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Post by zaphod on Apr 23, 2019 22:16:12 GMT
I am trying to figure out what the range of our 39i is under power. We have yet to consume a full tank of fuel since we got our boat last August, but this past weekend we put a pretty good dent in our fuel supply when we had to motor for a good 5 hrs. Of course the fuel gauge gives us inconsistent readings so it is not a reliable indicator.
I read somewhere that the engine burns 7 litres/hr at max continuous power, (2900rpm) so far I have found that 2500rpm is a sweet spot in terms of noise. At that rpm she does 7.5kts. If we push it to full throttle she will get up to 8.2kts.
Does anyone have a fuel consumption chart that shows fuel consumption at different rpms?
For anyone with the same engine, what have you found to be the best rpm to balance fuel consumption and boat speed?
I am curious what other people's experience is. This boat is a lot thirstier than my old boat for sure, and with the cost of fuel these days I want to get the most out of every tank!
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Post by hoppy on Apr 24, 2019 2:34:08 GMT
I had the 3JH4E in my old SO40 The Yanmar manual has a consumption/rpm graph. Easy enough to find online. For anyone with the same engine, what have you found to be the best rpm to balance fuel consumption and boat speed? There is no such thing as a best rpm. I once took the consumption chart and recorded the speeds at various rpms to create my own chart with rpm, speed, fuel consumption and range with the idea of finding the sweetspot. I don't think I went above 2700 rpm or much below 2000 when making the chart as they were the speeds I liked. The speed increase, range drop and fuel consumption increase remained roughly proportional. I'm sure that closer to 3000 and beyond the speed gain will drop right off. My conclusion is that there is no sweetspot. Every extra rpm will give you an x% in extra speed at a cost of y% more consumption. No extra 1000rpm was more costly than any other. If you want to save money or get extra range, go slower..... |
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Post by ianpowolny on Apr 24, 2019 8:23:59 GMT
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Post by rene460 on Apr 24, 2019 10:58:34 GMT
Hi Zaphod,
In my experience, the best way to determine fuel consumption is to start with a full tank, then cruise preferably for more than just a few hours, even if it is over several days, then measure fuel consumed by how much it takes to return the tank to full.
I go a step further, and each time I refuel, I include in my fuel log a shetch showing the fuel gauge needle position immediately before I refuel. This is intended to give me a calibration graph for the tank.
As others have noted, the gauge stays on full, just above 4/4 on the gauge, until nearly 25 litres are required to to refill the 50 litre tank.
As I have mentioned in another thread, my average consumption over the life of the engine is about 1.75 l/hr. You will have to determine the equivalent figure for yours. The last reading (last week) was 662 hours (we have the dreaded engine hour display fade). Our log is showing over 4000 nm. So about 0.16 hours of engine use per mile on average. You can see we do sail as much as possible on our lake system, but are not hesitant to use the “iron topsail” when the wind is absent. A long trip for us is typically about 17 miles each way, though there are longer possibilities.
But for motoring range, I would use that 1.75 l/hr. at 5 to 5.5 knots in calm winds and the fifty litre tank on our boat. If this is inadequate for a trip I want to do, I need to carry some jerry cans. We lashed them to the pushpit rails for the delivery trip to the lakes, especially as we had not had the chance to work out consumption before the delivery trip. In any case, things like wind direction, motor sailing and wave conditions make it pretty hard to be precise.
I am with Hoppy, that there is no sweet spot. Slower means better fuel consumption, but of course too slow means you never get anywhere. The general recommendation for Diesel engines is 80% rated rpm and load for at least 80% of the time. For my engine, rated at 3600 rpm this means about 2800 rpm which seems to give a pretty good speed with much less noise, wavemaking and fuel consumption than running at higher rpm. Because of the lower torque required at the lower rpm, much lower rpm means very low load which is generally not considered good for Diesel engines. On the other hand, full rpm and load uses a lot more fuel for very little extra speed, due to the hull wavemaking characteristic.
rene460
PS I am not sure what happened to the font size, iPad sometimes does these things to me with no obvious way to reverse.
