MurrayM
Guru
Very cool.
Fuel consumption at low speed?
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If you have answers, please help by responding to the unanswered posts.
Very cool.
AKDoug
Guru
We are (just about) all using nautical miles, without stating the nautical part. Still reads as mpg, instead of nmpg
No one I know uses statue miles on the water... No one I know uses kph or kpg either...
Rustybarge
Guru
The huge divergence in performance figures in this thread only illustrates how difficult it is getting accurate consumption numbers on diesels because of the fuel return pipe on the injector pump; a tiny error in the fuel flow meter can skew the results by a large margin.
To the original question I'd suggest a rule of thumb: at slow speeds on all boats smaller than 60' and larger than 32' is that it takes about 5hp to push a boat at 7-8kts or slightly less for every ton of weight.
So weight in tonnes x5 to give you the HP needed.
Diesels use 1 gal of diesel to produce 20 HP.
Divide the total HP needed by 20 to get gals/hr at 7-8kts.
I think you'll find the result surprisingly accurate!
To the original question I'd suggest a rule of thumb: at slow speeds on all boats smaller than 60' and larger than 32' is that it takes about 5hp to push a boat at 7-8kts or slightly less for every ton of weight.
So weight in tonnes x5 to give you the HP needed.
Diesels use 1 gal of diesel to produce 20 HP.
Divide the total HP needed by 20 to get gals/hr at 7-8kts.
I think you'll find the result surprisingly accurate!
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Except for the Canadian Government who have kilometers per hour signage in canals and harbours , never seen a guage in kph on a boat.... metric sharts drive me nuts too.
I'm a bit surprised at many of the answers because in boating we usually use GPH (gallons per hour) not MPG (miles per gallon). Why? because the water is moving under us (current) so miles through the water and miles over land are usually not the same. If you are travelling with the current, the miles through the water will be less than the miles over land. If you are travelling against the current, the miles through the water will be more than the miles over land.
At 2K RPM, my boat burns about 1.9 GPH. At that engine speed, my speed over ground (GPS) can vary from 5 knots to 9 knots, depending on the current. I've seen as low as 3.8 and as high as 12.4.
Peter:
User reported fuel consumption data is inaccurate for several reasons: Flowscan or similar meter inaccuracies as you note (does anyone rigorously calibrate their Flowscan?), Generator and docking time erroneously included in travel time vs fuel fill data, and probably most importantly- owner wishful thinking.
Your rule of thumb broadly works. I proposed a more comprehensive one that takes into consideration displacement, hull type and engine type in a related thread. See Trawler Forum - View Single Post - What is your mpg, gallons per hour etc... at various speeds?
By making the rule of thumb based on power to reach displacement speed, you automatically consider water line length. A longer water line leads to a higher displacement speed and better NM/gal. If you have two boats with the same displacement and hull type, the one with a longer water line length will achieve better fuel mileage.
But maybe the biggest variable is hull type. True displacement hulls are fairly consistent in hp per thousand pounds to reach displacement speed. The data shown in Bebe's book (edited by Leishman) shows a rather small variation of maybe 20%.
But semi-displacement hulls vary all over the map at low speeds. One poster on the referenced thread claimed he used twice the displacement hp or 3 hp per 1,000 lbs for a semi-displacement hull. My downeaster style Mainship Pilot 34 is notoriously inefficient at low and high speeds.
A less significant variable is engine type. No matter whether it is a 30 year old 5.8 liter 120 hp Lehman making 40 hp or a new 380 hp Cummins QSB making the same hp, they both make about 16 hp per gph of fuel. Only a modern smaller displacement engine like the 4.5 liter JD will do better because it has less iron to turn over and its fuel injection is optimized for the lower hp output.
So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption.
David
User reported fuel consumption data is inaccurate for several reasons: Flowscan or similar meter inaccuracies as you note (does anyone rigorously calibrate their Flowscan?), Generator and docking time erroneously included in travel time vs fuel fill data, and probably most importantly- owner wishful thinking.
Your rule of thumb broadly works. I proposed a more comprehensive one that takes into consideration displacement, hull type and engine type in a related thread. See Trawler Forum - View Single Post - What is your mpg, gallons per hour etc... at various speeds?
By making the rule of thumb based on power to reach displacement speed, you automatically consider water line length. A longer water line leads to a higher displacement speed and better NM/gal. If you have two boats with the same displacement and hull type, the one with a longer water line length will achieve better fuel mileage.
But maybe the biggest variable is hull type. True displacement hulls are fairly consistent in hp per thousand pounds to reach displacement speed. The data shown in Bebe's book (edited by Leishman) shows a rather small variation of maybe 20%.
But semi-displacement hulls vary all over the map at low speeds. One poster on the referenced thread claimed he used twice the displacement hp or 3 hp per 1,000 lbs for a semi-displacement hull. My downeaster style Mainship Pilot 34 is notoriously inefficient at low and high speeds.
