Views: 25 Author: UC Marine China Publish Time: 14-07-2014 Origin: www.ucmarine.com
What’s To Know About Liferafts
Life rafts come in many shapes and sizes with virtually no standardisation between manufacturers. Square, oval, hexagonal, octagonal, single buoyancy tube, two stacked tubes are just some of the choices. Cost, weight, intended purpose and people-carrying capacity are usually the determining factors. All life raft manufacturers try to satisfy the mutually exclusive aims of making their rafts and equipment the simplest and most foolproof possible while providing the most features. No-one ever expects to have to use their raft but it remains a salutary thought: your life and that of your crew may one day depend on it, so choose your life raft carefully.
Construction/build quality: Most life raft air tubes are made from either PVC, nylon reinforced butyl rubber or polyurethane. Some also have an outer cover of welded PVC that protects a PVC bladder. Quality rafts, including those to SOLAS (international shipping) standard, tend to be made from polyurethane, while cheaper versions are usually constructed of welded PVC. Whichever fabric the raft is made of, initially it needs to withstand being squashed into a tight container for an extended period. On launching, it must withstand rapid inflation from a gas that enters it at freezing temperatures. The International Organisation for Standardisation (ISO) has established a range of criteriadetailing all the materials used in the construction of life rafts and their performance requirements including; inflation, launching, material resistance, buoyancy and interior space. All SOLAS rafts must be certified and stamped to show that they meet these demanding construction and performance standards and for rafts found on coastal shipping and recreational vessels, the recently introduced ISO 9650 certified standard is highly recommended.
Buoyancy Tubes: Where single tubed life rafts were once found on coastal and blue water vessels, they are no longer used except for aircraft and some search-and-rescue applications. Two stacked buoyancy tube rafts are now mandatory because they provide a redundancy safety margin. Either of the two tubes is designed to support the raft’s maximum load on its own. Unlike a car tyre or even an inflatable boat, the tubes of a fully-inflated raft are maintained at a relatively low pressure of only about 2 psi. It’s the volume of air, not its pressure that supports the weight - so any leaks that may develop are reasonably easy to deal with, by either applying repair patches or inserting leak stoppers into any holes.
Shape: With a space allowance of only about 1.2 square metres per person, no-one can expect a life raft to be luxurious, but its shape can effect comfort and morale. The choice however, finally depends on certain trade-offs. For example, a rectangular or square floor may mean that several adults can stretch out and lie down. The raft’s centre of gravity is lowered which increases stability. But a round shape provides greater strength uniformly around its perimeter compared to a rectangular floor, but may only allow one or two persons to stretch out flat in the centre. Survivors sitting in a round raft will automatically tend to position themselves around the sides, with their backs against the tubes and feet inwards. Their weight will be distributed evenly.
Floor & Canopy: As well as protection from wind and sun, a raft’s other role is to keep you warm and dry. A single membrane floor and roof could allow rapid heat loss. This may become life-threatening very quickly, particularly in colder temperatures and higher latitudes. SOLAS life raft standards require an insulated roof and floor, but this can result in a bigger, heavier and more costly raft. Insulated floor choices available are: a sandwich of layers with a core of cellular foam, a fabric base with an aluminium film on top to reflect body heat, or an inflatable floor that needs to be independently pumped up after the raft is boarded. Insulated canopies usually have a double-skinned membrane that is self-erecting.
Weight and Manageability: Even a basic four-man raft is quite heavy while a so-called light-weight raft, primarily designed for weight conscious racing sailboats, could be half the weight of a standard version. Light-weights are often made by what is called bent tube construction, where a single continuous top and bottom tube is simply bent at each corner and welded. The floor is highly critical to the design to maintain overall shape and integrity. Light-weight rafts are a popular choice with coastal sailors, in that they are light and easy to manhandle and relatively inexpensive, but unfortunately it was such a raft that failed in the 1998 Sydney to Hobart disaster when its floor was cut. Regulations and demand for more features has seen significant changes to life raft design since then. Nevertheless in an extreme emergency and particularly when conditions are rough, getting a raft over the side can be a difficult task. Having a prearranged method in place where even the smallest crew member can remove a standard raft from its locker or storage place and prepare it for deployment is certainly good practice.
Access: Nearly all rafts larger than eight person have openings on opposite sides. Gaining access to a single opening raft in extreme conditions can be difficult, when stepping down from a boat while trying to rotate the raft. A double entry system is mandatory for larger commercial life rafts and standard on larger recreational versions, but the down-side is that it may lessen security and allow more water ingress.
Very often, rafts need to be accessed from the water and this can be difficult. Assistance is normally provided by a weighted multi-rung ladder and/or a semi rigid boarding ramp plus a webbing handhold to haul yourself over the buoyancy tubes. The coroner’s finding into the 1998 Sydney Hobart disaster stated that sailors who were cold, fatigued and weighted down with foul weather clothing and inflated lifejackets found it extremely difficult to board a life raft using a simple webbing ladder. To aid entering a raft, it is now accepted that single entry rafts and the second entry point on larger rafts should be fitted with a hard or semi-rigid boarding ramp. The Coroner was also highly critical of the methods of securing the opening of some raft types. Access flaps secured using fabric ties were impossible to undo and retie and are now discouraged in raft manufacture.
Stability: Life raft stability is influenced by a number of factors: raft shape, wind getting under the floor, canopy shape, loading of occupants, sea anchors and water ballast. But it is the combined force of wind and waves that cause rafts to capsize and water ballasted stability bags, firmly attached to the raft’s underside, help to resist this occurrence. They should be triangular in section and fill with water within seconds of inflation to increase stability and reduce the risk of capsize. SOLAS and ISO 9650 standards require that stability bags be weighted and each capable of containing at least 22 litres of water. The important role of the sea anchor or drogue should also not be overlooked. Numerous authoritative tests have shown that they deliver a powerful stabilising force that help prevent capsize. To be effective, they should be a tapered sleeve of heavy duty fabric, about twice as long as the mouth is wide.
In calm conditions, wind presses down on the canopy and helps to keep the raft level. In rough weather, rotational forces come into play whereby a breaking wave in combination with the wind on the canopy can cause the raft to lift, allowing more wind under the floor. If the occupants are thrown to the low side, the raft is likely to capsize.
Observation/ventilation port: Most people will be seasick if they are in a raft for any length of time. Raising the canopy in rough conditions for this or other reasons could allow much water ingress without this important feature. They also allow a watch for possible rescue activity.