Even if previously hollowed out, you'd need a monster lathe to turn it.
I recall that FWW once had an article on a poor man's router lathe,
basically a plywood box into the ends of which the workpiece was
mounted, as if between centers. A router with a dish cutting bit rode
the length of the box's sides. The workpiece was turned with a crank
mounted on one end. Tapers would be cut by raising or lowering the
mounting at one end.

I'm sure this would be slow with much wear and tear on the router. But
without specialized super heavy duty lathe equipment I don't know how
else you'd do this safely in the average generalist's woodshop.

J.

Michael wrote:
Hi,

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?

Thanks

Michael