Good question.
When the forums were originally set up many years ago, it was necessary balance the number of forums with interest groups to make the forums manageable. For a manufacturer like Catalina or Hunter there are way too many different models to have separate forums for each model.
For a forum to "work" there needs to be a critical mass of readers of that forum, if their forum is too narrowly focused that critical mass will not be reached. Additionally, many boats from different manufacturers use the same or similar systems. Reading through the forums you will notice that people who do not currently own Catalinas often have owned them in the past or understand the systems and issues with boats in general.
Boat builders do not reinvent the wheel when building boats of different sizes. If something worked on a Cat 27, it will probably work with minimal modifications on the 25 and 30. Some of the parts may even be interchangeable.
If you have an issue with your boat, asking the question in the forum that has the broadest readership will likely get you the best answer. But, please do not post the same question in multiple forums. It is our practice to merge those threads into one thread in the forum that has generated the most responses.