(covers information from several alternate timelines)
Ships of the class
When it came time to design the Y-class freighter, there weren't many potential influences to dictate how the ship should look. The design began with concept artist John Eaves, who produced three alternative sketches for the craft. All three illustrations shared the same basic architecture, featuring a central framework with a series of cargo pods on either side. (Star Trek: The Official Starships Collection, issue 49, pp. 10-11)
Much of the Y-class design was inspired by the script for "Fortunate Son". (Star Trek: The Official Starships Collection, issue 49, p. 11) In the first draft script of "Fortunate Son", the Y-class was initially described thus; "The large [...] cargo ship is comprised of a half dozen cargo modules strung together, each a hundred plus meters in length. A much smaller crew module is perched at the nose of the barge-like vessel." The same script draft established the Y-class as typically having had only one plasma cannon (when Travis Mayweather was serving on the ECS Horizon) but the ECS Fortunate as having a total of three plasma cannons and, in one ultimately unused line of dialogue, the Y-class's top speed was referred to as approximately three hundred times slower than that of the NX-class. As regards the initial description of the Y-class, the final draft of the teleplay changed the length of each of the cargo modules to "fifty plus meters" and added the sentence, "Each module is marked with a number: 1 through 8." As well as referring to the Y-class ECS Fortunate as a "huge" craft, the script was also inspirational in the design of the class by making it clear the Fortunate would have one of its cargo pods blown off in the course of the episode. John Eaves, as a result of these details, focused on producing slightly different versions of the cargo pods and the front part of the vessel. In one of his drawings, the "cab" at the front was similar to a tug boat, with twin warp nacelles on either side. However, the producers refused this approach, preferring the other two configurations. (Star Trek: The Official Starships Collection, issue 49, p. 11)
The selected pair of illustrations were given to the visual effects team, so the final design could be constructed as a CGI model. While the VFX of "Fortunate Son" were being supervised by Ronald B. Moore, his influence had a considerable effect on how the design turned out. (Star Trek: The Official Starships Collection, issue 49, p. 11) Once he received the two drawings, he discussed the Y-class with the CGI modeler to decide how they would like the ship to look. Moore's approach to the Y-class was to place importance on how the craft was to be used in the episode, taking note of some extremely specific shots described in the script. Also important to the VFX artists was giving the vessel a sense of scale as well as being capable of docking with other spacecraft. "I wanted to design something that would work for that [procedure] – which meant I had to make sure that I had a proper hatch, and there was room for it," said Moore. Additionally, he needed to have one of the cargo pods separate from the main body of the craft and float away into outer space. "So I varied the model a little bit to make all of that work as well as possible." (Star Trek: The Official Starships Collection, issue 49, p. 12)
Ultimately, the structure that the Y-class designers arrived at was basically a mix of the two approved concept images, which were on the same sheet as each other. (Star Trek: The Official Starships Collection, issue 49, pp. 12 & 13) "Either one of the drawings we had would have worked," Ron Moore remarked. The reasons why some sections were preferred to their equivalent components on the other illustration were varied. "Sometimes it was just because I liked one element better," admitted Moore, "but we had reasoning for some of it too." (Star Trek: The Official Starships Collection, issue 49, p. 12)
Ron Moore specifically took the nose section from the image on the right of the two illustrations. "I thought the nose on the one on the right gave us a little more bulk and we needed that [to show the Y-class docking] [....] I think also subconsciously it reminded me of the Leonov, which was the Russian ship in 2010, which I worked on. The nose section of the one on the left was more aerodynamic and I didn't feel that was all that necessary, especially for a freighter." (Star Trek: The Official Starships Collection, issue 49, pp. 12-13)
Conversely, Ron Moore incorporated, in the final design, the cargo pods from the sketch on the left. "The cargo modules on the right were probably more functional," he commented, "but I thought the symmetry of the ones on the left worked a little better [....] I remember I wanted to push them a little bit further apart so I could create this valley. That was something that was in the drawing on the left [....] We wanted to build it and get the spaces just right to make that work." (Star Trek: The Official Starships Collection, issue 49, p. 13)
As an additional step in the design process, Ron Moore and his modeler equipped the Y-class with a pair of warp nacelles. They were tucked into the underside of the ship, though that part of the vessel was never shown in canon. (Star Trek: The Official Starships Collection, issue 49, p. 13)
In retrospect, Ron Moore could hardly have been more pleased with the results of designing the Y-class freighter. "I liked building this ship," he reminisced, "and I really like the final design." (Star Trek: The Official Starships Collection, issue 49, p. 13)
For "Fortunate Son", the set for the Y-class bridge was built on Paramount Stage 8 whereas the other sets for the rest of the Y-class (including the ship's access corridor, infirmary, and cargo modules, including catwalks overlooking those cargo areas) were on Paramount Stage 18. ("Fortunate Son" shooting schedule)
The Y-class bridge of the Fortunate was a heavily modified reuse of the bridge set for the USS Defiant (NX-74205), from Star Trek: Deep Space Nine. (Star Trek: The Magazine Volume 2, Issue 9, p. 112) Some of the other sets used to depict the Y-class were redressed NX-class sets commonly shown as areas of Enterprise. The Y-class hallways were redressed corridors from Enterprise, with a new lighting scheme and several new additions to the walls. Likewise, the cargo bays of the Y-class were the launch bays of Enterprise, filled with several types of cargo containers.
Cargo module 4 aboard the Fortunate was scripted to be "optically enhanced," in order to make it look huge. The script also directed for a single hatch set piece to be redressed, so it could be used for the hatch to multiple individual cargo modules.
Several vessels of this class appear in the game Star Trek: Legacy.