Well, when surfaced you'd expect them to run all engines. If you want the boat to move at 1/3 speed, the other engines would be charging until the batteries were topped.
It's a little odd, there should be a difference between when you click the recharge batteries or not button. If set to recharge, I'd assume all engines would run, and any power produced NOT used to propel the boat at the ringed up speed would charge.
When the button is off, I'd expect the boat to only run as many diesel engines as needed to produce the power needed for the ringed up speed. If you have the boat set for 5 knots, I'd expect one engine running I think.
tater
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