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Post by alenka on Apr 24, 2019 11:05:07 GMT
I don't know how they measure boat/diesel engine fuel usage for their graphs, but as we all know car manufacturers MPG figures never represent real life. How can they? They're allowed to remove the alternator amongst other things and tests are performed on a rolling road in a lab so that drag is not an issue. Electric vehicles are no better. They are also conducted inside labs with batteries pre-heated, I believe, to around 25c.
Are boat diesel engine figures designed to sell engines or be a useful planning tool??
For the last four years I have maintained a fuel upload v's engine taco hours log. I have found that a normal cruising average, some fast passages, some meandering, works out at around 2.7 Lts/Hr. This is for a Yanmar 4JH3TE 75HP Turbo engine pushing along an SO 43DS. For noise consideration we prefer 6Kts but we are mindful to keep the turbo working so as not to soot up.
Before every uplift of fuel I calculate anticipated fuel burn (eng taco hrs x 2.7) and try to fill to the same point (foaming of fuel in the filler) and compare estimated with actual. There are obvious inaccuracies in the methodology but more often than not I am within 10% of the anticipated amount. Sometimes more sometimes less.
I refer to 'Taco' engine hours rather than a timed hour because Taco hours can sometimes differ with RPM. Perhaps not so much on very modern electronic ones but on ours it happens.
Drag is the biggest issue. Boats, cars, aeroplanes they are all affected. Once you start getting close to hull maximum speed you can pile on loads of extra power and gain very little in speed. Backing off just a little can really improve fuel burn. I suggest there will definitely be a sweet spot for your boat speed if not for your engine. I think 7.5kts may well be very close to your max theoretical speed.
However, this should not be taken in isolation. There are techniques to help save fuel. Keeping your hull free of weed is a big one. Also travelling at a higher speed against a foul tide/current is more efficient than a slower one. And vice versa a slower speed traveling with the current will probably save a bit of fuel.
Establishing a log for your individual boat not only helps in passage planning but it also helps to ensure your leg is not lifted by rogue fuel suppliers. They don't always set the counter to zero!!
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Post by zaphod on Apr 24, 2019 19:34:20 GMT
I don't know how they measure boat/diesel engine fuel usage for their graphs, but as we all know car manufacturers MPG figures never represent real life. How can they? They're allowed to remove the alternator amongst other things and tests are performed on a rolling road in a lab so that drag is not an issue. Electric vehicles are no better. They are also conducted inside labs with batteries pre-heated, I believe, to around 25c. Are boat diesel engine figures designed to sell engines or be a useful planning tool?? For the last four years I have maintained a fuel upload v's engine taco hours log. I have found that a normal cruising average, some fast passages, some meandering, works out at around 2.7 Lts/Hr. This is for a Yanmar 4JH3TE 75HP Turbo engine pushing along an SO 43DS. For noise consideration we prefer 6Kts but we are mindful to keep the turbo working so as not to soot up. Before every uplift of fuel I calculate anticipated fuel burn (eng taco hrs x 2.7) and try to fill to the same point (foaming of fuel in the filler) and compare estimated with actual. There are obvious inaccuracies in the methodology but more often than not I am within 10% of the anticipated amount. Sometimes more sometimes less. I refer to 'Taco' engine hours rather than a timed hour because Taco hours can sometimes differ with RPM. Perhaps not so much on very modern electronic ones but on ours it happens. Drag is the biggest issue. Boats, cars, aeroplanes they are all affected. Once you start getting close to hull maximum speed you can pile on loads of extra power and gain very little in speed. Backing off just a little can really improve fuel burn. I suggest there will definitely be a sweet spot for your boat speed if not for your engine. I think 7.5kts may well be very close to your max theoretical speed. However, this should not be taken in isolation. There are techniques to help save fuel. Keeping your hull free of weed is a big one. Also travelling at a higher speed against a foul tide/current is more efficient than a slower one. And vice versa a slower speed traveling with the current will