PREVIOUS | TABLE OF CONTENTS | NEXT

Chapter 3 - Buoyancy, Sub-Division and Hydrostatic Stability.

301. Definition.

1. In considering the extent of damage to be assumed in Sections 310 and 311 and in subsequent calculations, the following definitions shall be used :-
   
   "L" means the length, measured on the intact waterline when floating at maximum certified weight, between the forward and after extremities of the centre-line of structure within which spaces contributing to intact buoyancy are provided, and
   
   "B" means the breadth, measured on the intact waterline when floating at maximum certified weight, of structure at mid-length of the damage under consideration, which contains spaces contributing to intact buoyancy.
   
   
2. "Watertight", with respect to a space, means a space which satisfactorily meets the test requirement of Section 3207 of this Standard.

302. Intact Buoyancy Provision.

All A.C.V's designed to operate over water shall be provided with buoyancy which, when intact, provides at least 100% reserve when floating in fresh water of density 1 000 kg/m³ (62.5 lb/ft³) at maximum certified weight. The designer shall declare all spaces for which buoyancy is claimed, and the amount of buoyancy claimed for each space.

303. Intact Buoyancy Distribution.

Spaces claimed for intact buoyancy shall be evenly distributed over the vehicle plan-form so far as is practical, and shall be below a datum watertight deck or structure from which passenger emergency evacuation into survival craft could be effected.

304. Eligibility for Intact Buoyancy. 

Contributions to intact buoyancy shall be claimed only for:-

1. Spaces designed and provided exclusively for buoyancy;
   
   
2. Spaces designed to carry fuel or other liquids, only to the extent of buoyancy contribution when the spaces contain their designed quantity of liquid and the vehicle is floating at maximum certified weight.
   
   
3. Other spaces within the vehicle up to a level 76 mm. below the lowest point at which, when the vehicle is floating at maximum certified weight in still water, that water may enter the vehicle at any point.

No space above the level of the lowest opening through which progressive flooding may occur when the vehicle is prepared for emergency evacuation of passengers shall be claimed in calculating intact buoyancy. Volumes of any such spaces may be considered in calculations of damaged stability.

305. Buoyancy Media. 

If buoyancy is provided by other than structural voids, the buoyancy media and their method of application shall be approved. (see Appendix 1 to this Chapter.)

306. Buoyancy Sub-division.

Air spaces designed to contribute to intact buoyancy shall be watertight, and shall be sub-divided to ensure that the provisions of Sections 312, 313 and 314 are complied with.

307. Watertight Integrity. 

Any penetrations of boundaries of air spaces contributing to intact buoyancy shall be watertight; any piping or ducts passing through such spaces will require to be approved with specific reference to the possibility of flooding other compartments when considering the damage postulated in Sections 310 and 311, and the possibility of transfer of bilge water when bilge pumping.

308. Buoyancy Space Access. 

All spaces designed to contribute to intact buoyancy shall be provided with ready access for internal inspection, and air spaces shall be provided with means of detecting and removing accumulated liquids. Where access for detection of liquids by visual means on a regular basis is not practical or appropriate, buoyancy air spaces shall be provided with flood alarms giving indication in the compartment from which the vehicle is normally manoeuvred.

309. Intact Stability. 

The vehicle when floating in still water at maximum certified weight shall have positive stability, and shall not exceed an angle of 8° in any direction under any permitted loading condition.

310. Bottom Damage to be considered. 

In determining the distribution and sub-division of buoyancy spaces, the designer shall assume the following bottom damage:-

1. Damage resulting in loss of watertight integrity over the least of the following lengths measured parallel to the centre-line:-
   
   i) 0.1 L
   
   ii) 3m. + 0.03 L
   
   iii) 11 m.
   
   
2. Such damage shall be assumed to extend vertically the lesser of 0.02 B or 0.5 m.
   
   
3. The width of the damage shall be assumed to be the lesser of 0.2 B or 5.0 m.
   
   
4. The shape of the damage shall be assumed to be parallelepiped.

311. Side Damage to be considered. 

In determining the distribution and sub-division of buoyancy spaces, the designer shall assume the following side damage:-

1. Damage resulting in loss of watertight integrity over the least of the following lengths anywhere around the vehicle periphery:-
   
   i) 0.1 L
   
   ii) 3m. + 0.03 L
   
   iii) 11 m.
2. Such damage shall be assumed to extend vertically over the full depth of the buoyancy space which has been damaged.
   
