Small naval vessels often have to operate close to enemy coasts making them vulnerable to air attack. However, due to their small size, patrol boats and landing craft cannot be fitted with large air defence system, such as radar guided surface-to-air-missiles.

Air defence protection for small boat operations id usually provided by larger warships, which create protective “umbrellas” with their surface-to-air-missiles, or by fixed wing fighter aircraft flying combat air patrols.

There is always a risk of “leakage” through these air defence umbrellas, so smaller vessels are provided with their own close-in defensive weapons. New generations of air threats, such as mini-unmanned aerial vehicles (UAVs) and low flying helicopter gunships, are also often easily able to evade air defence designed to deal with large jets or supersonic missiles. To deal with them requires bespoke air defence capabilities.

Installing air defence systems on small vessels is often difficult because space issues limit ammunition storage, the installation of long range early warning sensors and structural integrity issues of securing rapid fire guns onto small decks.

More often than not, anti-aircraft crews on small vessels can only rely on their own eyes or other optical sensors to provide them with warning of air attack and give them a cues to employ their weapons in self defence. Simple machine guns and shoulder launched manportable surface-to-air missiles or MANPADs are the most common air defence weapons used to protect small vessels.

Basic gun mounts are very simple to install and are often provided with armour shields to give the gunners limited protection from return fire.

More sophisticated gun-based air defence systems are offered by many companies. The Rheinmetall Oerlikon Millennium Gun or Rheinmetall GDM-008 is a close-in weapon system designed by Rheinmetall Air Defence AG (formerly known as Oerlikon Contraves) for mounting on ships. It was developed from the 35/1000 revolver gun land-based air defence system and uses AHEAD ammunition.

A device at the muzzle end of the barrel measures the exact speed of each round as it is fired, and automatically sets the fuse to detonate AHEAD rounds as they approaches a pre-set distance from the target. Each round disperses 152 small tungsten projectiles weighing 3.3 grams each to form a lethal cone-shaped cloud to strike the incoming target. Whilst these are too small to do major damage in themselves, the accumulation of damage from multiple strikes is intended to destroy wings and control surfaces, sensors and aerodynamics of aircraft or missiles, causing the target to crash. Other firing modes are designed to be effective against surface targets such as small fast attack boats.

The weapon is designed to be controlled by an external fire-control system using either radar or electro-optical trackers. It is fitted with an optional onboard observation TV camera which relays imagery to an operator console from which it can also be aimed and fired in an emergency mode. The computer system uses an open architecture and is claimed to be compatible with many existing fire control systems. The gun's ammunition capacity allows it to engage 10 anti-ship missiles or 20 surface targets. A Rheinmetall GDM-008 systems weighs some 3000 kilograms so it can only be installed on larger patrol vessels. Danish, Indonesia and Venezuelan warships are fitted with the weapon.

Pantsir-M is a Russian jamming-resistant naval close-in weapon system (CIWS) which entered service in 2018. It is equipped with friend or foe identification system and armed with naval version of the Pantsir's 57E6 missiles and Hermes-K missiles. Its’ secondary armament are two six-barreled 30×165mm GSh-6-30K/AO-18KD rotary cannons, which have a range of five kilometres. The Pantsir-M is fully automated and can engage up to four targets simultaneously at a range of up to 20 kilometres with its missiles and can operate as a battery of up to four modules. It can intercept sea skimming missiles flying as low as two meters above the surface. If a target isn't sufficiently destroyed by Pantsir's missile attack it can automatically direct its cannons against it. Pantsir-M's phased array radar, electro-optical/infrared targeting and identification system are based on that of Pantsir's 1RS2-1.There are two versions: the Pantsir-M domestic version for the Russian navy and the Pantsir-ME export version.

Operating MANPADs from small patrol vessels does not require any modifications but the craft need to have a suitable space where the missile operator can stand-up to deploy his weapon. One of the most widely sold MANPADs is the Russian 9K38 Igla, which is the latest derivative of the classic Strella weapon that was first developed by the Soviet Union in the 1960s.

The 9K310 Igla-1 system and its 9M313 missile were accepted into service in the Soviet army in 1981. The main differences from the Strela family of MANPADs included an optional Identification Friend or Foe system to prevent firing on friendly aircraft, an automatic lead and super elevation to simplify shooting and reduce minimum firing range, a slightly larger rocket, reduced drag and better guidance system extend maximum range and improve performance against fast and manoeuvrable targets, an improved lethality on target achieved by a combination of delayed impact fuzing, terminal manoeuvre to hit the fuselage rather than jet nozzle, an additional charge to set off the remaining rocket fuel (if any) on impact, an improved resistance to infrared countermeasures and slightly improved seeker sensitivity.

Subsequent improvements over the basic Igla-1 included much improved resistance against flares and jamming, a more sensitive seeker, expanding forward-hemisphere engagement capability to include straight-approaching fighters (all-aspect capability) under favourable circumstances, a slightly longer range, a higher-impulse, shorter-burning rocket with higher peak velocity.

The naval variant of 9K38 Igla has been developed that mounts two launchers on a 360 degree rotating mounting to help the operator to rapidly bring the weapon into action.

A similar concept is used by the MBDA Simbad missile that incorporates two of the company’s Mistral MANPADs. A new version is the SIMBAD-RC is designed to provide a primary self defence capability on small warships or to complement the main air defences of larger classes of warships.

Featuring an automatic launcher remotely controlled from an interior terminal, SIMBAD-RC is ideally suited for ships with low crew numbers and for modern ship design requirements, particularly those relating to stealth and very fast craft concepts.

SIMBAD-RC provides an extremely effective defence capability against all threats including anti-ship missiles, combat aircraft, unmanned aerial vehicles, helicopters, as well as small surface threats such as those presented by “swarms” of small fast attack craft.

Depending on ship size and/or crew requirements, SIMBAD-RC offers a range of turret/SMU-RC configurations and system integration schemes. Being entirely operated by one interior-located operator, the SIMBAD-RC system offers high availability in all weather and sea conditions.

A key element in enhancing the effectiveness of air defence system on small naval vessels is a warning systems to allow gun or missile operators to being their weapons to bear in the direction of any threats. MANPADs have to be switched on and activated before they can engage targets, which can take several minutes.

The Leonardo RAN-30X 2D Medium Range Surveillance Radar is typical of smaller multi-mode 2D air/surface surveillance radars that can be installed onboard small and medium combatant vessels. The RAN-30X is manufactured in two different versions to be especially effective against surface targets as small ships, snorkels, low flying targets and against high diving air targets.

Another way to provide early warning of air attack is through the use of data links to allow the transmission of tactical information from larger warships or airborne early warning aircraft.

Effective air defence of small vessels can also be enhanced by multiple warships operating in concert to create barrages of gun fire or missiles at attacking aircraft. While shooting down an enemy aircraft, helicopter, missile or drone is welcome often force the attackers to abort their attack.

While hi-tech weapons bring a lot of capability help protect small vessels, often simple weapons can be the most effective because of reduced crew training requirements and speed at which they can be brought to bear.