probably save a bit of fuel. Establishing a log for your individual boat not only helps in passage planning but it also helps to ensure your leg is not lifted by rogue fuel suppliers. They don't always set the counter to zero!! I would expect that the manufacturer specifications for fuel consumption would be pretty accurate. They know the volume the injectors shoot per stroke, and how many ignition strokes per rpm etc. So assuming the engine is in good condition the fuel consumed for a given rpm should be pretty consistent. Where it becomes more difficult to predict is when you try to figure out fuel consumed per mile traveled. That's where variables such as propeller pitch, bottom fouling, sea conditions and even current come into play. That's why it is simpler just to think in terms of Litres per hour. My engine running at 2500rpm should burn around 5 l/hr, regardless of whether the boat is doing 5.5kts or 7.5kts. Looking at the fuel consumption chart for my engine, rpm and fuel consumption are not a linear relationship, it is a curve. Again according to the chart, my engine running at 3000rpm burns 8.5 l/hr! For my boat that means I would be burning an additional 3.5 l/hr to gain an additional 0.7kts of boatspeed. This is the data I am looking for! Alenka, you say your 75hp engine only burns 2.7 l/hr....that seems impossibly low for such a large boat. My engine burns that much at idle! |
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Post by alenka on Apr 24, 2019 21:22:06 GMT
With regards to my own engine I am only relaying what I have found from keeping accurate fuel logs. Two years ago when we tried hard to use up a big chunk of fuel so as to enable a repair inside the tank it took for ever. Maybe I am blessed with a good engine. I am not complaining.
If you are burning the same at idle your boat is indeed very thirsty!
Some years ago Jezza Clarkson of Top Gear fame demonstrated how he got more MPG out of a large engined BMW than a Toyota Prius. Of course it was down to nothing more than how it was driven.
Agree. Manufacturers fuel graphs should be accurate and maybe they are. I really don't know if they go for accuracy or a competitive edge to satisfy the marketing department.
Good luck with your quest.
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Post by buddyseattle on Apr 24, 2019 21:52:13 GMT
Greetings, I have some fuel efficiency and speed data that might be of interest. I have a 39I with the 3JH4E. I've logged my usage and done some loaded and near empty correlations to speed and RPM. Over time I've refined to be a good mean for what we use our boat for. It is from about 600 hours of logging. It ranges from day cruising near empty to fully loaded for four weeks of travel. So while it will vary, I've found this to be a useful guide. Nothing fancy, but servicable. I've found that when we are cruising, loaded, and calm water, the boat runs well between 2300 and 2500 RPM. The cost factor is really not much of a consideration, but it does help me determine when we need to find fuel when cruising some distance. We will vary the speed from 2300 to 2700, but mostly are around 2400. As others have suggested, a clean bottom and a prop in good condition are notable variables. As long as I don't weigh down the bow excessively, the variance in speed at a fixed RPM is marginal. Hope you find it of some value  www.23hq.com/BuddySeattle/photo/54103704/original www.23hq.com/BuddySeattle/photo/54103696/original |
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Post by zaphod on Apr 24, 2019 22:04:17 GMT
With regards to my own engine I am only relaying what I have found from keeping accurate fuel logs. Two years ago when we tried hard to use up a big chunk of fuel so as to enable a repair inside the tank it took for ever. Maybe I am blessed with a good engine. I am not complaining. If you are burning the same at idle your boat is indeed very thirsty! Some years ago Jezza Clarkson of Top Gear fame demonstrated how he got more MPG out of a large engined BMW than a Toyota Prius. Of course it was down to nothing more than how it was driven. Agree. Manufacturers fuel graphs should be accurate and maybe they are. I really don't know if they go for accuracy or a competitive edge to satisfy the marketing department. Good luck with your quest. You can't really compare fuel consumption metrics between cars and boats. With a car there are so many variables that go into the final MPG number. Air temp, road conditions, hills, how hard you accelerate, how much drag your car has, how much air in the tires etc etc. Marine engines are simpler. You set the throttle to a given speed and leave it there. According to Yanmar for your engine to be burning 2.7 l/hr you should be running about 1700 rpm. If that is all it takes to go the speed you want then you are indeed fortunate! If you were to put the throttle up to 2600 you would be burning 8 l/hr. That's why I am curious what speed other people choose to motor at, and what engine rpm it takes to achieve it. My wife loves that we can motor at 7.5kts because our last boat couldn't break 6kts flat out, but I am pretty sure we burn more than 2-3 l/hr to do it! Of course powerboaters would just laugh at the numbers we are throwing around! |
