The essential reason that the Mk3 is different is because it was designed by the Agusta side of the partnership (in the 1980s don't forget) as a basic and cost effective civil utility ac with a ramp, not a multi-capable battlefied helo, never mind one that could operate in all environments.
The Mk1 maritme ASW version on the other hand was designed specifically by Westland for the maritime battle environment to a very comprehensive (and eventually) a very very expensive RN spec. The integral marinisation was a natural consequence of that. (BTW it cost over £4.5Bn to develop and buy those 44 Mk1s!! Where as we spent about £750M on the original 22 Mk3s ..and that sum included about £100M to make them as good as possible at being compatible with the UK SH role rather than a civ Utility ac).
The Mk3 was never intended to go near the salty water when it was bought in 1995...but a few years later the Labour SDR98 created the JHC and also set in train the "go anywhere" policy that lead to all SH potentially being used at sea as well as on land.- even if thew were not marinised. More (UOR) money has now been spent on the Mk3 to make it a more capable SH (which wasn't available in 1995 when we purchased them) for Afghanistan but it still has not (and probably should not) had any cash spent on marinisation.
As to performance, the Mk3 has more powerful engines than the Mk1, but with full role equipment and heavier floor and armour (and despite no foldy bits) it struggles to have a good payload when enough fuel is added too. There has been a heavier take off clearance in recent times but this is by no means without its penalities in terms of cost of ownership..and thats before we consider tail rotor performance. Don't forget that the Mk1 performance was also a compromise against the original ASW spec as the basic design has a poor (for a modern helo design) payload to basic weight ratio. This was one of reasons the programme was almost cancelled in 1990 when the true performance was made apparent after flight testing - the poor ASW mission system performance being the other main issue at the time. The more modern 101 can be a capable and effective choice...but probably only in certain roles where its attributes can be efficiently employed...long range SAR or special ops being some of them
Many believe the Merlin should never have been purchased for SH...and in 1995 every study by MoD showed that the CH47 was always the most cost effective option given its similar price...the then Tory government went against all that advice and bought them anyway.....for every Chinook you need 21/2 Merlin Mk3s to carry the same payload equivalents for an aircraft that isn't much smaller ..although of course the Chinook can always carry much larger volumes and physically large sized loads or very heavy single ones...so which is the better investment, particualry when you consider the costs of running 2 ac types rather than one?
Given that understanding....there was much effort put in to getting any new CH47 purchase marinised so that they could replace some if not all the SK Mk4s in due course...but that proved too expensive and technically riisky with a possible set of folding tandem heads etc, and was binned as an option a while ago. In the absence of any such dedicated "CHF" Chinooks, and the lack of any money under SDSR to replace the Mk4 Sea Kings with a new order for "proper" CHF Merlins---we now face the prospect of the Mk3s being used, swapped or whatever to do the job instead...any port in a storm springs to mind I think....
Cheers
