Good and Poor Stability: Stiff, Tender and the Warning Signs

Read first: Flotation, Centres and Metacentric Height. This topic builds on G, B, M and GM.

GM and GZ: what good stability means

In short: a ship with good stability has a positive GM that is big enough but not too big, and a GZ curve that gives a strong righting lever over a wide range of heel.

• G = centre of gravity. M = metacentre. GM = the distance between them (metacentric height).
• Bigger GM = more stable at small angles. GM is the quick check of initial stability.
• GZ = the righting lever: the horizontal distance between G and the line of buoyancy when the ship heels.
• Bigger GZ = more force to come back upright. Righting moment = displacement × GZ.
• At small angles (about 10–15°), GZ ≈ GM × sin(heel). So GM tells you how the ship starts; the GZ curve tells you how she behaves at larger angles.
• Three states of equilibrium: stable (GM positive), neutral (GM = 0), unstable (GM negative).

The minimum figures (2008 IS Code, Part A, 2.2)

• Initial GM₀ ≥ 0.15 m.
• GZ ≥ 0.20 m at an angle of heel of 30° or more.
• Maximum GZ at an angle of not less than 25°.
• Area under the GZ curve: ≥ 0.055 m·rad up to 30°, ≥ 0.09 m·rad up to 40° (or the down-flooding angle if smaller), and ≥ 0.03 m·rad between 30° and 40°.
• These are minimums only. Meeting them does not make a ship safe from capsize; good seamanship still counts (IS Code Part B, 5.1.1).

The stiff vessel

In short: a stiff vessel has a large GM. She snaps back upright fast and hard.

• Very strong righting moment: she comes back upright quickly and with force.
• Short, quick, jerky roll. In heavy weather the rolls can be violent.
• Warships are often given a large GM to survive damage: a classic stiff hull.
• Advantage: a big reserve against heeling forces (wind, turning, weight shifts).
• Disadvantages: uncomfortable and tiring for crew and guests; heavy loads on lashings, masts and fittings; cargo and loose gear can break free; people get hurt.
• The IS Code sets no maximum GM, but advises avoiding excessive GM because of the large acceleration forces (IS Code Part B, 5.1.6).
• On the bridge: if she is too stiff, raise G a little (for example, move weight higher or use the upper tanks), within the stability booklet.

The tender vessel

In short: a tender vessel has a small GM. She rolls slowly and gently, but her margin of stability is small.

• Small righting moment: she comes back upright slowly and without force.
• Long, slow, easy roll. Comfortable, which can hide the danger.
• The closer G gets to M, the more tender the motion becomes. At GM = 0 she has no initial stability at all.
• Advantage: comfortable motion, low stress on structure and lashings.
• Disadvantages: heels a lot to wind, turning and weight shifts; small reserve if G rises further (ice, water on deck, slack tanks, fuel used from low tanks).

Stiff vs tender at a glance

• Roll period gets longer as GM gets smaller (the period depends on 1/√GM). Typical figures: about 12 s for a passenger ship built for comfort, 6–8 s for a tanker or cargo ship (Wikipedia, “Metacentric height”).

Warning signs of poor stability

In short: a ship losing stability tells you through the way she moves. Learn the signs and act early.

• Long, slow, lazy roll, longer than normal for that vessel.
• She hangs at the end of each roll before coming back.
• Heels more than usual to a gust, a small turn or a weight moved across the deck.
• A list that has no obvious cause, or a list that flops from one side to the other: a sign of angle of loll (GM negative; she floats at the angle where GZ = 0).
• A list that gets worse when you fill or empty a tank, or move fuel or water.
• Water staying on deck instead of draining away; ice building up on the upper works.
• On the bridge: if you see these signs, stop moving weight, check tank soundings and the loading condition, reduce free surface and lower G. Do not try to correct loll by moving weight across the ship.

Raising and lowering weights

In short: G moves towards a weight that is added, away from a weight that is removed, and in the same direction as a weight that is moved.

• Raising a weight raises G: smaller GM, more tender.
• Lowering a weight lowers G: bigger GM, stiffer.
• How far G moves: GG₁ = (weight × distance moved) ÷ displacement.
• Suspended weights: a load on a crane or davit acts at the head of the crane the moment it leaves the deck. Lifting a tender or a jet ski can raise G a lot.
• Using fuel or water from low tanks removes weight from low down: G rises and the tanks become slack.
• Adding weight high up (ice, wet sails and covers, toys on the upper deck, water in a swimming pool) raises G.
• Transferring weight or fuel: think stability first. Check the effect on GM, list and free surface before you start.

Low freeboard

In short: less freeboard means less reserve buoyancy and a smaller range of stability.

• The deck edge goes under at a smaller angle of heel, so the GZ curve peaks earlier and falls away sooner.
• Openings (doors, vents, air pipes) reach the water sooner: the down-flooding angle gets smaller.
• More green water comes on deck, adding weight high up.
• Keep a seaworthy freeboard in all loading conditions (IS Code Part B, 5.3.1). Never load beyond the load line marks.

Blocked freeing ports and scuppers

In short: freeing ports and scuppers let water off the deck. If they are blocked, that water stays aboard as weight high up with a large free surface.

• Trapped water raises G and adds free surface: GM falls, sometimes fast.
• The water runs to the low side and makes the list worse; in the worst case this leads to capsize.
• The IS Code names water trapped on deck as an influence that reduces stability (IS Code Part A, 2.1.4).
• If freeing ports cannot drain a well deck, reduce speed, alter course, or both. Freeing ports with closing appliances must always work and must never be locked (IS Code Part B, 5.3.5).
• On the bridge: before heavy weather, check that scuppers and freeing ports are clear of mooring lines, fenders, deck gear and rubbish. See also Heavy Weather Preparation and Seaworthiness.

Slack tanks and free surface effect

In short: a partly filled (slack) tank lets the liquid shift when the ship heels. This acts as a virtual rise of G, so it reduces GM.

• The free surface effect grows with:
  • the density of the liquid (denser liquid, bigger effect);
  • the breadth of the free surface: the effect goes with breadth cubed (a wide tank is far worse than a long, narrow one);
  • a smaller displacement (the effect is divided by the ship’s displacement).
• What matters is the size and shape of the free surface, not how high the tank sits in the ship. A tank nearly empty or nearly pressed up has a smaller free surface.
• How to reduce it:
  • keep tanks empty or pressed up (full), never slack;
  • keep the number of slack tanks to a minimum (IS Code Part B, 5.1.5);
  • use longitudinal divisions: two baffles that split a tank into thirds cut the effect by a factor of nine (Wikipedia, “Metacentric height”);
  • remember swimming pools and spa tanks: a filled pool has a large free surface (IS Code Part B, 5.1.5).
• Damage that lets water into a compartment creates a large free surface too; see Collision, Grounding, Flooding and Machinery Failure.