What happens if we have the fans running in order to quickly cool down the engine room and connect to shore power?
Most boats today have a battery charger for the purpose of connecting to shore power. Providing the battery charger is properly sized, the house batteries will be charged much more slowly whilst the fans are running. If the battery charger is undersized, the fans will still run at a speed proportionate to the voltage but the batteries will not be recharged. It is most unlikely that they will be even slightly discharged.