Silvertip
Supreme Mariner
- Joined
- Sep 22, 2003
- Messages
- 28,771
Here are some fuel economy and performance charts to study. My observations and contributions follow:
Many of you on this and other forums have asked about fuel consumption data for your outboard. The typical response is at wide open throttle your engine will burn in the vicinity of 10% of its rated horsepower. 100 HP = 10 g/hr (gallons per hour) or 75 HP = 7.5 g/hr. Hold that thought because in the following paragraphs, tests of my boat, an Alumacraft Navigator 165CS (75 HP Merc carbureted 2-stroke) will include data from my fuel computer, along with charts from Alumacraft (75 HP E-tec) and two charts from Yamaha. One Yamaha was a 70 HP 2-stroke on a PolarCraft Kodiak 165CS and the other a 75 HP Yamaha 4-stroke on a Alumacraft Navigator 165CS. Yamaha did not test the 70 HP 2-stroke on the Navigator so I selected the Kodiak as the closest to it in size, weight, and design. All are aluminum, single console, walleye-style boats. This is about as fair a comparison as one can get using data from three sources ? personal use, a boat manufacturer, and an engine manufacturer.
BOAT #1
Personal Boat - 75HP Mercury 3 cylinder, 2-stroke/Alumacraft Navigator 165CS
The data in the chart was gathered during testing using a Navman 2100 fuel flow monitor and a Garmin GPS12MAP GPS for speed measurement. The engine is fitted with an aluminum, three blade, 13-1/4 x 19 Quicksilver prop. Tests were run with two aboard, a MinnKota 55PD trolling motor, two batteries, approximately 100 pounds of gear and 19 gallons of fuel. Water temp was 76 degrees with a moderate chop and with 5 mph winds. The engine is mounted four holes up. As you might expect, wide open throttle on this combination results in a fuel flow rate of 7.5 g/hr so the 10% rule is alive and well with this engine/boat combination. Testing was difficult due to less than ideal water conditions and traffic.
BOAT #2
Alumacraft Boats - 75HP Yamaha 4-stroke/Navigator 165CS
This chart data was taken from the Yamaha Performance Test web page so these are Yamaha numbers, not something I came up with. A fuel burn rate of 10% of horsepower at wide open throttle holds true for this engine as well. Isn?t it ironic that the ?fuel efficient? 4-strokes still burn the same amount of fuel at wide open throttle as their 2-stroke elders. Midrange economy and a couple of terms that need to be understood which will help separate the chaff from the wheat in this fuel economy debate. You should also note that although the 4-stroke is getting 6.31 MPG at 3500 RPM, my carbed 2-stroke is going about 5 MPH faster at the same RPM.
BOAT #3
Yamaha Outboards ? 70 HP Yamaha 2-stroke/PolarCarft Kodiak 165CS
This chart data was also taken from the Yamaha Performance Test web page. Your eyes should immediately be drawn to the wide open throttle GPH and MPG figures. Although these two boats are obvious not identical, they are close enough that the numbers show some interesting traits. Note first that at wide open throttle the 70 Yamaha, as should be expected, burns slightly less fuel than the 4-stroke 75. This is not startling since it again proves the 10% rule. But note that the engine pushes the boat more than 3 MPH faster that the 4-stroke. That fact leads us to examine the MPG figure and you will see the 70 2-stroke got significantly better fuel economy (5.07 MPG vs 4.60 MPG) ? why? Because the engine was pushing the boat faster, thus traveling farther on the same amount of fuel.
BOAT #4
Alumacraft Boats ? 75 HP Evinrude E-tec 2-stroke/Navigator 165CS
This chart data was taken from the Alumacraft Boats Performance Test web page. These are therefore Alumacraft derived numbers, not Evinrude. Here again you will see the 10% value reinforced. Of interest is that at wide open throttle the E-tec burns very slightly less fuel than the Yamaha 4-stroke. As with the Yamaha 2-stroke, the E-tec at mid range rpm (3000 ? 4000 RPM) is pushing the boat significantly faster than the Yamaha 4-stroke. That translates to the same or better fuel economy than the Yamaha 4-stroke.
FUEL EFFICIENCY vs FUEL ECONOMY:
Here is where some definitions need to be applied. Fuel Efficiency is a measure of fuel consumption at a given RPM. Two engines can be compared side by side on this measure and in looking at the charts you will see the 4-stroke clearly burns less fuel than the 2-stroke at any RPM making it more efficient in those terms.
Fuel economy is a measure of how much fuel it takes to travel a given distance. Once on plane (3000 RPM and up) you will see the MPG figures for the 2-stroke are either very close to the 4-stroke but in many cases it is actually better than the 4-stroke. Wide open throttle on the 4-stroke returns 4.6 MPG and on the 2-stroke it is 5.07 MPG. Why the difference? It?s really quite simple! Look at the speeds (MPH) for the two engines. At any given RPM the 2-stroke is pushing the boat faster than the equivalent 4-stroke. Hence, better economy! Expressed in converse terms, if you ran both engines at 3000 RPM for one hour, the 4-stroke would burn less fuel than the 2-stroke. However the 2-stroke would have carried you farther on the same amount of fuel because at 3000 RPM it is pushing the boat faster.
