I would stay away from using foam for a large hull. It is going to need that much more wait to get it to still deep enough in the water. Super Tanker hulls are just ig boxes with stubby bows and sterns tacked on. Realy not hard to do a stand off hull. Use you imagination, glue and some plywood.
Good Luck

As cool a project as this would be, and do not get me wrong, I love the idea, building on out of foam may well cause you more headaches than it is worth.Sverval folks have built models of such and all, while not struggling with the issue, spent a lot of time figuring how to get the hull deep enough into the water, even for the prop to work. Then there is the issue of sail area. tankers have lot's of it, even loaded, ut without any mass to help hold them in place, they skit about like a pebble on smooth ice. There is a build of an ore carrier, in 1/96th scale, on RC Groups. I think that hull is around 8 ft. I might be worth looking at the build to see what was entailed.
A sheeted hull would not be difficult, after all they are big boxes with a stubby nose and tail tacked on.
Would not be hard to build. Good luck.

I have a couple of foam blocks around 6 foot long....

This might be the ideal project to 3D print the hull.there should be several parts which would be exactly the same.
Peter.

Maybe a stand off scale with hull carved from foam block?