The U-13, which entered fleet service in 2146, was one of the heaviest designs to see service in the Romulan fleet. It mated a hull based on that of the early U-7 class to a new wing structure mounting the new RFTL-3 warp nacelle. The U-13 was designed for a specific purpose; it was intended as a long-range patrol cruiser. The re-adoption of the large hull form allowed a bay to be fitted that carried one of the immense SB-2 star bombs and sufficient fuel for an extended individual patrol. 120 of the U-13As were built, and it performed sterling work in this role. Additionally, these ships carried out deep penetration raids with some success. While on patrol near Eta Leonis VI on 16 June 2156 , a U-13A received the ultimatum calling for the Romulan Government to stop the piratical attacks from Admiral Gunther.

The success of these penetration missions prompted the adoption of the U-13B. The U-13B (60 built) first commissioned in 2148. It carried three SB-2 star bombs in the weapons bay, but carried only half the fuel of the U-13A. These ships were employed as strike cruisers and as heavy weapon platforms alongside more conventionally armed lighter cruisers.

Desperate for a way to strike back at the Earth Forces the U-13C and U-13D were conceived in 2157. The U-13C carried no star bombs (but retained conventional weapons); the U-13D carried five star bombs, at the expense of a reduced range. The plan was to employ the U-13Cs as tankers to allow the U-13D to reach deep inside Allied territory. It was planned that two U-13Cs would support each U-13D. In total 50 U-13Cs and 24 U-13Ds were converted (40 U-13Cs and 6 U-13Ds were converted from U-13As, 10 U-13Cs and 18 U-13Ds were converted from U-13Bs).

The first mission of the U-13C/D combination saw 48 U-13Cs stage the fleet to a safe distance from Alpha Omega B. The 24 U-13Ds and 24 escorting U-13As proceeded to that planet and attacked on 25 July 2159 . The ensuing raid saw the fleet bombard the planet with 120 star bombs from the U-13Ds (and a further 24 from the U-13As in a diversionary attack). The Diversionary U-13As went in first and were wiped out by local defences, but the U-13Ds went into the gap left in the defences and struck. Over 20,000 military and civilian personnel were killed, and the planet was rendered uninhabitable. This proved to be the deepest incursion into Allied space.

The raiding force suffered a major loss while returning to base, during an in flight  refuelling operation one of the U-13Ds exploded, destroying the U-13C it was linked to, and taking a further 12 U-13Cs with it in the explosion that resulted. Fortunately, all but one of these had already dispensed their fuel loads, allowing the surviving ships to return to base. A raid on Earth was in preparation, when on 25 November 2159 an Allied force raided the Romulan shipyards at Manaraam, this was the deepest Allied incursion of the war, and was in response to the attack on Alpha Omega B. Ironically 18 of the ships destroyed in the raid were U-13Ds. This attack and the loss of the 13 tankers in one accident was effectively the death knell for the program, as insufficient numbers of U-13s remained to convert to U-13C or D standards. The remaining U-13Cs were later modified to carry two externally mounted Star bombs as U-13Es, but the war ended before they could be perfected.

The first U-11 commissioned in 2144. It was a standard U-9 with a stretched hull. The hull extension allowed the fitment of weapon bay that allowed the internal storage of two star-bomb missiles.

Internally the U-11A mounted the same systems as the U-9A, and the warp drive system was the same as the earlier vessel (RFTL-2). The additional weight of the star-bomb missile launch system meant that the U-11A was slower and less efficient than its smaller cousin was.

Production of the U-11A was limited, as the U-13B introduced in 2148 did the same role much more effectively. In all just 40 ships were built. In 2157, the surviving 21 ships were fitted with RFTL-5 engines and became U-11B in a program that ran alongside the U-9B program.

The U-11 was employed mainly to interdict Allied Capital ships on penetration raids. The firepower given by the Starbombs made it devastating for a single salvo, but woefully under armed as soon as the two missiles had been fired if the target was not crippled or destroyed. Operation experience led to the ships being deployed in packs of three of four vessels to ensure a killer blow could be swiftly struck. Against individual targets this worked, but if engaging a capital ship with escorts, the end result was normally the loss of the pack.

The U-11B is notable for being the first ship class to use the then experimental R2L-1, a year before it’s formal acceptance into service.

The U-9 class started to commission in 2142 the ship soon gained a fearsome reputation, but despite this the Earth crews dubbed it the flying peanut.

The first confirmed action involving U-9s against an Earth ship came on 18 October 2144, when two U-9s attacked the SS Amaretto within Earth controlled space. The U-9s broke off when a patrol of four Cavalry class destroyers appeared on the scene.

09 June 2148 saw a major reversal of fortune for the Romulan fleet, when the Earth’s Twenty Fourth Tactical Anti-Piracy Group cornered a U-9. This group consisted of eight Messier class cruisers and a Cavalry class destroyer. The U-9 exchanged fire with the ships, damaging the UES Oort. After sustaining substantial damage from a missile strike from the UES Hubbell the U-9, following standard Romulan policy, self-destructed to avoid capture.

On 04 May 2150, three U-9 class cruisers attacked a liner as part of the ongoing series of ‘pirate’ attacks. Almost 600 crew and passengers lost their lives in the raid, and the removal of the cargo completed the impression of piracy. The Romulans retained only the strategic materials and equipment from the cargo; they simply destroyed the remainder of it, as they had no need for it. The liner in question was the SS Diana.

The U-9 was also involved on 21 November 2151 when a mixed squadron of U-7s and U-9s destroyed eleven ships at Outer System Defence Outpost 4. The same squadron, still with U-9s in service scored a further triumph on 02 November 2154, when it attacked and destroyed Outer Sector Defence Outpost One.

Two U-9s attacked and destroyed a series of mining outposts near Sigma Canori on 16 August 2155, these ships then confronted two Horizon class cruisers of the Earth Starfleet. The U-9s managed to damage severely the UES Matador, but the UES Excelsior (commanded by Captain William Gustavus Larson) destroyed one of the U-9s and managed to drive away the other one.

The loss of the U-9 to the inferior Horizon class ship prompted an upgrade to U-9B standards. 96 ships were upgraded to U-9B standards (of 120 U-9s built). The basic U-9 had mounted the same engine system as the U-7 (RFTL-2), the lighter hull structure giving increased speed and enhanced manoeuvrability. The U-9B adopted the new RFTL-4 as used on the U-15. This engine gave the same power levels and performance, but was 20% lighter, carrying an enhanced weapons load. By January 2157, only the U-9B was in service following completion of the upgrade program. The U-9 adopted a destroyer role, acting as escorts to the larger cruisers then entering service.