Stability and Damage Control

Prerequisites

This lesson assumes Yachtmaster Coastal knowledge of boat handling in heavy weather, basic yacht construction (keel, ballast, through-hull fittings, seacocks) and the use of bilge pumps. It links directly to the Offshore lessons on heavy weather tactics (heaving-to, running off, drogues and knockdown recovery) and on abandoning ship. No mathematics beyond multiplication and division is needed. If your yacht has a stability booklet, an RCD/ISO 12217 design category or a STIX number, have it to hand: you will learn what those figures mean.

Learning Objectives

By the end of this lesson you should be able to:

• Define centre of gravity (G), centre of buoyancy (B), metacentre (M), metacentric height (GM) and righting lever (GZ).
• Read a GZ curve and identify initial stability, maximum GZ, the angle of vanishing stability, the downflooding angle and the area under the curve.
• Explain how loading, deck weights and partially filled tanks (free surface effect) change stability, and calculate the effect of moving a weight.
• Explain why waves and breaking seas can capsize a yacht that is statically stable.
• Explain floodable length, damage stability and the limits of counter-flooding.
• Carry out a damage control sequence: find the leak, stop or slow the water, pump, assess stability and decide.
• Estimate water ingress through a hole and compare it with realistic pump capacity.
• Map your knowledge to the RYA Yachtmaster Offshore syllabus (G158) items on stability and damage control: the factors affecting a yacht's stability and seaworthiness, the capsize risk in heavy weather, the effect of weights and free surface, flooding and damage control, using pumps, stopping leaks and deciding when to abandon, as the examiner will test them in the oral and in practical emergency drills.
• Use the capsize screening formula and the stability figures on the builder's plate to judge a yacht's suitability for offshore work.
• Distinguish a list from an angle of loll, and act correctly on each.
• Calculate the effect of wind strength on heeling moment, and explain why reefing early is a stability decision.

The Basic Forces

A floating yacht is acted on by two equal and opposite forces:

• Weight, acting vertically downwards through the centre of gravity, G. The position of G depends on how weight is distributed: hull, ballast keel, rig, engine, tanks, stores and crew.
• Buoyancy, acting vertically upwards through the centre of buoyancy, B, the geometric centre of the underwater volume.

Upright, G and B lie on the same vertical line and the boat is in equilibrium. When the yacht heels, the underwater shape changes: more volume is immersed on the low side, so B moves towards the low side. G does not move (unless something inside shifts). The two forces now form a couple, separated by a horizontal distance called the righting lever, GZ. The righting moment is the displacement multiplied by GZ.

Righting moment = displacement x GZ

A yacht of 8 tonnes with a GZ of 0.5 m at 30 degrees has a righting moment of 4 tonne-metres. Anything trying to heel her further (wind in the sails, a wave, crew on the leeward rail) must overcome that moment.

The metacentre and GM

At small angles of heel (up to about 10 to 15 degrees), the vertical line through the new centre of buoyancy crosses the centreline at a nearly fixed point called the metacentre, M. The distance from G up to M is the metacentric height, GM.

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For small angles, GZ is approximately GM multiplied by the sine of the heel angle. So GM governs initial stability: how stiff or tender a boat feels when she first heels.

• Large GM: stiff boat, resists heeling, but has a quick, jerky roll that is uncomfortable and tiring and loads the rig.
• Small GM: tender boat, heels easily, slow comfortable roll.
• Negative GM (G above M): the boat will not float upright but lolls to one side. This should never occur on a yacht in normal condition; if it does, something serious has happened (flooding, ice, heavy weight added high).

A beamy, flat-bottomed modern cruiser gets much of its initial stability from form (hull shape: B moves a long way sideways as she heels). A narrow, deep-keeled classic yacht gets more of her stability from weight (a low G). This matters at large angles, as you will see.

The GZ Curve

The GZ curve (curve of statical stability) plots the righting lever against the angle of heel, from upright to capsize. It is calculated by the designer for a stated loading condition, usually "minimum operating" (empty tanks, minimal crew), which is the worst case.

Read these features from the curve:

Angle of vanishing stability

The AVS is the most quoted single figure, and the one examiners ask about.

