her listed angle.

• Causes: cargo or stores loaded on one side, fuel or water used from one side only, a slack tank draining to one side, a leak on one side.
• G moves off the centreline towards the extra weight. The ship heels until B is right under the new G. She then floats there in equilibrium.
• She still rolls normally, but about the listed position, and she always comes back to the same side.
• The size of the list depends on the shift of G and on GM: tan list = GG₁ ÷ GM, with GG₁ = weight × distance moved ÷ displacement (small angles). The MCA syllabus asks you to calculate it (MCA Master (Yachts) Stability syllabus 2.2.1.3–2.2.1.4).
• The same weight gives a bigger list when GM is small. A big list for a small weight is a warning that GM is low.

Worked example: a list from moving one weight

• Displacement 1,000 t, GM 0.50 m. A 10 t tender is moved 6 m to port.
• GG₁ = 10 × 6 ÷ 1,000 = 0.06 m. tan list = 0.06 ÷ 0.50 = 0.12, so the list is about 7° to port.
• Same move with GM 0.25 m: tan list = 0.24, list about 13.5°. Half the GM, about double the list.

Angle of loll

In short: loll happens when G rises above M. GM is negative, so the ship has no stability upright. She leans over until she finds a little stability, and floats at the angle where GZ = 0.

• Upright she is unstable: any small push makes her heel. As she heels, the shape of the underwater hull changes and B moves out. At some angle B comes back under G: righting lever GZ = 0. That is the angle of loll.
• She can loll to either side. In a seaway she can hang to port, then flop over and hang to starboard. Wind or waves can flip her suddenly, which is quite unlike a normal roll (Wärtsilä Encyclopedia, “Angle of loll”).
• At the angle of loll she looks stable, but the margin is tiny. Any further rise of G, and she capsizes. Very dangerous: act at once.
• Typical causes, all of which raise G or cut GM:
  • top weight loaded (deck cargo, tenders, stores high up);
  • low tanks used up (fuel or water from the double bottom) without ballasting;
  • free surface in slack tanks, or flooding after damage (Wikipedia, “Angle of loll”);
  • weight added high up at sea: ice on the superstructure, water trapped on deck, deck cargo soaking up water.
• Watch the wording: loll is negative GM, not “neutral” GM. Neutral equilibrium is GM = 0.

Angle of heel

In short: heel is a temporary transverse lean caused by an outside force. When the force stops, the ship comes back upright.

• Forces that heel a ship: wind on the beam, turning (the centrifugal effect in the turn), waves, a tow line pulling to one side.
• G does not move and GM does not change. The ship is pushed over and her righting moment pushes back.
• The 2008 IS Code checks heel from a steady wind and gusts (the weather criterion) and, for passenger ships, heel when turning (IMO MSC.267(85), 2008 IS Code Part A).
• On the bridge: a list or a loll adds to any heel. A ship listed to starboard that turns to port heels further to starboard. With small GM, slow down before a large alteration of course.
• Common trap: heel is not a longitudinal movement. Fore-and-aft is trim.

Telling list from loll on board

In short: a list leans always to the same side and the ship rolls normally. A loll makes her more and more tender, she hangs on one side, then on the other.

• Signs of loll:
  • the ship becomes increasingly tender: long, slow, lazy rolls;
  • she hangs at an angle, then flops over and hangs on the other side;
  • the lean can change side after a small push: a gust, a wave, a turn.
• Signs of list:
  • normal motion, rolling about the listed position;
  • always the same side;
  • the angle matches a weight you can find (a tank, cargo, a tender).
• Alongside: ask what was loaded, discharged or used just before. Deck cargo loaded? Double-bottom tanks pumped out or used? Ballast changed? A tank left slack?
• Check the soundings of all tanks: a hidden slack tank or a leak changes the answer.
• If in doubt, treat it as loll. The cure for a list (moving weight across the ship) is dangerous with loll.

Correcting a list

In short: first rule out loll. Then put back the balance with an equal and opposite moment.

• Make sure it is a list: positive GM, always the same side, a known cause.
• Then:
  • remove weight from the low side; or
  • add weight on the high side; or
  • shift weight from the low side to the high side.
• Size the correction with the same formula: the moment you add must cancel weight × distance of the cause.
• Find the cause first: if the list is growing, think of a leak or a tank filling by itself, not just poor loading.

Correcting loll

In short: lower G with no sideways moments. Never move weight across the ship. Act early.

• Moving weight across a lolling ship does not help: G is still above M. She may flop over and hang at the same angle on the other side, or further.
• Use the steps in this order:
  1. Move weights down, starting on the low side.
  2. Remove or jettison top weight, starting on the high side.
  3. Ballast low tanks (double bottom), starting on the low side. Use small or subdivided tanks, one at a time, and press each one up.
• Why this order: steps 1 and 2 lower G straight away. Every step is chosen so that she stays on the side she already leans to and never swings across.
• Why ballast comes last: it is fast, but a tank being filled is slack at first. Its free surface raises G (virtually) before the added low weight lowers it, so the loll first gets worse. The effect soon passes and the heel then falls quickly (Wärtsilä Encyclopedia, “Angle of loll”).
• A tank divided at the centreline helps: fill only the half on the low side (MCA Master (Yachts) Stability syllabus 2.4.1.5).
• Cut existing free surface as well: few slack tanks, the rest pressed up or empty.
• Act early. Loll is the warning that GM is already negative. One more rise of G (fuel used, ice, water on deck) and she can capsize.