At displacement speeds FD is more efficient than the main ship box shape. Hull shape, taper, fairness & size and load among other things determines the
fuel consumption based on how many HP needed to push it through the water.
In my discussion here I am looking at Fuel usage at displacement speeds only at this time so that is what we are talking about here.
Efficiency of the propeller, how many blades, pitch, etc. also makes a difference as do a lots of other things like prismatic coefficient & beam, angle of the prop & shaft, & even a big flat stern has a huge effect on Fuel usage, how much of a wake the boat makes too, etc.
So their are a lot of variables. To many to list here. BTW - Water line length makes a huge difference as it changes the S/L ratio for a given hull speed as measured in knots. Beam in the water makes a big difference as you have to push the water farther to get it out of the way for the boat to move forward. We all generalize, but their are truly many variables.
NOTE: Every boat is different.
Here are some approximate example numbers for my Kadey-Krogen 42.
being an approximate 42 foot boat overall hull length with approximately a 39 foot 6 inch water line length. Lets look at the approximate numbers.
Starting point is to get find out what the Square Root of water line length is, for this example it is approximately 6.25 & at a S/L factor of: S/L ratio of 1.0 it works out to be about 6.25 knots.
in this example remember S/L ratio of 1.0 gives better fuel consumption that S/L ratios of 1.1, or 1.2 or 1.3 , 1.34 etc... Higher the S/L ratio the more fuel consumed for that particular individual boat design & shaping at that speed through the water on that individual boat design.
so KK 42 Displacement Speed length Ratio's are approximately as follows
S/L Ratio - 1.0 = 6.25 knots
S/L Ratio - 1.1 = 6.875 knots
S/L Ratio - 1.2 = 7.5 knots
S/L Ratio - 1.3 = 8.125 knots
S/L Ratio - 1.34 = 8.375 knots
Full displacement boat is not going much above these hull speeds by any significant amount with out pegging your fuel usage meter.
Advantage of a SD boat is it can move up into the planning speeds & FD can't efficiently do that.
I have seen with my own eyes that a similar dimension FD boat & same water line length SD boat with flat sides & no taper to the rear will burn maybe for example 4 GPH at 7.8 knots & FD burns maybe 2 GPH at 7.8 knots. All due primarily to different hull shaping, but other stuff too.
Like I said - every boat design is different here & there & it adds up.
Kadey-Krogen 42 has a true full displacement hull with a wine glass stern & leaves very little wake. Here is an example Fuel consumption curve with the Ford Lehman 135 - for my KK 42 the fuel usage curve is as follows:
Fuel Usage Curve – 700 Gal
Kadey Krogen 42 - 148 w/ Ford Lehman Sp135 & 4 Blade Prop
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RPM--- Gal/HR--- Spd Kts--- Range
1800 ------- 2.00 ----- 7.8 ----- 2730
1700 ---- 1.75--- 7.2--- 2880
1600 ---- 1.55--- 6.6 --- 2981
1500 ---- 1.45--- 6.2--- 2993
1400 -----1.15--- 5.7--- 3470
1300 -----1.1-----5.5 --- 3500
1200 ----- 0.80--- 4.8--- 4200
1100 --- 0.70 --- 4.6--- 4600
900 ----- 0.50 --- 3.8 --- 5320
So there is an example where at displacement speeds the SD still used more that the true FD.
But the FD is stuck at maybe under 9.0 knots no matter what they do HP wise.
SD can get up & plane at higher speeds like 12, 14, 16 etc or more knots, but uses more fuel & power to do that.
Alfa Mike