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As a rule of thumb, an offshore yacht should have an AVS of at least 120 degrees, and an ocean-going yacht 130 degrees or more. Design categories under the Recreational Craft Directive and ISO 12217-2 use a combination of AVS, displacement and the STIX (stability index) number:

Note that the category's minimum AVS depends on the boat's displacement: a light boat in category A needs a larger AVS than a heavy one. A beamy, light boat with a high freeboard can have a very large maximum GZ but a relatively small AVS and a large negative area, meaning she may remain inverted for some time after a capsize.

Area under the curve: dynamic stability

Static stability (the height of the curve) tells you how much steady heeling moment the boat can resist. But capsize in heavy weather is caused by the impact of a breaking wave, which delivers energy, not a steady push. The energy a yacht can absorb before capsizing is the area under the GZ curve multiplied by the displacement.

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Two boats with the same maximum GZ can have very different capsize resistance: the one whose curve stays positive over a wider range of angles has more area and will absorb more wave energy. Displacement also matters: for the same GZ curve, a heavier boat has a greater righting moment and absorbs more energy.

Downflooding angle

The GZ curve assumes the hull is watertight. In reality, at some angle of heel water starts to pour in through openings: the companionway, open hatches, cockpit lockers, ventilators, the engine air intake. This is the downflooding angle.

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Beyond it, water floods in, adding weight high and with a free surface, and the real stability falls away much faster than the curve suggests. Practical consequences:

• In heavy weather, close and secure all hatches, put in the washboards (secured so that they cannot fall out if inverted), close ventilators and seacocks not in use, and lock cockpit locker lids.
• A yacht knocked down to 90 degrees with the main hatch open can take on hundreds of litres in seconds. Several yachts in the 1979 Fastnet and 1998 Sydney Hobart races were lost or nearly lost to downflooding after knockdowns.
• Hatches that cannot be closed from both inside and outside, or washboards that are not secured, effectively lower the downflooding angle.

How Loading Changes Stability

Weight high and weight low

Raising any weight raises G, which shortens GZ at every angle. The effect on the curve is shown below.

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Common offshore culprits:

• A dinghy or outboard on davits or on the foredeck.
• Solar panels, radar and wind generators on arches and the mast.
• Spare fuel and water in jerricans lashed to the guardrails.
• In-mast furling mainsails, heavy roller-reefing gear and extra halyards aloft (weight aloft has a very large effect because of the lever).
• Anchors and chain stowed in the bow locker (high and at the end of the boat, also increasing pitching).
• Crew sitting on the coachroof.

Rules of thumb: stow heavy items low and close to the centre of the boat; carry the second anchor and spare chain in the bilge near the mast; keep the deck clear before heavy weather; never stack heavy stores on high shelves.

Calculating the shift in G

When a weight is moved within the boat, G moves parallel to the direction the weight moved:

Shift of G (GG1) = (weight moved x distance moved) / total displacement

The same formula, with weight added or removed, gives a good approximation if the weight is small compared with the displacement.

Free Surface Effect

When a tank is partly full, the liquid surface stays level as the boat heels. The liquid flows to the low side, moving its centre of gravity outward, which has the same effect as raising G. This is called the virtual rise in G due to free surface.

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Key points:

• The effect depends on the width of the free surface cubed, not on the amount of liquid. A half-full wide tank is far worse than a narrow, deep one. Splitting a tank with a longitudinal baffle into two halves reduces the effect to a quarter.
• A tank that is completely full (pressed up) or completely empty has no free surface.
• Bilge water sloshing across a wide, shallow bilge is the worst case of all: wide, unconstrained and often with debris.

For a rectangular tank, the virtual rise in G is approximately:

Virtual rise = (length x breadth cubed / 12) x density of liquid / displacement

Example. A water tank 1.0 m long and 1.2 m wide under the saloon sole, half full, on an 8 tonne yacht: (1.0 x 1.2 x 1.2 x 1.2 / 12) x 1.0 / 8 = 0.144 / 8 = 0.018 m. Now imagine 500 litres of flood water in a bilge 2.0 m wide and 3.0 m long: (3.0 x 8.0 / 12) x 1.025 / 8 = 2.0 x 1.025 / 8 = 0.26 m. That is a third of a typical yacht's GM, gone, before counting the weight of the water.

