Fuel Burn for Marine engines.

[CmdrDick]

I often use the following link to check fuel consumption.
It lists numerous engine types and their fuel burn at RPM's from idle up to WOT. I've found it be be quite accurate.
Remember, it is the engine that burns the fuel, not the boat. The boat only determines at what speed and how far you will go . Also, the chart is based upon ONE engine. For dual installations, your burn is double the shown rate.
https://www.boat-fuel-economy.com/
Using this chart and knowing my speed at any RPM, allowed me to make a burn chart that only needs correction for water flow and wind. The GPS gives us that data and makes accurate burn projections quite easy.

 

[smitty477]

I often use the following link to check fuel consumption.
It lists numerous engine types and their fuel burn at RPM's from idle up to WOT. I've found it be be quite accurate.
Remember, it is the engine that burns the fuel, not the boat. The boat only determines at what speed and how far you will go . Also, the chart is based upon ONE engine. For dual installations, your burn is double the shown rate.
https://www.boat-fuel-economy.com/
Using this chart and knowing my speed at any RPM, allowed me to make a burn chart that only needs correction for water flow and wind. The GPS gives us that data and makes accurate burn projections quite easy.

"Remember, it is the engine that burns the fuel, not the boat."
Actually I have found that the boat determines the fuel burn and not the engine. The speed that you drive the boat is key and to a lesser effect the hull design, running gear, weight, sea water type and conditions etc all come into play.
The boats size, speed, and other items determine the amount of HP required to drive the boat at various speeds.
Whether you choose to supply that power with engines powered by gasoline, diesel, electricty or nuclear does not matter ....it is the boat that determines how much of that power is required for the application.

 

[CmdrDick]

I believe in the "keep it simple" way of thinking. You can make something as complicated as you choose, or find a simpler path to a solution. Once you have a "burn" chart for YOUR boat, the consumption will always be the same for any specific speed.

 

[lwarden]

Fuel burn is also dependent on how it's propped. Over propped and it's going to burn more than the standard curve.

 

[smitty477]

I believe in the "keep it simple" way of thinking. You can make something as complicated as you choose, or find a simpler path to a solution. Once you have a "burn" chart for YOUR boat, the consumption will always be the same for any specific speed.

"Once you have a "burn" chart for YOUR boat, the consumption will always be the same for any specific speed"
As long as there is no change in ....
- any water current
- sea states
- wind
- weight
- fresh to salt or salt to fresh water
- hull cleanliness
- prop condition
- engine condition
Then the consumption will always be the same for any specific speed.

 

[SteveK]

I often use the following link to check fuel consumption.
It lists numerous engine types and their fuel burn at RPM's from idle up to WOT. I've found it be be quite accurate.
Remember, it is the engine that burns the fuel, not the boat. The boat only determines at what speed and how far you will go . Also, the chart is based upon ONE engine. For dual installations, your burn is double the shown rate.
https://www.boat-fuel-economy.com/
Using this chart and knowing my speed at any RPM, allowed me to make a burn chart that only needs correction for water flow and wind. The GPS gives us that data and makes accurate burn projections quite easy.

what engine(s) do you have and how does it relate to mercruiser and volvo in the link. Those are the only two I see in link. I do not see Ford Lehman.

 

[CmdrDick]

As stated, the GPS erases everything you mentioned. You compare your actual speed over the earth to your chart and get a real consumption that eliminates all of the variables.

 

[smitty477]

As stated, the GPS erases everything you mentioned. You compare your actual speed over the earth to your chart and get a real consumption that eliminates all of the variables.

Your GPS erases all of these....
As long as there is no change in:
- any water current
- sea states
- wind
- weight
- fresh to salt or salt to fresh water
- hull cleanliness
- prop condition
- engine condition

Which make and model of GPS do you have?

