Jonty, I believe that you are correct, but I think that it depended upon the class of ship, as well as when it was built, ie, attached with rivets rather than welded. I enclose a photo of the USS Kidd, which is a memorial ship ship here in the States. She is in almost exact late war configuration. It illustrates the sweep of the bilge keel, which again varies per class, as well as its apparent means of attachment, welded. it does not appear to be an I beam, although it could be a L beam, although with that sweep, I doubt it. I found that US "four pipers" of which 50 were sent to the RN during the war, used T beams which were 12" x 5" x 31.5" in dimension. Being applied in sections would allow the T beam to appear to bend to the necessary curve of the hull. ( Info from "Anatomy of the Ship: The Destroyer Cambeltown", by Al Ross.
As for using this or a similar approach on a model, the joints of sections could always be soldered to fill the small gap created between sections.