Tank management offshore:

1. Before heavy weather, keep tanks either full or empty: transfer fuel or water to press up one tank and empty another.
2. Draw from paired port and starboard tanks equally to avoid a list, or empty one tank completely before starting the next.
3. Pump bilges dry; keep limber holes clear so water reaches the pump.
4. Do not carry large quantities of fuel or water in flexible bladders or jerricans on deck.

Worked Example: Stability Calculation

The diagram below summarises the calculation that follows.

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A 12 m yacht displaces 8,000 kg (including the dinghy) and has a GM of 0.80 m. For an offshore passage the owner lifts the 100 kg dinghy from the cockpit sole onto stern davits, raising its centre of gravity by 2.5 m.

1. Shift of G: GG1 = (100 x 2.5) / 8,000 = 0.031 m.
2. New GM = 0.80 - 0.031 = 0.769 m.
3. The water tanks are each half full, giving a further virtual rise of G of 0.05 m.
4. Effective GM = 0.769 - 0.05 = 0.719 m, a 10 per cent reduction.

What does that mean? At 10 degrees of heel, GZ is approximately GM x sin 10 degrees (0.174):

The same G rise shortens GZ at every angle, so the whole curve shrinks, the maximum GZ drops and the AVS comes closer. Add jerricans on the rail, a wet in-mast mainsail and a heavy anchor in the bow locker, and the losses accumulate. Each decision on its own seems trivial; together they can turn a 125-degree AVS into a 110-degree one.

Heeling Moments: Why the Wind Matters So Much

Stability is only half the story. The other half is what is trying to heel the boat. The heeling moment from the wind on the sails is approximately:

Heeling moment = force on the sails x height of the centre of effort above the centre of lateral resistance

The force on the sails is proportional to the square of the wind speed. A doubling of wind speed means four times the force.

Worked example. A 10 m cruising yacht carries 55 square metres of sail. In a 15 knot breeze (7.7 m/s, Force 4), the force on the sails when close-hauled is about 0.5 x 1.2 x 7.7 squared x 1.0 x 55 = about 1,960 N (roughly 200 kg-force). With the centre of effort 5 m above the centre of lateral resistance, the heeling moment is about 1,000 kg-m. In 30 knots (15.4 m/s, Force 7) the same sails, if still set, would produce four times the force, about 4,000 kg-m. If the yacht's maximum righting moment is 8 tonnes x 0.65 m = 5,200 kg-m, then a Force 7 with full sail takes her up to 77 per cent of her maximum, with no reserve for a gust, a wave or a mistake. Halving the sail area by reefing halves the heeling moment and also lowers the centre of effort: the effect is more than the sail area alone suggests.

Equilibrium. The yacht heels until the righting moment equals the heeling moment. Look at the GZ curve: the steady heel angle is where GZ times displacement equals the heeling moment. A stiff boat (high GM) heels less for the same wind. A boat that is already heeled at 35 to 40 degrees in gusts is near the top of her curve and has little margin: reef earlier.

Gusts and squalls. A gust is a sudden increase in wind, often by 50 per cent or more, and the heeling moment rises by the square of the ratio: a 50 per cent increase in wind gives 2.25 times the force. A yacht near her maximum righting moment at the steady wind will be knocked down by the gust. The reef must go in before the squall, not during it. Easing sheets, luffing and releasing the mainsheet or the traveller dump the heeling force in a second; the crew must be ready to do it.

The effect of heel on the sails

As the yacht heels, the sails present less area to the wind and the centre of effort moves to leeward, so the heeling moment falls a little with the cosine of the heel angle. This helps, but not by much. At 60 degrees of heel the mainsail is nearly horizontal, the boat is almost knocked down, and the force falls sharply, which is why knockdowns often recover when the sails spill the wind. Do not rely on it.

Worked GZ Curve and Dynamic Stability

The table below shows GZ values for a typical 8 tonne offshore cruiser in the minimum operating condition.