SUMMARY:
If the 4-strokes aren?t more economical, why should I buy one? Good question but there is merit to viewing efficiency as long as you disregard distance and speed in the consideration. The charts show that speed is the enemy with a 4-stroke. At any given rpm a 2-stroke is simply pushing the boat faster hence the MPG are better even though the engine may be consuming more fuel per hour.
If you are a walleye fisherperson for example, you may want to back-troll for hours at a time. The E-tec at 1000 rpm burns 0.37 g/hr, the Yamaha 2-stroke burns 0.9 g/hr (yikes), and the Yamaha 4-stroke burns 0.4 g/hr. In this example you ignore how far you traveled so MPG is not an issue. Lets then assume you want to cruise for two hours at a relatively constant speed of 22 MPH which translates to 4000 RPM on the Yamaha 4-stroke. At that speed you would burn 3.8 g/hr. On the Yamaha 2-stroke you would only need to run the engine about 3500 RPM to run 22 MPH and you would be burning very nearly the same amount of fuel -- because the engine is running slower to provide 22 MPH. On the E-tec, you would burn significantly less than the 4-stroke because the engine would be running about 3400 rpm and burning about 3.6 g/hr. In comparison, my 75 Merc 2-stroke would burn 4.4 g/hr. At wide open throttle all of this is moot because they all burn the same amount of fuel.
When engaged in 4-stroke, 2-stroke and direct fuel injection 2-stroke (Optimax/E-tec) discussions make sure what you are saying and hearing is actually what you mean to say or what you want to hear. There is no disputing the fact that at any given RPM a 4-stroke will generally burn less fuel than a carbureted 2-stroke. But equate that rpm to boat speed and you now have a totally different set of parameters. 4-stroke engines make power on every other revolution so they should in theory, burn ? the amount of fuel a 2-stroke. However a 2-stroke makes power on every revolution so they generally have better hole shot, top end, and provide faster boat speeds at the same rpm as a 4-stroke. How can that be? 4-strokes tend to need lower gear ratios in the lower unit to get out of the hole. They also run slightly less pitch on the props. That translates directly to less speed.
After preparing this study I am more convinced than ever that I will run a my carbed 2-stroke until it dies or is outlawed, at which time I will be faced with the same decision most new boat owners face ? 2-stroke or 4-stroke! But I think you know which way I lean.
Many of you on this and other forums have asked about fuel consumption data for your outboard. The typical response is at wide open throttle your engine will burn in the vicinity of 10% of its rated horsepower. 100 HP = 10 g/hr (gallons per hour) or 75 HP = 7.5 g/hr. Hold that thought because in the following paragraphs, tests of my boat, an Alumacraft Navigator 165CS (75 HP Merc carbureted 2-stroke) will include data from my fuel computer, along with charts from Alumacraft (75 HP E-tec) and two charts from Yamaha. One Yamaha was a 70 HP 2-stroke on a PolarCraft Kodiak 165CS and the other a 75 HP Yamaha 4-stroke on a Alumacraft Navigator 165CS. Yamaha did not test the 70 HP 2-stroke on the Navigator so I selected the Kodiak as the closest to it in size, weight, and design. All are aluminum, single console, walleye-style boats. This is about as fair a comparison as one can get using data from three sources ? personal use, a boat manufacturer, and an engine manufacturer.
BOAT #1
Personal Boat - 75HP Mercury 3 cylinder, 2-stroke/Alumacraft Navigator 165CS
The data in the chart was gathered during testing using a Navman 2100 fuel flow monitor and a Garmin GPS12MAP GPS for speed measurement. The engine is fitted with an aluminum, three blade, 13-1/4 x 19 Quicksilver prop. Tests were run with two aboard, a MinnKota 55PD trolling motor, two batteries, approximately 100 pounds of gear and 19 gallons of fuel. Water temp was 76 degrees with a moderate chop and with 5 mph winds. The engine is mounted four holes up. As you might expect, wide open throttle on this combination results in a fuel flow rate of 7.5 g/hr so the 10% rule is alive and well with this engine/boat combination. Testing was difficult due to less than ideal water conditions and traffic.
BOAT #2
Alumacraft Boats - 75HP Yamaha 4-stroke/Navigator 165CS
This chart data was taken from the Yamaha Performance Test web page so these are Yamaha numbers, not something I came up with. A fuel burn rate of 10% of horsepower at wide open throttle holds true for this engine as well. Isn?t it ironic that the ?fuel efficient? 4-strokes still burn the same amount of fuel at wide open throttle as their 2-stroke elders. Midrange economy and a couple of terms that need to be understood which will help separate the chaff from the wheat in this fuel economy debate. You should also note that although the 4-stroke is getting 6.31 MPG at 3500 RPM, my carbed 2-stroke is going about 5 MPH faster at the same RPM.
