Outrage 18 with twin Evinrude 70: Fuel Economy

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Author Topic:   Outrage 18 with twin Evinrude 70: Fuel Economy

jar15sp1 posted 08-12-2012 08:33 PM ET (US)
I seem to be experiencing poor fuel economy with my recently purchased 1987 Outrage 18 with twin Evinrude 70 outboards. I took the boat offshore for about 50 miles. I was surprised to and sad to see that the trip consumed a whooping 40 gallons for the trip. The boat had loaded with four passengers and a tank full of fuel. I was running at an average of 4,000-RPM. I seem to have a serious problem here. Anybody else here have twins on there Outrage 18? What kind of fuel economy should i expect with this setup?
jimh posted 08-12-2012 08:57 PM ET (US)
If you run a 70-HP two-cycle outboard engine at 4,000-RPM you can expect to be producing at least 50-HP. The fuel flow rate will roughly be 5-GPH. For twin engines that is 10-GPH.
Buckda posted 08-12-2012 09:00 PM ET (US)
We'll need some more information. How long did you troll? How much time at no-wake from the ramp to the ocean, and then how many miles on plane, and what were the conditions like?
When trolling and/or idling into and out from the ramp, did you idle on one engine, or two?
What was the actual mileage logged on your GPS?
Carb'd two stroke engines are at their absolute WORST fuel economy at idle speed, and you can spend a lot of time at idle.

jimh posted 08-12-2012 09:56 PM ET (US)
If you burned 40-gallons of fuel at flow rate of 10-GPH the time implied is thus four hours. If you covered 100-miles (that is, 50-miles offshore and return), your average speed was 25-MPH. This means your average fuel economy was 2.5-MPG. I'd say that is not bad for an 18-footer with twin old-fashioned two-cycle engines.
If you want really good fuel economy get a 115-HP four-cycle engine. You can probably double your present fuel economy to about 5-MPG.
L H G posted 08-13-2012 12:04 AM ET (US)
For twin 70's, I'd say that is not great fuel economy. My guess is they have to work a little too hard to push the 18. I consistently got the same 2.5 MPG on my 18 with twin Mercury 115 in-line 6's, which were able to cruise at much lower RPM and high 23" pitch props. Now, with newer 3 cylinder twin 90's, I am getting a little over 3.0 MPG under the same conditions, running 22" Laser II props. Not great, but I'll take it, and it's all that I expect from the security of having twins on the boat.
I doubt if there is much you can do to improve your fuel economy with those engines.
jimh posted 08-13-2012 07:18 AM ET (US)
The fuel flow rate of an engine is related to its brake specific fuel consumption (BSFC). For most two-cycle outboard engines the BFSC is going to be very similar. BSFC varies with engine speeds and loads. On a 70-HP Evinrude I would suspect that the BSFC probably peaks around 4,000-RPM--this is just based on the nice sound the engine makes when running at that speed. It seems very pleasant.
If you spend $3,000 more on each engine to get a larger engine, it will take you a long time to recover the extra $6,000 you spend in any sort of fuel saving, if you actually do get any sort of fuel saving.
Again, if you want to improve your fuel economy, change to a single engine and get a modern outboard engine in the 115-HP range. You can probably double the MPG you will get. When I changed from a classic carburetor two-cycle outboard engine to a modern direct-injection outboard engine my MPG increased about 65-percent. It did not quite double, but in your case, going to a single from twins will probably add more savings.
jar15sp1 posted 08-14-2012 09:43 PM ET (US)
Thanks for the replies. this forum is awesome!
irishcreamer posted 08-22-2012 09:26 PM ET (US)
Running a modern 115HP Four Stroke or DFI Two Stroke will likely yield the best fuel economy for you. However, if you plan to run offshore in the Pacific often you should consider more HP. When the swells and/or chop are large you will definitely want the extra HP. Most Outrage 18/19 that I have seen are powered with the max 150 HP. I run an Optimax 150 HP on my Outrage 18 and mid-range cruising is at about 5mpg in calm conditions.
IC
Peter posted 08-23-2012 10:23 AM ET (US)
The difference in fuel economy between a modern 115 or a 150 will be insubstantial. This has been shown a number of times over the years on a variety of boats.

