I don't pretend to be an expert, & others may not be either, so please excuse me from posting these details from the RAFs own web pages (yes, I know this may be different from operational reality slightly):
Chinook:
Engines: Two Textron Lycoming T55-L712F turboshafts Thrust: 3,148shp each Max speed: 160kts Length: 30.18m Max altitude: 15,000ft Span: 18.29m Aircrew: 4
The HC2 and HC2A aircraft are used primarily for trooping and for carrying internal and/or underslung loads and can carry up to 55 troops or 10 tonnes of freight. The cabin is large enough to accommodate two Land Rovers, while the three underslung load hooks allow a huge flexibility in the type and number of loads that can be carried externally. Secondary roles include search and rescue and casualty evacuation, in which role a total of 24 stretchers can be carried.
Merlin:
Engines: Three Rolls-Royce Turbomeca RTM 322 turbines Thrust: 2263shp each Max speed: 167kts Length: 22.8m Max altitude: 15,000ft Span: 18.6m Aircrew: 4
The Merlin is able to carry a diverse range of bulky cargo, either internally or under-slung. Cargo can include artillery, Land Rovers or light-strike vehicles and over five tonnes of freight. The spacious cabin can also accommodate up to 24 fully equipped combat troops and, when required, will convert to carry 16 stretchers for casualty evacuation or during humanitarian and disaster relief operations.
Puma:
Engines: Two Turbomeca Turmo 3-C4 turbines Thrust: 1300shp each Max speed: 147kts Length: 14.08m Max altitude: 17,000ft Span: 15.09ft Aircrew: 3
The aircraft can carry 16 fully-equipped troops, or up to two tonnes of freight carried either internally or as an underslung load. The other major role is that of casualty or medical evacuation support, for which up to six stretchers can be fitted.
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Seems to me that there are significant differences between all 3 & no doubt each type has their place in the scheme of things but am I right in saying that only the Merlin can be deployed at sea for long periods (inc having the capability to be rotors folded automatically & struck down in to a ships hangar)?
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As an aside, these specs are from a US Army Site:
BlackHawk S70A:
Main Rotor Diameter 53ft 8in Maximum Gross Weight 22,000lb
The S-70 is qualified as a launch platform for the laser-guided Hellfire anti-armour missile. The Black Hawk can carry 16 Hellfire missiles using the external stores support system (ESSS). The ESSS has the capability of carrying a 10,000lb payload of missiles, rockets, cannons and electronic countermeasures pods. The S-70 can mount 7.62mm or .50-calibre machine guns in the windows.
The cabin provides accommodation for 11 fully equipped troops or four litters (stretcher patients) with a medical officer for medical evacuation missions.
The S-70A can carry external loads up to 9,000lb (4,072kg) on the cargo hook - for example, a 155mm howitzer.
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The Puma stacks up pretty well on this basic comparison, although the ability to displace troops & carry hellfire is a significant advantage for me.
More Chinooks then, with Puma replaced by Blackhawks, and the Merlin transferred to 845/6.
OK, I know its not that simple & its dreamland.