Fuel Economy Monitoring?

[Jeff F]

Looking ahead think of the Loop. You can go 6000 miles or 8000. Let's go with 6000 nm. If you're getting 2 nmpg, that's 3000 gallons or 3 nmpg that's 2000 gallons. At $3 per gallon, it's $9000 or $6000.

Now, let's look at a Beneteau Swift Trawler as a representative of semi-planing. At 7 knots, you get 2.1 nmpg, at 10 knots you get 0.7 nmpg, at anything from 12 to 21.3 knots, you get 0.5 nmpg. On the performance curve for that boat you see huge differences. So you could do 6000 nm with only 2857 gallons but if you decided to spend half your time at 10 knots you'd use 4285 gallons. And, if you decided 1/3 of your time at 7 knots, 1/3 at 10 knots and 1/3 at 15 knots, now you're more like 5485 gallons. So, obviously on this boat you can impact your fuel cost by as much as $7,884. Suddenly the insignificant fuel cost may not be as insignificant.

I'd add that what nobody talks about is what the number for the ST would be at 6 knots. If it's 2.1 @7.0 I'd wager it's > 3.0 nmpg @6.0. Maybe that's outside design specs for that particular boat, but there are huge economies in traveling at no wake speeds. But few are patient enough to do it.

 

[BandB]

I'd add that what nobody talks about is what the number for the ST would be at 6 knots. If it's 2.1 @7.0 I'd wager it's > 3.0 nmpg @6.0. Maybe that's outside design specs for that particular boat, but there are huge economies in traveling at no wake speeds. But few are patient enough to do it.

At 6.3 knots, it's 3.3 nmpg. It's the same at 5 knots.

 

[Insequent]

Here is the story of my last 12 months with 3,500 nm, in three parts.

Part 1 was 1,000 nm north from Brisbane to half way up the Great Barrier Reef. I was impatient, and had the 'throttle' levers pushed forward a fair way....

Part 2 is the 1,000 nm return. Mostly, er...what is the rush? But with some pushing on long days to cover as much distance as possible when winds were favourable.

Part 3 is 1,500 nm mostly in SE Queensland, mainly in Moreton Bay. Ebb and flood tides can influence speed by +/- 1.4 kn, and more often than not I 'ride the tide' so speeds vary from about 6.7 kn to 9.5 kn when operating at 1600 rpm, 10 litres per hour per engine.

Next year I plan on taking 5-6 months and going further north than last years trip. If the total trip is 4,000 nm then slowing down a bit -target 8 kn cruise - will be very much worthwhile in terms of fuel cost/efficiency.

 

[markpierce]

...

Seems to me that in general, former sailors and those of relatively modest means, such as myself, tend to purchase boats that use less fuel and continue to consider fuel consumption carefully. I'm not unusual in creating a table of rpm/gph/speed and use it to determine cruising speed trying to optimize that mix.

Fits my profile.

Depreciation doesn't figure in my calculations. Purchase of my then-new boat wasn't an investment. (I'd been much wealthier, financially, if the money was invested in Hershey Chocolates. About doubled my small investment in that company.) The boat is a toy for the joy. Figure I'll get 25 cents on the dollar when I give up the boat.

 

[Conall63]

I have a chart showing fuel burn per hour at various RPM...trust the chart, but verify. Every day after we travel, I measure and calculate how much fuel we used by measuring my tanks and transferring. The fuel burn chart is always within a gallon or two once compared to engine hours.

Conall

 

[Bigsfish]

I look at my boat as my reward for working hard and saving my money, I don't care about depreciation unless I can deduct it from my income tax. Fuel is an expense to have fun, sometimes I go slow sometimes fast, the cost of fuel isn't a factor. I use to do all my own maintenance because I knew it would be done right, I enjoyed doing it plus it did save money. Today I have those jobs done for me. I do have a fuel-rpm-speed chart that I refer to as my memory isn't as good as it once was.

 

[jleonard]

Fuel cost is not a factor for me. When I run at 6.5 knots (1600 rpm) I use about 1.75 GPH. If I run "fast" at 7 knots (1750 rpm) I use about 2 GPH.
I don't need to be more accurate than that.

