Unit machinery arrangement

This is the system in which a ship's propulsion plant is divided into two (or for larger ships four) units capable of functioning independently. This provides redundancy in case of mechanical problems or combat damage and is a significant asset to a warship's survivability. In most ships it involved alternating boiler and engine rooms. I will further opine that a true unit system meant half (or 1/4) of the boilers in each unit. There were some ships that had say three boiler rooms, forward engine room, one more boiler room, aft engine room, therefore normally requiring the aft engines to get some of their steam from the forward unit - a degree of redundancy but far from ideal.

Side note, there were also ships which had all their engines together but the boilers divided into two groups, not always equally, fore and aft of the engine rooms. This was mainly in the coal era and was particular popular for French and Italian battleships and cruisers, including some sold to other navies. This meant that the coal bunkers, a large variable weight depending on how full they were, could be balanced fore and aft to help maintain trim as the coal was expended.

Back to topic, as far as I know the first ships to have a true unit system were the otherwise unremarkable Omaha class light cruisers of the US Navy. Can anyone cite an earlier example?

The first unit system destroyers appear to have been - surprise - the Navigatori class of the Regia Marina; however the Italians reverted to the standard boilers, then engines setup in subsequent classes. The unit arrangement reappeared in the American Benson and later classes, then in the Japanese Matsu class escort destroyers, although their fleet destroyers retained the more vulnerable conventional arrangement.

The unit system had a cost in that the forward propellor shaft(s) were longer, therefore heavier, and took up space passing through the aft boiler room(s). Thus the system was abandoned in some ships like the New Orleans class CAs which sought to conserve space and length in order to maximize protection. On the other hand, there were examples like the modified Leander class and the St. Louis (modified Brooklyn) which transitioned from conventional to unit systems with little if any impact on ship size.

 

Most Italian cruisers had unit systen arrangements for the propulsion system, this didn't prevent a single torpedo from disabling propulsion in more than one instance.
IMO the Navigatori large destroyers were much better ships than the later classes.

 

Can't say I know much about British cruisers, but, the US St. Louis class benefited from more modern boilers that were smaller, but operated at a higher temperature & higher pressure than previous boilers. These were not available for the previous cruiser classes.

 

The Italians had some odd arrangements, for example the Zara class cruisers had two shafts and eight boilers arranged, fore to aft:
#1 boiler room - two boilers
Forward engine room - one set turbines, one boiler
#2 boiler room - two boilers
#3 boiler room - two boilers
Aft engine room - turbines, one boiler
So for split plant operation at full power, #2 boiler room would be supplying both plants, one boiler to each. A reasonable degree of redundancy, but not an ideal unit system. The Zaras were another design that achieved good armor protection; the engineering plant was likely arranged to conserve space.

IMO the Navigatori large destroyers were much better ships than the later classes.

I agree. The midships gun mount seemed a bit odd the first time I saw it, but the blind arc would only be about 20 degrees fore and aft, and it avoids the chance of more than one mount being taken out by a single hit; another survivability feature.

 

As the CL's USS Houston and USS Canberra showed, even one torpedo hit was enough to knock out all propulsion.

 

A sufficiently catastrophic event could make the question academic. Houston was hit while she was heeling in a turn; the explosion was almost underneath the ship. The torpedo hit under the forward engine room and the impact caused bulkheads at both ends to fail, flooding both the forward and after boiler rooms and depriving the ship of power. Flooding subsequently spread to the aft engine room as well.

Canberra, a CA, had a somewhat similar experience. She was hit in #4 boiler room and both adjacent compartments - #3 boiler and the aft engine room - flooded also. The propellor shafts from the forward engine room of course pass through the aft spaces and were sufficiently distorted to allow water to spread in the forward engine room as well. She still had #1 and#2 boiler rooms, which in that class each contained a ship's service turbogenerator, so she had steam and electrical power but was immobilized.

On the other hand on many occasions the system functioned as intended, allowing a ship to retain some motive and auxiliary power. A couple of British cruisers even managed to have three shafts disabled by hits in the aft engine room, which knocked out the two sets of turbines there and one of the shafts passing through.

 

I thought both of these were heavy cruisers (CA's in US parlance).

 

It gets a bit confusing. Since gromit801 mentioned them together, I assume he was referring to the Cleveland class Houston (CL-81) which was hit by an aerial torpedo October 14, 1944, during Halsey's attacks on Formosa. Canberra (CA-70) had been hit the previous day, and as noted, both completely lost propulsion despite their unit machinery and had to be towed out of the battle zone. They had to be towed by other cruisers for a few days, until fleet tugs could meet them and take over. The ever belligerent Halsey supposedly hoped that "Crip Div One" would attract Japanese forces trying to finish them off which could then be attacked.

Houston took a second hit while under tow and was fortunate to survive; at one point this nominal 10,000 ship was estimated to be displacing 20,900 tons.

Houston commemorated the earlier Houston (CA-30) which was sunk in the Far East the day after the Java Sea battle. That Houston appears to have remained mobile after one torpedo hit but received three more plus numerous 5- and 8-inch shell hits.

USS Canberra commemorated the Australian cruiser sunk along with three of ours at Savo Island. She holds the interesting (to some) distinction of being the only USN ship named for a foreign capital city.

p.s. one more thing, the early "tinclad" treaty cruisers like Houston were originally designated CLs to distinguish them from the early 1900s armored cruisers which were still in commission. The London Treaty of 1930 established a distinction between 6" and 8" cruisers and imposed tonnage limits, which led to the old CAs finally being retired. CA was then used for the 8" ships, which is why "A" apparently means "heavy" in Navy parlance. At that point the only CLs were the Omaha class, but few years later we had the Brooklyn class CLs which were more heavily armored than "heavy" cruisers like Houston.