(Our) Real World Fuel Burn
Oct 18, 2019 19:04:00 GMT
Post by alenka on Oct 18, 2019 19:04:00 GMT
At the start of the season a question was posted on the forum asking if there was a sweet spot for fuel burn v’s speed. The boat in question wasn’t a 43DS but when I put forward that we use a calculation of 2.7 lts/hr it raised some scepticism to say the least.
Generally we start the day with no set destination in mind and go wherever the wind blows us keeping engine usage to a minimum. In a perfect world this means we start it for nothing more than getting in and out of harbours, on and off the quayside.
Our plans this year however took us far over the horizon, beyond our normal cruising ground and on an adventure close to 1,000nms.
With places to go and people to meet up with this meant we had to keep to a pretty tight itinerary.
Some legs required keeping up a fast pace to avoid night sails - My crew always prefer being in a taverna when the sun goes down rather than keeping night watch so if our plans required fast motoring that’s what was going to happen.
In short, it was a good opportunity to re-asses fuel burn on an extended cruise that involved a cross section of cruising styles - Fast motoring, motor sailing, and sailing.
There was just one initial problem…
Haul out at the end of the previous year was a bit of an ordeal to say the least. Firstly, we were unable to top up our tanks fully. Then we were waved away from the hoist on three occasions due to shoreside difficulties. Four hours of pushing against the current elapsed before we were eventually called in. Additionally, running the engine ashore for both winterisation and re commissioning ate into our diesel, so just how much of our 200 lt capacity we actually had at launch was difficult to say. The problem was compounded because desperately needed spare parts (which hadn’t arrived at the marina) had to picked up in Nidri and that required an immediate cast off and high speed (7kt+) dash under power to make the canal opening at Lefkas. There just wasn’t time to stop by the refuelling dock.
The following day, was a dawn departure to cover 81nms to Trizonia in the gulf of Corinth. Again, it started out purely on engine at 7kts but by mid afternoon we were motor cruising before eventually running downwind under sail at 8.2kts to 9.2kts in over 30kts of wind from behind.
Leg three was another long one. 7kts motoring in calm conditions. 53nms saw us through the Corinth Canal and to our first refuelling since launch.
If the tank had been full at launch, which it wasn’t, our standard fuel planning estimate of 2,7lts/hr suggested 58lts to full. It turned out to be 71lts (3.3lts/hr). But a mere 13 lts difference didn’t seem right and, in truth, less than expected.
The day ended with another dash to reach Epidavros before nightfall, only then could we could relax into a far more leisurely cruising regime. For the next few days at least.
Six days later we were in Athens topping up once more. Using the 2.7lts/hr figure we estimated 41lts to full - It was a whopping 71 lts. Which meant we had burned 4.76lts/hr. Or had we?
Refuelling in this Athens marina is a two man operation and it transpired my crew had had to tell one operative that the counter wasn’t at zero, but with a lot going on they were unsure if it was acted upon appropriately. Checking back in the log it seemed we had an equally, unexplained, high uplift from the same guys the last time we used their services. Needless to say we had some doubts as to just how much fuel actually went into the tank compared to what was indicating on the gauge.
For the sake of brevity I will not go through every uplift but these initial uncertainties threw our calculations somewhat awry.
Returning back to the Ionian a month latter we topped up to a full tank again at the Corinth canal and over the next couple of days fought our way to Sami on Kefalonia through some big seas in 30kt winds that were often more of an hindrance than help.
George is one of the few mini-tankers I trust 110% on both quality and quantity so when our estimated uplift of 65.7 lts turned out to be 112 lts we were both somewhat surprised at the large discrepancy. “Your estimates are normally within a few litres. What’s happened?” he asked.
Some investigation work was definitely needed.
Going over the log a pattern emerged. Every fuel uplift at the canal was always lower than expected and the following one always higher, prompting George to ask “Was the filler nozzle they used long or short? Is it gravity feed or pumped??” At the end of the questions he came to the conclusion that the nozzle/pump pressure they were using at Corinth didn’t suit our boat and causing excessive foaming that makes the tank appear full when it isn’t.
Several days of gentle motor sailing saw us back into our home berth and a final top up before leaving for a few weeks at home. We estimated 25.1its - we couldn’t get anymore than 15lts in!
Anyway, to cut to the chase.
Our first six weeks on the water saw us motoring at much higher speeds than our norm and venturing out in some pretty big winds and swells. Total uplift divided by engine hours worked out at 3.7 lts/hr. Or 3.57 lts/hr if we assume that we started the journey 20 lts short of a full tank.
The second part of our season found us back meandering around our local cruising area with no big distances to cover and no need for high speed motoring, It came as no surprise to us that our average fuel burn over these weeks worked out at 2.86 lts/hr.
