Perhaps if we got off the term hull speed or displacement speed as what we're really asking about it "Economy cruising speed". In fact, many boats, especially larger semi-displacement and planing ones, list three speeds in their data. They list WOT, Wide Open Throttle. They list Cruising Speed (some refer to as Fast Cruising Speed), which typically is a sweet spot around 70-80% load and they list Economy Cruising Speed which is the speed which gives the greatest range and is typically one knot or so below a calculated hull or displacement speed. In most performance charts of semi-displacement or planing boats the curve looks something like this. A very significant savings somewhere around 70-80% load vs. 100% throttle. Then only a small improvement in nmpg and range as one reduces load. Then at some slow point a dramatic improvement in efficiency occurs.
I pulled up a couple of performance charts.
A Hatteras 60 with twin 1135 hp CAT's.
Range at WOT (31.5 knots) is 236 nm.
Range at 70% load (25.4 knots) is 279 nm.
You get very little improvement until you drop to about 33% load
Range at 33% load (15.9 knots) is 358 nm.
Range at 10% load (10.3 knots) is 773 nm.
Range at 5% load (7.2 knots) is 1083 nm.
LWL is 53'4"
So using 1.2 instead of 1.34 is 8.7 knots and the 7.2 knots is near what most would be calling 1-1.5 knots below hull speed but very close to 1 x sq rt LWL which is 7.3 knots.
So WOT 236 nm
Cruising speed 279 nm
Economy cruise would likely be represented at 773 nm but maximum range or displacement speed using 1 x sq rt is 1083 nm or using 1 knot below 1.1 x sq rt. Either way this is your slow cruising speed and maximum range.
So the three ranges listed are WOT 236 nm, Cruising 279 nm, Economy 1083 nm.
Using a Beneteau ST 44 with Twin 300 hp Volvo
Range at WOT (25.6 knots) is 303 nm
Range at 70% load (19.6 knots) is 343 nm
Very little improvement until about 30%
Range at 27% load (10.0 knots) is 419 nm
Range at 10% load (7.4 knots) is 980 nm
Range at 4% load (5.7 knots) is 1733 nm
Now, while the Hatteras is a planing hull, this is a semi-displacement and it shows in the performance curve as it acts like a planing hull from 10 knots up but then the curve gets very displacement like and with each speed reduction the range changes significantly.
I'm guessing calculated "displacement speed" is between 5.7 and 7.7 knots
This might be represented as
WOT 303 nm
Cruising range 343 nm
Economy range 980 nm
Maximum range 1733 nm
I should note that on a displacement boat you see a smoother curve as every speed reduction results in fuel savings.
Looking at a KK 48 with a 225 JD.
At WOT (9.6 knots) range is 783 nm
At 80% load (9.3 knots) is 956 nm
At 40% load (8.4 knots) is 1786 nm
At 12% load (5.7 knots) is 3913 nm
At 8% load (4.7 knots) is 4739 nm
LWL is 45'5"
So using 1-1.34 then "displacement speed" is 6.7 - 9.0 knots.
On this combination though where you're pushing through the water, there is range increase and improved efficiency at every point along the scale. It becomes just how slow one wants to go. Most probably run between 6 and 8 knots using the lower speed if they're trying to cross oceans. I imagine Richard's curve on Dauntless looks very much like this. He reduces speed to the distance he needs to cover.
These are just three examples of three very different boats with very different hull types and characteristics. Reality is there are thousands of different types and no boat perfectly fits any label.
However, in a displacement type boat one is going to see a change in fuel consumption at every speed. In a semi displacement, this will be seen at slow speeds, but then once on plane it will be fairly level up to about 70% load then a big change. On a planing hull, below planing speed a change at every speed, but once on plane very little change until you pass 70-80% load.
We all have to learn the characteristics of our boats. Then we choose the speeds to run. We have one boat that has identical range from 15 knots to 36 knots. 266 nm, 0.35 nmpg. Most boats that plane in any fashion have very slight differences in nmpg between 30% and 70% load but large differences above and below. Boats referred to as displacement hulls generally have changes in nmpg at every load and every speed.
You are somewhat correct. Today I filled the tank after a 13.5 hour run from Wrangell AK to Ketchikan AK. I filled in Wrangell to the absolute top. The gallonage today was 24 gallons on the nose. This works out to 1.77777 gallons per hour or 1.8 gallons vs the older 1.5 gallon at 1450-1500 RPM to now 1650 RPM.