 

[CmdrDick]

SteveK
"what engine(s) do you have and how does it relate to mercruiser and volvo in the link. Those are the only two I see in link. I do not see Ford Lehman."
I have Crusaders that have been rebuilt and upgraded to 383/6.2L specs. I tweaked the numbers to align with my history log.
I'd love to have diesels, but alas, I have gas engines and all of the negatives that go with those.
One problem with oil burners is the "return" fuel which requires costly fuel flow monitoring equipment to compute (burn less return).
Even so, we keep boats for a long time and every trip gives us another set of data that can be used to generate a reliable burn chart.

 

[CmdrDick]

SteveK
I have two GPS. One in the Raymarine set and a second that I removed from my ex airplane when I upgraded. That gives me a quick accuracy check. Gotta go, mom calls.

 

[SteveK]

CMDRDICK, how do you use the link in post 1 for those engines?

 

[DavidM]

Despite all of the critiques above, that website does seem to give good real world numbers, ie actual fuel burn based on prop hp requirements at various rpms. Sure your boat has to be propped to reach the wot rpms and if it isn't, either high or low, the results won't be right.

How fast your boat will go is not addressed. That depends on a lot of factors: size, weight, hull type.

David

 

[koliver]

Fuel burn is also dependent on how it's propped. Over propped and it's going to burn more than the standard curve.

Way more complicated than that statement suggests.

I re-powered from 150hp TMD40s to 200hp TAMD41s in my 8 knot boat. In order to IMPROVE fuel economy, my mechanic recommended adding pitch to the props, thus lowering the rpm at cruise. I added 4", taking 13" to 17", lowering cruise rpm from 2700 to 2000, IMPROVING fuel economy by 5 to 10%. That was over 20 yrs ago, and my fuel savings alone have long ago paid for all of those changes.

If you are over propping a boat that is always at or near WOT, then you are asking too much of your engine and will likely see very different results.

 

[rgano]

I agree about the costly fuel flow monitoring for diesels which is why when I had twin diesels in my trawler I refrained from installing monitors and instead depended on calibrated sight gauges on my tanks to give quite adequate usage data. However, when I lucked into a very affordable never-installed NIB Floscan for my current single diesel, I jumped at it. This gadget is wonderful for the high-speed Yanmar I have with its highly variable fuel usage on this much smaller boat, but I think for a cruising trawler like I had run at 1600 RPM all day long its usefulness would have been quite limited.

 

[lwarden]

Way more complicated than that statement suggests.

I re-powered from 150hp TMD40s to 200hp TAMD41s in my 8 knot boat. In order to IMPROVE fuel economy, my mechanic recommended adding pitch to the props, thus lowering the rpm at cruise. I added 4", taking 13" to 17", lowering cruise rpm from 2700 to 2000, IMPROVING fuel economy by 5 to 10%. That was over 20 yrs ago, and my fuel savings alone have long ago paid for all of those changes.

If you are over propping a boat that is always at or near WOT, then you are asking too much of your engine and will likely see very different results.

You added HP so it would be expected you would need to add pitch (or diameter if you can swing it) to get the correct WOT for the extra 50hp. I was referring to over-propping a boat where the diameter or pitch does not allow WOT. In that case the fuel burn curve will be heavy for a given RPM and your economy will suffer, not to mention engine longevity.

 

[DDW]

Despite all of the critiques above, that website does seem to give good real world numbers, ie actual fuel burn based on prop hp requirements at various rpms.
David

Was not even remotely close for my boat. About on for WOT, progressively worse at lower rpm peaking at about 100% error.

 

[ancora]

A GPS is too inaccurate to determine actual speed. Running a measured mile is far more accurate...ask any Predicted Log contestant.

 

[rslifkin]

A GPS is too inaccurate to determine actual speed. Running a measured mile is far more accurate...ask any Predicted Log contestant.

It depends... Do you want to know how fast the boat is actually moving, or just how fast you're getting closer to your destination?

 

[DavidM]

Was not even remotely close for my boat. About on for WOT, progressively worse at lower rpm peaking at about 100% error.

