What is the most efficient cruising speed?

S

SurfDory

New member
Ahoy! New 22 cruiser owner here. Curious about where the boat operates most efficiently, one of the first mods I did was to add a GFS10 fuel flow sensor to the Garmin MFD that came with the boat. My girlfriend and I did about 50 miles today and I was able to run an experiment where I collected fuel flow data at various RPMs/Speed. Each data point was collected with the motor and tabs trimmed for highest speed at that RPM. I first trimmed motor with permatrim plate for highest speed, then played with trim tabs. I found that the boat actually went a little faster with about 1/2 bow down trim on tabs. They seemed to "lift" the stern some and help the boat ride flat on the flat section of the hull.
I entered this data into a spreadsheet, calculated MPG, and then plotted Speed, Burn Rate and MPG vs. RPM hoping to find a speed with the best efficiency. Obviously, slower should be better but I was curious to see if there was a dip in efficiency at semi-planing speeds (where you throw the biggest wake). Surprisingly, fuel economy seemed pretty consistent at around 4 MPGs from about 8mph all the way up to full throttle around 30mph.

I hope some of you find this interesting or helpful. Has anyone else measured similar results?

Now if I could only be smart enough to figure out how to access and upload photos to my album on here... Here's a link to the FB post I made on the Cdory owners group with a picture of the plot and data...

https://www.facebook.com/share/p/ugFAGu4fb8TV1y4T/
 
SurfDory, looked at your FB link numbers for fuel economy. Do you have a number for fuel economy at 1500 rpm? I would think that your fuel economy mpg would be quite a bit higher at an even slower speed. Being able to slowboat and save fuel is a great benefit of owning a C-Dory!
 
8 mph is probably one of the worst speeds you can go MPG-wise while still not on step for fuel economy. You're probably pushing a big bow wave forward (displacing a lot of water), and the boat is barely not able to ride over it and plane at this speed. Here are my numbers for a new Honda 100 on a moderately-loaded CD22. Note that mpg is pretty high, almost 10 mpg until we get close to hull speed of 7.3 mph, where it drops. In his long journey in Hudson Bay (Arctic Solitare book), Paul Souders typically cruised at 5 mph or less since he needed to increase the distances he could go between fueling in these remote areas. 5 mph should be about as good as you can get, and much after that you start of push more water instead of boat. As you get onto step, mpg increases a little from 4.6 to 5.5 mpg, and then at higher speeds, you get a small drop with higher speeds. 31 mph was not WOT for this motor, but was getting a little scary for my boat, so I don't think I will push it to WOT very often, if ever. Most of my boating is in kayaks, canoes and sailboats, so I have little need for speed, but I love high MPG.

MPH MPG
3.5 9.0
4.8 9.9
7.0 4.9
8.4 4.6
12.9 5.5
16.1 5.5
19.1 4.4
22.4 4.4
28.3 4.3
31.0 3.5
 
On my CD22 the MPG curve is pretty flat once it gets fully on plane. I use 3 MPG for planning purposes. This is sufficiently below the actual MPG that a decent reserve is built in.

Yes, above 30 MPH, the CD22s get squirrelly. I've had mine up to about 35 on very flat water, but the handling gets pretty risky when going that fast.
 
I forgot to include the RPM data, so will post below. Most of this is based on runs of a few miles at given RPMs with NMEA 2000 data from the motor. I did check the fuel consumption at 1500 and 3500 RPM for 10 minutes by using a portable tank and weighing on a scale. The NMEA was spot on...incredible, because you can see instantly how tilt, trim, and loading, even wind and tide, affect mpg. When I start, I put the amount of gallons of fuel in tanks, and the display even gives me estimated ranges at various speeds, etc.

RPM MPH MPG
1000 3.5 9.0
1500 4.8 9.9
2000 7.0 4.9
2500 8.4 4.6
3000 12.9 5.5
3500 16.1 5.5
4000 19.1 4.4
4500 22.4 4.4
5000 28.3 4.3
5500 31.0 3.5
 
I was not able to view your data on a Facebook CDory boat owners group--despite my membership in another Facebook CDory owner's group. I believe our community as a whole would be best served if the information from "Surf Dory"'
s study would be posted here. When and If I am "approved" for membership in this second Facebook (actually the 3rd, because factory has a Facebook page also.), I will bring that data over to this forum.