BOAT #3
Yamaha Outboards ? 70 HP Yamaha 2-stroke/PolarCarft Kodiak 165CS
This chart data was also taken from the Yamaha Performance Test web page. Your eyes should immediately be drawn to the wide open throttle GPH and MPG figures. Although these two boats are obvious not identical, they are close enough that the numbers show some interesting traits. Note first that at wide open throttle the 70 Yamaha, as should be expected, burns slightly less fuel than the 4-stroke 75. This is not startling since it again proves the 10% rule. But note that the engine pushes the boat more than 3 MPH faster that the 4-stroke. That fact leads us to examine the MPG figure and you will see the 70 2-stroke got significantly better fuel economy (5.07 MPG vs 4.60 MPG) ? why? Because the engine was pushing the boat faster, thus traveling farther on the same amount of fuel.
BOAT #4
Alumacraft Boats ? 75 HP Evinrude E-tec 2-stroke/Navigator 165CS
This chart data was taken from the Alumacraft Boats Performance Test web page. These are therefore Alumacraft derived numbers, not Evinrude. Here again you will see the 10% value reinforced. Of interest is that at wide open throttle the E-tec burns very slightly less fuel than the Yamaha 4-stroke. As with the Yamaha 2-stroke, the E-tec at mid range rpm (3000 ? 4000 RPM) is pushing the boat significantly faster than the Yamaha 4-stroke. That translates to the same or better fuel economy than the Yamaha 4-stroke.
FUEL EFFICIENCY vs FUEL ECONOMY:
Here is where some definitions need to be applied. Fuel Efficiency is a measure of fuel consumption at a given RPM. Two engines can be compared side by side on this measure and in looking at the charts you will see the 4-stroke clearly burns less fuel than the 2-stroke at any RPM making it more efficient in those terms.
Fuel economy is a measure of how much fuel it takes to travel a given distance. Once on plane (3000 RPM and up) you will see the MPG figures for the 2-stroke are either very close to the 4-stroke but in many cases it is actually better than the 4-stroke. Wide open throttle on the 4-stroke returns 4.6 MPG and on the 2-stroke it is 5.07 MPG. Why the difference? It?s really quite simple! Look at the speeds (MPH) for the two engines. At any given RPM the 2-stroke is pushing the boat faster than the equivalent 4-stroke. Hence, better economy! Expressed in converse terms, if you ran both engines at 3000 RPM for one hour, the 4-stroke would burn less fuel than the 2-stroke. However the 2-stroke would have carried you farther on the same amount of fuel because at 3000 RPM it is pushing the boat faster.
SUMMARY:
If the 4-strokes aren?t more economical, why should I buy one? Good question but there is merit to viewing efficiency as long as you disregard distance and speed in the consideration. The charts show that speed is the enemy with a 4-stroke. At any given rpm a 2-stroke is simply pushing the boat faster hence the MPG are better even though the engine may be consuming more fuel per hour.
If you are a walleye fisherperson for example, you may want to back-troll for hours at a time. The E-tec at 1000 rpm burns 0.37 g/hr, the Yamaha 2-stroke burns 0.9 g/hr (yikes), and the Yamaha 4-stroke burns 0.4 g/hr. In this example you ignore how far you traveled so MPG is not an issue. Lets then assume you want to cruise for two hours at a relatively constant speed of 22 MPH which translates to 4000 RPM on the Yamaha 4-stroke. At that speed you would burn 3.8 g/hr. On the Yamaha 2-stroke you would only need to run the engine about 3500 RPM to run 22 MPH and you would be burning very nearly the same amount of fuel -- because the engine is running slower to provide 22 MPH. On the E-tec, you would burn significantly less than the 4-stroke because the engine would be running about 3400 rpm and burning about 3.6 g/hr. In comparison, my 75 Merc 2-stroke would burn 4.4 g/hr. At wide open throttle all of this is moot because they all burn the same amount of fuel.
When engaged in 4-stroke, 2-stroke and direct fuel injection 2-stroke (Optimax/E-tec) discussions make sure what you are saying and hearing is actually what you mean to say or what you want to hear. There is no disputing the fact that at any given RPM a 4-stroke will generally burn less fuel than a carbureted 2-stroke. But equate that rpm to boat speed and you now have a totally different set of parameters. 4-stroke engines make power on every other revolution so they should in theory, burn ? the amount of fuel a 2-stroke. However a 2-stroke makes power on every revolution so they generally have better hole shot, top end, and provide faster boat speeds at the same rpm as a 4-stroke. How can that be? 4-strokes tend to need lower gear ratios in the lower unit to get out of the hole. They also run slightly less pitch on the props. That translates directly to less speed.
After preparing this study I am more convinced than ever that I will run a my carbed 2-stroke until it dies or is outlawed, at which time I will be faced with the same decision most new boat owners face ? 2-stroke or 4-stroke! But I think you know which way I lean.
and testing