• Maximum GZ is 0.65 m at 45 degrees. The maximum righting moment is 8,000 x 0.65 = 5,200 kg-m.
• AVS is 130 degrees.
• Initial GM is roughly GZ at 15 degrees divided by the sine of 15 degrees (0.259): 0.30 / 0.259 = about 1.16 m. This is approximate because GZ at 15 degrees is already a little below the tangent from the origin.
• Area under the curve. Use the trapezoid rule over each interval: 15 x (0 + 0.30)/2 = 2.25; 15 x (0.30 + 0.55)/2 = 6.4; 15 x (0.55 + 0.65)/2 = 9.0; 15 x (0.65 + 0.60)/2 = 9.4; 30 x (0.60 + 0.30)/2 = 13.5; 20 x (0.30 + 0.10)/2 = 4.0; 20 x (0.10 + 0)/2 = 1.0. The total is about 45.5 metre-degrees, or 45.5 x 0.01745 = 0.79 metre-radians.
• Energy to capsize (to the AVS) is displacement x gravity x area = 8,000 x 9.81 x 0.79 = about 62,000 joules (62 kJ). That sounds like a lot, but a breaking wave carries much more energy than this over the length of a hull. That is why a yacht's static stability does not make her immune from capsize in a breaking sea.

Now see the effect of adding 300 kg to the top of the mast and rigging (for example, a heavy radar, in-mast furling and a wet mainsail), 9 m above G. GG1 = 300 x 9 / 8,300 = 0.33 m rise in G. The GZ at every angle falls by GG1 times the sine of the angle: at 45 degrees by 0.33 x 0.707 = 0.23 m, which takes GZ from 0.65 to about 0.42 m. At 90 degrees GZ falls from 0.30 to about 0 (0.30 - 0.33). The AVS drops from 130 degrees to nearly 90 degrees. This simplified calculation (the correction is GG1 sin heel for a vertical shift of G) shows why weight aloft is so dangerous and why a modest-looking addition can turn a safe offshore boat into a marginal one. Use this as an upper-end estimate because other effects, such as a change in displacement, are ignored, but the lesson holds.

Capsize Screening and the Numbers on the Plate

The capsize screening formula

A quick indicator of a yacht's resistance to capsize in breaking seas, used in offshore racing rules, is the Capsize Screening Formula (CSF):

CSF = maximum beam (m) / (displacement volume in cubic metres) to the power of one third

Displacement volume is the displacement mass in tonnes divided by 1.025 (the density of seawater). A value of 2.0 or less is regarded as acceptable for offshore use; beamy, light yachts have higher values and a greater susceptibility to capsize and to remaining inverted.

Example. An 8 tonne yacht has a maximum beam of 3.8 m. Volume = 8 / 1.025 = 7.8 cubic metres; the cube root is 1.98. CSF = 3.8 / 1.98 = 1.92. This is below 2.0, so the yacht passes the screen. A lighter, wider yacht with a beam of 4.0 m and a displacement of 6 tonnes has a volume of 5.85 cubic metres, a cube root of 1.80 and a CSF of 2.2: not as suitable for hard offshore conditions.

The CSF is a screening tool, not a guarantee. It does not include ballast, keel depth or AVS.

Other figures

• Stability index (STIX) from ISO 12217-2 combines length, beam, displacement, AVS, downflooding height, ballast and other factors in one number that gives the design category. Check the builder's plate and owner's manual.
• Ballast ratio, the keel weight divided by the total displacement, is typically 30 to 40 per cent for a cruising yacht. It is an indication of weight-based stability but not a measure.
• Righting moment per degree of heel at 1 degree, given in the manual of some yachts, lets you compare boats.
• Design categories A to D tell you for what conditions the yacht was designed, not for what she is capable if overloaded.

List, Loll and Negative GM

A yacht that is heeled when she should be upright is either listing or lolling, and the cure is different.

An angle of loll is dangerous because the boat has very little stability near upright. It is a sign that something serious is wrong: flooding with free surface, severe ice or an enormous weight aloft. Check with a slow roll: a yacht lolling to port will slowly roll to starboard past the vertical and settle at a similar angle to starboard. Treat it as an emergency, reduce the free surface and lower G at once.