The study by robhwa is very tropical, for a fairly light boat. A lot of the MPG will depend on the weight of the load--those of us who run the boat loaded for a month's cruise, with two refrigeration units, generator, food, clothing etc may well have at least 1000 lbs more gear and supplies aboard. Those numbers will be somewhat lower.

On Rob's data, I suspect that the boat went on a plane somewhere between 8.4 mph and 12.9 mph. Many run their boats at about 17mph and find that an excellent "goldilocks" speed. It is also important to differentiate statute miles per hour Vs nautical miles per hour. (same for the mileage). Although there are many ways of writing the nautical mph, often it is noted as "knots per hour" or knmph vs smph or statute miles per hour. One might ask why many do not use the nautical miles per hour. On the ICWs and Rivers the mileage is in statute miles. At sea and offshore, the mileage distant measurement is usually in nautical miles.
 
I've always questioned some of these MPG figures on 22's and 25's. Using my digital fuel gauges hooked in to the motors, my current 25 with the Yamaha F150 gets around 4.5mpg at hull speeds, and drops to around 2mpg on plane. I don't remember exactly what my 22 with a Mercury 115 got, but I believe it was around 6mpg at hull speed, and less than 4mpg on plane. Actual fuel usage measured at the tank pretty much supports those numbers as well. I do believe the OP's numbers are probably pretty accurate from what I think my numbers were on my 22 with the Merc 115 4-stroke. Colby P.s. that was statute miles per gallon.
 
This is the OP's chart from FB. BTW, the flattening of the curve is pretty much what I've found on both my 22 and 25. Best fuel flow is at hull speed or slower. After that, the fuel flow is pretty much the same. Current can play havoc with numbers. Colby

454852052_10232629493862575_5199966193398604960_n.jpg
 
This is great data I can use to compare actual energy efficiency between gas and electric. First off I should mention that my top speed is only 6.8 mph, so I have to use the slower speeds. At 4.8 mph Rob was using a tenth of a gallon per mile. Since a gallon of gas holds 33,000 watt hours of energy (33kWh ) he was using about 3.3 kilowatt-hour per mile. With my two electric outboard motors I am running at1.6 kW per hour at 4.8 mph or 0.33kWh per mile. So the electric outboard is10 times as efficient as gas at the slower speeds.
 
Tom Hruby":1cl1tohw said:
This is great data I can use to compare actual energy efficiency between gas and electric. First off I should mention that my top speed is only 6.8 mph, so I have to use the slower speeds. At 4.8 mph Rob was using a tenth of a gallon per mile. Since a gallon of gas holds 33,000 watt hours of energy (33kWh ) he was using about 3.3 kilowatt-hour per mile. With my two electric outboard motors I am running at1.6 kW per hour at 4.8 mph or 0.33kWh per mile. So the electric outboard is10 times as efficient as gas at the slower speeds.
Click to expand...

I would not put much in the way of bets, These flow meters are notoriously inaccurate at the low flow rates. For example the engine appeared to have 9.0 1000 RPM and 9.9 at 1500 RPM. The timed consumption of fuel from a calibrated container is far more accurate for the low speeds.
 
I just checked my Mercury vessel view (bluetooth gateway to motor data) screenshots from my 22 w/ new Mercury 115, 4 blade nemisis 15P and the higher RPM numbers are almost identical.

However, it looks like my peak planing-speed MPG was in a different spot. I confirmed across several logs that my peak MPG was about 1000 RPM lower than OP's while moving quite a bit faster:

5.2 MPG at 3500 RPM 18 MPH.

With my new 25 I get excited any time the gauge reads higher than 2 MPG :(
 
Here is the data on our 2007 C Dory 25 by prior owner. We got almost the same exact "mileage";graph_data.jpg

Our boat was moderately loaded with supplies for several weeks onboard.
 
Keep in mind the data given is no more than the result with a particular boat
at a particular time (or average times) in a particular place.

Because you may have the same make/model boat and engine as another who
posts their data does not signify your boat will do the same. Why?

Two identical boats and power sources will differ in efficiency due to
* different propellers
* different boat loading/weight distribution
* different engine tune and/or age
* different amount of hull growth/cleanliness
* different sea condition; salt vs fresh water, current, wind
* different altitude above sea level
* probably more.

Maybe comparisons would be close; in the same ballpark. But technically, no cigar.

Aye.
Grandpa used to say, "Boats are kinda like people. None are the same. None are
perfect. But most are fun and interesting to spend time with."
 