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Post by zaphod on Apr 25, 2019 1:26:47 GMT
Greetings, I have some fuel efficiency and speed data that might be of interest. I have a 39I with the 3JH4E. I've logged my usage and done some loaded and near empty correlations to speed and RPM. Over time I've refined to be a good mean for what we use our boat for. It is from about 600 hours of logging. It ranges from day cruising near empty to fully loaded for four weeks of travel. So while it will vary, I've found this to be a useful guide. Nothing fancy, but servicable. I've found that when we are cruising, loaded, and calm water, the boat runs well between 2300 and 2500 RPM. The cost factor is really not much of a consideration, but it does help me determine when we need to find fuel when cruising some distance. We will vary the speed from 2300 to 2700, but mostly are around 2400. As others have suggested, a clean bottom and a prop in good condition are notable variables. As long as I don't weigh down the bow excessively, the variance in speed at a fixed RPM is marginal. Hope you find it of some value www.23hq.com/BuddySeattle/photo/54103704/originalwww.23hq.com/BuddySeattle/photo/54103696/originalThat is the same boat we have. What is your boatspeed at your chosen rpm? Whatever the pics you posted are, the links are broken for me. |
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Post by ianpowolny on Apr 25, 2019 6:28:19 GMT
Zaphod,
The group here did discuss this topic a number of months ago. I was advised that there was a minimum engine speed, I can’t remember why, but either oiling the engine correctly or engaging the turbo.
We don’t work on nautical miles to litres but hours we can run. With a full tank we calculate we can run for 48 hours at 2200rpm. This gives us about 7 knots. When we are on a long run, in excess of 24 hours I always carry 3 x 10 litre cans of diesel, just in case.
Ian
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Post by rene460 on Apr 25, 2019 10:58:09 GMT
There seems to be a few questions about those performance curves and how they relate to boat performance. I hope a few thoughts based on my experience using performance curves for similar machines during my working life might be helpful.
They are neither sales hype nor are they a planning tool. They are expected performance data for use by the naval engineers selecting an engine for a hull design. They are for Yanmar marine engines, not Jeanneau specific.
The really big difference between automotive and marine performance is that there is no direct relationship between engine/propellor rpm and boat speed equivalent to the circumference of a car tyre directly geared to the engine. When a marine engine runs at a constant rpm, the boat speed can be zero if you still have the ropes attached, or quite high if you are motor sailing, or somewhere between depending on hull form, fouling, wind, waves. The boat speed depends on the hull resistance, not engine rpm.
The curves are produced under standardised conditions, in this case ISO8665/3046 is quoted for the 3JH4E. That code will tell you the specific test conditions, or probably which code applies to each aspect of the test and the allowable tolerance on results. You can see two rated power figures which apply for different fuel temperatures. It is very normal to adjust performance to a standard ambient temperature, probably 15 C as noted for the fuel density specification.
The power curve shows the maximum power available over the speed range. It stops at about 1500 rpm. I am not sure what idle speed is recommended for the engine but it is reasonable to assume that 1500 rpm to 3000 rpm is the useful range of the engine. You can also see that the maximum continuous speed is 2907 rpm, not 3000. General recommendation is at least 80% of rated speed for eighty percent of the time, so at least 2400 rpm for this engine. With a propellor load, there is not much load if you are running down to 50% rated rpm.
