Special effects, miniature or real u-boat?

I agree with most of the other contributors; this is a superb war film. I've had the Director's Cut on video for some years but I recently got the DVD of the "consolidated" mini-series with a 282 minutes running time and it's even better.
Don't watch it if you're claustrophobic!
Does anyone know if the exterior shots of the u-boat sailing on the surface were of an actual full-sized operational boat or if they were of a miniature? It certainly looks authentic; the bow wave and propellor wash look accurate but I've not spotted anyone in the conning tower in those scenes so it may be a model ~ albeit a marvellously detailed one.
The boat's interior is a studio mock-up and I would imagine the exterior underwater bits were filmed in a studio tank.
Cheers.