After a little bit of thought, I think I see how to do it.
The membership database that is part of this website is fully searchable, and I can add more fields to it in all the usual forms - text, numbers, "radio buttons"(choose one) etc. At the moment we have a membership directory that covers both member and boat. We can split this into two, and have a Member Directory that is just about the people and their interests, and a Boat Directory that is just about the boats. These are simply "reports" taken from the full database.
New members would see a lot more questions to answer than at present. Existing members would have to be asked to go to their profile and edit in all the data about their mast construction and size, their yard construction and size, etc., etc.
Would this do what we want it to? I think it would.