Running a tight ship

Andrew Blyth and Tom Nighy review RCD requirements for cockpits, windows, hatches and doors.

In previous articles we have discussed the stability and buoyancy requirements of the ISO Standard 12217. Clearly the stability and buoyancy properties required by this standard are only achieved if the boat can shed water sufficiently quickly if swamped, and apertures in the hull or superstructure are sufficiently watertight when closed.

For this reason, this standard is supported by two other important standards containing detailed technical requirements that affect the ease of draining and the watertight integrity of the hull and superstructure. These are:

• ISO 11812 — Watertight cockpits and quick-draining cockpits.
  
• ISO 12216 — Windows, portlights, hatches, deadlights and doors.

The principal requirements of both these standards are described later, but the actual text of the standards should always be consulted.

Cockpits
Swamping of a cockpit or other recess in the deck of a boat may cause water to flood below and, because of the added weight of water and its movement, may make the boat vulnerable to further swamping. Therefore, it is important where water may accumulate that such areas have the following:

• Watertight boundaries
  
• Sufficiently high sills to doors and openings
  
• Drains and height of sole that enable the water to drain at an appropriate speed
  Although the title of this standard only refers to cockpits, ISO 12217 — Stability invokes the same requirements for all recesses that may hold water, however briefly. Other such recesses might include decked areas bounded by bulwarks and anchor wells. ISO 12217 requires all boats intended for Design Categories A and B to be ‘fully decked, the definition of which is a boat in which the horizontal projection of the sheerline area comprises any combination of:
  
• Watertight deck and superstructure, and/or
  
• Quick-draining recesses which comply with ISO 11812, and/or
  
• Watertight recesses complying with ISO 11812 with a restricted total volume
  ISO 11812 defines requirements for both watertight cockpits and quick-draining cockpits.

Watertight cockpits
Watertight cockpits do not have to meet the drainage time requirements applied to quick-draining cockpits. For this reason, ISO 12217 restricts the total volume of watertight cockpits and other recesses to (boat length x beam x freeboard amidships)/40 for operation in Design Categories A and B. This is a very small volume, so most boats are designed to the quick-draining requirements.

Quick-draining cockpits
What is this new word — quick-draining?
It was invented by the ISO Working Group to make a clear distinction between a cockpit that meets the rapid drainage requirements of ISO 11812 and a cockpit with drains that might only be sufficient to clear rain water or light spray. Because the term ‘self-draining’ was already well established, it was necessary to make this distinction immediately clear.

To be ‘quick-draining’ a recess or cockpit must have a minimum height of the sole above the loaded waterline (see Figure 1) and a maximum time to drain from full to a remaining depth of 0.1m. Recesses such as decks with bulwarks up to 0.1m high are considered to be quick-draining, even if they have no drains! The standard requires a maximum draining time that varies according to cockpit volume (given by the coefficient kC which equals the ratio of cockpit volume to the product of (hull length x maximum beam x freeboard amidships)) — see Figure 2. That is, recesses that are smaller in relation to the above-water volume of the boat are permitted longer to drain. The maximum drainage time for any cockpit is five minutes.

At least two drains must be fitted to a cockpit, positioned so that at least 98 per cent of the cockpit volume will drain when a boat is upright, and 90 per cent will drain when heeled in either direction. Non-sailing boats and sailing multihulls are considered when heeled at 100, and sailing monohulls at 300 heel or when the side deck first becomes immersed if this happens sooner.

Compliance with the drainage time requirement can be demonstrated by test or calculation. (Annex C of ISO 11812 describes how to assess complex cockpit geometries). However, if the cross-sectional area of drains (in m2) exceeds five per cent of the cockpit volume (in m3) then the drains are deemed to be satisfactory without further detailed calculation.

There are additional requirements regarding number and minimum dimensions of drains, and allowing for blockage caused by debris guards.

