through hull drain tubes in the floor

[77SceptreOB]

That's because your missing one small but important detail. There are small wedges attached to the hull just in front of the scupper tubes which create SIGNIFICANT suction. It pulls the water out much faster than just depending on gravity!

I think I had temporarily removed them for better access when flaring the end of tube, so that's why you didn't see them in photos in my old post.

Here's a picture