 

[dhays]

Fuel cost is not a factor for me. When I run at 6.5 knots (1600 rpm) I use about 1.75 GPH. If I run "fast" at 7 knots (1750 rpm) I use about 2 GPH.

I don't need to be more accurate than that.

Given your range of fuel burn, of course it isn't an issue for you! I would submit, that if your range of fuel burn varied from 1.5 gph to 20 gph, you may think about it a bit more.

 

[Jeff F]

.

There's a recent post from a guy who bought a new boat with the same hull as mine but had a special low RPM, low HP engine installed in it. He posted his fuel calculations and at 7 knots (just slightly above the theoretical hull speed), his fuel burn was the same as mine.

I'd say the biggest revelation for me as I've learned about this whole subject is that the motor itself almost doesn't matter. People think that a boat with a 300 hp diesel is going to use a lot more fuel than a sistership with a 100 hp motor. If both boats travel at the same speed using 40-80 hp for example there's not going to be a lot of difference between the fuel burn. Less difference than adjusting speed by a fraction of a knot. You can look at a range of motors capable of operating at the same output and fuel usage doesn't vary much between them. Optimization of gears/prop has some influence too of course, but really it boils down to how much power is required to drive the boat at a given speed, and with diesels the fuel maps to the power requirement in a surprisingly (to me) linear fashion.

 

[twistedtree]

Bingo!

Lots of people NEVER grasp the concepts that you just outlined.

 

[Jeff F]

Bingo!

Lots of people NEVER grasp the concepts that you just outlined.

It took me a while to get there

 

[kev_rm]

Hello,

I'm new to these forums, but not to boats and electronics. I've been out of the marine electronics world for a while now, and am planning my Great Loop adventure to begin over the next couple of years.

As I have been reading, one of the most talked about topics is fuel efficiency.

I'm curious if there are specific fuel economy monitoring solutions on the market today, and if anyone is using something that provides a good indication of "real-time" gallons-per-hour and miles-per-gallon? It would also be good to have a profile for a boat that allows you to see the predicted trade-offs of speed and fuel efficiency.

Doing my research, it seems that if a good system had access to the fuel consumption rate (simple fuel-line flow sensor) and then your speed-through-water (SWT), and speed-over-ground (SOG), then you could easily collect the data required to create a boat profile, and offer suggestions on optimal fuel economy. Eventually, you could also create a throttle control integration that would allow you to set the throttle based on desired fuel efficiency, instead of RPMs, etc.

Overall, this would provide a very good idea of the characteristics of the engines and boat, and all anyone to understand the trade-offs of speed vs. fuel consumption.

I know that many people would ask "Why bother?" ... but this is what I do for a living, and I love technology and boats.

Anyhow ... I was curious about inexpensive solutions that would do this, if any exist, and also to hear feedback from boaters who might have thoughts about this more.

Scott

My boat came with analog fuel flow meters, they are extremely accurate. I call them "throttles".

 

[FlyWright]

My boat has 3 speeds...slow, slower and stopped. I'm ignoring all current and tidal influences in the basic speeds.

Slow (7.5 Kts) is fast enough to get there with reasonable fuel economy. Slower (6-6.5 kts) feels like the speed of smell...slow torture! It takes too long to travel and the effects of wave action seem magnified on my hull, but in all honest, maybe that's my imagination. I've not traveled at that speed long enough to compute fuel burn, but my consistent average fuel burn at 7.5 Kts/1800 RPM has been 3.2 GPH over the 10 years I've owned the boat and been keeping records.

So it feels like there's one speed that fits my boat and my uses and that's fine for me. If I was to buy another boat with larger engines, I'd have more choices which I lack now and then a fuel flow system could be beneficial. But for now, I'm pleased with 7.5 Kts and 3.2 GPH which gives me 2.3 MPG.

 

[AusCan]

I always monitor my fuel use per hour rather than by distance. It removes the variables of current, sail assistance, sea condition, and hull cleanliness, so the resulting figure is very consistent at a fixed rpm. I normally cruise at 1800 rpm @ 3.0 litres/hr or sometimes 2000 rpm @ 3.5 litres/hour.