   
3.  Damage shall be assumed to penetrate horizontally for 0.2 B or 5.0 m., whichever is the less, provided that such penetration in from the outer rigid structural boundary of the vehicle is at least 12% of the buoyancy tank width at the station considered.

312. Stability in the Damaged Condition.

Following damage to the extent considered in either Section 310 or 311, the vehicle shall have sufficient buoyancy and positive stability to ensure that when floating in still water at maximum certified weight:-

1. The final waterline at any point is at least 76 mm. below the lowest point of any opening through which progressive flooding may occur, and
   
   
2. The final floating attitude does not exceed 8° in any direction with fuel, ballast and payload positioned for normal operational trim, but making due allowance for movements of fluids resulting from the damaged attitude, following initial angles of attitude of up to 16°, and
   
   
3. Passenger evacuation is not impeded by flooding.

313. The design should ensure that, in the worst environmental conditions for which certification is sought, the vehicle when floating following the assumed damage shall maintain a positive metacentric height. 

314. The designer shall examine the buoyancy and stability provisions to determine any more severe effects which may result from lesser damage than that considered in Sections 310 and 311 

315. Damaged Integrity. 

The combined structural integrity and post-damage buoyancy and stability should ensure that, in the worst environmental conditions for which certification is sought, the vehicle will remain afloat, with minimal impairment of access to and operation of escape routes, survival craft and emergency equipment, for the lesser of 30 minutes or 3 times the demonstrated evacuation time plus 7 minutes.

316. Icing. 

In all design considerations related to floating and cushion-borne roll stability, the designer shall take due account of icing. Icing allowances to be considered are provided in Appendix 2 of this chapter.

317. Design Substantiation. 

Buoyancy and stability design calculations shall be prepared and submitted to demonstrate quantitative compliance with the intact and damaged buoyancy and stability requirements of this Chapter. Representative model testing may be required.

318. Sidewall Vehicles. 

The design of underwater structure or appendages of sidewall A.C.V's shall include provisions to minimize damage and consequent reduction in buoyancy and stability resulting from collision with underwater or floating objects.

319. Passenger Loading. 

For the purposes of stability calculations, it shall be assumed that each passenger has a mass of 75 kg., with a centre of gravity 300 mm. above the seat cushion when seated, or 1 m. above the deck when standing.

Asymmetric loading to provide the most unfavourable rolling moment in the damaged condition shall be assumed, with passengers standing and distributed at 4 persons per square metre.

Appendix to Chapter 3 - Buoyancy Media. ^

Low Density Plastics. 

Low density plastic media are frequently proposed for use as buoyancy. While there is no objection to this in principle, there is a very wide variety of such media, and designers should be aware that not all such media are acceptable as buoyancy. In general, foam plastics formed from multi-part mixes and poured in place are very sensitive to environmental conditions when mixing and pouring, and to vibration under operating conditions. Structural voids and poor generation of a closed-cell foam may result from inadequate mixing or pouring conditions, and vibration may result in rapid deterioration of the foam structure. These foams cannot therefore be relied upon as buoyancy, and will NOT, in general, be approved.

Expanded polyethylene foam has been shown to have excellent long-term performance as a buoyancy medium, and other pre-formed expanded or foamed plastics will be considered provided that supporting specifications and tests results are submitted.

In selecting buoyancy media, designers should ensure, and submit data to show, that they are:-

1. Structurally stable under service conditions;
   
   
2. Impervious to water absorbtion, and
   
   
3. Fire-retardant or self-extinguishing.

Appendix 2 to Chapter 3. ^

Icing Allowance. 

1. In vehicle designs intended for year-round operation in Canadian waters, at least the following icing allowance should be considered in all stability and buoyancy analyses.

1. 60 kg./m² on all exposed horizontal surfaces below a height equal to the maximum significant wave height in which the vehicle is designed to operate, measured from the cushion-borne waterline at maximum certified weight. NOTE - The hull structure within the cushion is "exposed".
   
   
2. 37 kg/m² on all surfaces other than vertical.
   
   
3. 30 kg per running metre of all guardrails, stanchions, antennae, rigging, etc., less than 6.1 m. above the normal floating waterline at maximum certified weight.

2. The above values have been derived based upon year-round operation of A.C.V's in Canadian waters. 60 kg/m² equates approximately to an ice thickness of 7 cms., which, as an average may be generally experienced. Ice accretion is naturally a function of local operating conditions, and seasonal or temporary operating restrictions may be imposed upon vehicles which cannot meet these allowances or when conditions are likely to cause them to be exceeded.

PREVIOUS | TABLE OF CONTENTS | NEXT