What is the difference between a fully decked and an open vessel?

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A vessel that can take on water and get rid of it quickly can operate in more unfavourable conditions than one that can’t. The assessment criteria for fully decked vessels are considered valid for operation in waves up to 2 metres in height, whereas the criteria for open vessels are considered valid for waves up to 50 cm high. If your vessel meets the following criteria, it qualifies as a fully decked vessel.

Fully decked vessels have:

• a watertight ¹ deck located at least the distance from Table 1² above the waterline when the vessel is fully loaded, and either:
• freeing ports ³ with total area not less than 4 per cent- of bulwark area; or
• watertight ¹ cockpits/recesses with a combined volume not more than L × B × F /40 (length × breadth × freeboard/40) ⁴.

Note 1: A deck, cockpit or recess is watertight if you can spray it directly for three minutes with a hose that has a flow of at least 10 litres per minute and no more than 5 centilitres of water (less than a quarter cup) passes through.

Note 2: May be reduced to not less than 75 mm (3”) above the waterline when the vessel is fully loaded when equipped with effective non-return devices. Consult your local Transport Canada Centre.

Note 3: At a minimum, two freeing ports (one port and one starboard) may be accepted each having a clear area of at least 225 cm² – the ports may be fitted in the transom on vessels where the shipping of water will not result in a trim by the head, preventing the deck from draining.

Note 4: Freeboard ( F ) is used to determine whether a cockpit/recess can be ignored from the perspective of swamping by calculating its volume as a percentage of reserve buoyancy. If a vessel has a well deck with scuppers only, then F would be measured from the top of the gunwale. If, however, there are large freeing ports (minimum 4 per cent of bulwark area) or flush deck with rails only, F should be measured from the deck.

Table 1

For lengths between 6, 9 and 12 metres, calculate the appropriate distance.

An open vessel is any vessel that is not full decked.

Preparing for the assessment

A little preparation and some readily available equipment are all that’s needed to carry out the assessment. Before you start, you will need:

• a measuring tape;
• weights representing the full complement of passengers and crew (75 kg per person) and, if applicable, cargo and diving equipment (36 kg per diver). Weights may consist of: water-filled drums; jerry cans; fish boxes; sandbags, etc. As a last resort, people may be used if lifejackets are worn and the testing is carried out with caution;
• a scale to measure the weights if their weight is not known and cannot be established by other means; and
• if you have not already determined whether the vessel qualifies as fully decked, a garden hose, with jet nozzle, capable of a continuous flow of a least 10 litres per minute (pressure = 200 kPa /30 psi ).
• equipment for measuring the angle of heel: a pendulum (string and weight) and marker; or a clinometer - a simple instrument for measuring the angle from vertical.

On the day of the assessment, prepare the vessel as follows:

• moored in calm water, mooring lines slack;
• fully loaded at design trim;
• scuppers and downflooding openings that may be immersed during offset load test temporarily sealed as a precautionary measure to prevent water from entering the vessel during the test;
• tanks for carrying liquids filled to at least three-quarters full;
• the helmsperson at helming position; and
• weights distributed to represent the typical operational distribution of the complement, with the vertical centre of gravity ( CG ) for simulated passenger weights as close as practical to 75 cm above the deck. Note: For multi-deck vessels, place the weights so that they simulate the distribution of the complement among the decks with maximum capacity on upper decks.

Carrying out the assessment step by step

1. Determine the passing marks

Use the tables in the Appendixes to determine the minimum downflooding and residual downflooding heights and the maximum heel angle based on the overall length of your vessel. Write them down on the Record of Stability and Buoyancy Assessment in the following section.

As you carry out the assessment, you will be able to tell whether your vessel meets the criteria.

2. Measure the downflooding height

With the vessel upright and at the design trim in the fully loaded condition, measure the height in metres from the waterline and the downflooding opening (see below). This is known as the downflooding height. Record the outcome.

Determining the downflooding opening

The downflooding opening is the lowest point that could let a significant amount of water into the interior or bilge of a vessel. It can be an opening in the hull, or the deck for vessels being assessed as fully decked, such as a vent, non-watertight hatch, window or door. Openings that are equipped with non-opening appliances or watertight opening appliances that are clearly marked “Watertight Closure – Keep Shut While Underway” or that would not cause the vessel to sink if the affected compartments were flooded are not downflooding openings.

Engine space louvered vents (intakes) located just below the top of the gunwale on an open vessel may also be disregarded provided a high water alarm and efficient bilge pumping system are fitted.

Use the following as guidance to determine which openings are downflooding openings:

• cross sectional area in square mm bigger than 50 times the length in metres squared; or
• cross sectional area in square mm bigger than 660 times the fully loaded displacement in tonnes.

If displacement is available, use the smaller of the two values.

See Appendix 1 for examples of downflooding height measurement on vessels of varying configurations.

3. Measure the heel angle and downflooding height in the offset position

To measure the angle of heel, you can use a measuring tape, a pendulum, or a clinometer. If you have a clinometer, this is the easiest method and needs no further explanation. Start with the vessel in the upright position.

Measuring Tape method: Measure the distance from the centreline to the top of the gunwale, and from the top of the gunwale to the waterline. Record these in the record of assessment in the following section.

Pendulum method: Create a pendulum on the centreline by setting up a string with a weight at the end. Make it as long as possible and mark the resting position of the weight on the deck. Measure the length of the pendulum and record it in the record of assessment. Suspending the pendulum weight in a liquid such as oil will dampen excessive movement and make measurement easier and more exact.

Taking care to ensure that the vessel’s stability remains within acceptable limits, shift weights to one side of the vessel simulating realistic passenger and crew shift to the side and re-measure the distance from top of the gunwale to water (measuring tape method), the distance the pendulum moved (pendulum method) or angle of heel (clinometer method). Re-measure the downflooding height. Record the results, calculate the angle of heel and record it as well.

Shift the weights to the other side of vessel and repeat the measurements.

Note: The weights are to be placed so that they represent the most extreme situation that may occur. If all passengers are likely to rush to one side of the vessel if a whale is sighted, for example, move only the weights representing passengers but leave the cargo weights in place, assuming the cargo will be secured and will not shift as the passengers change position.

If your vessel does not pass

If the vessel doesn’t meet the pass criteria, you can remove weights totalling 75 kg at a time until the vessel meets the criteria. You will then know how many people the vessel can carry and still meet the Simplified Assessment criteria.

Other options are also available. Assessment against a full stability standard – ISO , US or UK – is recommended. Because the assessment is more extensive, the full standards have less stringent criteria than the Simplified Assessment and should result in less restrictive operating constraints for your vessel.

For more information on the options available, check the Small Vessels webpage on the Transport Canada Marine Safety website at www.tc.gc.ca/smallvessels.

Capsizing and swamping are the most likely of all marine incidents to result in fatality. - Transportation Safety Board

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