Thursday, February 15, 2007

The only way is up

The only way is up (part 1 of 2) An article posted on European Boat Builder online concerning boat stability (referencing the use of clinometers or inclinometers)

Andrew Blyth and Tom Nighy review the RCD’s stability and buoyancy requirements, which they both helped to draft as part of an ISO Small Craft Committee working group.

Ever since man first started building boats it will have been obvious that there is a lot more to successful design than merely staying afloat. Staying the right way up and keeping more water outside the boat than in have always been fundamentals.

However, the need to prove that a boat has adequate stability for its intended use to the satisfaction of an inspecting body is something relatively new to this industry. While adequate ‘rules’ and ‘standards’ for various types or classes of craft have existed for many years, it took the arrival of the EU’s Recreational Craft Directive to force a comprehensive standard.

The task of establishing a Stability Standard within the scope of the directive and its design categories fell to an ISO Small Craft Committee working group made up of delegates from 13 countries, including most EU and EEA countries, the USA, Canada and Japan. The result of that work has now virtually become law in the EU. Builders or distributors of any craft built to the EU RCD (94/25/EC) must be able to show that it has ‘sufficient stability and freeboard’.

The Stability Standard
In 2002 the Stability Standard was published as an International and a European Standard (Norme) No 12217. It has three parts:

· Part 1. Non-sailing boats of hull length greater than or equal to 6m

· Part 2. Sailing boats of hull length greater than or equal to 6m

· Part 3. Boats of hull length less than 6m

The Regulation
The RCD Essential Safety Requirements state that ‘The craft shall have sufficient stability and freeboard considering its design category… and the manufacturer’s maximum recommended load’, and ‘The craft shall be constructed to ensure that it has buoyancy characteristics appropriate to its design category… and the manufacturer’s maximum recommended load.’

The RCD introduced Design Categories to ensure that craft have acceptable minimum stability, freeboard and buoyancy to cope with the environmental conditions for which the boat is designed.

· Design Category A ‘Ocean’. Designed for extended voyages where conditions may exceed Force 8 (Beaufort Scale) winds and significant wave heights of 4m and above, and vessels are largely self-sufficient.

· Design Category B ‘Offshore’. Designed for offshore voyages where conditions up to, and including, Force 8 winds and significant wave heights up to, and including, 4m may be experienced.

· Design Category C ‘Inshore’. Designed for voyages in coastal waters, large bays, estuaries, lakes and rivers where conditions up to, and including, Force 6 winds and significant wave heights up to, and including, 2m may be experienced.

· Design Category D ‘Sheltered Waters’. Designed for voyages on small lakes, rivers and canals, where conditions up to, and including, Force 4 winds and significant wave heights up to, and including, 0.5m may be experienced. (Note: In the planned amendment to the RCD the 0.5m wave height in Category D will be changed from ‘significant’ to ‘maximum’, which is a reduction of 47 per cent. The Stability Standard anticipates this change and uses the lower height, which is already being used by builders.)

Interpretation
Despite the labels of Ocean, Offshore and Coastal, it is the wind and sea conditions that matter most. Large waves and strong winds can occur close to shore. For example, the
English Channel may have Category C or even D conditions during summer months, yet be Category A waters in a bad winter gale.

Therefore, ISO 12217 provides graduated stability requirements according to the Design Category taking account of the following hazards: Boats used in Categories A and B may encounter ‘steep breaking waves’ with a height greater than their beam. Such waves have the capability to invert the boat, they can also induce heavy rolling that can cause progressive flooding when openings become submerged.
For many boats the ‘angle of heel’ induced when all the people on board crowd to one side may be a critical feature. This may either result in excessive heel with the risk of people losing their footing, or the risk of flooding due to openings becoming submerged. Sailing boats particularly must be designed and equipped to resist the ‘capsizing effect of the wind’. And this can be significant for some non-sailing boats too. For small open boats the ‘risk of swamping’ may be countered by having sufficient freeboard relative to the sea state, or by installing sufficient flotation material to ensure that the boat will float and support the crew when swamped.

Manufacturer’s Maximum Load
The stability standard is also used to determine the maximum safe load (meaning people and any carry-on items) for a given boat in a specific Design Category. The maximum safe load must be displayed on the builder’s plate to meet the regulation.

The Physics
Floating — as Archimedes discovered 2,200 years ago, any floating body will displace a volume of water equal to its own weight. If, as is the case with all surface craft, the total volume of the boat exceeds the volume needed to support its weight, it will have a reserve of buoyancy. Thus the shape, which defines the hull volume, and the weight are both intimately involved. Changing either of these changes the buoyancy (and stability) properties of the boat. If water is kept out of the boat , either by sufficient freeboard or a watertight deck, the buoyancy derived from the volume of the hull will cause a boat to float even if it is built of heavy materials such as metals.
A boat is said to possess ‘swamped flotation’ if the materials from which it is built have sufficient volume in relation to the total weight enabling it to float when completely filled with water. Many small open boats incorporate sufficient low-density buoyancy materials or dedicated buoyancy compartments (air tanks) to enable the boat to support itself and the crew when swamped.

Stability — a boat is said to be stable if, when disturbed from its initial position, it has a natural tendency to return to that position. The greater the forces that may attempt to capsize the boat, the stronger that needs to be. So an appropriate minimum amount of stability depends on the hazards that a particular boat is likely to experience. For example, an ocean-going boat needs different stability properties to a vessel of similar size for use on a sheltered river or lake.

For Part 2 click here

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