Off the top of my head a Merlin has a cruising speed of 150 knots and an endurance of five hours
I recall in the 80s when Merlin numbers were being debated, the rule of thumb handed down to us was 5 would do the job of 8 Sea King Mk5s. (Mk6 didn't exist then). The beancounters asked how many Mk5s we had (and prove it). The answer was - we're contracting the conversion of 82 to Mk6 - which was a little optimistic but raised the Merlin numbers so the inevitable cut would bring us down to something acceptable. At the same time, we knew the SUS/DUS fleet was being reduced from 50 to 32. It must be under 20 now.