Capsize - understanding the risks

A skipper should know how their boat will cope with rough seas. By working within known limits and understanding the risks, then the chances of a capsize occurring are much reduced.

Safety is all about improving the odds. When considering the odds of a boat capsizing, knowing the limitations of its design and stability are critical. In order to do this, it helps to understand the basic principles of how a boat remains upright.

Basic principles

A boat remains upright because of the way its weight and buoyancy interact. The basic principle of buoyancy is that the upward buoyant force on a body immersed in fluid is equal and opposite to the weight of the fluid that the body displaces. The weight of the fluid displaced is known as displacement and the displaced water has an up thrust, or buoyancy, which is equal to the weight of the boat. The displaced water has a central point, or centre of buoyancy, which varies according to the shape of a boat’s hull and keel.

The centre of buoyancy is not to be mistaken for the centre of gravity. The weight of a boat is distributed along its length, pushing the entire vessel downwards. All the weight acts downwards through a central point, or centre of gravity, which is similar to the fulcrum or central point of a seesaw. All the structure and the distribution of weight aboard contribute to a boat’s centre of gravity.

To keep a boat stable in the water and prevent it from toppling over requires the centre of gravity to be low, which is greatly helped by having a deep, heavy keel and an engine below the waterline.

Angle of heel

If a sailing boat heels over in a strong gust of wind or is forced over by a big wave, then it will right itself once the gust or wave has passed. When a boat is upright then the force of gravity is directly opposed to the force of buoyancy. As the boat heels over the centre of buoyancy moves outwards and acts as a lever does, pushing upwards with an increasing force. This is fine up to a point, but eventually as the boat continues to heel the righting lever effect reduces and eventually is lost and then the boat will capsize and float upside down. This point is known as the Angle of Vanishing Stability (AVS).

Boats with a high AVS will resist becoming inverted and return to the upright position quickly in the event of a knockdown. These include narrow, heavy displacement boats with a deep draft which can heel to 120º or more. Once capsized, only a small amount of further rolling moves the hull into the positive righting area and the boat comes back upright. Boats with wide beams and shallow drafts tend to have high initial stability but may capsize at 90º of heel and will not always be self-righting.

Righting moment curve

Boat manufacturers publish righting moment curves of their yachts to show the stability characteristics of their designs. In Europe the Recreational Craft Directive (RCD) states that pleasure yachts between 2.5m and 24m must carry builders’ plates to categorize their boats in either Category A (Ocean), B (Offshore) or C (Inshore) and meet minimum standards of stability.

Breaking waves

Rules and regulations are one thing, but the force of steep breaking waves can knock any yacht down in coastal waters, especially if it is caught beam-on. Research has shown that the most significant factor in capsize is whether a wave is breaking or not. If the wave is greater in height than the beam of the boat, then it can easily knock the boat over. Tests carried out at Southampton University in England have shown that almost any boat can be capsized by a wave equal to 55% of the boat’s overall length. Such waves may occur where the seabed suddenly shelves towards the coast, or where wind is blowing against tide.

This research points to the fact that yachts seeking shelter often find themselves in greater danger when approaching harbours than when coping with a storm further out to sea.

Being prepared

If you are well offshore in rough weather, consider your options. If needs be, heave to and ride out a storm as the boat will be more stable and comfortable, but check you have sufficient sea room to drift downwind and are not approaching a lee shore. Another option is to lie ahull, with no sail up and the helm tied to leeward. If conditions worsen then the next stage is to lie to a sea anchor or drogue, which will prevent the boat from meeting waves beam on and reduce the vessel’s drift rate.

Don’t automatically head for the nearest harbour or your intended destination. Check first what the conditions are likely to be there, by considering the state of the tide, wind direction and whether there are danger areas such as headlands and sand bars to contend with. Check out all the alternatives and be prepared to alter your plans in order to opt for a safe option.

Tips to prevent capsize:

Know your boat’s limitations.
Don’t overload the boat.
Pump the bilges regularly.
Keep a generous margin of safety.
Know when it is best to yield to conditions, rather than fight them.
Avoid areas known for overfalls and tide rips.
Avoid being caught beam on to breaking waves.

Boat engine cooling systems

Some boat engine breakdowns are unavoidable but those caused by lack of maintenance or regular checks can be avoided. Failure to maintain an engine’s cooling system is a well known example of this, so it is well worth spending time checking over the cooling system both when the boat is ashore and afloat.

Antifouling for leisure boats – Part 4

Applying antifouling. Antifouling is best applied on a dry, calm day. It is best to apply the antifouling in the middle of the day to ensure the hull is dry and as warm as possible.

What boating skills should you have before you buy a yacht?

Many people dream of owning a yacht and sailing off into the blue yonder. What boating skills should you have before you buy...

Marine engine oil system maintenance

The regular maintenance of a marine diesel is key to preventing engine failure at sea. This means doing regular checks of the fuel, cooling, electrical and oil systems.

Hourly Checks when sailing or motoring

Hourly Checks Get into the habit of carrying out these checks and both yourself, your crew and your boat will be...

How to operate a winch

Winches are drum shaped mechanical devices used to handle halyards, sheets and control lines. One of the important crew...

Marine toilets – care and maintenance

There are a number of different types of marine toilet, or heads. They fall into one of three categories – manual, electric and vacuum, the most common being the manual, hand pumped type. These have double acting piston-pumps which both discharge the waste and flush the toilet with sea water.

First Aid Afloat – fish spine injury

First Aid Afloat - Here is what to do if somebody stands on a fish spine: • Check for dangers. Is it safe for you to enter...

Boat engine basics

Boat engines come in all shapes and sizes and include inboards, outboards, petrol, diesel, electric and hybrid systems. Some engines are...

Dripless shaft seals

Dripless shaft seals are designed to completely stop water from entering a boat’s hull via the stern tube. There are two main types of dripless seals known as face seals and lip seals which many boat manufacturers now fit to production boats.

Essential Knots: Figure of eight

Essential Knots: Figure of eight Use: Stopper knot, prevents a rope from being pulled through a hole e.g. through a block or...

Boat Engine Failure – what to check

Engine failure If your engine fails or is overheating there are a number of things to check immediately: • Air filter...

Five dangers a vessel may encounter at sea

What are the main dangers a vessel may face at sea and what should skippers do to reduce the risk of these happening?

Passage planning and pilotage

Passage planning and pilotage help skippers navigate safely from one port to another. A passage plan takes into account all...

Wooden Hulls – Part 1

Traditional wooden boats have a plank on frame construction, a centuries old boat building method that is still in use today. Variations of the traditional method include carvel, clinker and strip planking, which all relate to the way the planking is attached to the frame.

Essential Boat Safety Briefing

Skippers Responsibilities Skippers are obliged to give a safety briefing to the crew even if they are a regular crew. At...

Boating emergency – how to broadcast a MAYDAY emergency call

How to broadcast a MAYDAY emergency call How to broadcast a MAYDAY emergency call if a vessel or person is in grave...

Rudders and steering systems – Part 1

Rudders and steering systems. A rudder is one of the most critical parts of a boat. Rudder failure is a common occurrence on neglected or overworked boats and a very unpleasant and potentially dangerous thing to happen when you are out at sea.

Sail boat rig checks – Part 1

Sailing boat rigs need to be checked regularly to reduce the risk of rig failure at sea. In part one of Sail boat rig checks we run through a series of useful checks that owners and skippers can carry out.