When planning a cruise it is easy to work out the exact fuel usage if I keep to a planned engine speed. If I want to keep to an exact time frame, then the fuel usage can change substantially.

 

[Simi 60]

. People think that a boat with a 300 hp diesel is going to use a lot more fuel than a sistership with a 100 hp motor. If both boats travel at the same speed using 40-80 hp for example there's not going to be a lot of difference between the fuel burn. .

There will be some difference in engine wear.
Our sister ship has a 127hp Garner vs our 315hp cummins.
I reckon they'll be running near full noise doing 8 knots where as we are just plodding along at 1200rpm

 

[dhays]

I am still trying to vary engine speed with my new engine. Right now running at 1460 rpm and burning 2.0 gph. That is giving me about 7.0 knots (6.8 -7.2 with small current fluxuations). The engine says that is 27% load.

A bit earlier I was running a while at 2000rpm burning 6.0 gph. This gave me about 2 knots more speed.

 

[AusCan]

There will be some difference in engine wear.
Our sister ship has a 127hp Garner vs our 315hp cummins.
I reckon they'll be running near full noise doing 8 knots where as we are just plodding along at 1200rpm

Actually, the 127 hp Gardner is putting out over 100 hp @ only 1200 rpm, but you are right- it would be working at the upper range of its 1500 rpm limit.

 

[Nomad Willy]

I'd say the biggest revelation for me as I've learned about this whole subject is that the motor itself almost doesn't matter. People think that a boat with a 300 hp diesel is going to use a lot more fuel than a sistership with a 100 hp motor. If both boats travel at the same speed using 40-80 hp for example there's not going to be a lot of difference between the fuel burn. Less difference than adjusting speed by a fraction of a knot. You can look at a range of motors capable of operating at the same output and fuel usage doesn't vary much between them. Optimization of gears/prop has some influence too of course, but really it boils down to how much power is required to drive the boat at a given speed, and with diesels the fuel maps to the power requirement in a surprisingly (to me) linear fashion.

That's usually true but the bigger engine has much more metal exposed to heat that will be sapped out of the combustion air reducing fuel efficiency. The reason it comes close w many installations is that w a diesel engine (combustion) tempetures are very low at low loads. But if a smaller engine was used less heat loss would make it more efficient. Also the friction of the bigger moving parts is greater the bigger engine suffer again. But then the smaller engine's parts move much faster.

BUT the smaller engine is subjected higher temps so the outcome is similar. There is the efficiency at any specific rpm to vary efficiency as well.

The best fuel efficiency will probably be a small engine running at about 80% load. Not mentioned is the underloading "issue" and IMO engine manufacturers have the say on that by insisting their engines be loaded at max hp rpm.

 

[Simi 60]

The best fuel efficiency will probably be a small engine running at about 80% load. Not mentioned is the underloading "issue" and IMO engine manufacturers have the say on that by insisting their engines be loaded at max hp rpm.

This from Tony Athens.



To me, the easiest way to gage whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings..The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less..Yes, there are many other parts of the equation that leads to the life of a diesel engine, but I know from experience that running them slow (i.e. cruising for days on end at hull speeds or less) is NOT a cause for concern

In closing, I’ll mention that although this topic is brought up quite often and many people preach that you’ve got to use a diesel hard if you want it to last, I’m still waiting to find one that was rebuilt before its time due to low speed use..Just the opposite seems to be always the norm.

https://www.sbmar.com/articles/low-speed-running-break-in-of-marine-diesels/

 

[Art]

This from Tony Athens.



To me, the easiest way to gage whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings..The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less..Yes, there are many other parts of the equation that leads to the life of a diesel engine, but I know from experience that running them slow (i.e. cruising for days on end at hull speeds or less) is NOT a cause for concern

In closing, I’ll mention that although this topic is brought up quite often and many people preach that you’ve got to use a diesel hard if you want it to last, I’m still waiting to find one that was rebuilt before its time due to low speed use..Just the opposite seems to be always the norm.

https://www.sbmar.com/articles/low-speed-running-break-in-of-marine-diesels/

Actually, quite logical too! Thanks for posting that input.

