I was reading about how some of the larger ocean liners, such as Titanic and Mauretania, were constructed and launched. They slid the finished hull down a slipway into the water, and they were concerned about it tipping over before the entire hull was in water. Harland and Wolff did have the graving drydock for fitting out, so they could fill and drain the dock, so why couldn't they build a second graving dock and construct the ships entirely in there? When ready to launch they'd calculate the afloat drafts and fill the dock with water until it floats. That's a large part of what I do for a living for ships coming in for maintenance, and it's a much slow and more controlled (and probably much safer) process than sliding it down soap and animal fat.