90-HP Mercury Four-stroke Uses Excessive Fuel

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Author Topic:   90-HP Mercury Four-stroke Uses Excessive Fuel

Rik posted 08-21-2006 06:20 PM ET (US)
After being a long time sailor I bought a power boat last year for the first time - it was a 350hp inboard runabout. One day and $100 worth of gas we sold it as too expensive to own - and that was on $2.00 a gallon gas.
I had always love the looks of Whalers and did a lot of research. I was told that a 90hp [revealed much later in the discussion to be a four-stroke and thus obviating many of the comments that follow] would only "sip" fuel. OK so we bought a 2001 16 Dauntless.
Took it out last week. Never went over 28 mph and much of the time in no-wake zones under 10 mph. We cruised for about 3 hours and went through 20 gallons.
Is this normal fuel use - my wife does not consider this "sipping" ?
Rik
dino54904 posted 08-21-2006 06:28 PM ET (US)
Hi Rik,
My 1983 Montauk has a 6 cyl 90hp Merc on it and it doesn't use anywhere near that amount of fuel. We water ski behind it for three hours and only use about 10 gallons of gas. That is a lot of starting and stopping and running nearly full out. Sounds like you're using more gas than I do.
Buckda posted 08-21-2006 06:34 PM ET (US)
The only engines that "sip" in no wake zones are DFI 2-Strokes and 4-Strokes. You can expect to see about 6-8 MPG economy in those zones with those engines, as opposed to the gulp gurgle of classic 2-strokes at around 2-3 MPG economy.
The 90 2-stroke (which is what I'm assuming you have) is best at about 2,800 - 3,300 RPM, which should return around 4.5 MPG economy.
That, by powerboat standards, IS sipping fuel when compared to the 1.8 MPG economy you would have with a 22' Outrage or Revenge, or the 3 MPG economy you'd see with an 18' Outrage.
Given that you and your wife hail from the sailing world, where your propulsion is free (albeit a little unreliable at certain times), your definition of "sipping" fuel is somewhat different than a person who has downsized from a larger powervessel.
Perhaps JimH will chime in here with his experiences and feelings about all this since he also came from the sailing world.
Perhaps I've read too much into his sail logs, but his frequent mention of fuel prices lead me to believe that this is generally not an easy transition for any sailboater!

Good luck.
Dave
ConB posted 08-21-2006 07:49 PM ET (US)
As a life long sailor I know what you are feeling on the fuel use. I had good information from these folks here what the fuel usage would be.
When I think about the seasonal slip fees, launch, and haul out fees at the boat yard that I've saved I feel better. And then I trashed a crank shaft bearing.
A hole in the water in which you throw money.
But we sure have fun.
Con
jimh posted 08-21-2006 08:01 PM ET (US)
If you have a classic two-stroke motor with a carburetor, and I think that is the case, then there is bad news for you: idle and slow speeds are the WORST fuel economy in terms of miles-per-gallon. With a classic two-stroke motor there will be a narrow window of boat speed where you will get good fuel economy, usually around 4,000-RPM. If you are motoring at slow speed against an opposing current, the fuel economy of a two-stroke classic motor can often drop to below ONE MILE PER GALLON.
Investing some significant money in a new motor will remedy this, but the economics of this decision are influenced by the amount of use of the boat and by the kind of speeds you operate at which you operate. If you troll for ten hours a day, seven days a week, get a new motor. If you used the boat 50-hours a year and run around on plane most of the time, it will take a lifetime to pay off the new motor in fuel savings.
The good news in all of this: you probably won't use your boat enough to make this economic burden a deal-killer. I know a lot of guys who spend $200 to play a round of golf--that will buy me enough gas to do a lot of boating.
David Livingstone posted 08-21-2006 08:10 PM ET (US)
Rik, I also read your other post and your problem is that you may be operating in the worst, I mean worst part of the power curve. When you operate a planing hull at a displacement speed you might as well throw your wallet overboard. For a planning hull there are only two fuel efficient speeds - idle (2-3mph) and planning. At 4 - 15 mph you will be pouring the fuel through it.
