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above illustrate the required minimum height from the waterline
to each potential downflooding. Figures in parentheses show
the assessment option. |
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RIBs/boats with buoyant collars
These are treated as un-decked boats with flotation except that the downflooding
height test does not have to be applied in Categories C and D if, when
swamped, the boat can remain level supporting one third more than the
maximum total load. Such boats of over 6m (19.7ft) may attain Design Category
B if they also meet the relevant downflooding height and ‘wave and
wind’ requirements (Part 1, Option 3).
Requirements and tests used
Wherever possible either a physical test or a calculation may be used
to satisfy the requirements. In general practical tests are most suitable
for the smaller under-6m (19.7ft) boats and calculation is preferable
for the majority of boats over this length.
Downflooding Height
The boat is loaded with the manufacturer’s maximum recommended load
such that it floats aFt the designers intended trim. The height from the
waterline to each potential downflooding opening is then measured
The required minimum height can be derived in two ways. The first is a
simple function of boat length as shown in the diagram above. It varies
according to the Design Category and design option being used. This diagram
is a simplification of the one used in Part 3 of the standard. The second
is a more sophisticated and complex method given in an Annex, but which
often results in a lower required height. It produces a minimum height
for each individual opening according to its position on the boat.
Boats with minimal or no decking must have increased freeboard over the
forward third of the length. Conversely a reduced height is possible for
small openings in the aft portion of a boat or around the mounting of
an outboard.
Boats under 6m (19.7ft) fitted with an outboard motor have a separate
freeboard requirement to take account of the situation when one person
on board is starting the outboard, as this is sometimes more critical
than when fully loaded.
The diagrams above illustrate the way down- flooding height varies with
boat length and assessment option. The Stability Standard must be used
to determine these values accurately.
Downflooding Angle
For boats over 6m (19.7ft) length (see Part 1) this angle is usually calculated
and is greater than the heel angle measured during the Offset Load Test
by an amount which increases with the severity of the conditions that
may be experienced. There is an overall minimum for each Design Category.
A simplified method of calculation is provided in Annex C using the height
of the opening above the waterline and the distance off the centreline.
For boats under 6m (19.7ft) length (see Part 3) this requirement is expressed
as a minimum residual freeboard measurement during the Offset Load Test,
which varies from 10mm to 250mm (3.9in-9.8in).
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| Keeping
it watertight: boatbuilders must meet strict stability and buoyancy
standards for non sailing boats. |
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Offset Load
This measures the maximum heel angle resulting when people on board the
boat crowd towards one side. This may occur when the maximum number of
people is used, but if they cannot all be located to one side of the centreline,
a lesser number may produce a larger heel.
In Part 3 this is simply tested by positioning test weights, or people
of known weight as specified, to one side and measuring the residual freeboard.
The actual inclination angle is required to be checked only if
Option 2 (fully decked boats) is being used.
In boats under 4.8m (15.8ft) length a progressively reduced test weight
is used for each person to be represented, to correlate the test with
practical experience for smaller craft.
In Part 1, because the people can often be distributed in a variety of
ways, a formalised calculation routine is used to derive a test heeling
moment to be applied. The heel angle must be less than a sliding scale
that varies from 20° at 6m (19.7ft) to 10° at 24m (79ft) length.
Waves and wind
Applicable only to boats for Design Categories A and B, this assessment
requires a righting moment curve for the boat, which means careful derivation
of the height of the centre-of-gravity and detailed calculations for the
individual hull form — nowadays almost invariably conducted by computer.
The requirements take the form of minimum areas under the righting moment
curve. A naval architect will normally be required to perform such calculations.
Heel due to wind
Boats of Categories C and D with a high profile in relation to their length
are required to show that the heel angle under a steady wind is not more
than half the limit permitted for the Offset Load Test. Compliance is
established by a simple calculation based on the Offset Load Test results.
Level flotation
This is a practical test in which it is shown that the boat will support
a minimum weight — which is based on but not equal to the actual
crew weight — when the boat is swamped and that the boat will float
approximately level with at least two-thirds of the periphery above the
surface.
Boats under 4.8m (15.8ft) also must be capable being pumped or bailed
dry from the swamped condition with one person on board.
The stability in the swamped condition is also tested by suspending a
weight over the gunwale in each of four places in turn.
Basic flotation
This requirement demonstrates that a swamped boat will not sink, but not
necessarily float level. It can either be satisfied by testing or by a
calculation described in an annex that shows that the total buoyancy of
the boat exceeds the requisite weight.
Pro forma worksheets
The Stability Standard includes a series of pro forma worksheets to guide
designers and builders through the process of assessing a boat, starting
with the intended manufacturer’s maximum recommended load. Each required
entry identifies the relevant paragraphs of the standard for reference.
In this way a complex process is simplified as much as possible.
Copies of the various ISO standards referred to above are obtainable
from all national standards organisations — BSI, AINSI, AFNOR, DIN
and so on — and possibly through Marine Industry Associations. The
French marine industry association Fédération des Industries
Nautiques (FIN) offers all the relevant small craft ISO standards on a
CD ROM – check out www.fin.fr for details.
Note: The next issue of European Boatbuilder (February 2004) will look
at stability and buoyancy requirements for sailing boats.
For Part 1 click here
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