An arc through the middle can cause no stilts or propellers!

https://oskarstalberg.com/Townscaper/#MzJODmWAdn_ETTo_gSLput...

(and, for posterity's sake, the shape with propellers is a donut).

you can even stack them! https://oskarstalberg.com/Townscaper/#MzJODmugKVfngggBBBC_TM...

Huh. Adding one sea level block to the outside of that doughnut leads to all the propellers disappearing but no stilts. Resulting structure does not look very likely. :)

Ahaha jeez, he really did think of everything.