Why turtleback armour would be difficult on a first rate

Could armour schemes be transferred between construction types, such as giving a first rate turtleback armour?

In theory, but the armour's position and the resulting stability problems would make the proposed arrangement impractical.

Transferring a turtleback arrangement onto a first rate is treated as theoretically possible but impractical. The required armour position would fit poorly with the hull and waterline. The added arrangement would also create serious rolling and stability problems. HMS Captain is invoked as a cautionary comparison, without a detailed account of her design here.