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Introduction

Stability is a fundamental aspect of a vessel’s safety, but often it is not really understood. With terms like GZ, GM, righting lever and heeling moment, it’s no wonder the science of stability seems complicated. In the end, it all comes down to whether your vessel will come back to the vertical position.

The Canada Shipping Act (S 391) requires the owner and the master to ensure that the vessel is seaworthy – including having adequate stability – both before setting out and for the duration of the voyage. While most operators have a feel for their vessel, this is generally based on operating in less than the most extreme conditions the vessel may encounter. How then can you show that the vessel has an adequate level of stability?

New vessels: In February 2005, Transport Canada established minimum stability criteria for new small commercial vessels – those built after March 31, 2005. New vessels must be assessed using the International Standards Organization standard ISO 12217-1 – Small Craft Stability and Buoyancy Assessment and Categorization or STAB 6 of the Stability, Subdivision and Load Line Standards (TP 7301) to determine the maximum wave height and wind speed the vessel can be expected to handle safely given its stability characteristics.

Existing vessels: For existing vessels – vessels built on or before March 31, 2005 - Transport Canada has taken steps to help owners and operators move beyond relying on “feel” in assessing their vessels’ stability characteristics by identifying recognized stability standards that are appropriate for small non-pleasure vessels.

Who can use this guide? ^

The simplified assessment described in this guide is not for all vessels. To be able to use it, a vessel:

MUST

• have been built before April 1, 2005; and
• be more than 6 metres in length overall.

MUST NOT

• be more than 15 tons gross tonnage, or if not measured for tonnage, not be more than 12 metres in length overall;
• carry more than 12 passengers and/or 1,000 kilograms of cargo;
• travel further than 25 nautical miles from shore;
• be inspected annually;
• make international voyages;
• operate in waves greater than 2 metres (6.5 feet) high if fully-decked or 1.2 metres (4 feet) high if open (fully-decked and open are defined on page 5);
• be a commercial fishing vessel; a sailing, inflatable or multi-hulled vessel; a vessel engaged in towing, dredging or lifting operations; or any other vessel of non-conventional design or usage.

If your vessel falls under the “must not”, you can refer to other standards, as outlined in the table below.

 If requirements for your vessel are not listed above, contact your local Transport Canada Centre or a marine consultant.

Scope of the Simplified Assessment ^

Transport Canada developed the Simplified Assessment of Intact Stability & Buoyancy of Small Non-pleasure Vessels from the International Organization for Standardization’s (ISO) standard for stability and buoyancy for small craft.

The most important aspects of stability are taken into account – the resistance to swamping (watertight integrity; downflooding height) and the effect on the vessel of moving its load to one side (offset heel angle and residual downflooding height).

Because the Simplified Assessment does not include all the calculations of the ISO standard, the values for the criteria (e.g. maximum heel angle) are more conservative. In addition, where the ISO standard sets out four categories of environmental conditions (maximum wave height and wind speed) that a vessel can operate in, the Simplified Assessment is not for vessels that travel more than 25 nautical miles from shore and restricts fully decked vessels to operation in maximum wave heights of 2 metres. Open vessels are restricted to operating in waves of no more than 1.2 metres. The difference between fully-decked and open vessels is described later in this guide.

If these conditions are too restrictive for your operation, you should assess your vessel against a full stability standard.

Definitions ^

(L) Length - distance in metres from the tip of the bow to the furthest point aft on the stern, excluding removable parts that can be detached in a non-destructive manner and without affecting the structural integrity of the craft (e.g. spars, bowsprits, pulpits at either end of the craft, stemhead fittings, rudders, outdrives, outboard motors and their mounting brackets and plates, diving platforms, boarding platforms, rubbing strakes and fenders).

(B) Breadth – distance in metres between the outermost permanently fixed parts of the hull, including all structural or integral parts of the craft such as extensions of the hull, hull/deck joints and bulwarks. Breadth excludes removable parts that can be detached in a non-destructive manner and without affecting the integrity of the craft (e.g. rubbing strakes, fenders, guardrails and stanchions extending beyond the craft's side, and other similar equipment), but does not exclude detachable parts of the hull which act as hydrostatic or dynamic support when the craft is at rest or underway.

(F) Freeboard – distance in metres, measured amidships, between the waterline and:

1. the intersection between deck and hull; or
2. for rounded deck edges, the natural intersection, or,
3. where no deck is fitted or the hull extends above the deck (bulwark), the upper edge of the craft's hull.

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