My fuel consumption study-for analytical geeks only!

I have a 1991 19' Reinell 190hp. It has a 4.3L OMC Cobra. Owned a boat for 6 years and the Fuel guage hasn't worked since I bought it so I finally did a "fuel study" last weekend to help me better estimate my fuel situation. Was wondering what other people get for mileage. I had heard that this OMC motor was pretty good for mileage. I realize there's a HUGE difference if you're mostly WOT or if you're hanging out alot without the motor running. We spent about 11-12 hours on the boat that weekend. A couple hours of hard water skiing, tubing, etc. Another couple hours of "cruising the lake" at higher speeds. Half the time was probably "hard time" and the other half was idle, no engine running, etc. Probably a pretty typical way my boating time is spent.

When I went to fuel up the next weekend, it took 14 gallons. Therefore, it was about 1.2 gallons per hour spent on the boat. Again, I realize this could swing dramatically depending on the throttle usage. The tank is (I think) about 26 gallons. So I should be able to get about 20 hours of boating per tank. At $3/gallon, that would be $78/tank and about $4/hour to run the boat(gas expense only of course!).

Curious if others could estimate their "gallons per hour" usage.

Re: My fuel consumption study-for analytical geeks only!

I have been tracking MPG. I use my GPS to estimate differences and determine the gasoline by refilling the 2 cycle oil tank. On my 21' Dual Console w/ 150 Johnny, I see a range between 1.7 MPG and 3.3 MPG.

Re: My fuel consumption study-for analytical geeks only!

Depending on throttle position I can burn as little as 6gph at a low cruise or 13+gph at WOTeither way it's about the same for me

80 mile round trip/24mph=3.333 hours traveled at 6gph=19.9 gallons of fuel

80 mile round trip/50mph=1.6 hours traveled at 14 gph= 22.4 gallons of fuel

thats a 2.5 gallon difference or about 1 extra mile at 24 mph so why not open it up and have a little fun.

Re: My fuel consumption study-for analytical geeks only!

Chris1956 said:

I have been tracking MPG. I use my GPS to estimate differences and determine the gasoline by refilling the 2 cycle oil tank. On my 21' Dual Console w/ 150 Johnny, I see a range between 1.7 MPG and 3.3 MPG.

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Isn't calculating Miles per Gallon on a boat not very accurate? Even though your distance over land between point A and Point B are always the same, there are other variables that affect boats unlike cars, such as current and wind direction. I thought that's why on boats Gallons per Hour is the standard. I'm sure the difference wouldn't be incredibly different, but I'm sure mpg on a flat, smooth, lake is going to give a fairly considerable rate of consumption then the same distance against the current (or even with) the current on a river, and again additional differences on a larger body of water with rolling waves.

Knowing average MPG is nice, but I think if I'm going offshore, I'd rather know my average GPH. Time is always consistent.

Re: My fuel consumption study-for analytical geeks only!

Oh - oh!!!! The old MPG vs GPH discussion is heating up. MPG is a great number to know but it is an average that can vary significantly. GPH is a very useful number and it can be an instantaneous or average number if one has a fuel flow monitor (most boats don't but they should.) Within 500 feet of the dock one can tell whether there is a fuel consumption issue whereas MPG cannot be calculated until the trip is over and you've refueled, or unless you have an integrated fuel flow monitor and gps system that can run that calcuation. The fuel flow monitor quickly allows you to see where the optimum throttle setting is. It also allows much better management of actual fuel consumption rather than having to rely on a gas gauge that is not a very accurate instrument. If you can see shore any time you are on the water neither MPG or GPH are of importance. If you are a long distance traveler on big water then every bit of information you have available may save your life.

Re: My fuel consumption study-for analytical geeks only!

The fuel flow monitor quickly allows you to see where the optimum throttle setting is.

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Exactly how do you accomplish this? My fuel flow gauge shows 2 GPH at idle, 7 GPH at 2000 RPM and 40 GPH at WOT. Where is my optimum throttle setting?

Please explain how you find your optimum throttle setting using only fuel flow. I will guarantee you that your lowest fuel flow will be at idle on any engine made.

MPG is a great number to know but it is an average that can vary significantly.

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MPG cannot be calculated until the trip is over and you've refueled

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MPG is no more an average than fuel flow. If you want your average fuel flow you must wait till you fill up then divide the amount of fuel you added by the hours the motor ran.

On the other hand, most people are capable of calculating that if you're going 40 MPH and flowing 10 GPH, you're getting 4 MPG. If you speed up to 45 and start burning 15 GPH, you're getting 3 MPG. The only way you're going to find your optimum throttle setting is by computing instantaneous MPG.

Fuel flow is useless without MPH. If you're looking for the most efficient throttle setting, is it where you're burning 6 GPH or 8 GPH? How do you know without knowing the speed at each flow? Once you know the speed at each flow, you're figuring MPG to determine optimum throttle setting ....

In answer to the original poster's question, last weekend we went out 6 miles at 9 AM, anchored and spent all day watching para-sailers, wind surfers, etc, then came back 6 miles at 6 PM. Burned about 3 gallons of gas. So I guess my fuel burn by your method is 1/3 of a gallon per hour.

