An immersion suit, also called a survival suit, is a full-body protective suit intended to limit water ingress and body heat loss during immersion and after maritime isolation. Its protection comes from the closed outer shell, its seals and the thermal insulation specified for that model. The particular suit design determines whether a separate lifejacket is required or an integral buoyancy system is used.
Thermal protection and sealing
Cold water takes heat from skin and clothing; currents and movement carry warmed water away and replace it with colder water. A closed suit shell limits this direct contact and the flushing of clothing underneath. Above the surface, it also provides a barrier against wind and spray. Waterproof fabric alone, however, provides little insulation: an insulating material construction, a fitted liner or the required underclothing must slow heat flow outwards. The suit does not generate heat.
Water resistance depends on several parts working together. Alongside the material and its seams, the waterproof zip, neck and wrist seals or the face and hand closures fitted to that model must seal properly. Attached feet or socks close the leg sections. An open closure or poorly fitting seal provides an entry route even if the rest of the suit is undamaged. Water ingress wets the inner layers and reduces part of their insulating effect. Sweat can also soak them; closing the outer shell does not dry those layers again.
Designs and use
A suit donned specifically for abandoning a ship is used differently from a constant-wear suit already worn during work. In a suit worn for longer periods, sealing and thermal protection must allow movement and equipment operation. Aircrew also need to accommodate cockpit seating, access to controls and the combination with other personal equipment. The US Air Force uses the term anti-exposure suit for pilots' cold-water protection. The suit performs a separate thermal function alongside a lifejacket, survival vest and liferaft; a G-suit instead protects against acceleration loads in flight.
Suit designations can overlap. The VIKING Hurricane, for example, is described as an immersion suit, anti-exposure suit and constant-wear suit, with the liner configuration changing its thermal classification. An aviation model such as the Survitec 1000S combines a breathable, flame-retardant outer shell with a specific thermal insulating garment and a suitable lifejacket. The outer shell alone is therefore only part of the intended protective combination. An ordinary waterproof jacket provides neither the sealed body openings nor the tested combination of such a suit.
Check the suit actually available
Before donning, the model, size and donning instructions must be identifiable on the actual suit. A bag may contain a different suit from the one described on its label. Unfold it on a surface as clean as possible, check inside and outside for wetness, tears, punctures and damaged seals, and check that the zip moves throughout its full travel. A jammed closure is unlikely to become easier to close in the water with cold hands. Only the specified lubricant is suitable for its care; checking operation is no reason to force a sticking closure.
The required liner and specified underclothing layers must actually be present. The size, leg and torso length and neck size must accommodate those layers. The wearer must be able to bend, move arms and legs and reach the necessary closures and equipment they actually carry. A suit that is too tight restricts these movements; an overly loose neck or wrist seal can admit water. With a suit fitted with gloves, the check must also establish whether they allow the wearer to operate their own equipment's small controls.
Check any fitted buoyancy components for damage and correct attachment. An inflatable system must show the readiness indications specified for it, and its activation handle must be accessible. A hood and gloves may be attached, detachable or stored in dedicated pockets; they must be found and used before hand function deteriorates. Reflective material makes the wearer easier to see when illuminated, while a fitted light or whistle provides an additional local signal. These features also increase visibility to an adversary.
Donning and fully closing the suit
A suit intended to be pulled on for immersion should, wherever possible, be donned over its required inner clothing before entering the water. Legs, torso and arms must be fully in their intended sections before the waterproof closure is closed and seal fit checked. The model's instructions determine whether shoes and existing headgear must first be removed, how a chest fastening is secured and when the hood, gloves and buoyancy equipment are donned. The Hurricane, for example, requires footwear and headgear to be removed; that sequence must not be transferred to another suit with a different foot and neck construction.
The zip must be moved fully into its intended closed end position. Clothing must not hold a seal open. Putting on the hood does not replace correct seating of the neck seal. With an aviation or working suit already being worn, closures opened for ventilation or more comfortable wear must also be fully closed before possible immersion. The intended operating handles and equipment must remain accessible afterwards. Cutting a seal or modifying the suit would alter its fit and sealing function and forms no part of these actions.
Buoyancy and the airway
Thermal protection, flotation and keeping the airway clear are separate functions. Buoyancy provided by a suit or its auxiliary chamber does not establish whether it will turn an exhausted or unconscious wearer into a favourable position. The Hurricane PS4190 requires an approved, compatible lifejacket. The Hurricane+ PS4191 instead has an integral inflatable buoyancy aid and must not be used with an additional lifejacket. Neither suit version will automatically turn an unconscious wearer into a face-up position.
An arbitrary additional lifejacket can worsen the floating position: alongside total buoyancy, its distribution around the body matters. An unsuitable combination can incline the wearer head-down. Preparation for the particular system therefore includes the approved suit and lifejacket combination. In the water, the wearer must assess their own position and ability to breathe freely. An insulating hood leaves the mouth and nose open for breathing; a purpose-designed lifejacket sprayhood performs the separate task of keeping wave spray away from the airway.
Cold, leaks and damage
Sudden cold-water immersion can initially cause uncontrolled breathing. Protective clothing slows skin cooling and can reduce this response. Once breathing is controlled again, necessary actions must be taken early: close accessible closures, use the hood and hand protection and reach any available signalling devices. Cooling of nerves and muscles can weaken grip and swimming ability before a substantial fall in core body temperature has occurred. Thick or permanently attached gloves can also make small switches harder to operate; this limitation needs to be identified during preparation.
If water ingress is felt, first check an accessible closure that is still open or the fit of an accessible seal, as far as breathing and floating position permit. If the suit continues to leak with its closures shut and its seals correctly fitted, incoming water reduces the intended protection. A genuinely reachable floating object or platform onto which the wearer can move wholly or partly reduces further heat loss in the water. Opening or taking off a suit in the water in an attempt to correct a fault creates additional water contact.
An apparently intact surface does not rule out hidden defects. Seams, bonded joints and closures can age and leak without a conspicuous tear. Visual inspection identifies obvious damage; it does not replace the specified leak testing and servicing. The original protective performance of a damaged suit therefore cannot be assumed in subsequent decisions. Improvised pressure testing of the suit shell and leak repairs by gluing or sewing do not replace the specified repair and test procedure.
Protection outside the water
On a reachable liferaft, a supporting floating object or ashore, protection against wind and spray remains important. Wet inner clothing can continue to cool the wearer outside the water, particularly through evaporation and wind. A sufficiently sheltered place must therefore be available before the shell is opened; waves or spray must not soak the clothing again. Someone only partly out of the water must still account for the immersed body areas and their own stable position.
In an enclosed, warmer shelter or during strenuous activity, extensive insulation can instead trap heat and sweat. Clothing can be opened or adjusted there only as far as the surroundings allow and without risking fresh water ingress. A thermal class or test duration marked on a suit refers to a particular combination of suit, liner and underclothing under defined test conditions. Different wetness, movement, wind and personal physical characteristics change the heat balance. The figure does not tell the wearer how much survival time remains or justify delaying their own protective actions.