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Introduction
Stability is too important to leave to intuitionStability 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.
Ship Safety Bulletin 07/2006 sets out five standards that owners may choose from
if
they wish to assess their vessel. The Transport Canada Simplified Assessment of
Intact
Stability & Buoyancy of Small Non-pleasure Vessels is the easiest of the
standards to use.
With a few simple measurements, operators can assure themselves that their
vessel is
likely to stay right side up within defined operating conditions. It is
important to
remember though, that compliance with any stability standard does not
guarantee freedom from risk of capsizing or sinking. This guide explains how to carry out the Transport Canada Simplified Assessment
of Intact
Stability & Buoyancy of Small Non-pleasure Vessels. Read on to learn
more.
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Intact Stability defined:
The vessel’s stability in the intact (normal or undamaged)
condition.
Stability – capability to return to the upright condition after
heeling due to waves, wind, etc. |
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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.
Less than 6 metres |
Construction Standards for Small Vessels (TP 1332) – Section
4 |
More than 15 gross tons, or 12 metres if
not measured for tonnage, or carrying
more than 12 passengers and/or 1,000 kg of
cargo |
Stability, Subdivision and Load Line Standards (TP 7301)-
STAB 5 or STAB 6 Passenger Vessels only may use Standards for the Construction
and Inspection of Small Passenger Vessels (TP 11717)
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Going further than 25 nautical miles
from shore or making international voyages |
Select from one of several recognized stability standards (see
Ship Safety Bulletin 07/2006) |
Built on or after April 1, 2005 |
ISO 12217-1 – Stability and buoyancy assessment
and
categorization or Stability, Subdivision and Load Line Standards –
(TP 7301) - STAB 6 |
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.
In 2004, a workboat capsized. One person died.
The Transportation Safety Board found that a modification made to the vessel – the wheelhouse was raised 1.25 metres – was a contributing factor in the incident. |
Stability can change:
If you modify your vessel, its stability can change. Carry out the stability assessment again after you have made any modifications. |
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:
- the intersection between deck and hull; or
- for rounded deck edges, the natural intersection, or,
- 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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