Shorepower: 480 VS 100amp three phase, or 100 single

I'm reading ; your not using all 3 phases, Just one leg (phase) of the three per cable.
Neutral is not needed from the dock, the Atlas makes that itself.
The boat does well on a single phase 480Vac(48Kw), not well on 2 x 208Vac (41.6Kw).

I feel the atlas is getting 2 different phases (of the 3).
One cable getting Vac from Black-Red the other cable getting VAc from Red-Blue.
The Atlas is using its transformer to make work. In doing so, your drawing a lot of of shore current and popping the breaker.
Probably an old/abused dock breaker & outlet also.

Your 208/460 converter could be re-strapped to get both lines on the same phase.
Improving the in-efficiency of phase correction at the Atlas transformer.

Fly me in, I can do it for you.
I'll bring my O scope and show you the 120° phase delay between the legs.

Atlas takes incoming AC, be it 3 phase or single phase, converts it to DC and then discharges 60 Hertz of 120 per line. The voltage drop isn't relative to what is happening within Atlas, and Atlas does use some, but rather out on the dock.

The dock is delivering 480 to the pedestal, but from a meaningful distance, and then the shore connection and cable aboard create more loss. In many single phase dockside setups, they are fed 480 but deliver 208 per line to the pedestal because they use a straight transformation of 480 to 208 delta setup rather than to boost the output to 240 per line. Why? Because neither the marina nor the power company want to pay the amp hour cost of making that boost conversion. They want the customer to pay for it.

Atlas uses two of the dockside lines or legs. It converts them into 600v DC, then transforms the DC back into 60 Hertz and 120v AC per line for load. This both provides flexibility of voltage input as well as cycles and delivers clean power to the ship. You can plug her into 480-3, 277-3, 208-3, 240-1, 208-1, etc and get teh desired useful outcome.

Like any other device, it doesn't car much for low power, and these big loads are often interpreted as low incoming voltage. Atlas is pulling amps through the dockside breaker necessary to both provide the power as well as the voltage outcomes. I find myself at some marinas where I just default to genny when time to fire up the kitchen as to not stress the system nor to trip the dockside breaker.

Yep, 270 to ground that would be at neutral potential.
Your tied to 2 of the hots that make one of the 3 phases.

The 208 works the same way.

Questions for all;
What happens to the boat when that one pedestal breaker pops?

Are there two separate input transformers converting the two leads to DCv?

I fear only one lead is in use and the other (out of phase) is rejected. This theory may be re-enforced if Pascal reports the boat goes dark.
I'm thinking you can not combine (stack) two out of phase inputs on the same Atlas transformer primary.

So, How do you know your drawing from both leads? I'm still worried you were drawing only from one pole.

This just messed me up; Your not consuming all 3 phases, only one of the three.

Do you have a PDF of your Atlas model, theory or tech manual I could read.
I really need to get my mind in phase.
I'm not in sync with you kids or I have took a wrong turn in Albuquerque again.