There was considerable variation between breweries in the length of boil and between different beers in the same brewery.
During both wars the government struggled to keep coal output at a sufficient level to meet demand. As things like kettles were generally powered by coal, a shortage of it caused acute difficulties. Even more so in breweries where everything was steam powered.
"The shortage of coal created further difficulties which were often aggravated by its poor quality. This need to conserve coal raised the question of copper boiling, and brewers found they had to reduce their boiling period almost to the limit of safety. It was also often difficult, with the low-quality of coal available in some districts, to maintain a sufficient head of steam on the boilers to obtain a suitable boiling temperature. When the coal situation became difficult brewers were asked to take the necessary steps to conserve fuel as much as possible. The experience of the last war, however, had taught them a lot about the saving of fuel, and its rising price in the intervening years had encouraged efficiency in this direction, so that there were not many improvements that could be carried out. Notwithstanding this they were subjected to frequent visits from inspectors and advisers from the Ministry of Fuel, many of whom knew little more than the theory of fuel consumption."
Journal of the Institute of Brewing Volume 52, Issue 3, May-June, 1946, pages 124 - 125.
Not getting a decent boil could be as problematic as too short a boil. A brewer’s life wasn’t an easy one in wartime.
I have found brewers who did adapt their boiling practices. Whitbread providing a good example of brewery which reduced its boiling times during the war.
|Whitbread Mild Ale 1938 - 1947|
|Date||Year||Beer||OG||FG||ABV||App. Atten-uation||lbs hops/ qtr||hops lb/brl||boil time (hours)||boil time (hours)||Pitch temp|
|Whitbread brewing records held at the London Metropolitan Archives, document numbers LMA/4453/D/01/105, LMA/4453/D/01/107, LMA/4453/D/01/108, LMA/4453/D/01/109, LMA/4453/D/01/110, LMA/4453/D/01/111, LMA/4453/D/01/112, LMA/4453/D/01/114.|
At the start of the war, the boil time was already quite short at 85 minutes. The mere 60 minutes to which it fell in 1942 is extremely short. Probably at the limit of safety as mentioned above.
Similar reductions were made at Barclay Perkins, though starting from a somewhat higher level:
|Barclay Perkins XX Mild Ale 1939 - 1947|
|Year||OG||FG||ABV||App. Atten-uation||lbs hops/ qtr||hops lb/brl||boil time (hours)||boil time (hours)||boil time (hours)|
|Barclay Perkins brewing records held at the London Metropolitan Archives, document numbers ACC/2305/01/623, ACC/2305/01/624, ACC/2305/01/625, ACC/2305/01/626 and ACC/2305/01/627.|
Over the whole period the length of the boil was about halved.