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.

Engine failure at sea – common causes and how to avoid them

Many engine failures are caused by lack of maintenance, resulting in fuel filter blockages, water pump failures, overheating and other breakdowns. Indeed, one of the most common reasons for marine rescue service call outs is for one of the most basic reasons possible – boats that have run out of fuel.

Learning about diesel engine maintenance

Marine diesel engines are internal combustion engines that are designed specifically for use in maritime applications. These engines are commonly used in a variety of watercraft, ranging from small boats and yachts to large ships and vessels. Learn about marine diesel engine maintenance courses.

Wooden Hulls – Part 2

It is important to ensure the essential hull maintenance of a wooden boat is done, even if you are paying others to look after your boat for you. The priority is to prevent rot from taking hold. The protective layers of paint and varnish over wood are far more critical than on GRP boats, where the topsides are painted more for cosmetic reasons.

Sterndrive maintenance

Sterndrives are a popular form of propulsion in the powerboat market, but require a fair amount of care and maintenance. The main factors to be aware of are salt water corrosion, lubrication and regular inspection of the bellows, the condition of which is vital to prevent water from entering into the hull.

Sending distress signals

In an emergency situation at sea, it is a top priority is to know how to send and receive emergency radio calls and alert others of your predicament. Likewise, if you receive a distress signal, you must be ready to go to the help of others.

Weather forecasting resources and tips

All competent sailors need to have a good basic understanding of how to interpret a weather forecast. They also need to be able to interpret the actual conditions they are experiencing.

Steel and Aluminium Hulls

The two metals used for hull construction are steel and aluminium. These are both very strong materials and will last a long time as long as they are cared for, which primarily means protecting steel boats from rust and aluminium boats from electrolytic action.

Estimating your position at sea

Navigators use a combination of techniques to estimate their position at sea. The primary method used today is GPS (Global Positioning System), a network of 24 satellites that became fully operational in the 1990s and was originally restricted for use by the United States military.

Propeller care and maintenance

Propellers are complicated and repairs should be done by specialists but owners can carry out checks and some routine maintenance themselves when the boat is in the boatyard. A propeller is critical to a boat’s performance, fuel consumption and ride, so it makes sense to keep a propeller in good working order.

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.

Keel design – options to consider when choosing a yacht

Keel design is constantly evolving and nowhere is this more apparent than in modern racing yachts such as the Imoca Open 60...

Medical Emergency at Sea

How to deal with a medical emergency afloat If you are planning a boating trip, it is important to have at least one...

Fire safety advice at sea from the Marine & Coastguard Agency

Fire safety advice for boaters Top fire safety advice at sea: 1. Fit smoke alarms, carbon monoxide and gas detectors 2. Turn...

ColRegs Rule 14 – Head-on Situation

ColRegs Rule 14: Head-on Situation (a) When two power-driven vessels are meeting on reciprocal or nearly reciprocal...

Finding your way at sea: waypoints

Any sea voyage needs a certain amount of planning before it is undertaken. It makes sense to think about where you are going, how you will get there and what factors might influence your plan. Planning the route itself is also critical. One of the essential parts of modern navigation is the use of waypoints.

Pleasure craft safety equipment recommendations

Safety equipment is an important part of boat preparation and it is advisable for all pleasure craft skippers to check their vessel is...

Diesel engine winterisation

An inactive boat engine needs to be protected from corrosion during the winter, caused by the rising humidity levels through the cold months and the salty coastal air. This applies whether the boat is left afloat or hauled out over the winter. Read here about the two important stages of winterisaton for a diesel boat engine.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 3 – Preparations

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the third of a 10-part post where solo sailor, Bernie Branfield, shares his first hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Feeling anxious at sea

Some people feel anxious at sea. Will they be seasick? What if they get caught in a violent storm? Could the boat...

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...

Boating Etiquette: Do’s and Don’ts on the Water

Boating etiquette is essential for ensuring the safety, enjoyment, and harmony of everyone on the water. Whether you’re a seasoned boater or new to the boating community, it’s important to be aware of the do’s and don’ts of boating etiquette. Take a look at our guidelines:

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?

Keeping boat records and doing checks

A boat’s records should provide information about maintenance schedules, when major work was done and when equipment was replaced or added to the boat. Without this information you are left guessing when things are likely to need replacing in the future and also what the costs are likely to be.

The Round The Island Race 2023

This year’s Round the Island Race turned out to be a really thrilling and competitive event. 1100 boats entered the race, which took place on Saturday 1st July. The race started from the world famous Royal Yacht Squadron line in Cowes UK, with the largest and fastest boats starting first, leaving the rest of us to wait our turn.