Foggy":at5vfc9i said:
Keep in mind the data given is no more than the result with a particular boat
at a particular time (or average times) in a particular place.

Because you may have the same make/model boat and engine as another who
posts their data does not signify your boat will do the same. Why?
Click to expand...

All of what Foggy says is incredibly true. The reasons I was able to post the data I did was partly because I repowered from an 2003, high-hours carbureted Honda 90 to a brand new Honda 100 that had integrated NMEA 2000. The Honda 100 is lighter, but not significantly.

I also removed two things...a heavy starboard 9.9 kicker with tilt/trim/etc. that maybe put an extra 100 lb on the starbord stern. I also had an electric trolling motor mounted above the antiventillation plate on the Honda 90. All of these made my combination starboard and stern heavy unless I loaded up the front and port, which I did for trim and tilt. In the new combination, I changed to a bow-mounted electric trolling motor. The stern rides higher.

Also note that my MPG were statute miles, not nautical miles. I should probably have posted both.
 
Thanks for the interesting data. I have experimented with my boat on every trip as I work out fuel consumption on every cruise. I do it the old fashioned way, by looking at the fuel tanks and my distance records on the GPS, so my numbers are less accurate than yours are. I tend to run a fairly heavy boat, with two kayaks on top. Most often I am loaded with supplies for the average two week journeys that we go on. I have found that there are about two speeds that we feel comfortable travelling at. For cruising I tend to travel at about 2000 RPM which has me moving at about 5.5 knots. When I need to or want to pick up the speed, I go on plane at about 4400 RPM, which gets me about 12 -14 knots depending on conditions. This is with my 90 HP Honda. These are speeds I like to travel at. I also like to give 'Gretzky" my Honda 9.9 HP Kicker a workout at least every other day and will spend an hour or more travelling on it at WOT which will move me at near 5 Knots. Fuel consumption is really low with the kicker. For me a normal fuel load is about 200 litres with my two tanks and my 45 litre slip tank. I'm Canadian, so we no longer use miles or gallons, and our gallons were bigger than your gallons. My current range estimate based on travelling with 90 HP at 2000 RPM 75% of the time, 25% on plane and also using the kicker for an hour or so every other day. I now use the 'easy math' that if I have 200 litres of fuel, I should be able to travel about 200 NM.
 
Yes there will always be variability between boats, motors etc. We can however try to normalize to variables that are universal. David Gerr in his Propeller Handbook attempted to do that using his knowledge and empirical data. Using his equation I calculate that it takes 2 shaft horsepower (1500 watts) to move 1000 lbs of boat at hull speed, and there was not much difference at this speed between displacement and planing hulls. Based on the data I have seen this number is +- 15% of what folks have found. The power needed at lower speeds is reduced much faster than the speed (a hyperbolic curve).

So, based on this equation it should take 6000 watts (8 shaft hp) to move a 4000 lb 22ft C-Dory at hull speed and 2000 watts at 5mph. This comes out to be 400 watt hours per mile or the equivalent of about 1.6 Oz of gasoline per mile (a gallon of gas holds 33,000 watt hours of energy). Any higher rates of consumption are a result of the ineffiencies of the gas motor. I have found that Gerr's equation does provide a reasonable value of the actual electrical energy I have needed to power the four electric boats I have had over the last 26 years .
 
CDfolks; for the sake of international and nautical folks...here is a reinterpretation of my data, including the estimates for the 90. I only did two serious estimates based on using an external tank, warming things up, moving with no/low tides or significant wind, and placing the external tank on an accurate scale. I used 6.07 lb/gallon as density for gasoline.

Again, remember that this is:

1) just my boat, relatively lightly loaded, just me and no gas in the main tanks
2) I tried to keep under low wind and tide
3) I expect less mpg when boat is well loaded

I'd be thrilled to see similar data from other folks. If you have it, you can use your multifunction display to adjust rpm/speed, etc. You really don't need to know exactly.

CDory22statuteMPG.jpg
 
I have logged an hour or two on my boat and found your numbers to be perfectly reasonable. Thanks for sharing them. As somebody noted up in the thread, 17-18 mph does seem to be the best cruising speed, in regards to efficiency, for my boat too. I have an idea why....maybe because of a sweet spot in cam grind, conservative rpms, and a light throttle setting resulting in a more efficient curve. Pure speculation on my part.....
 
Back
Top