I would consider a mono powerboat, or mono sail if I had to down-size. Better than nothing.
But if I had to downsize I might get a pocket
trawler around 30-34 feet for vacations and keep a small cat or performance planing sailboat for fun daysailing. The pocket
trawler idea appeals because it could be towed hundreds of miles per day at 65mph down the interstates, and launched anywhere there's a
ramp or
lift. I don't have that kind of range on my sailboat, in the limited time available for vacations.
But getting back to the original question -- all trade-offs considered (and there are some) I will always look at a mono as a step down compared to a
trimaran or cat. If I buy another mono it will be because I'm sacrificing sailing performance and level-sailing comfort for
interior space and/or cost.
This is my third tri. Before I bought this one I considered a 36 foot mono "performance cruiser". It had all the go-fast stuff, CF
mast, sprit, go-fast
sails etc. I went for a test sail and we started out in about 10 knots and the
boat sailed nicely at about 6k and I was thinking this is OK but then the
wind went up to about 15k and I thought now it should get interesting but it didn't. The
boat just leaned more. It didn't go any faster! On my 33ft. tri (prior boat I owned at the time) it would not have leaned any more --it would have hardened up and gone faster (probably 9 or 10k depending on
sails and relative
wind angle). The mono was like a weeble. Blows more.... it leans more but no faster to windward or reaching, much more pitching and rolling downwind. Awful motion and slow sailing by comparison, like being stuck in quicksand.
Multihull -- more wind more speed, no rolling, less pitching, no rolling at
anchor, neutral
helm more comfort and no slipping and sliding means better crew
safety without significant heeling.
PS -- I own a few mono dinghies, does that count?