I am amazed at how easily hard science intrudes into almost any subject. A few years ago I edited a volume of New Zealand naval memories for Random House – stories from participants in our Second World War sea battles. One of the accounts proudly explained that our light cruiser Achilles had been good for 36 knots, and pushed towards that at the Battle of the River Plate on 13 December 1939.
From a science perspective that didn’t add up, literally, because Achilles was designed for 32.5 knots. I knew that the draconian physics of ship propulsion made it impossible for Achilles to achieve 36. But the claim was emotionally genuine, and that made me wonder, so I set to work to figure out how Achilles actually performed at the River Plate – and why her crew had that belief in her speed.
That battle pitted Achilles, her sister ship Ajax, and the heavy cruiser Exeter against the German ‘pocket battleship’ Admiral Graf Spee. In theory the Graf Spee had it all over the British. Actually, the Germans were sent packing; and one of the ways the two light cruisers avoided much damage was by ramping up to maximum speed, well above Graf Spee’s modest 26 knots.
The eyewitness, in short, was right about the need for speed, even if his numbers were out.
But I wanted the numbers. I knew the speed Commodore Henry Harwood ordered the cruisers to make: 31 knots, essentially max tactical speed, with bit in hand for station-keeping. But was it feasible? First off in my analysis was design data. Achilles, a Leander class cruiser, was designed for 32.5 knots at 72,000 horsepower. However, the physics of marine propulsion are dismal. Ship hulls are typically optimised for cruising speed, and as a rule of thumb, power demand goes up by the cube of the speed. For Achilles, cruising at 16 knots demanded just 1/8 of the power needed to reach 32. I calculated that she would have had to generate roughly 115,500 horsepower to make 36 knots – quite impossible for her steam plant, even if it was pushed until it exploded.
That wasn’t all, either. For Achilles to reach even her theoretical top speed of 32.5 knots demanded ideal conditions – not what was found off the Plate. Parameters that make practical maximum speed differ from design speed include actual displacement at the moment (variable, depending on consumables aboard), cleanliness of the hull, state of the propulsion plant, and especially depth of water.
Conveniently, Achilles‘ maintenance record is held by Archives New Zealand and the other data was also to hand. So! When battle opened Achilles was four months out of dock, well within the life of her anti-fouling paint. Barnacles and weed were not an issue. But she had just refuelled; and with full fuel tanks and munitions her displacement was close on maximum war load, affecting the water-plane area and cutting top speed below her peacetime design maximum. She was also two months overdue for a ‘two year’ refit, major maintenance intended to include ‘wear and waste’ tests on her boilers. She had been worked hard since the outbreak of war despite the delay. When the refit was eventually completed in June 1940, some 781 superheater tubes had to be renewed.
All this hampered the ability of the ship to reach her design speed – but the main complication was that the battle was fought in shallow water. The thing about water is that (a) it has high mass for volume, demanding a lot of energy to move it; and (b) it’s incompressible in any practical sense, meaning that it acts as a solid object as far as energy transmission is concerned. What this means is that a ship in shallow water expends stupid amounts of propulsion power pushing a train of water from the sea floor to the surface – creating a huge wake – instead of pushing itself forwards. The wake phenomenon was observed by the crew at the time, and is visible in photographs. According to one account, when closer to the Plate, Achilles was unable to achieve more than 24-25 knots for this reason.
What happened during the battle was all the more extraordinary as a result. Achilles entered battle at around 14 knots, reaching 28 knots around 6.40 a.m. and working up to 31 knots by about 6.50 a.m. To do this under shallow-water hydrodynamic conditions meant radically over-stressing a tired steam plant – and in fact Achilles’ engineers managed to achieve 82,000 hp and 283 revolutions, well above design limits. What this added up to was that, irrespective of the speed actually reached, the ship was pushed absolutely flat out, a superlative achievement on the part of her engine-room crews. They kept it up through the battle in spite of fact that flames roared out of the furnace grilles and across the boiler room spaces with the shock of every salvo. Think about it. The word you want is ‘heroism’.
And that, my friends, explains the pride with which the performance was then remembered.
I put my reconstruction of what had happened into the book – Torpedo (Random House, 2007), with the science…and nobody noticed. Sigh. But you can read my other account of the battle, right now, in my book Blue Water Kiwis.
Copyright © Matthew Wright 2015