All in all we came to the following conclusions regarding fuel burn.
Accurately refuelling too the same level is almost impossible due to the different techniques used at different facilities and how each operative considers a tank to be full.
We found the best way to always fill to the same level was to use a battery operated hand pump and a jerry can of diesel for the last few litres.
It is probably more accurate to ask is there a sweet spot for rpm v’s fuel burn rather than speed v’s fuel burn, but let’s face it we all passage plan using speed not rpm.
If setting a speed to meet deadlines expect variations on RPM to achieve the desired speed. We found minor tweaks are required hour to hour let alone day to day. If you keep a steady rpm (which is not desirable over a long period) then except variations in speed. Wind, waves and no doubt loading of the boat all contribute.
At the end of the season I am still pretty confident that our ‘relaxed’ style of cruising and motoring at around 5 - 6 kts, plus low rev berthing will give us a fuel burn of around 2.7 lts/hr. Coping with bigger seas and speeds up to 7 - 8kts will see this rise 3.6 lts/hr. I would be happy to accept 3.2 lts/hr for planning purposes when we expect a mixture of styles.
Did we find a sweet spot? Well 7.5 kts at 3.6 lts gives a ratio of around 2.1 nms to the ltr, whereas 5.5 kts at 2.7 lts surprisingly returns a ratio of just 2.0nms to the ltr. Bearing in mind all inevitable minor inaccuracies it is perhaps just too close to call.
It is important to remember that speeds close to or above max theoretical hull speed will cause a rise in drag and drag is the enemy of fuel efficiency. Our theoretical max hull speed is around 8.2kts and 7kts represents 85% of that speed.
Based on these figures will we be motoring all the time at higher speeds in future? No!
Once we go through 6kts engine noise and vibration increase and the experience becomes less enjoyable for us.
Technical bit.
We cruise two to three people up in our 2004 43DS.
We uplift water whenever it is freely available so for the most of the time are always close to 500 lts.
Our Yanmar 4JH4-TE 75HP turbocharged engine has less than 1,600 hours of use.
The original prop was swapped out for a ‘Variprop’ feathering prop. I cannot say what the pitch is because it is user adjustable and was set by engineers before I purchased the boat.
The season is always started with a fresh coat of anti-foul.
There are just so many variables that I dare say that other 43DS owners will have fuel burns better or worse than ours.
Of course don’t just assume our figures will work for your boat even if it is a 43DS. You need to compile your own database over a fairly long period of time.
Generally we start the day with no set destination in mind and go wherever the wind blows us keeping engine usage to a minimum. In a perfect world this means we start it for nothing more than getting in and out of harbours, on and off the quayside.
Our plans this year however took us far over the horizon, beyond our normal cruising ground and on an adventure close to 1,000nms.
With places to go and people to meet up with this meant we had to keep to a pretty tight itinerary.
Some legs required keeping up a fast pace to avoid night sails - My crew always prefer being in a taverna when the sun goes down rather than keeping night watch so if our plans required fast motoring that’s what was going to happen.
In short, it was a good opportunity to re-asses fuel burn on an extended cruise that involved a cross section of cruising styles - Fast motoring, motor sailing, and sailing.
There was just one initial problem…
Haul out at the end of the previous year was a bit of an ordeal to say the least. Firstly, we were unable to top up our tanks fully. Then we were waved away from the hoist on three occasions due to shoreside difficulties. Four hours of pushing against the current elapsed before we were eventually called in. Additionally, running the engine ashore for both winterisation and re commissioning ate into our diesel, so just how much of our 200 lt capacity we actually had at launch was difficult to say. The problem was compounded because desperately needed spare parts (which hadn’t arrived at the marina) had to picked up in Nidri and that required an immediate cast off and high speed (7kt+) dash under power to make the canal opening at Lefkas. There just wasn’t time to stop by the refuelling dock.
The following day, was a dawn departure to cover 81nms to Trizonia in the gulf of Corinth. Again, it started out purely on engine at 7kts but by mid afternoon we were motor cruising before eventually running downwind under sail at 8.2kts to 9.2kts in over 30kts of wind from behind.
Leg three was another long one. 7kts motoring in calm conditions. 53nms saw us through the Corinth Canal and to our first refuelling since launch.
If the tank had been full at launch, which it wasn’t, our standard fuel planning estimate of 2,7lts/hr suggested 58lts to full. It turned out to be 71lts (3.3lts/hr). But a mere 13 lts difference didn’t seem right and, in truth, less than expected.
The day ended with another dash to reach Epidavros before nightfall, only then could we could relax into a far more leisurely cruising regime. For the next few days at least.
