Dirk,
You may not be talking about a wooden yacht from the 1970's now, but in 5 short months your proposed category would start to include them! Around my parts, Seattle, wooden Chris Craft older than that are so worthless they are being used as homeless encampments (not joking, it was in the news last week). I've seen photos showing the same story up in your waters.
And why is a wooden vessel any more interesting than a steel or fiberglass one? No other category on the site is dependent on the material of construction, so why should this one be? In my opinion, the fiberglass yachts of the 1950's can be works of art, featuring car-inspired fins and other jet age extravagance. In comparison, 1950's wooden yachts are stodgy and hardly different from those of the 1940's or 1960's.
And that's exactly the point I was trying to make, interest in small vessels is incredibly arbitrary. Would you consider one of those moldy Chris Craft I mentioned above as interesting now as you would have considered a 1930's wooden yacht 20 years ago? Remember, even harder than judging interest by age is judging interest by condition, so a category open to nicely restored vessels would be equally open to those in a near-wreck state.
I'm not saying we shouldn't look at changing the category allowances in favor of smaller older vessels. But "opening up" some category that is already a dumping grounds of sort, like the ill-defined Ancient Motor Vessels, or adding some category with no lower boundary don't seem like wise options.
Perhaps expanding Motor Yachts into two parts would be a better solution. For example a " Motor Yachts Newer than 50 years (20 m and over)" category alongside a "Motor Yachts Older than 50 years (12 m and over)" category. That sort of compromise could expand the interesting vessels that are added, as well as weed out the potentially "over-abundant" forms of debatable-classic vessels no one would probably look at.
As for the Ancient Motor Vessels category itself, perhaps it would best be served as a home to working vessels converted to yachts, retired working vessels, or something of that sort. Apart from old yachts, which by site guidelines should be in the yachts category, that seems to be the primary focus, anyways.