 

[O C Diver]

That can be very misleading. It's one thing to say that you can get comparable fuel economy out of an engine running at 30 to 50% of rated HP. It's another thing to say you can run an engine at 10% of rated HP with comparable economy and have good engine longevity. My boat originally had a 450 HP Cummins. It now has a 135 HP John Deere. It takes 40 HP to push the boat at 7 knots. Fuel consumption with the JD is little more than half of the Cummins.

Ted

 

[Art]

That can be very misleading. It's one thing to say that you can get comparable fuel economy out of an engine running at 30 to 50% of rated HP. It's another thing to say you can run an engine at 10% of rated HP with comparable economy and have good engine longevity. My boat originally had a 450 HP Cummins. It now has a 135 HP John Deere. It takes 40 HP to push the boat at 7 knots. Fuel consumption with the JD is little more than half of the Cummins.

Ted

Hi Ted - Am I understanding correctly...

The 450 hp. Cummings used nearly 2X fuel amount to produce same 40 hp. need to do 7 knots... as compared to the 135 hp. JD fuel amount? If so, would that near 2X increased fuel usage occur because the Cumming's need to simply move its own massive internal parts at that low power output level; or is because the JD is actually that more efficient?

 

[Bigsfish]

Also, how did the top end speed change?

 

[Jeff F]

Hi Ted - Am I understanding correctly...

The 450 hp. Cummings used nearly 2X fuel amount to produce same 40 hp. need to do 7 knots... as compared to the 135 hp. JD fuel amount? If so, would that near 2X increased fuel usage occur because the Cumming's need to simply move its own massive internal parts at that low power output level; or is because the JD is actually that more efficient?

I'm guessing that a large part of the difference is in a better optimized gear/prop. Agree that steady operation at < 10% rating is far from optimal, and for your use the smaller motor is absolutely the better choice. Would be interested in comparing mfg power curve between the two to see how much difference there is in overlapping power outputs.

 

[O C Diver]

Hi Ted - Am I understanding correctly...

The 450 hp. Cummings used nearly 2X fuel amount to produce same 40 hp. need to do 7 knots... as compared to the 135 hp. JD fuel amount? If so, would that near 2X increased fuel usage occur because the Cumming's need to simply move its own massive internal parts at that low power output level; or is because the JD is actually that more efficient?

The JD burns 2 GPH at 7 knots. It probably is more efficient as it's operating near peak torque where the Cummins was at the bottom of its operating range. I'm sure the new QSC Cummins with common rail injection would be better than my old C engine. But there is simply a lot more engine to turn which takes fuel. The one that I don't know the answer to is whether the air to fuel mixture ratio at that low RPM is a factor.

Also, how did the top end speed change?

Goes twice as fast with the JD.

No longer a semi planing boat, displacement speed only.

Ted

 

[Bigsfish]

Ted

You'll have to explain to me over beers how a lower hp can get more speed. I don't doubt a thing you say but that sounds strange.

 

[Nomad Willy]

This from Tony Athens.



To me, the easiest way to gage whether slow speed running is detrimental over years and years of operation is to look at commercial fishing vessels with older designed engines from Detroit, Cat, Cummins, etc… Revisiting the “Detroit” mystique again, its longevity was built on engines rated to run at 1900-2100 RPM and above, but could only last for 30+ yrs when operated continuously at 1100-1600 RPM (again, well under 50% of rated HP)..These same engines in a “crew” boat used in the off-shore oil industry, would go through “top-ends” (or worse) just about yearly when run at close to their governor settings..The longest-lived engines that I’ve been involved with (hrs and yrs wise), have been engines in commercial or recreational trawler type applications run at 50% of rated HP or less..Yes, there are many other parts of the equation that leads to the life of a diesel engine, but I know from experience that running them slow (i.e. cruising for days on end at hull speeds or less) is NOT a cause for concern

In closing, I’ll mention that although this topic is brought up quite often and many people preach that you’ve got to use a diesel hard if you want it to last, I’m still waiting to find one that was rebuilt before its time due to low speed use..Just the opposite seems to be always the norm.

https://www.sbmar.com/articles/low-speed-running-break-in-of-marine-diesels/

Yup and may to most followed AD to the letter. Tony whatever but I'd put stock in what the manufacturers say. Most of their info comes from engineers and they are very interested in their engines performing well and lasting a long time.

