Martin,
What a question! I am possibly putting my neck on the block to be shot down by other marine engineers on the Forum but here goes.
If we are talking 'thermal efficiency', then the earliest puffer engines were single-cylinder, double-acting units exhausting to atmosphere and probably their efficiency was about - I stress the word "about" - 10% and possibly less. When these were replaced with compounded two-cylinder, double-acting engines with the LP exhausting to a condenser, thermal efficiency possibly rose to about 15%. (I believe there was an interim period when some compound engines still exhausted to atmosphere.)
Whatever the aesthetic appeal of steam engines, whether reciprocating or turbine, and even with regenerative condensers, reheat etc., marine steam plant never achieved efficiencies exceeding about 34% for turbine plant and considerably less for reciprocating. The huge heat loss to cooling water circulating through the condenser could never be recouped.
The expansion of the puffers' trading areas from canal to open sea also brought its own technical demands. While operating in the Forth & Clyde canals, the puffers took the boiler feed water they required from the canal itself. Once operations were extended to sea water, feed water had to be carried on board as boilers do not like sea water and will quickly scale-up. (Yes, I know that some early boilers were operated on salt-water feed with heavy blow-down to try and reduce the boiler salinity but it was never successful, always dangerous and led to even more heat loss.) Possibly some shipowners did not like feed water carried on board as it reduced space for cargo.
As for how much money the shipowner saved in coal, I couldn’t put a figure on it partly because I’m unfamiliar with 19th C./20th C. coal prices and consumption could vary widely not least because of bunker quality. What I would say is that nothing attracts a shipowners attention like an innovation that reduces his overheads.
A Puffers coal consumption might also vary because of the unofficial access hatch that sometimes appeared between cargo hold and engine room. It was not unknown for a cargo of coal to mysteriously reduce on passage and, allegedly twelve tons went AWOL while one vessel was navigating from Troon to Coll. En route, the puffer put into Tobermory supposedly to trim cargo but shortly after the local coal merchant, at a time of post-war coal shortage, had ample stocks to sell. The case went to court but nothing was proved.
It was a different world.
Barry M
