Alister,
With a question like this my first answer would always be to wait for Umi's wisdom and do that
How about sailing the plug? If you have gone to the trouble of making the plug to the right shape you can sail what you carved after giving it a surface finish which will depend on the use you envisage and the dinging it will get. Surface treatments in order of increasing robustness could be:
Nothing
Paint - any that doesn't eat your foam
Brown paper applied with white glue
Papie mache glued with emulsion paint
Fibreglass
Plank with balsa, ply sheets as large as the hull permits, or anything else cheap and durable
Then smooth, finish as required.
As Umi says the load part of a VLCC is totally parallel, so can be made with sheets of ply, etc, and you only need to shape the pointy ends. Carving in foam is a good plan - then you can cover/finish as above and leave all or most of the foam in place.
Strength to your arm, and pics, please, as you go
andrew