Thanks, yes strange setup.
Just a fan unit mounted at the rear of the engine bay – as set up would just create some air circulation within the bay but not draw from or vent overboard.
Regarding the 100mm hose that goes aft to a vent through the the stern and its forward end in the engine bay near to the engine air intake – presumably this is just a fresh air supply to the engine air intake – My previous boat did not have this.
On the assumption that the fan is needed I will leave it wired up!
ditto the hose from the transom to the engine bay – will leave as is