You mentioned an Irish Sea Car ferry some time ago and I am aware that Model Boats Magazine has published many articles and given free plans for such craft.
Back copies are available and I believe they still supply the plans.
In 2015 Model Boats Issued a Special edition marking 65 years of model boating. The centre og the Magazine has a full size pull out plan ofBute a twin screw Passenger Car ferry at 1:75 scale by James A Pottinger originally presented free with their April 2007 Magazine.
Sarik now hold the copyright and I found the plan at
May be this might be what you are looking for.
E-bay sometimes has back copies of Model Boats mag and if someone has the April 2007 copy on offer it may also contain the plan.