Designing ships is fun, but there are some things that need more attention than currently is provided.
Internal space: Barbettes and Turbines & Boilers. Citadel length paired with armor weights.
Currently, the only major hinderance you come across is weight and cost.
So I just want more attention to be paid at what isn't seen on the surface of the ship, but what's inside.
Switch the slider to a button to place the turbines and boilers yourself, where the turret barbettes hinder space available...
Though it doesn't have to be individually placed, could just be "Assign engine space" (Where you click and drag a box to place engines, and ctrl+click to remove), and have the game calculate speed based on how much is available, and quality.
((From an older game, but it shows what I mean. But I understand you want to keep things rather simple))
Lastly, just wanted to say the game should take into account citadel length for the Belt, Deck and Extended Belt, Extended Deck.
(Something like: Citadel Length ##% of ship length)
This percent could be calculated automatically by the length end points of barbettes and engines.
This would help players that want to keep weights down by creating condensed citadel designs.