Practical Offshore Preparation

Before leaving, a skipper should carry out a stability check as part of the safety brief:

1. Know the numbers. The design category, the AVS and the CSF (if available) of your yacht. If you do not know them, find them.
2. Stow low and central. Heavy items on the cabin sole, over the keel and as near the centre of the boat as possible.
3. Keep the deck clear. Dinghy lashed securely and as low as possible (if not on davits), jerricans in the cockpit lockers or stowed below, nothing loose.
4. Check the downflooding openings. Hatches, ports, vents, cockpit lockers. Washboards secured with lanyards. Seacocks closed when not in use.
5. Plan the tanks. Full or empty. If the starboard tank is used first, switch to the port tank before a long tack.
6. Check the reefing arrangements and be ready to reef before the wind rises. Trim the sails to the yacht's needs.
7. Monitor as the passage proceeds. As fuel and water are used, the yacht becomes lighter and G may rise. Wet sails, ice and spray add weight aloft. Consider a spare half tank of fuel in a low tank as a ballast.

Dismasting and Hull Damage

A dismasting is a stability event as well as a rigging event. A rig that has gone over the side is still attached by the shrouds and is a battering ram against the hull. The sea moves it and can hole the topsides in a few minutes.

• Act at once. Cut it clear with bolt croppers, a hacksaw or by removing the clevis pins, whichever is quickest. Do not delay in the hope of saving it: the lives of the crew come first. Make sure all crew are accounted for and no one is hurt.
• Protect the propeller and rudder. Do not start the engine until you are sure that no lines are in the water. A fouled propeller is the usual reason a dismasted yacht cannot motor.
• Rig a jury rig from a boom, a spinnaker pole or the remains of the mast if you can, and anchor or stream a drogue if the yacht is drifting to a lee shore.
• Call for assistance (Pan-Pan) and give your position and your intentions.
• Check the hull for damage where the rig hit and for leaks at the chainplates and at the deck where the mast stood.

Damage kit

A well-prepared offshore yacht carries the following, in a known, dry place:

The checks that make it work are practice (each crew member should know where the bungs, seacocks and pumps are) and a seacock drill: walk around the boat and touch every seacock, and name its fitting and the way to close it.

Worked Example: Knockdown and Downflooding

At 2300, in a Force 8 gale, your 12 m yacht is running under bare poles when a breaking wave strikes her on the quarter and rolls her to 100 degrees. The mast is in the water for a few seconds. She comes back upright. The companionway washboards were out for the ventilation, and 200 litres of water have come below. The crew are shaken, one is hurt, and the batteries are wet. The water is sloshing from side to side in the saloon.

1. Immediate priorities. Account for everyone, bring them into the cockpit clipped on, and stop the water: fit the washboards and close the hatch. Put on the lights.
2. Assess the free surface. 200 litres is about 0.2 tonnes: 2.5 per cent of the displacement. But on a wide saloon sole the free surface has a large effect: for a 3 m by 2 m surface, the virtual rise in G is (3 x 2 cubed / 12) x 1.025 / 8 = 2.0 x 1.025 / 8 = 0.26 m, which is a third of a typical GM. A second knockdown could be far worse. Get the water out fast.
3. Pump and bail. Use the manual and electric pumps and buckets. Check the strainers.
4. Reduce exposure. Stream a drogue to keep the stern to the seas and slow the boat, or heave to if you have searoom.
5. Check for damage. Rig, hull, companionway and electrics. Check the crew for injuries.
6. Tell the coastguard. A Pan-Pan with position, damage and intentions. Consider asking for a lifeboat to stand by.
7. Review the tactics. The wave was breaking at a size comparable with the hull length, and the yacht was running fast. Slow down further and keep the stern to the waves.

The key lesson is that the stability reserve is made up of the curve and the watertight integrity, and both must be maintained before the wave arrives.

Examiner Expectations

• Be able to define and sketch G, B, M, GM and GZ, and a GZ curve with the AVS marked.
• Answer: "what happens to stability if you fill that half-empty tank?" (the free surface disappears, stability rises) and "where is your range of positive stability?" (to the AVS, a figure you should know for your boat).
• State the damage control sequence from memory and explain each step. The RYA expects a systematic response: find the hole, stop the water, pump out, assess stability, decide.
• Know where all the seacocks and the bilge pumps are on the exam boat, and show it.
• Be realistic about pump capacity and the limits of the boat.

