Michael wrote:

I need to turn a piece of wood thats 4m (12') long by 50cm (20") wide
(diameter). Would a piece of wood this size be suitable for turning on a
lathe, if I was to make one especially for this purpose?

The item I'll be turning will be a (model) submarine hull.

Does anyone have any better suggestions?

I'd suggest that you tell us a bit more about what exactly you're
doing.
Is there a particular reason the sub can't be built up in sections?
Will the hull be hollowed out?
Will it be painted?
What's the intended purpose?

R