From an old sailor who use to use 5 gal a year on a 25 foot fixed keel sailboat. I use to sail June to September, with the Montauk, I`m in the water from April to December, but I still miss the sailboat.
All the best,
David
Rik posted 08-21-2006 08:15 PM ET (US)
It is a 90hp 4-stoke
davej14 posted 08-21-2006 09:59 PM ET (US)
If you went 2 hrs @ 28 mph and 1 hr at 10 mph that would be a total distance of 66 mi. Using 20 gal of fuel would be an average of 3.30 MPG. From your comments it is likely that this is an optimistic average. It seems very low for a 4-stroke but about right for a 2-stroke.
What is the specific year and model of your motor?
jimh posted 08-21-2006 10:29 PM ET (US)
Check on the factory website to see if they have a performance report on this model and engine combination. My guess is you ought to average about 4-MPG, and better if a lot of low speed operation. A four-stroke motor should run very efficiently at idle speeds, much more so than a two-stroke.
jimh posted 08-22-2006 09:27 AM ET (US)
I can run my much larger boat, with 2.5-times as much horsepower and an old two-stroke motor, and still get around 2-MPG. Your smaller boat, smaller motor, and better four-stroke fuel economy ought to get you at least double that.
As a rough estimate, you can figure that an engine will consume fuel in proportion to its horsepower at a rate of 1:10. That is, a 90-HP motor will consume about 9-gallons per hour when being run at 90-HP.
tarbaby posted 08-22-2006 09:50 AM ET (US)
David Livingstone is right on. Pick up the speed and get her on plane and then pull back on the throttle a bit and that should be your sweet spot. +- 3,500- 4,000 RPM. Boat should not feel like it is lugging. If you are moving a bunch of water out of the way you are using a ton of fuel.
The Judge posted 08-22-2006 11:48 AM ET (US)
My 115 suzuki 4 stroke would have burned about 10-12 gallons doing what you did. Something is wrong OR....you filled it up on th trailer and it was not completely full, then it took more gas when you filled it up again. Can't go by the fuel gauge and one time outing. Try a 6 gallon tank and see how long it lasts, my 70hp suzuki would not burn a 6 gal tank doing what you did.
Rik posted 08-22-2006 08:46 PM ET (US)
[The anti-ventilation plate of the motor] has [been modified by the addition of] a Stingray Hydrofoil[.] [W]ould that [cause the engine to] burn more fuel[?]
Rik
jimh posted 08-22-2006 11:14 PM ET (US)
If you ask the marketing department of the manufacturer of the anti-ventilation plate extension which has been bolted onto your motor, I am certain that they will tell you that it always improves fuel economy--it could never cause an increase in fuel consumption.
There is some anecdotal evidence that the DAUNTLESS 16 hull benefits by adding such a foil extension. The only way to really determine its effect is to make careful measurements of the fuel consumption rate with and without.
Marlin posted 08-23-2006 11:18 AM ET (US)
I would have to work very hard indeed to burn 20 gallons of gas in 3 hours on my 160 Dauntless with a 115 EFI 4-stroke. It would probably require a WOT run.
A few weeks ago we trailered up to Lake Champlain, and I filled the tank when we arrived. We spent a few hours cruising and skiing one day, had a 3-hour nighttime cruise (mostly slow) the next night, spent the following day skiing, trailered home again and went out skiing for a few more hours. Then I filled the tank with 24 gallons.
Last weekend we cruised the Patapsco River and Baltimore's Inner Harbor. Lots of very interesting (and very large) commercial shipping. About 4 hours by the clock, and about 5 gallons by the gas guage. Burned 3 times that in the Explorer to get to the ramp.
For the entire 2005 season, I put about 75 hours on the engine clock and averaged 1.7 gallons per hour. Granted some of that was trolling, and some of it was slow cruising or no wake zones, but a lot of it was 3500 RPM. Granted, this engine is EFI and I presume Rik's is carburated, but I think there must be something fundamentally wrong with Rik's numbers.
