Using the ISO Small Craft Stability Standard ISO 12217-1

Introduction

Stability is a fundamental aspect of vessel safety. To provide a level of assurance that new vessels have an accepted level of stability, amendments to the Small Vessel Regulations and Section 5 of the Construction Standards for Small VesselsTP 1332 (2004) introduced in February 2005 require power-driven non-pleasure vessels more than 6 metres in length overall and not more than 15 tons gross tonnage (or 12 metres in length if not measured for tonnage) to comply with the requirements of ISO (International Organization for Standardization) standard 12217-1 Small craft – Stability and buoyancy assessment and categorization if their construction began after March 31, 2005, unless they are: sailing vessels; multi-hulls, inflatable craft; built or converted for towing, dredging or lifting; or intended to carry more than 12 passengers or 1,000 kg of cargo.

Purpose

This document is intended to explain the concepts underlying the standard and how to select from the assessment option for your vessel from the various options found in ISO 12217-1.

Word of caution

Compliance with ISO 12217-1, or any stability standard, provides a level of assurance that a vessel will retain its stability if operated within the prescribed operating conditions, however it does not guarantee total safety or freedom from risk of capsize or sinking.

Assessing the proposed design against the requirements of ISO 12217-1 before starting construction will help ensure that the design is suitable for the vessel’s intended operation.

Accessing the standard

Copies of ISO 12217-1 – Small craft – Stability and buoyancy assessment and categorization and the two primary supporting documents – ISO 12216 – Small craft – Windows, portlights, hatches, deadlights and doors – Strength and tightness requirements and ISO 11812 – Watertight cockpits and quick-draining cockpits can be purchased from IHS Global, distributor of ISO documents.  Alternatively, you may visit your nearest Transport Canada Centre to view the documents and make notes, although due to copyright restrictions you may not photocopy any part of the standards.

References to other standards

ISO standards refer to other standards to make it easier to update reference information. In addition to ISO 12216 and ISO 11812, ISO 12217-1 refers a number of other standards that you are unlikely to need unless you are using unusual materials. The standards referred to in ISO 12217-1 can be obtained from IHS Global or by contacting your local Transport Canada Centre.

Establishing stability the ISO way

The basic concept behind the ISO stability assessment is determining a vessel’s resistance to taking on water, i.e. its ability to keep water out or shed water once on board, and comparing that to the minimum requirements for various operating conditions.

ISO defines four categories of operating conditions, called design categories, according to significant wave height and wind force (see below).  A vessel is considered sufficiently stable to operate in the maximum conditions of a design category if it meets or exceeds the minimum requirements of a design category for resistance to taking on water.

When assessing a vessel to ISO 12217-1, you examine it to determine whether it is fully-decked, partially-decked and “any amount” of decking,. (categories of ability to take on and shed water) then carry out a series of measurements and compare them to the corresponding requirements for the design category.

Sections 1 to 7 of the standard systematically explain ISO 12217-1, from scope and definitions to the assessment procedure and interpreting the results.  Annexes A to G describe how to carry out the various measurements and tests referred to in the standard.

Not all the measurements need to be made, however. One of six assessment options (combinations of requirements) is selected based on the desired operating category and the vessel’s resistance to taking on water. Annex H summarizes the requirements for each option.

Design Categories

The demands on a vessel to right itself increase as its operating environment becomes more extreme. ISO has established four design categories based on wave height and wind force.  A vessel is assigned the highest design category for which it meets all of the requirements.

Design Category A B C D
Wave height (significant / maximum) up to 7 metres significant 4 metres significant 2 metres significant 0.5 metre maximum
Typical Beaufort wind force Up to 10 Up to 8 Up to 6 Up to 4
Wind gusts to 100 km/h 60 km/h 60 km/h 45 km/h

Gust speeds rounded

Significant wave height - average height of the highest one-third of the waves.  Some waves will be twice the significant wave height.  Note that the limit for Category D is one-half metre maximum, not significant, wave height.

Fully-decked, partially – decked and open vessels

The requirements for downflooding height (height from the waterline to any opening that may let water into the vessel) and /or flotation increase as the vessel’s resistance to taking on water decreases.

ISO 12217-1 refers to standards ISO 12216 (determines degree of watertightness) and ISO 11812 (determines ability to shed water).  Use all three standards to categorize your vessel as fully-decked, partially-decked or “any amount” – essentially a vessel open to swamping.

Method of determination – mathematical or physical

Vessels may be assessed by way of calculation if all required information can be determined from the plans.  If not, then the assessment will be by physical test. 

Navigating the standard

To guide you through the procedure of assessing your vessel, the standard includes ten calculation worksheets.

Complete Worksheet 1 to arrive at the values required in future calculations.

The answers to the first two sections of Worksheet 2 allow you to determine which of the six options your vessel is eligible to use.  Select the desired option and complete the worksheets indicated. 

Example: The vessel is partially decked

Options 1 and 2 cannot be selected as they call for a fully-decked vessel. 

If design category B is desired, select option 3 and complete worksheets 3, 4, 5, 6, 8 and 9.

If design category C or D is desired, any of options 4, 5 or 6 can be selected.  The likely choice is option 5, as option 4 requires a swamping test and the requirements of option 5 are less stringent than those of option 6.  Complete worksheets 3, 4, 5 and 9 and possibly worksheet 7.

Determining assigned design category

Each worksheet determines the design category that can be assigned. 

Once all worksheets have been completed, the vessel is assigned the lowest design category appearing on any worksheet. 

Word of advice

Sit down and review the requirements of the standards carefully before starting construction.  Knowing what you have to do before you start out – including understanding of the terms used in the standards, such as downflooding height/opening.  By planning ahead with meeting the requirements in mind, you can avoid after the fact alterations that can be expensive and detract from the look of the vessel.  For example if sills are not high enough for the design category, you are looking at a retrofit - a shortened door and extended sill.  You can also save yourself time and money by using the approved appliances found on the International Marine Certification Institute website since custom built appliances must be tested and independently witnessed to ensure they comply with the standards.

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