Except it is actually a thing. Large ships have a separate emergency steering hydraulic circuit driven by its own generator, and operated by hand, commands given from the bridge by radio or telephone.
https://www.imo.org/en/About/Conventions/Pages/International...
Namely that every tanker, chemical tanker or gas carrier of 10,000 gross tonnage and upwards or every other ship of 70,000 gross tonnage and upwards, the main steering gear shall comprise two or more identical power units. Theres no requirement for separate circuits in these large applications. "power units" meaning we just duplicate the engine/partial drivetrain and slave it to the SCADA system as a standby unit. these standby's can be started by using residual air in the compressor system (if available) or by diverting charge air from the compressor system to the standby.
remember: we've been hacked, so compressor valves are likely to be locked shut (or worse, destroyed) until someone can get down to the engine room and force-open the valve manually.
ships will often "flip" between engines for service intervals, so it can be useful for the SOC team during triaging the problem, but the failover likely wouldnt provide much help.
to answer the question "couldnt we steer using air?" and yes you could, but it would be glacially slow. you might only have enough power air to move 5-10 degrees.
I’m in an adjacent industry, with less risk of death or commercial loss, and the compressor backups only output to SCADA. The pressure regulation is all relay based and the on switch is a manual secondary contactor.
Also another problem I noticed is the engines are often on the ships controls (network maybe?) As the manufacturer name. Something like MANN1 and MANN2 or MITSU1 is a dead giveaway: thats propulsion.
I suspect that at least some ships/designs are done right ‘right’.
(1) the engine controllers are internal safety limits and have very controlled input ranges. Cummins engines as an example.
(2) the network has a ‘Battlestar’ mode where you can just cut the wire. People would still need to connect laptops or jumpers locally to control devices in anything beyond a ‘max’ vs ‘idle’ vs ‘off’ mode… but 100%, ready, and off should be enough in an emergency.