On the other hand, if the boat is running, it's burning between 8 and 10 gallons per hour.

Re: My fuel consumption study-for analytical geeks only!

Silly me, I thought 45 would settle in on the the OPs 1.2 GPH average. Instead he has taken up my fuel flow position which is: GPH is worthless information without speed. Add to that, if you add in speed what you have is MPG . . .

Since this thread is "for analytical geeks"' I analyze the OP's 1.2 GPH thing as also worthless, and at worst dangerous. If anybody is calc'ing fuel needs and also leaves the proximity of the launch ramp, they're going to be seriously bummed

Welcome aboard BoiseBoater!! We may be a relatively mild bunch, but this topic brings lot's of opinions which I think is what you wanted

Re: My fuel consumption study-for analytical geeks only!

Chris1956 said:

I have been tracking MPG. I use my GPS to estimate differences and determine the gasoline by refilling the 2 cycle oil tank. On my 21' Dual Console w/ 150 Johnny, I see a range between 1.7 MPG and 3.3 MPG.

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most oil injection systems are variable, and fairly linear between 100:1 at idle up to 50:1 at WOT. In other words, judging fuel consumption based on oil consumption is useless.

Re: My fuel consumption study-for analytical geeks only!

QC said:

Silly me, I thought 45 would settle in on the the OPs 1.2 GPH average.

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I just wanted to show that my .33 GPH average was better than his 1.2, so I can do 4 times as much boating on the same amount of fuel!

I agree, it's pretty much a useless number, which is why I added in the last line in my post:

45auto said:

On the other hand, if the boat is running, it's burning between 8 and 10 gallons per hour.

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I was kind of interested to see how many people would also understand that my 8-10 gallon number is also useless because I didn't tell you how fast the boat was going!

Re: My fuel consumption study-for analytical geeks only!

Geesh. Guess I swatted the proverbial hornet's nest! Good info from everyone though.

Was more just looking for answers that explained how many gallons are used for your typical outing. Tempered with some adjustments for how often you have WOT and how often you're just idling or trolling or whatever.

Re: My fuel consumption study-for analytical geeks only!

It doesn't matter how fast the boat is going if you're relying on GPS for the speed.

In my neck of the woods the boat likes to turn 1200 rpm in each engine and runs 3 mph upstream and 11 mph downstream according to the GPS. The river's current is about 4 mph. That's an average of 7 mph.

More to the point, the boat burns roughly 6 gallons per hour at that engine RPM, which equates to a rule-of-thumb of 1.3 mpg.

It doesn't matter if I'm on the lake in 6-foot seas against the wind, or cruising down a calm canal. It's 6 gallons per hour.

I'm carrying 200 gallons, so I know that 1/3 out = 66 gallons = 11 hours. 1/3 back to port = another 11 hours. With 1/3, or 66 gallons, held in reserve.

If I run on plane the boat's fuel efficiency drops to 25 gph @ 25 mph. Not a whole lot of calculating there: 1 mile per gallon. Adjust run time accordingly -- and still have 1/3 in reserve.

Re: My fuel consumption study-for analytical geeks only!

Our "family" boat ( a 140 HP I/O) is often used for the same trip on a Sunday. We're out from about 9AM to 9PM. The one-way voyage is 43 miles. Part of it is a no-wake zone at 6 mph. Part is in an area of heavy wakes, so we have to slow way down. The majority is in open seas and I most always run at 3000 rpm, which gives me between 25-30 mph, depending on trim and size of waves. I almost never go WOT. I trim out till the hull breaks and the steering torque is nil (that's where my trim tab is set for). If it gets a bit rough, I trim in all the way. We stop at a couple of fishing holes along the way, then meet friends in a cove for lunch, finally making it to the beach for the afternoon. The return trip is along the same path, so where we ran against the current one way, we ran with it the other. So, this total 86 mile trip will take 12 gallons of gas, or roughly 7 mpg. I could calculate the gph, but frankly I never look at the hour meter on a daily basis. From past experience I know that my tank will be half full when we get home.

One of my other boats is a 14' Starcraft aluminum with a 25 HP 2007 model 2-stroke. I only have one 6 gal. tank so to me gph is more important, thus I DO watch the hour meter. My "typical" fishing day with this boat consists of once again running a bit in a no-wake zone, running 3/4 throttle in the river for a portion of the day, and maybe doing some light trolling. She uses almost exactly 1 1/4 gph on average every time out.

In contrast, the 1952 25HP 2-stroke on my woodie requires that I have a minimum of 2 tanks in the boat. We often participate in a classic boat parade, which travels along a no-wake zone for 6 miles at 6 mph. She gets a whopping 2 mpg, smoking profusely all the way...LOL.

Re: My fuel consumption study-for analytical geeks only!

Smokingcrator, They way I understand the OMC OMS oil injection system, is that it is not variable. You will always see a pretty consistant gasil mix.

The older OMC systems and the Merc systems are indeed vailable ratio.

Re: My fuel consumption study-for analytical geeks only!

The MPG numbers I am collecting is for my own reference. I know about how far I travel on a given fishing trip, or pleasure cruise. What I am trying to figure out is the effect of boat load, propeller changes, trim settings etc. If I get an MPG range figured out, I can use it to plan, a bit better.