Stability in Waves

The GZ curve is calculated in calm water. At sea:

• Wave crest amidships: the bow and stern are less immersed and the waterplane at the middle narrows on many hull forms, so stability can drop markedly, just as the yacht is on top of the wave and most exposed to the wind and the breaking crest.
• Breaking waves deliver a sudden impact. A breaking wave higher than about 55 per cent of the hull length, striking the boat beam-on, can roll most yachts regardless of their static curve. This is why lying ahull in breaking seas is so dangerous and why active tactics (running off with a drogue, heaving-to, motoring into the seas) are preferred.
• Broaching: surfing down a wave face, the yacht can slew beam-on and be laid over, then struck by the following crest.
• Rolling resonance: if the period of the waves matches the boat's natural roll period, rolls build up. Altering course or speed changes the encounter period and breaks the cycle.
• Water on deck and in the cockpit adds weight high up with a free surface. Cockpit drains must be large enough to empty a flooded cockpit quickly.

Damage Stability

Floodable length

If a compartment floods, the yacht settles and trims towards it. The floodable length is the length of the hull that can be flooded without the boat sinking beyond a safety limit (the margin line, a line just below the deck edge). Watertight bulkheads divide the hull into compartments; if any one compartment is no longer than its floodable length, the yacht survives the flooding of that compartment.

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Most cruising yachts have no true watertight bulkheads: water from a hole forward runs aft through the bilge to the whole boat. Some offshore and racing yachts have a watertight collision bulkhead forward (the crash box), and some have watertight doors to the forepeak and lazarette. If your boat has them, keep the doors shut on passage; they buy time.

Damage stability assumptions

Commercial vessels, and MCA-coded vessels in some categories, must show by calculation that they survive defined damage.

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The key ideas carry over to any yacht:

• Permeability is the proportion of a compartment that water can actually fill. An empty space is about 95 per cent permeable; a space packed with stores, sails and machinery is less, so a full forepeak floods to a lower level than an empty one.
• After flooding, the yacht must have residual stability: a positive GZ curve with enough area to survive the sea state while you deal with the damage.
• Heel after damage must be limited, typically to 15 to 25 degrees, so that crew can work and pumps and hatches function.
• The flood water inside the boat has a large free surface. Even if the yacht stays afloat, her stability is greatly reduced.

Counter-flooding

Large ships sometimes deliberately flood a compartment on the opposite side to correct a list. On a yacht this is almost never appropriate. It reduces the heel, but adds more weight, lowers the freeboard everywhere, reduces the reserve buoyancy and adds a second free surface. Only consider it if you have calculated the final displacement and freeboard and are certain that the list itself is the main danger. Correcting a list by moving stores, crew or transferring fuel is usually better.

Damage Control

Sources of water

Find out where water is coming from before deciding what to do. Common sources, in rough order of likelihood:

1. Seacocks, hoses and hose clips (heads, engine intake, cockpit drains, galley sink).
2. Stern gland and propeller shaft, or saildrive seal.
3. Rudder stock bearing, or loss of the rudder.
4. Deck leaks, windows, hatches, chainplates and the anchor locker in heavy seas.
5. Keel bolts after grounding (a "keel strike" can crack the hull around the keel root).
6. Collision with a container, log or another vessel.
7. Tank leaks, water system failure (fresh water pumping into the bilge).

Taste the water: fresh water suggests a tank or the water system; salt means the hull or deck.

How fast water comes in

The flow through a hole depends on its area and on the depth of water above it (pressure head). Approximately:

Flow per second = 0.6 x hole area x square root of (2 x 9.81 x depth)

The depth relationship matters: a hole twice as deep lets in about 40 per cent more. Heeling the boat to lift a hole out of the water (tacking onto the other tack, shifting weight) can be the single most effective first action.

Pump capacity

Pump ratings are measured with no lift and clean water. On a yacht, the pump lifts water a metre or more to an outlet, through long, narrow hose, from a strainer that clogs with debris. Assume real output is about half the rated figure.

Compare this with the table above. Even a one-inch hole can overwhelm the bilge pumps of a typical cruiser. This is why stopping the water is always the priority, and pumping only buys time.