What you describe indicates that that particular engine was showing wot fuel burn not prop fuel burn, in other words the throttle is wide open, no matter what the rpm is. Yes that means that it would be way off at lower rpms.

I checked a gasser and the Merc/Cummins diesel. Both looked like they followed the prop curve values not the wot values. So it looks like it is a mixed bag.

What engine do you have?

David

 

[HTurner]

In the old days you used this for speed:
Knotstick Sailboat Speedometers


And you used a sextant for position.


If you fast forward to today you use a gps for both.

 

[Lepke]

A GPS is too inaccurate to determine actual speed. Running a measured mile is far more accurate...ask any Predicted Log contestant.

Gps over any distance is accurate. A measured mile is a small sample with only one sea/tide condition.

 

[catalinajack]

Why is this so difficult? Fill the tanks. Run the boat for a few hundred miles. Fill the tanks. Arithmetic. Sure, changing conditions will cause some changes up or down but the difference, over time, will average out. Yes, generator time will have some affect but really not much to make a difference in arriving at a useful number.

 

[rslifkin]

Why is this so difficult? Fill the tanks. Run the boat for a few hundred miles. Fill the tanks. Arithmetic. Sure, changing conditions will cause some changes up or down but the difference, over time, will average out. Yes, generator time will have some affect but really not much to make a difference in arriving at a useful number.

It's easy if you always cruise at the same speed. But if you don't, then it's a bit harder to figure out without flow meters.

 

[Takoradi]

I often use the following link to check fuel consumption.

It lists numerous engine types and their fuel burn at RPM's from idle up to WOT. I've found it be be quite accurate.

Remember, it is the engine that burns the fuel, not the boat. The boat only determines at what speed and how far you will go . Also, the chart is based upon ONE engine. For dual installations, your burn is double the shown rate.

https://www.boat-fuel-economy.com/

Using this chart and knowing my speed at any RPM, allowed me to make a burn chart that only needs correction for water flow and wind. The GPS gives us that data and makes accurate burn projections quite easy.

Aw maaaaan! No Crusaders! [emoji23]

 

[rwidman]

A GPS is too inaccurate to determine actual speed. Running a measured mile is far more accurate...ask any Predicted Log contestant.

A GPS is about as accurate as you can get. Typically within ten feet.

 

[rwidman]

It's pointless to know the fuel burn of an engine sitting on a test stand.

Conditions in a boat are entirely different than on a test stand and different boats, differently loaded, will produce different fuel burns.

For example, a long thin boat will move through the water more easily and with less fuel burn than a short, wide boat.

Want to know the fuel boat for your boat? Keep records and figure it out.

 

[omoore]

I agree with you Rich. I installed Floscans on my 58 Hat MY with 8v96 TA engines. Once I had the numbers for displacement , slow plane and fast plane I never used it again. Could have saved serious money just creating a burn rate chart. Oh, I did WOT once. Scared me to deathP: 77 GPH.

Oliver

 

[Tiffany]

Not a very useful tool for most trawlers as it deals with only a very limited range of makes and outboard engines.

 

[rgano]

Gps over any distance is accurate. A measured mile is a small sample with only one sea/tide condition.

A measured mile done properly accounts for sea condition (current) by being run in opposing directions for each RPM. It would not be very useful to run the MM in bad conditions. GPS and a Floscan, accurate instantaneously everywhere.

 

[rgano]

I agree with you Rich. I installed Floscans on my 58 Hat MY with 8v96 TA engines. Once I had the numbers for displacement , slow plane and fast plane I never used it again. Could have saved serious money just creating a burn rate chart. Oh, I did WOT once. Scared me to deathP: 77 GPH.

Oliver

I bought the FS for its digital tach, but I also like the fuel totalizer which I reset each trip or after fueling to double check the fuel gauge as the fuel level goes down. I use the GPH display to look for consistency at my cruising RPM.