It's just guru's and dock talkers that put way too much power in a boat underloading it for better fuel consumption and most probably overload it at times. The real deal is the right way.

Commercial boats are usually propperly powered and most here would say their engines are screaming ... but diesel engines make noise.

The FL should probably be propperly propped and run at about 2000rpm but almost nobody does.

I worked for years in a machine shop and many of the machinists left their lathes ect run all the time they ate lunch. I don't like noise that much but propper sound insulating can even make a DD rather quiet.

But most here don't get the opportunity to choose the power for their boat because few repower even when it should be done.

There was a GB36 on Yachtworld w two 55hp Yanmars. Re this thread I wish we had that boat here on the forum to compare it to a stock GB36. Just the reduction in weight should make a noticeable difference in fuel burn.

 

[dhays]

Yup and may to most followed AD to the letter. Tony whatever but I'd put stock in what the manufacturers say. Most of their info comes from engineers and they are very interested in their engines performing well and lasting a long time.

It's just guru's and dock talkers that put way too much power in a boat underloading it for better fuel consumption and most probably overload it at times. The real deal is the right way.

I have looked through the original Cummins manuals that I have. They talk a lot about how to get maximum life and performance out of the Cummins QSB380 HO engine. The manuals discuss not running the engine at idle for extended periods of time at temperatures before normal operating temps. They do recommend idling the engine for 3-5 minutes before gradually increase rpm and load. They are issue very strong warnings about operating the engine at max throttle for an extended period of time. Nowhere in the manual to they recommend or warn against running the engine at a low power setting.

In fact, one of the options that was available (not sure it is available now as it seems like a silly idea) was a "Marine Cruise Control". This was an electronic control that had two, preset rpms for "Cruise 1 and Cruise 2". The factory preset for Cruise 2 was 1200 rpm. In this engine, 1400 rpm is about 25% load. 1200 would be significantly less. Cummins seems to believe that operating this engine at a very low power setting is just fine.

So, while Tony Athens may be opinionated, a bit abrasive, and derisively dismissive of engineers, he is also an extremely experienced Cummins mechanic. He has no problems calling out what he sees as bad ideas on the part of Cummins, but in this case he and the engineers seem to agree.

There was a GB36 on Yachtworld w two 55hp Yanmars. Re this thread I wish we had that boat here on the forum to compare it to a stock GB36. Just the reduction in weight should make a noticeable difference in fuel burn.

A while back there was a Bayliner 4788 available for sale in BC. If I recall it was repowered with two low-power Yanmar engines. I remember thinking that for my purposes, that would be a nice combination. I also recall seeing a Great Harbor 37 that was powered with Yanmar 4jh4 56hp engines (same great engine that is in my sailboat).

Turns out the Bayliner was repowered (and had a completely new electrical system as well as new carpeting, cushions etc) because it had sunk at the dock one winter.

 

[Insequent]

For my JD's there was a run-in process for the first 100 hours using a special (non-detergent?) oil. After that, running for long periods at low load/low rpm is not an issue according to the dealer or anything else that I can find. They do say avoid prolonged idling however.

I tend to look at the power curves for the engines. They invariably start at several hundred rpm above idle, and my guess is that where they start is about the lowest rpm to operate at for any significant period of time.

My engines are rated at 2600 rpm, and from memory can be operated continuously at 2200 rpm. But I run at 1600 or thereabouts most of the time as that is speed that is the sweet spot for my hull. Its 8 kn, or 1.2 x sqrt waterline length.

 

[BandB]

Every engine has it's own recommendations. However, I haven't found a moderate horsepower engine yet that says not to run at low load after the break in period. In fact, looking at higher hp engines, all they say generally is to take it to cruise briefly every few hours.

It's not just propulsion engines with all the misinformation, the same with generators. Each manufacturer has their guidelines but I hear people saying you must run at least 50% load and I don't know a current generator manufacturer who says that. The most common recommendations I'm aware of say between 25% and 75% or other ranges and may advise against running it excessively without load on it. Get the information from the manufacturer for the unit you're talking about.