First, I would like to say that I myself think that all the presented designs so far have been excellent, and that I am looking forward to showing my own after it is completed.
As for rationalizing the existing NX-01, I have always chosen to believe that the grey surfacing of the NCC-1701 was a replaceable, easily manufactured composite sheathing over the hull plates to protect against debris for brief periods of time to give the ships, long range explorers, time to come to a halt when travelling at higher speeds if the deflectors were to fail. My rationalization for the disappearance of said sheathing is the Enterprise refit.
When the vessel was hurriedly put into service for the V'Ger crisis, the ship was rushed out of dock before she was fully fitted out. They rushed the process by concentrating on operational systems such as the weapons and sensors. The hull was already marked because Starfleet was using her refit as a PR stunt, probably putting up images of her everywhere, and since it was already marked, they sent her out without the sheathing, which wasn't absolutely neccessary. After her performance in the V'Ger crisis, and considering the new deflector system which seemed more robust than the original, the sheathing was deemed entirely unneccessary.
As for the NX-01's overall more streamlined and 'fast' appearance, I believe that she was a high-speed thoroughbred instead of a workhorse, a showpiece designed specifically for the "Warp Five" engine, which, in reality, could barely attain and hold that speed with the sleekest warp profile they could develop. After further development, the second generation of exploratory starships were introduced, an NX-style thoroughbred capable of warp seven, and the workhorse Daedaelus class, meant for easy and rapid construction, simple maintenance, and warp five capability using the same engine as in the Warp Seven ship. These vessels started at NCC-100, in my accounting, to make room for the existing fleets of the various federation members to recieve registration numbers under the newly unified system.
As for rationalizing the existing NX-01, I have always chosen to believe that the grey surfacing of the NCC-1701 was a replaceable, easily manufactured composite sheathing over the hull plates to protect against debris for brief periods of time to give the ships, long range explorers, time to come to a halt when travelling at higher speeds if the deflectors were to fail. My rationalization for the disappearance of said sheathing is the Enterprise refit.
When the vessel was hurriedly put into service for the V'Ger crisis, the ship was rushed out of dock before she was fully fitted out. They rushed the process by concentrating on operational systems such as the weapons and sensors. The hull was already marked because Starfleet was using her refit as a PR stunt, probably putting up images of her everywhere, and since it was already marked, they sent her out without the sheathing, which wasn't absolutely neccessary. After her performance in the V'Ger crisis, and considering the new deflector system which seemed more robust than the original, the sheathing was deemed entirely unneccessary.
As for the NX-01's overall more streamlined and 'fast' appearance, I believe that she was a high-speed thoroughbred instead of a workhorse, a showpiece designed specifically for the "Warp Five" engine, which, in reality, could barely attain and hold that speed with the sleekest warp profile they could develop. After further development, the second generation of exploratory starships were introduced, an NX-style thoroughbred capable of warp seven, and the workhorse Daedaelus class, meant for easy and rapid construction, simple maintenance, and warp five capability using the same engine as in the Warp Seven ship. These vessels started at NCC-100, in my accounting, to make room for the existing fleets of the various federation members to recieve registration numbers under the newly unified system.