Select Page

Capsize a:w

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

Stability 1

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).

Stabilty 2

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

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 fuel system

If engines are installed and serviced correctly then most marine engines are very reliable, but one of the most important parts of the engine to check and service is the fuel system.

Rudders and steering systems – Part 2

One thing all rudders have in common is that they have three main parts that need to be checked: the rudder, or a steerable drive leg in the case of many power boats; the system that joins the rudder to the steering; the steering control itself.

The Boatyard Book – a boat owner’s guide to yacht maintenance, repair and refitting

The Boatyard Book is a fully illustrated 224 page practical reference manual that provides advice for boat owners on planning and carrying out annual maintenance, repairs, upgrades and refits of sailing yachts and motorboats, up to 20 metres in length.

Stern gear maintenance

The stern gear of a boat needs to be checked carefully when the boat is ashore as this is something that can only be done when it is out of the water. The same applies for any maintenance and repairs that may need doing, so it is best to check it all over as soon after an end of season lift out as possible.

Preparing for sailboat cruising

Preparing for a sailing trip entails a lot of planning. In this blog, we take a look at some of the many safety aspects that a skipper needs to consider before heading off on a cruise.

Marine engine electrical system

The typical basic electrical system associated with a marine engine includes a dedicated engine starting battery, a starter motor, a charger in the form of an alternator, a solenoid and some engine sensors and instruments.

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.

How to operate a winch

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

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...

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.

The give-way hierarchy at sea – who gives way to whom?

Whatever their size or type, all skippers have a responsibility to avoid collisions with other boats at sea.  It is...

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.

ColRegs – avoiding collisions at sea

ColRegs - avoiding collisions at sea ColRegs Rule 8: Action to avoid collision (a) Any action taken to avoid collision shall...

Antifouling for leisure boats – Part 1

Boats that are kept afloat can very quickly become a home for small marine organisms such as barnacles, weed and slime. Applying an antifouling paint to your hull is necessary to protect it from these micro-organisms, as a fouled hull can cause problems and will slow down a boat’s maximum speed considerably if left unchecked.

Top 5 Reasons Why an Inflatable SUP Should Be Your Next Yacht Accessory

In this article, inflatable paddle board expert Jason Paul gives the top 5 reasons why an inflatable SUP should be your next...

VHF DSC radio – how best to communicate at sea

There are many ways to communicate with others at sea. What makes the VHF DSC radio the best form of short range...

Competent crew skills: arriving and leaving a berth

Skilled boat handling is needed when entering or leaving harbour. Crew tasks include preparing the mooring lines and fenders...

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.

Points of Sailing

The course on which a boat is sailing can be described by its angle to the wind, not to be confused with its compass...

Nautical paper charts – a reminder of the basics

The nautical chart is an indispensable tool for navigation. A chart is a graphic representation of an area of the sea which might also include coastlines, estuaries and islands. All cruising leisure boats should carry up-to-date paper charts.

Boat surveys

A full boat survey assesses the condition of the hull, mechanical gear and means of propulsion. The survey is carried out with the boat...

Seacock maintenance

If seacocks are always left open and neglected they can eventually seize which will prove a serious threat to boat safety should a connecting hose fail and the seacock refuses to close. There are three main types of seacock – ball valves, cone valves and gate valves.

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...

Top Ten Tips For Learning The ColRegs Boating Rules Of The Road

Colregs Boating Rules Of The Road Skippers struggle to learn and remember the ColRegs Yachtmaster and Day Skipper course...

Know your Navlights & Shapes – essential for all skippers

Know your Navlights & Shapes International Regulations for Preventing Collisions at Sea (ColRegs) Anyone who is...