The damage control sequence

1. Raise the alarm and allocate tasks. One person helms and navigates; one starts the pumps; the rest search for the leak. Consider a Pan-Pan (or Mayday if sinking) early, so that help is on its way if the situation deteriorates.
2. Find the leak. Lift floorboards, open lockers, follow the water uphill. Check every seacock and the stern gland.
3. Stop or slow it. Close the seacock. Hammer a tapered softwood bung into a failed fitting (keep one tied to each seacock). For larger holes, stuff with cushions, sails or clothing, and brace with a floorboard and a wedge or a spinnaker pole. Underwater epoxy putty and sheets of plywood or neoprene can be screwed or braced over a hole from inside. Externally, a collision mat or a sail drawn under the hull with lines will be pressed against the hole by the water pressure and can greatly reduce inflow.
4. Reduce the pressure. Slow down, heel the boat to raise the hole, or change course to put the damaged side up and to leeward.
5. Pump. All pumps, and buckets. Keep strainers clear. Run the engine as long as possible for charging, but watch for water reaching the air intake and alternator.
6. Assess stability and buoyancy. Is the water level rising, steady or falling? Is the yacht listing or trimming? Are the inflow and pump rate stable? Close watertight doors. Secure loose weights.
7. Decide. Head for the nearest safe haven, request escort or a salvage pump (the coastguard and RNLI can drop or deliver pumps), or prepare to abandon. Do not abandon to the liferaft while the yacht is still afloat and you can still keep her so: "step up into the liferaft" means only leave when the boat is actually sinking beneath you.

Worked Example: Grounding and Flooding

At 0200, your 11 m yacht strikes a rock at 6 knots, 15 miles from the nearest harbour, in a moderate sea. Crew of four. Water is coming in forward at an estimated 50 litres per minute. Combined realistic pump capacity is about 80 litres per minute (manual plus electric). The bow is trimming down by about 5 degrees.

Immediate actions.

1. Stop the boat clear of the rock and check the depth: you do not want to strike again.
2. Allocate roles: skipper on deck in command, one crew pumping manually, one searching forward, one on the radio.
3. Send a Pan-Pan with position, nature of the problem, intentions and persons on board. Ask the coastguard to stand by and consider tasking a lifeboat with a salvage pump.

Assessment. Pumps exceed inflow by about 30 litres per minute, so the water level should fall, provided the pumps keep working and the leak does not grow. But the bow-down trim shows water is accumulating forward faster than it reaches the pump, probably because the forward limber holes are blocked.

Actions.

1. Find the source: a split in the hull at the forefoot. Stuff it with a cushion and brace it with a floorboard and the boathook.
2. Move crew and heavy gear (spare anchor, toolbox, water jerricans) aft to raise the bow and reduce pressure on the hole.
3. If you have a watertight bulkhead or forward crash box, close it.
4. Rig a sail as a collision mat over the bow if conditions allow.
5. Proceed to harbour at reduced speed (3 to 4 knots) to cut the water pressure at the bow. At 3.5 knots the 15 miles take about 4 hours 20 minutes. At a net gain of 30 litres per minute the pumps are winning, but a clogged strainer or an exhausted crew on the manual pump would reverse that. Rotate the pumpers every 15 minutes.
6. Monitor the bilge level and the inflow every 15 minutes, and report to the coastguard at agreed intervals.
7. Keep the liferaft and grab bag ready. If the water starts to gain, consider beaching on a sheltered shore rather than sinking in deep water.

Common Mistakes

• Overestimating pump capacity. Assume half the rated output, falling with crew fatigue and debris.
• Pumping before looking for the leak. Pumps buy time; stopping the water saves the boat.
• Leaving tanks half full in heavy weather. Press them up or empty them before the weather arrives.
• Weight creep: dinghies on davits, jerricans on rails, heavy anchors in the bow, gear on the coachroof.
• Open hatches and unsecured washboards in heavy weather, which lower the downflooding angle.
• Counter-flooding to cure a list without calculating the effect on freeboard and stability.
• Abandoning too early. Many crews have been lost from liferafts while their yachts were later found afloat.
• No bungs: softwood bungs should be tied to every seacock, and the crew should know where the seacocks are.
• Not knowing your boat's figures: AVS, design category and the location of every through-hull.

