Why not go slow with the Steyrs? We've been traveling at 6-6.5 knots (1300-1400 RPM) in our heavy 26-footer, powered by a Volvo
KAD44P 260hp rated for 3800-3900 RPM
, for 90+% of the last 3,000 hours or so. We get 3X the NMPG at 6 knots vs planing cruise
speed of 18. 5100 hours so far, and the engine
runs like a top.
Added: We run fast enough to make the engine work
a bit - somewhat above our hull speed
. You do need to keep the engines at a proper temperature, not poke along so slow they get too cool.