…moved from grounding topic…
I am not making any assumptions that the current installation on that boat has been sufficiently thought through on the subject of correct grounding (nor any other electrical subject for that matter, hence the upgrade 
).
Could I ask guys to have look at what I’m trying to achieve and how that relates to ground wiring @jrmartin505 @chrigu @pwfarnell @kevind1 ?
PS, regulatory requirements are EU/NL. From what I can see, current installation doesn’t ground anything to the metal boat hull, not the 48V propulsion battery, not the 48V house battery, not the 12V basic loads battery. On shore the shore ground connects to the onboard AC outlets and the 3-phase propulsion battery charger, off shore the (non Victron) Inverter ground is also connected to the onboard AC outlets but not to neutral and neither to the hull. All in all everything seems to be isolated from everything else.