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Fuel Consumption


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Hey guys, Here in the past few weeks or so, i've notice that we are burning through gas at a really rapid pace. Like a month or so ago, i could fill the entire tank (03 RLX), fill up all fat sacs (almost 2500#), head out to where we ride which is literally a 5 min cruising ride, then drop the wedge and start riding and make it almost all day of constant hard riding. But the past couple of weeks its been a lot worse. Take yesterday for example, Filled the tank (maybe 35 gals?), rode the 5 mins or so to the beach where we filled up the fat sacs, then started wakeboarding right there, we didn't make it but 4 hours and had to go back for more gas and we were taking our time between riders, even stopped back at the beach to talk to people, filled it up and it took almost an entire 35 gals again. I'm not expecting 30 GPH with putting 2500#s in a boat and wakeboarding but i know that it was much better a few weeks ago. Is there something i need to check or something? There are no fuel leaks i've check for that already. Thanks guys....

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Are you using the same gas, I know there are different brands of gas you can buy and won't mention them but you can get much better gas mileage with you more common name brand products. Ex: Safeway, 7/11 and Exxon (EC emissions fuel really sucks) gas I never pump, 76, Chevron, Shell and once in a while Valero (only cause they are local) is what I use.

Edit: you may also want to do a tune up if you are using the same gas or at least plugs

Edited by 68Slalom
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Do you notice a change in the smell of the exhaust? i.e. could you be running extremely rich? Could be something as simple as a sensor not letting your engine think it's warmed up, thereby increasing the fuel flow? I'm assuming you have a Monsoon.

Then again, burning close to 35 gallons in 4 hours (less than 9 GPH) in a weighted down boat might not be off the mark by much. I figure I burn 5 gallons an hour or so, and that's with no weight, no wedge (99% of the time) and plenty of 'down time' during that hour.

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How certain are you about how much gas you put back in? How did you know you needed more fuel?.. based on the gauge?

I'm asking because your comments left some room for doubt that you burned that much gas. For example, are you sure it was full when you started? Could it have not been quite full? And did you check your actual time spent, or are you sorta guessing about how much time the engine was running?

Not nit picking here, just checking before we all send you off tearing your boat appart. Personally, I would fill the boat up, verify it is full (look at the tank) and note the hrs, then use it.. and when it needs more gas, fill it up again, again, double checking and note the hrs... Then you'll be certain you have an issue.

I'd be surprised if you're burning more than 6gph, and if so, you've got some sort of issh' ;) Trying to save you the goose chase here

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I've been using the same gas that i use every year. A company comes out and fills up my tank, its 93 octane. I have like a 800 gal or so tank with a ticker on it that tells me how much i am pumping at the time and how much is left in the tank. I put right at 30 gals in the tank before we went out today, which topped it off, and left with no sacs filled yet at 3pm, drove about 10 mins going right at 30 mph to the sandbar where we stopped and filled up all the fat sacs, started wakeboarding with full sacs and wedge at 230, wakeboarded (in no hurry, taking our time between riders, just joking around) until right at 8pm. then we started idling back and draining fat sacs. When we we got about 200 yards from my house it started acting like it was going to run out and the gauge was on E. Pulled it in the boat house and filled it back up just to see how much it used, and I put in 33 gals! about a month ago, i remember us goin out and riding almost all day without having to fill up. It just seems to be going so much quicker for some reason.

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I've been using the same gas that i use every year. A company comes out and fills up my tank, its 93 octane. I have like a 800 gal or so tank with a ticker on it that tells me how much i am pumping at the time and how much is left in the tank. I put right at 30 gals in the tank before we went out today, which topped it off, and left with no sacs filled yet at 3pm, drove about 10 mins going right at 30 mph to the sandbar where we stopped and filled up all the fat sacs, started wakeboarding with full sacs and wedge at 230, wakeboarded (in no hurry, taking our time between riders, just joking around) until right at 8pm. then we started idling back and draining fat sacs. When we we got about 200 yards from my house it started acting like it was going to run out and the gauge was on E. Pulled it in the boat house and filled it back up just to see how much it used, and I put in 33 gals! about a month ago, i remember us goin out and riding almost all day without having to fill up. It just seems to be going so much quicker for some reason.

Maybe you had so much fun since it's the beginning of the year that you lost track of how much you actually ran the engine :)

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I've been using the same gas that i use every year. A company comes out and fills up my tank, its 93 octane. I have like a 800 gal or so tank with a ticker on it that tells me how much i am pumping at the time and how much is left in the tank. I put right at 30 gals in the tank before we went out today, which topped it off, and left with no sacs filled yet at 3pm, drove about 10 mins going right at 30 mph to the sandbar where we stopped and filled up all the fat sacs, started wakeboarding with full sacs and wedge at 230, wakeboarded (in no hurry, taking our time between riders, just joking around) until right at 8pm. then we started idling back and draining fat sacs. When we we got about 200 yards from my house it started acting like it was going to run out and the gauge was on E. Pulled it in the boat house and filled it back up just to see how much it used, and I put in 33 gals! about a month ago, i remember us goin out and riding almost all day without having to fill up. It just seems to be going so much quicker for some reason.

Maybe you had so much fun since it's the beginning of the year that you lost track of how much you actually ran the engine :)

How old is the gas in the tank? How often do you go through the 800 gallons?

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A while back, I learned that fuel consumption is very empirical and can be calculated with simple math. You can apply this formula to just about any gas engine.

