Age old fuel consumption question - with a twist

N

Nwheeler

New member
I’msure this is recorded somewhere on the site, but…..
….Coming from 45 years of sailing, fuel was never a big issue. Then we switched to a 34’ slow trawler and ran the ICW……and I paid more attention. But being a heavy displacement boat,the variables were limited to “go slow and save, go faster (plow more) and watch the inevitable curve to the bottom of the tank. But our new boat, a 2007 CD25 brings in the added option of planing. We just repowered from our old Suzuki 150 to a new 200 and I am now on a quest to get some additional data points - or, as Johnny 5 says in Short Circuit; “more input”.

I know that there are a number of ways to gauge fuel usage, but the one I’m trying to nail down is approx MPG. Specifically how it fluctuates up, and then down, and then up again, based on rpm and whether on plane or not. I noticed on the charts from the engine manufacturers, the common knowledge is that the fuel use is best at idle, then decreases up to a point (usually around 2500 rpm), then it increases up until around 3500 (the sweet spot), only to start the draining of the tank in earnest.

So here’s my question: has anyone out there done a real life study on their CD25, using either a 150 or 200hp engine? So far I have found that my boat, at 2200 rpm, runs at a real 3.3 mpg. From the Suzuki charts, I can see that I should actually get better than that by going on plane at 3500.
 
Do you have one of the Suzuki SMG4 Multi-Function Gauges? If you don't I would get one. That gauge with a good MFD and a NMEA2000 network will give you MPG, Fuel Flow, Range, Trip, RPM's and Hour meters. Pat Anderson has a 25 with a Suzuki 200 and the gauge. He should be able to give something close. Weight plays a big part in the picture.
 
Jody, that would be nice, but no; I have no smart gauges. I’m just looking to get the info “one time” and then I’ll know my planing sweet spot (assuming my weight stays fairly constant.
 
Right, weight will make a big difference so does sea state and wind. I gave up on the quest for ultimate fuel consumption efficiency and go for what feels right
I often wish I had a 200 though,,,
 
Take two identical CD 25s powered the same; owned and operated by two
different people.

Tracking and computing the 2 boat MPG will be different. Why?

* boat loads (weight) will differ
* condition of underwater hull cleanliness will differ
* sea conditions, wind and weather will differ
* operator helmsmanship will differ
* miscellaneous differences: engine tune, fuel, accessories (little things add up)

Hence, another's MPG figures apply to the other, not you. It may
approximate your MPG. If that isn't good enough for you and the difference is
important to you, you need to figure out your own MPG.

Aye.
 
Here is the table made by a former owner of the 2007 C Dory 25, with fiberglass interior, 150 hp Honda--and fairly well loaded:

graph_data.jpg

I did flow studies and found the average of the two was very close. The boat had a fuel flow transducer. Sea conditions and weight have as much to do as anything, Both were done with autopilot--which often is better than most helmsmen can achieve. The boat was trimmed for best speed and
RPM.

The numbers at 2000./2500 are very close to yours and is at the upper end of displacement speeds (the old 1.34 x sq rt LWL). I also found that my fuel gauge worked well. No I don't think you will find better numbers on plane, even thought the C Dory is a very efficient bot on plane.

As you can see the fuel usage begins to come down after about 12 knots--and the boat fully on plane--which is higher than 3500 RPM. The props on the Hondas and Yamahas will be different than the Suzuki due to the different gear ratios. I have found the Suzuki tends to push many of these boats faster. load you boat for the maximum weight and see what your WOT RPM and GPS speed is.

As noted. you can pull fuel usage off the engine computer--as well as calculate fuel used, and range left. I believe the relative small cost of the gauge is a good "investment" if you are going to do serious cruising. Places where this is critical are as in AK where fuel is only available in limited places. I would run at displacement speeds until I was sure that I could made the next fuel stop on plane.


Welcome aboard--many of us are "ex sailboaters".
 
Nwheeler":33775grp said:
Jody, that would be nice, but no; I have no smart gauges. I’m just looking to get the info “one time” and then I’ll know my planing sweet spot (assuming my weight stays fairly constant.
Click to expand...

