That might work to see the level of interest. However, the issue is still there. Tractors, lawn mowers, bicycles, 4-wheelers, snow mobiles, etc don't really have their own "place" yet owners want to show them off as well. I'm not as active in the challenges as I used to be, so I'm not sure if that is still an issue.
And please don't misunderstand me either. I think it's great that people want to show off their stuff. Let's just get a place where the can show it off. I also know that there's got to be a limit on it as well. Who's to say it should stop there...why not make a portal for trucks too? So with that in mind, I would make the following recommendations:
Keep the existing portals as they currently are:
Cars- for all vehicles requiring registration to drive on a road
Boats- for all boats, wave runners, etc
Planes- also self explanitory
I would then add one portal for vehicles that do not require registation to operate. Again, I would call this portal "recreational vehicles."
I understand the thought behind making an enthusiast group. I see the need to see how much demand there is for a new portal before you commit the resources required to create the new portal. But for those who do not regularly visit motortopia, the word might not reach them. If you add it to the newsletter that would definitely help. There are just so many enthusiasts groups that it's hard to sift through them all. I know that I've found out about groups only because I noticed them on others' pages.