-Bob

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Author	Topic: 90-HP Mercury Four-stroke Uses Excessive Fuel
Rik	posted 08-21-2006 06:20 PM ET (US) After being a long time sailor I bought a power boat last year for the first time - it was a 350hp inboard runabout. One day and $100 worth of gas we sold it as too expensive to own - and that was on $2.00 a gallon gas. I had always love the looks of Whalers and did a lot of research. I was told that a 90hp [revealed much later in the discussion to be a four-stroke and thus obviating many of the comments that follow] would only "sip" fuel. OK so we bought a 2001 16 Dauntless. Took it out last week. Never went over 28 mph and much of the time in no-wake zones under 10 mph. We cruised for about 3 hours and went through 20 gallons. Is this normal fuel use - my wife does not consider this "sipping" ? Rik
dino54904	posted 08-21-2006 06:28 PM ET (US) Hi Rik, My 1983 Montauk has a 6 cyl 90hp Merc on it and it doesn't use anywhere near that amount of fuel. We water ski behind it for three hours and only use about 10 gallons of gas. That is a lot of starting and stopping and running nearly full out. Sounds like you're using more gas than I do.
Buckda	posted 08-21-2006 06:34 PM ET (US) The only engines that "sip" in no wake zones are DFI 2-Strokes and 4-Strokes. You can expect to see about 6-8 MPG economy in those zones with those engines, as opposed to the gulp gurgle of classic 2-strokes at around 2-3 MPG economy. The 90 2-stroke (which is what I'm assuming you have) is best at about 2,800 - 3,300 RPM, which should return around 4.5 MPG economy. That, by powerboat standards, IS sipping fuel when compared to the 1.8 MPG economy you would have with a 22' Outrage or Revenge, or the 3 MPG economy you'd see with an 18' Outrage. Given that you and your wife hail from the sailing world, where your propulsion is free (albeit a little unreliable at certain times), your definition of "sipping" fuel is somewhat different than a person who has downsized from a larger powervessel. Perhaps JimH will chime in here with his experiences and feelings about all this since he also came from the sailing world. Perhaps I've read too much into his sail logs, but his frequent mention of fuel prices lead me to believe that this is generally not an easy transition for any sailboater! Good luck. Dave
ConB	posted 08-21-2006 07:49 PM ET (US) As a life long sailor I know what you are feeling on the fuel use. I had good information from these folks here what the fuel usage would be. When I think about the seasonal slip fees, launch, and haul out fees at the boat yard that I've saved I feel better. And then I trashed a crank shaft bearing. A hole in the water in which you throw money. But we sure have fun. Con
jimh	posted 08-21-2006 08:01 PM ET (US) If you have a classic two-stroke motor with a carburetor, and I think that is the case, then there is bad news for you: idle and slow speeds are the WORST fuel economy in terms of miles-per-gallon. With a classic two-stroke motor there will be a narrow window of boat speed where you will get good fuel economy, usually around 4,000-RPM. If you are motoring at slow speed against an opposing current, the fuel economy of a two-stroke classic motor can often drop to below ONE MILE PER GALLON. Investing some significant money in a new motor will remedy this, but the economics of this decision are influenced by the amount of use of the boat and by the kind of speeds you operate at which you operate. If you troll for ten hours a day, seven days a week, get a new motor. If you used the boat 50-hours a year and run around on plane most of the time, it will take a lifetime to pay off the new motor in fuel savings. The good news in all of this: you probably won't use your boat enough to make this economic burden a deal-killer. I know a lot of guys who spend $200 to play a round of golf--that will buy me enough gas to do a lot of boating.