They have also shown the power drawn by a propellor. The actual propellor rpm depends on which of the available gears is selected, but the curve shows the power at the engine allowing for the gear ratio. Propellor power is proportional to the rpm cubed. Nothing is implied about pitch or diameter of blade form, so long as the power required is just a little less than the power available at the rated engine speed to allow for the tolerance on available power and to allow the engine to just exceed the rated rpm at wide open throttle.
You can see the propellor requires considerably less power than available at lower rpm than rated. This tells me there is a governor mechanism in the engine control system because it is basic physics that if the engine power is more than the power absorbed by the propellor, the system will speed up, quite quickly in fact.
You can see the engine torque available increases at reduced rpm. The propellor torque requirement is not shown, but you can draw your own if you like. It matches the engine torque (or just a little less) at rated speed, then is a parabola, torque proportional to rpm squared. You can see the engine always has sufficient power and torque to drive the propellor at all speeds below rated.
The fuel consumption is ambiguous to me, I would expect as it is an engine specification sheet, it is fuel consumption at maximum power, and this would be considerably higher than power driving a propellor at rpm below rated. Because the engine efficiency is not shown, it is not easy to guess which is intended.
None of the above says anything about what your boat will do, or any other boat with that engine. You can construct a drag curve for your hull but easier is to assume the propellor thrust is roughly proportional to rpm, so a plot of boat speed and rpm in still water, no wind is approximately a hull resistance curve. You will quickly deduce that increasing engine rpm from 80% (2400 rpm) to 100% (3000 rpm) does not give a commensurate increase in boat speed, and what speed you get comes with a big increase in fuel consumption. You can also estimate the effect on hull drag of waves or wind by how many extra rpm is needed to achieve a given speed in more adverse conditions.
I hope that sheds some light on those curves without boring everyone.
Rene460
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Post by buddyseattle on Apr 27, 2019 2:19:45 GMT
Sorry that my images weren’t appearing. I think I have fixed them now. See above.
I hope they help you.
If you would like the worksheet, them message me directly.
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Post by rene460 on Apr 27, 2019 3:48:55 GMT
Hi buddy Seattle, looks like your fix has been successful. They are all visible now.
Very interesting graphs and data.
Thank you
rene460
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Post by saltymetals on Dec 2, 2019 12:23:10 GMT
I have the 3JH4 TE which is the 75hp version with the turbo but my consumption records show that my engine uses 2.7 - 3.0 l/hr at 2,000-2,200 rpm and gives me about 6 kn. This has been measured over considerable distances and many weeks. Speed obviously depends on what prop you have, state of cleanliness of the hull, whether the boat is light or fully loaded. The Yanmar curves tend to suggest that you get the best efficiency for the turbo model for hp produced at around 2,200-2,300 rpm. If you desperately want to do >7 kn long distance then maybe you shld have got a stinkpot instead of a sailing boat!  Andrew |
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Post by alenka on Dec 4, 2019 9:08:32 GMT
I have the 3JH4 TE which is the 75hp version with the turbo but my consumption records show that my engine uses 2.7 - 3.0 l/hr at 2,000-2,200 rpm and gives me about 6 kn. This has been measured over considerable distances and many weeks. Speed obviously depends on what prop you have, state of cleanliness of the hull, whether the boat is light or fully loaded. The Yanmar curves tend to suggest that you get the best efficiency for the turbo model for hp produced at around 2,200-2,300 rpm. If you desperately want to do >7 kn long distance then maybe you shld have got a stinkpot instead of a sailing boat! Andrew Andrew, Allowing for all the variables you quite rightly outline our fuel burn is probably very similar to yours. The Yanmar fuel curves are not taking into account any of the above or indeed hull design, keel design and even sea state. The big thing to remember is that fuel burn per hour is not the real issue it is the ratio of fuel burned to miles covered that's important if you are looking to save money. I posted a fairly lengthy thread on this issue some weeks ago. jeanneau.proboards.com/thread/7746/real-world-fuel-burn |
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