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Author	Topic: Outrage 18 with twin Evinrude 70: Fuel Economy
jar15sp1	posted 08-12-2012 08:33 PM ET (US) I seem to be experiencing poor fuel economy with my recently purchased 1987 Outrage 18 with twin Evinrude 70 outboards. I took the boat offshore for about 50 miles. I was surprised to and sad to see that the trip consumed a whooping 40 gallons for the trip. The boat had loaded with four passengers and a tank full of fuel. I was running at an average of 4,000-RPM. I seem to have a serious problem here. Anybody else here have twins on there Outrage 18? What kind of fuel economy should i expect with this setup?
jimh	posted 08-12-2012 08:57 PM ET (US) If you run a 70-HP two-cycle outboard engine at 4,000-RPM you can expect to be producing at least 50-HP. The fuel flow rate will roughly be 5-GPH. For twin engines that is 10-GPH.
Buckda	posted 08-12-2012 09:00 PM ET (US) We'll need some more information. How long did you troll? How much time at no-wake from the ramp to the ocean, and then how many miles on plane, and what were the conditions like? When trolling and/or idling into and out from the ramp, did you idle on one engine, or two? What was the actual mileage logged on your GPS? Carb'd two stroke engines are at their absolute WORST fuel economy at idle speed, and you can spend a lot of time at idle.
jimh	posted 08-12-2012 09:56 PM ET (US) If you burned 40-gallons of fuel at flow rate of 10-GPH the time implied is thus four hours. If you covered 100-miles (that is, 50-miles offshore and return), your average speed was 25-MPH. This means your average fuel economy was 2.5-MPG. I'd say that is not bad for an 18-footer with twin old-fashioned two-cycle engines. If you want really good fuel economy get a 115-HP four-cycle engine. You can probably double your present fuel economy to about 5-MPG.
L H G	posted 08-13-2012 12:04 AM ET (US) For twin 70's, I'd say that is not great fuel economy. My guess is they have to work a little too hard to push the 18. I consistently got the same 2.5 MPG on my 18 with twin Mercury 115 in-line 6's, which were able to cruise at much lower RPM and high 23" pitch props. Now, with newer 3 cylinder twin 90's, I am getting a little over 3.0 MPG under the same conditions, running 22" Laser II props. Not great, but I'll take it, and it's all that I expect from the security of having twins on the boat. I doubt if there is much you can do to improve your fuel economy with those engines.
jimh	posted 08-13-2012 07:18 AM ET (US) The fuel flow rate of an engine is related to its brake specific fuel consumption (BSFC). For most two-cycle outboard engines the BFSC is going to be very similar. BSFC varies with engine speeds and loads. On a 70-HP Evinrude I would suspect that the BSFC probably peaks around 4,000-RPM--this is just based on the nice sound the engine makes when running at that speed. It seems very pleasant. If you spend $3,000 more on each engine to get a larger engine, it will take you a long time to recover the extra $6,000 you spend in any sort of fuel saving, if you actually do get any sort of fuel saving. Again, if you want to improve your fuel economy, change to a single engine and get a modern outboard engine in the 115-HP range. You can probably double the MPG you will get. When I changed from a classic carburetor two-cycle outboard engine to a modern direct-injection outboard engine my MPG increased about 65-percent. It did not quite double, but in your case, going to a single from twins will probably add more savings.
jar15sp1	posted 08-14-2012 09:43 PM ET (US) Thanks for the replies. this forum is awesome!
irishcreamer	posted 08-22-2012 09:26 PM ET (US) Running a modern 115HP Four Stroke or DFI Two Stroke will likely yield the best fuel economy for you. However, if you plan to run offshore in the Pacific often you should consider more HP. When the swells and/or chop are large you will definitely want the extra HP. Most Outrage 18/19 that I have seen are powered with the max 150 HP. I run an Optimax 150 HP on my Outrage 18 and mid-range cruising is at about 5mpg in calm conditions. IC
Peter	posted 08-23-2012 10:23 AM ET (US) The difference in fuel economy between a modern 115 or a 150 will be insubstantial. This has been shown a number of times over the years on a variety of boats.