Sills
Both watertight and quick-draining cockpits have minimum requirements for the height of companionway sills above the recess sole. See Figure 3. Where a sill has a permanently attached moveable part (eg a hinged flap), it must satisfy Figure 3 when the flap is in the closed position, and the top of the fixed part of the sill must exceed half the heights shown.

Watertightness
Four degrees of watertightness are defined, and are also used in ISO 12216 in relation to hatches, doors and windows: Degree 1 is an impermeable structure giving protection against continuous immersion (namely the hull).

Degree 2 gives protection against temporary immersion in water, obtained when less than 0.05lt of water is admitted during a defined three-minute hose test. This is required on the bottom and horizontal surfaces of cockpits, and up the sides to the level of the minimum sill height in Figure 3.

Degree 3 protects against splashing of water, defined as when less than 0.5lt is admitted during the standard hose test. This is required on the sides between 100 per cent and 200 per cent of the minimum sill height of Figure 3.

Degree 4 protects against water falling at up to 15o from the vertical, tested with a spray nozzle mounted 2m above the centre of the appliance, less than 0.5lt to be admitted in three minutes. This is needed for structure above twice the minimum sill height.
Annex E of the standard describes how to test the degree of watertightness.

Windows, portlights, hatches, deadlights and doors
ISO 12216 specifies requirements for the strength and watertightness of windows, portlights, hatches, deadlights and doors, because inadequacy in these respects would prejudice the integrity of the hull and superstructure and hence the safety of the boat. Such fittings are termed ‘appliances’.
Both the degree of watertightness, and the design pressure to be used in assessing the strength required are dependent on the location of the appliance on the boat, some areas being more vulnerable than others. These areas are described in Figure 4 . The definition of Area I is further explained in Figure 4.

Watertightness
The minimum required degree of watertightness (as described earlier) required for appliances in these areas is shown in Figure 5. Sliding appliances must not be used in Area I or on the decks of the outrigger hulls of trimarans. These requirements do not override those for the watertightness of cockpits.
Opening appliances must have a positive way of closing to prevent inadvertent opening.

Strength
Basic design pressures for the strength of appliances vary from 6 kiloPascals to 70 KiloPascals according to appliance location area, boat type and Design Category.

Formulae are given for calculating the minimum thickness of appliance panels of a variety of materials, considering both allowable bending stress and permissible deflection. These formulae also take into account panel size, aspect ratio (ie: length to breadth ratio) and curvature. The materials covered include:

• Polymethylmethacrylate
• Polycarbonate
• Monolithic tempered glass
• Laminated glass
• Mahogany plywood
• GRP mat with 30 per cent glass
• GRP mat and roving with 30 per cent glass
• Aluminium alloy 5083-H111
• Mild steel

Minimum safety factors and thicknesses are specified for each material, the latter also being dependent upon the appliance location area. (Figure 4).

Opening deck hatches must also be subjected to a ‘unintentional stepping test’ and a ‘rope jamming test’, so as to avoid undue vulnerability to accidental misuse. A ‘hinge strength test’ is also prescribed.

Pre-calculated tables are provided (in Annex F) for the two common glazing materials polymethylmethacrylate and tempered glass, so that the required thickness is easily obtained for a given panel size and design pressure depending on the method of securing the glazing.

Design Details
ISO 12216 also includes requirements for a number of design details, such as the design of sliding appliances, the fixture of glazing into frames, stops, locking systems, multihull escape hatches, and the design and positioning of hinges. Annex D of ISO 12216 includes how to test mechanical or glued joints.

Many boatbuilders will have no need to delve into the finer details of this standard, especially the calculation of minimum thicknesses, as commercially available appliances are required to be supplied with the following details:

• Highest applicable Design Category
• Boat type
• Permitted location area

Reputable suppliers will thus have already completed the detailed calculations, and it only remains to ensure that suitable appliances are fitted in the appropriate appliance location areas (I, II III and IV).

In European Boatbuilder July 2004 we look at the Scantlings Standard.

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