I don't think a trunnion table would be practical with a model 1500mm long and with those overhangs on the stern.Breaking the model into a number of sections to be assembled is certainly a possibility,should one have the experience of 3D modelling.Its worth keeping in mind that the machining can be used to create holes for dowels that may be used to align sections and the same sort of techniques that bricklayers use to cover joints with the next block above can also be used to help alignment and reinforce the joint.

The 400mm Z is the killer as few hobby level machines will have any hope of getting near this height and even if they could,you can't get down the side of a tall block because the motor body and backplate are likely to foul.A fourth axis might help with some aspects provided the blank has sufficient rigidity to resist the combined forces of gravity and machining-but where do you find a machine with a 400mm Z axis?You would need rails with a length of 900mm for the Z,about 1100mm for the Y and 1800mm for the X so you would need a fair amount of workshop space.

Personally,unless the machine building challenge is as important as the creation of the model,I'd be looking on Ebay for something with a 600X400X70 envelope and extending my modelling skills.Then I'd be running as many machining simulations as possible so that when I wanted to produce something I could be confident that the parts I wanted would be coming off the machine.

On a different note,I would be curious about where you find models of boat hulls with that much detail as it looks very professional.Any hobbyist capable of that level of surfacing and detail has already covered a lot of the ground.