At peak horsepower - gasoline 4 stroke engines burn between .3 and .4 pounds of fuel per hour per unit of horsepower. If you have 300hp, that's 90 to 120 pounds of fuel per hour at peak horsepower. Put 2500 additional pounds, and you'll be pushing the torque curve toward peak hp cross in a hurry. Gasoline weighs 6.25 pounds per gallon. So 90 to 120 pounds of fuel... figure 14 - 20 gallons per hour at peak horsepower. At 80%, you'll use 66% of the fuel. So if you are at 80% of peak horsepower, 10 - 13 gallons per hour. So to me - burning through 35 gallons in 3 hard hours of watersports with the wedge down plus ballast, seems perfectly believable. I might even call it 'good'.

Not sure what caused it to be so much better in the past - but doesn't sound to me like there's anything wrong.

If you are wondering why cars get such good mileage compared to a boat - boats are pushing through water which is many times more dense than air, and cars coast a lot. Really a lot. Plus the transmission puts you at cruising speed often under 2,000 rpm which puts the engine at about 33% of the max horsepower. Boats can't coast. Neither can lawn mowers, generators or snow blowers. So if you are wondering how much fuel an engine burns - you can math it out.

Edited by JeffS
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A while back, I learned that fuel consumption is very empirical and can be calculated with simple math. You can apply this formula to just about any gas engine.

At peak horsepower - gasoline 4 stroke engines burn between .3 and .4 pounds of fuel per hour per unit of horsepower. If you have 300hp, that's 90 to 120 pounds of fuel per hour at peak horsepower. Put 2500 additional pounds, and you'll be pushing the torque curve toward peak hp cross in a hurry. Gasoline weighs 6.25 pounds per gallon. So 90 to 120 pounds of fuel... figure 14 - 20 gallons per hour at peak horsepower. At 80%, you'll use 66% of the fuel. So if you are at 80% of peak horsepower, 10 - 13 gallons per hour. So to me - burning through 35 gallons in 3 hard hours of watersports with the wedge down plus ballast, seems perfectly believable. I might even call it 'good'.

Not sure what caused it to be so much better in the past - but doesn't sound to me like there's anything wrong.

If you are wondering why cars get such good mileage compared to a boat - boats are pushing through water which is many times more dense than air, and cars coast a lot. Really a lot. Plus the transmission puts you at cruising speed often under 2,000 rpm which puts the engine at about 33% of the max horsepower. Boats can't coast. Neither can lawn mowers, generators or snow blowers. So if you are wondering how much fuel an engine burns - you can math it out.

Interesting details.. 1) I don't think he was spending 3hrs near 80% of max output.. 2) I really think this is a case of either inaccurate measurement coming from his home fuel delivery system, and/or inaccurate measure of how much time spent on the water.

I still say - you have an hour meter, use it - and see what you're really burning. Also, might want to check what you're using that says you took 30gal from your 800 gal tank. Sounds like too many loose variables to me to start tearing into the engine to figure out what's going on.

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  • 2 weeks later...

Ok, first thanks for the replies. Second, adding that much weight and the wedge obviously makes the engine work harder however it's repropped with an acme 911 so torque isn't a problem. To validate that I have a problem this morning I filled up 5, 5 gallon tanks at the store and put it all in the boat. It was pretty close to empty but not all the way. We went to the sandbar with no weight and it took us exactly 10 minutes to get there cruising at 28mph. We sat at the sandbar for a while, drove back same speed took 10 minutes. Went back to the sandbar same speed and no weight again no skiing or wakeboarding or towing of anything of any kind, took 10 minutes again. Sat there talking to people with engine off again for about 45 minutes, headed back at 28 again and got litterally 200 yards from my house and ran out of gas. Engine hours didn't even clock 1 full hour. It has a brand new belt, brand new impeller, new fuel filter. I just don't get it. I did notice when pulling the plug last night that I have an oil leak somewhere that I have to find but oil level is good. Any suggestions.

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...I filled up 5, 5 gallon tanks at the store and put it all in the boat...ran out of gas...Engine hours didn't even clock 1 full hour...

Shocking.gif >25 GPH !! Sheesh.

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The days are longest at this time of year are you using time? I know it is sometimes after 10:00 pm when I am coming in from our last set of skiing. Tease.gif

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Yea man, litterally I had a stop watch on how long it took us to get to the sandbar and back each time and that's all we did. I'm going to look now at the engine and stuff and see if there is a leak.

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Yea man, litterally I had a stop watch on how long it took us to get to the sandbar and back each time and that's all we did. I'm going to look now at the engine and stuff and see if there is a leak.

If there is a fuel leak you should smell it as soon as you open any hatches, or stop. The exhause will smell very bad if the mixture is not right. Check this while the engin is warmed up. the computer may think you are still trying to warm the engine up. Also check the plugs for carbon fouling. if they are covered in dry carbon the mixture may be the issue. a computer adjustment or replacement of a sensor may do the trick.

rew

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Yea man, litterally I had a stop watch on how long it took us to get to the sandbar and back each time and that's all we did. I'm going to look now at the engine and stuff and see if there is a leak.

If there is a fuel leak you should smell it as soon as you open any hatches, or stop. The exhause will smell very bad if the mixture is not right. Check this while the engin is warmed up. the computer may think you are still trying to warm the engine up. Also check the plugs for carbon fouling. if they are covered in dry carbon the mixture may be the issue. a computer adjustment or replacement of a sensor may do the trick.

rew

Just curous where are you putting 2500# of ballast bags Dontknow.gif

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