Would a third shaft for Nelson and Rodney have required thinner armour?

Would a third shaft for Nelson and Rodney have required thinner armour?

The suggested alternative is to use the roughly 1,200 tons between their stated 33,800-ton displacement and the 35,000-ton limit.

Nelson and Rodney are given a standard displacement of about 33,800 tons against a treaty maximum of 35,000 tons. That leaves roughly 1,200 tons for additional machinery within the limit. The proposed allowance could have accommodated a third shaft if it had been designed in from the start. On that estimate, thinning the armour would not have been necessary.