You're confusing recreational trawlers with boats that can readily exceed hull speed. Most so-called but mislabeled trawlers are motorboats that have overnight accommodations. Most all such motorboats have several times the horsepower of a trawler.
This is very true. As such they are not designed for slow speed efficiency. Unfortunately, some aren't built properly for high speed efficiency, either!
Before NWD we owned Great Escape, a 1977 Reinell B-307, designed as Reinell's largest planing flybridge cruiser. She was 30 ft with an 11 ft beam. Ours was (WAY UNDER-) powered with OMC 307 engines, at 230 HP each. The boat allegedly weighed about 12,000 lbs dry. Many other examples of that boat were powered by at least 350 CU engines if not big blocks.
Completely empty with a freshly rebuilt port engine, my son and I got it up to 22 knots one day. Loaded with fuel, water, equipment and supplies for a family of 5 I could only ever get it to plow water at 16-17 knots with both engines at WOT. We could "semi plane" at 11-12 knots but we were into the secondaries of the carburetors, and fuel economy was extremely poor. (<.5 nm/gal) We reviewed our props and performance with The Prop Shop and really could not see a way to eek anything more; they agreed the boat was underpowered for what we were doing with it. (family of 5 and all living supplies, 10 ft Livingston dinghy, etc on board.)
Therefor we operated it as a trawler almost all the time, finding that as long as we stayed under 8 kts we hovered around 1 nm/gal for fuel economy. 6 or 6.5 knots was a little better than 1 nm/gal, 8 knots a little less than 1. I usually aimed for an engine rpm that would get us 6.5 to 7 kts in current free water. This translated into around 6 gph of fuel burn. Fuel costs were high enough that it made sense to move the boat to a more expensive marina 10 miles closer to our cruising grounds because the annual fuel cost saved cutting out that 10 miles was more than the increased moorage!
NWD, built to stay at trawler/displacement speeds, has 1/2 the total HP and 3 times the fuel economy of Great Escape, despite outweighing her by nearly 20,000 lbs. We are typically seeing 3 nm/gal going the exact same speeds as the gas boat we had.
While we may have been able to get more reasonable planing cruise with more powerful engines in Great Escape, I don't know that we would have seen anything better than 1 nm/gal in that boat. It was built when they believed more glass was better. But I don't think you could ever expect much better economy in a twin v-8 gas boat operating at trawler speeds - I basically had the smallest displacement V-8's you can come across.
