Ultimately (sorry) the Ultras/Ultimates were pretty nasty things if my memory serves me right. The cube square law was against them and the numbers weren't very good - ratios similar to a pre amalg UK 14 and largely worse than a Cherub IIRC. As I recall the US boats, which were box rule/development, collapsed when the 14 based designs were completely horizoned by a couple of antipodean design boats, and the UK class fell apart when they ran out of willing sponsors to pay the exorbitant running costs of such big lumps. If you want a big heavy boat the 18s were better in any way you care to think of.