David Livingstone	posted 08-21-2006 08:10 PM ET (US) Rik, I also read your other post and your problem is that you may be operating in the worst, I mean worst part of the power curve. When you operate a planing hull at a displacement speed you might as well throw your wallet overboard. For a planning hull there are only two fuel efficient speeds - idle (2-3mph) and planning. At 4 - 15 mph you will be pouring the fuel through it. From an old sailor who use to use 5 gal a year on a 25 foot fixed keel sailboat. I use to sail June to September, with the Montauk, I`m in the water from April to December, but I still miss the sailboat. All the best, David
Rik	posted 08-21-2006 08:15 PM ET (US) It is a 90hp 4-stoke
davej14	posted 08-21-2006 09:59 PM ET (US) If you went 2 hrs @ 28 mph and 1 hr at 10 mph that would be a total distance of 66 mi. Using 20 gal of fuel would be an average of 3.30 MPG. From your comments it is likely that this is an optimistic average. It seems very low for a 4-stroke but about right for a 2-stroke. What is the specific year and model of your motor?
jimh	posted 08-21-2006 10:29 PM ET (US) Check on the factory website to see if they have a performance report on this model and engine combination. My guess is you ought to average about 4-MPG, and better if a lot of low speed operation. A four-stroke motor should run very efficiently at idle speeds, much more so than a two-stroke.
jimh	posted 08-22-2006 09:27 AM ET (US) I can run my much larger boat, with 2.5-times as much horsepower and an old two-stroke motor, and still get around 2-MPG. Your smaller boat, smaller motor, and better four-stroke fuel economy ought to get you at least double that. As a rough estimate, you can figure that an engine will consume fuel in proportion to its horsepower at a rate of 1:10. That is, a 90-HP motor will consume about 9-gallons per hour when being run at 90-HP.
tarbaby	posted 08-22-2006 09:50 AM ET (US) David Livingstone is right on. Pick up the speed and get her on plane and then pull back on the throttle a bit and that should be your sweet spot. +- 3,500- 4,000 RPM. Boat should not feel like it is lugging. If you are moving a bunch of water out of the way you are using a ton of fuel.
The Judge	posted 08-22-2006 11:48 AM ET (US) My 115 suzuki 4 stroke would have burned about 10-12 gallons doing what you did. Something is wrong OR....you filled it up on th trailer and it was not completely full, then it took more gas when you filled it up again. Can't go by the fuel gauge and one time outing. Try a 6 gallon tank and see how long it lasts, my 70hp suzuki would not burn a 6 gal tank doing what you did.
Rik	posted 08-22-2006 08:46 PM ET (US) [The anti-ventilation plate of the motor] has [been modified by the addition of] a Stingray Hydrofoil[.] [W]ould that [cause the engine to] burn more fuel[?] Rik
jimh	posted 08-22-2006 11:14 PM ET (US) If you ask the marketing department of the manufacturer of the anti-ventilation plate extension which has been bolted onto your motor, I am certain that they will tell you that it always improves fuel economy--it could never cause an increase in fuel consumption. There is some anecdotal evidence that the DAUNTLESS 16 hull benefits by adding such a foil extension. The only way to really determine its effect is to make careful measurements of the fuel consumption rate with and without.
Marlin	posted 08-23-2006 11:18 AM ET (US) I would have to work very hard indeed to burn 20 gallons of gas in 3 hours on my 160 Dauntless with a 115 EFI 4-stroke. It would probably require a WOT run. A few weeks ago we trailered up to Lake Champlain, and I filled the tank when we arrived. We spent a few hours cruising and skiing one day, had a 3-hour nighttime cruise (mostly slow) the next night, spent the following day skiing, trailered home again and went out skiing for a few more hours. Then I filled the tank with 24 gallons. Last weekend we cruised the Patapsco River and Baltimore's Inner Harbor. Lots of very interesting (and very large) commercial shipping. About 4 hours by the clock, and about 5 gallons by the gas guage. Burned 3 times that in the Explorer to get to the ramp. For the entire 2005 season, I put about 75 hours on the engine clock and averaged 1.7 gallons per hour. Granted some of that was trolling, and some of it was slow cruising or no wake zones, but a lot of it was 3500 RPM. Granted, this engine is EFI and I presume Rik's is carburated, but I think there must be something fundamentally wrong with Rik's numbers. -Bob