Six days later we were in Athens topping up once more. Using the 2.7lts/hr figure we estimated 41lts to full - It was a whopping 71 lts. Which meant we had burned 4.76lts/hr. Or had we?
Refuelling in this Athens marina is a two man operation and it transpired my crew had had to tell one operative that the counter wasn’t at zero, but with a lot going on they were unsure if it was acted upon appropriately. Checking back in the log it seemed we had an equally, unexplained, high uplift from the same guys the last time we used their services. Needless to say we had some doubts as to just how much fuel actually went into the tank compared to what was indicating on the gauge.
For the sake of brevity I will not go through every uplift but these initial uncertainties threw our calculations somewhat awry.
Returning back to the Ionian a month latter we topped up to a full tank again at the Corinth canal and over the next couple of days fought our way to Sami on Kefalonia through some big seas in 30kt winds that were often more of an hindrance than help.
George is one of the few mini-tankers I trust 110% on both quality and quantity so when our estimated uplift of 65.7 lts turned out to be 112 lts we were both somewhat surprised at the large discrepancy. “Your estimates are normally within a few litres. What’s happened?” he asked.
Some investigation work was definitely needed.
Going over the log a pattern emerged. Every fuel uplift at the canal was always lower than expected and the following one always higher, prompting George to ask “Was the filler nozzle they used long or short? Is it gravity feed or pumped??” At the end of the questions he came to the conclusion that the nozzle/pump pressure they were using at Corinth didn’t suit our boat and causing excessive foaming that makes the tank appear full when it isn’t.
Several days of gentle motor sailing saw us back into our home berth and a final top up before leaving for a few weeks at home. We estimated 25.1its - we couldn’t get anymore than 15lts in!
Anyway, to cut to the chase.
Our first six weeks on the water saw us motoring at much higher speeds than our norm and venturing out in some pretty big winds and swells. Total uplift divided by engine hours worked out at 3.7 lts/hr. Or 3.57 lts/hr if we assume that we started the journey 20 lts short of a full tank.
The second part of our season found us back meandering around our local cruising area with no big distances to cover and no need for high speed motoring, It came as no surprise to us that our average fuel burn over these weeks worked out at 2.86 lts/hr.
All in all we came to the following conclusions regarding fuel burn.
Accurately refuelling too the same level is almost impossible due to the different techniques used at different facilities and how each operative considers a tank to be full.
We found the best way to always fill to the same level was to use a battery operated hand pump and a jerry can of diesel for the last few litres.
It is probably more accurate to ask is there a sweet spot for rpm v’s fuel burn rather than speed v’s fuel burn, but let’s face it we all passage plan using speed not rpm.
If setting a speed to meet deadlines expect variations on RPM to achieve the desired speed. We found minor tweaks are required hour to hour let alone day to day. If you keep a steady rpm (which is not desirable over a long period) then except variations in speed. Wind, waves and no doubt loading of the boat all contribute.
At the end of the season I am still pretty confident that our ‘relaxed’ style of cruising and motoring at around 5 - 6 kts, plus low rev berthing will give us a fuel burn of around 2.7 lts/hr. Coping with bigger seas and speeds up to 7 - 8kts will see this rise 3.6 lts/hr. I would be happy to accept 3.2 lts/hr for planning purposes when we expect a mixture of styles.
Did we find a sweet spot? Well 7.5 kts at 3.6 lts gives a ratio of around 2.1 nms to the ltr, whereas 5.5 kts at 2.7 lts surprisingly returns a ratio of just 2.0nms to the ltr. Bearing in mind all inevitable minor inaccuracies it is perhaps just too close to call.
It is important to remember that speeds close to or above max theoretical hull speed will cause a rise in drag and drag is the enemy of fuel efficiency. Our theoretical max hull speed is around 8.2kts and 7kts represents 85% of that speed.
Based on these figures will we be motoring all the time at higher speeds in future? No!
Once we go through 6kts engine noise and vibration increase and the experience becomes less enjoyable for us.
Technical bit.
We cruise two to three people up in our 2004 43DS.
We uplift water whenever it is freely available so for the most of the time are always close to 500 lts.
Our Yanmar 4JH4-TE 75HP turbocharged engine has less than 1,600 hours of use.
The original prop was swapped out for a ‘Variprop’ feathering prop. I cannot say what the pitch is because it is user adjustable and was set by engineers before I purchased the boat.
The season is always started with a fresh coat of anti-foul.
There are just so many variables that I dare say that other 43DS owners will have fuel burns better or worse than ours.
Of course don’t just assume our figures will work for your boat even if it is a 43DS. You need to compile your own database over a fairly long period of time.
I am very impressed with the fuel burn rate of that 43DS.
