I was considering this issue as well. Some sources say that a transom adds some degree of improvement to a ship's drag. Whatever the case, the inclusion of this feature on Imperia is purely to provide a flat and wide surface for the placement of azipods and to accommodate a wider stern. M.S Imperia has a larger portion of the hull underwater than Queen Mary 2, which is quite flat, yet wide and tall. Therefore, there is indeed space enough for the placement of something like the transom on Imperia below the waterline, not to gain the benefits of reduced drag or improved flotation, but simply for the reasons mentioned above and to improve aesthetic qualities.
Nevertheless, I am quite interested in hearing what other purposes the transom on such a ship like QM2 serves, and what the benefits are of having it above the waterline.
Kind regards,
Mike.
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Illustrator at Liner Designs
https://www.linerdesigns.com/