I would see if the dealer who did the motor change would loan you one or rent it to you. Then you still have to deal with wind and tides.
 
As you can see from the chart, once the boat is fully planing, the faster you go, the better the mileage (or the MPG curve gets very flat). My 22 does the same thing.

If you want to measure your FF, try the Optio Fuel Flow sensor. Works via Bluetooth. It's not that cheap, but is comparable in price to other FF sensor installations (sensor, wiring, display) and is much easier to install.

https://panbo.com/interactio-optio-fuel-easy-installation-and-accurate-fuel-flow/
 
I have a digital readout on my Yamaha 150 on my CD-25. On my recent Erie Canal trip, I was running 1600 rpm, getting around 5 kts. Going into the minimal current I was getting about 4.7-5.0mpg. Coming back I was getting about 5.0-5.5mpg. Other times I believe on plane (around 4500 rpm, 21 kts or so?) I think I'm in the 2.9mpg range. I haven't really made up a chart. There are a lot of other factors that will affect the fuel efficiency. Trim settings, weight, current, temperature, prop size, etc. When cruising fast or slow, I try to trim for the best speed/fuel flow after I set the throttle where I want. In the end, I guess while I try to be fuel efficient, fuel costs don't change my desire to go boating. :) Colby
 
I can speak from a ton of hours....but only with my 22 and twin Honda engines. At 4500 rpms and 20 mph, they consistently get 4.5-5 mpg. Go 24-25 MPH and about 5k rpms I will get very close to 4 mpg. Go 30-32 mph, which is basically full throttle at 6k rpms, I will get around 3 mpg. So...in my experience, you will lose mpg after a certain threshold. And that threshold depends on hull design, the load, and so many other variables, but every boat I have owned has a tipping point in that regard. In my boat, 4.5k rpms (18-22 mph) seems to be the perfect balance of speed and MPGs in my 22. I am not saying any published numbers are incorrect, but what I shared has been my experience.
 
As always, many thanks to all! And why am I not surprised that Bob would have a full chart “coming and going”! It’s interesting to see that going on plane doesn’t seem to positively impact mpg (like I assumed), but it’s also notable that once you’re “up” it’s better to increase rpm in order to obtain the best mpg in that mode.

I’m aware that all boats will be different, dependent on many moving variables……but the info that I gleaned from all of this gives me an indication that the “sweet” spots are either at the very slowest of the displacement speeds - or about 80% of WOT when on plane. Thank you all.
 
IME with a C-Dory you want to trim it to be running fairly flat for best mpg. Nose too high and the stern settles too much for best mpg. Even with a light load, I find that I need to have some tab in to get the boat to run best. Best solution is to trim the boat with ballast (i.e. move the load around) instead of using the trim tabs (they create some drag).
 
I see that planing flattens the curve so to speak. It also seems true that the lowest speed that keeps you on place is the best mileage.(?)

I think the reason planing doesn't make a big difference over displacement speeds is that the C-Dory hull design is already giving you a benefit at displacement by "displacing less" than a displacement hull would.. Agree?
 
I see that planing flattens the curve so to speak. It also seems true that the lowest speed that keeps you on place is the best mileage.(?)
Click to expand...

nmpg_graph.jpg

Is this born out in the graphic representation of what I presented in tabular form above? The other posters numbers appear to validate out this graphic.
 
Wood Zeppelin":2s96vlg4 said:
I see that planing flattens the curve so to speak. It also seems true that the lowest speed that keeps you on place is the best mileage.(?)
Click to expand...

Best speeds on the graph for MPG is below 2200rpm or WOT. Below 2200 is probably not planing.
 
So horse of a different color but here you go. My 27 fter and absolute flat bottom with a 225 honda (1600 hrs) gets on plane at 14 mph and gets 2.25 mpg at that speed and any speed until 28 mph. After that mpg goes down fast and the boat gets a little squirrelly if she is lite on fuel. We pick speed by comfort not mpg as it just seems not to matter once on plane.
 
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