Summary

• Stability results from the couple between weight (through G) and buoyancy (through B). The righting lever is GZ; righting moment is displacement times GZ.
• GM governs initial stiffness; the GZ curve shows the whole picture: maximum GZ, AVS, range of stability and the area (energy) under the curve.
• An offshore yacht should have an AVS of at least 120 degrees; design category and STIX number summarise suitability.
• Raising weight, adding deck gear and partially filled tanks all reduce GZ at every angle. Free surface effect depends on the width cubed: keep tanks full or empty.
• Waves, breaking crests and downflooding make real stability worse than the calm-water curve.
• Damage control priorities: raise the alarm, find the leak, stop it, reduce pressure, pump, assess stability, decide.
• A small hole lets in a lot of water; realistic pump capacity is about half its rating.

Check Your Understanding

1. Define GZ and explain how the righting moment is calculated.

Answer: GZ is the horizontal distance between the line of action of weight through G and the line of action of buoyancy through B when the yacht is heeled. Righting moment equals displacement multiplied by GZ.

1. What is the angle of vanishing stability, and what minimum would you want for an offshore yacht?

Answer: The angle of heel at which GZ becomes zero; beyond it the yacht will continue to capsize. At least 120 degrees for offshore sailing, ideally 130 degrees or more for ocean passages.

1. Why is the area under the GZ curve more important than maximum GZ when considering capsize by breaking waves?

Answer: A breaking wave delivers energy as an impact. The area under the curve (multiplied by displacement) represents the energy the yacht can absorb before capsizing; a boat with a broader curve absorbs more energy even if her maximum GZ is the same.

1. A 10 tonne yacht has 200 kg of chain moved from the bow locker down into the bilge near the mast, 1.5 m lower. By how much does G move, and in which direction?

Answer: GG1 = (200 x 1.5) / 10,000 = 0.03 m downwards (and also aft if the chain moved aft), increasing GM by 0.03 m.

1. Why is a half-full wide tank more dangerous than a half-full narrow one of the same capacity?

Answer: The free surface effect depends on the width of the free surface cubed. Doubling the width increases the virtual rise in G eightfold, regardless of the amount of liquid.

1. What is the downflooding angle, and how can the crew increase it in heavy weather?

Answer: The angle of heel at which openings such as the companionway, hatches and ventilators become submerged and water floods in. The crew effectively raise it by closing and securing all hatches, fitting and securing washboards, closing ventilators and locking cockpit lockers.

1. A 25 mm seacock fails 0.5 m below the waterline. Roughly how fast does water come in, and how does that compare with a typical manual bilge pump?

Answer: About 55 litres per minute. A large manual pump rated at 60 to 70 litres per minute may deliver only 30 to 40 litres per minute in practice, so the pump alone cannot keep up. Closing the seacock or driving in a softwood bung is the priority.

1. List the damage control sequence in order.

Answer: Raise the alarm and allocate tasks (including a Pan-Pan or Mayday as appropriate); find the leak; stop or slow it; reduce the pressure (slow down, heel to lift the hole); pump with everything available; assess stability, trim and the trend in the water level; decide whether to make for harbour, request assistance, beach or prepare to abandon.

1. Why is counter-flooding rarely appropriate on a yacht?

Answer: Although it reduces the list, it adds weight, lowers freeboard everywhere, reduces reserve buoyancy and adds another free surface, so overall stability and survivability can get worse. Moving stores and crew or transferring fuel is usually better.

1. Why might a yacht be more vulnerable to capsize on a wave crest than in calm water at the same angle of heel?

Answer: With the crest amidships the hull's waterplane and immersed shape change, reducing stability, while the yacht is exposed to the full force of the wind and to the breaking part of the wave. The calm-water GZ curve overstates the available righting moment.

11. A yacht has a maximum beam of 3.6 m and a displacement of 6.5 tonnes. Calculate the capsize screening value and say whether it passes.

Answer: Volume = 6.5 / 1.025 = 6.34 cubic metres; cube root = 1.85. CSF = 3.6 / 1.85 = 1.95, just under 2.0, so it passes the screen but with little margin.

12. What is the difference between a list and an angle of loll, and how do you treat each?

Answer: A list is a steady heel to one side with positive GM, caused by off-centre weight; move the weight to the centreline. An angle of loll is a heel caused by negative GM; the yacht may flop to the other side. Cure it by lowering G (remove weight aloft, pump out free surface, add low ballast on the centreline), never by shifting weight to the high side.