A less significant variable is engine type. No matter whether it is a 30 year old 5.8 liter 120 hp Lehman making 40 hp or a new 380 hp Cummins QSB making the same hp, they both make about 16 hp per gph of fuel. Only a modern smaller displacement engine like the 4.5 liter JD will do better because it has less iron to turn over and its fuel injection is optimized for the lower hp output.
So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption.
David
Nomad Willy
Guru
David,
You keep talking about "reaching hull speed". I thought we made that expression illegal. It's valid however for SD hulls and about 93% here have SD. Many more have planing hulls than FD hulls. So it may increase clarity to state "(SD)" when you talk about power to reach hull speed. That's the best cruising speed for most SD boats but a FD skipper would be absolutely stupid to run at HS. And if not he clearly doesn't have a FD hull.
Rusty says;
"Diesels use 1 gal of diesel to produce 20 HP."
I think 15hp per gallon is closer to most of the engines on this forum.
WesK,
MPG is totally foreign to me on the boat.
GPH.
Paul Swanson wrote;
"Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased."
It is more efficient because the turbo pumps more air through the engine with less effort than the pistons do as a reciprocating air pump. And the CR is actually decreased to prevent over charging the combustion chamber w air creating too much pressure at compression.
David also wrote;
"So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption."
I don't even give fuel burn numbers that much credit on TF.
You keep talking about "reaching hull speed". I thought we made that expression illegal. It's valid however for SD hulls and about 93% here have SD. Many more have planing hulls than FD hulls. So it may increase clarity to state "(SD)" when you talk about power to reach hull speed. That's the best cruising speed for most SD boats but a FD skipper would be absolutely stupid to run at HS. And if not he clearly doesn't have a FD hull.
Rusty says;
"Diesels use 1 gal of diesel to produce 20 HP."
I think 15hp per gallon is closer to most of the engines on this forum.
WesK,
MPG is totally foreign to me on the boat.
GPH.
Paul Swanson wrote;
"Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased."
It is more efficient because the turbo pumps more air through the engine with less effort than the pistons do as a reciprocating air pump. And the CR is actually decreased to prevent over charging the combustion chamber w air creating too much pressure at compression.
David also wrote;
"So, we have seen this and another thread where members were asked what their fuel consumption numbers were, and lots replied. I take all of this data with a big grain of salt and prefer to use the foregoing to estimate low speed fuel consumption."
I don't even give fuel burn numbers that much credit on TF.
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Ski in NC
Technical Guru
I'll test Peter's math on my ride: 7.5t x 5hp/t = 38hp. My big engine is not very efficient at that low power, so figure 17hp/gph: 38/15 = 2.5gph.
At 7.5 kts my dipstick and tankfill gph avgs 1.9gph, well established. Although a few times we ran 8kts, and it was like 2.3gph.
We won't talk about running 20kts, that is the other speed of this dual purpose sled.
I might be better than most as the hull is relatively skinny, 38x12, and only about 10.5' at water line.
Or it could be my awsome job getting the bottom fair in construction!!??
I'd say Peter's math passes the sanity test.
Edit: most of our diesels are going to run around 15-17hp/gph depending on where we run them. The 20hp/gph is about the limit, only acheived when it is an efficient engine AND run at it's sweet spot on the BSFC curve. Really large engines of many thousands of HP can beat 20, but not ours in the 50-500hp class.
At 7.5 kts my dipstick and tankfill gph avgs 1.9gph, well established. Although a few times we ran 8kts, and it was like 2.3gph.
We won't talk about running 20kts, that is the other speed of this dual purpose sled.
I might be better than most as the hull is relatively skinny, 38x12, and only about 10.5' at water line.
Or it could be my awsome job getting the bottom fair in construction!!??
I'd say Peter's math passes the sanity test.
Edit: most of our diesels are going to run around 15-17hp/gph depending on where we run them. The 20hp/gph is about the limit, only acheived when it is an efficient engine AND run at it's sweet spot on the BSFC curve. Really large engines of many thousands of HP can beat 20, but not ours in the 50-500hp class.
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Eric, it is pretty much equally disadvantageous for an SD boat to run near hull speed as it is for a FD boat. David is speaking theoretically.
I have a question......why in the heck are Floscans so damn inaccurate when it measures the fuel being burned as well as the fuel being returned???
Ski in NC
Technical Guru
Because of subtraction.
Say supply is 11gph and instrument is +/- 2%. that means measured flow is between 10.78 and 11.22.
Say return flow is 10gph and instrument is +/- 2%. that means measured flow is between 9.8 and 10.2.
Let's say the instrument error is on the high side of the supply instrument and the low side of the return instrument.
Supply is reading 11.22. Ret reading 9.8. Calc'd flow is 1.42gph. Real flow is 1.0gph.
So two instruments with 2% error each can result in a total error of 42%.
Part of the reason I'm not a fan of flowscans, et al. You can tweak them for one spot, but how good are they at other spots???
They're not when properly maintained and interpreted correctly.
AKDoug
Guru
Floscans have an adjustable set of switches on the back of the unit for fine tuning that are intimidating enough that I think most people just accept whatever inaccuracy the unit has when it comes pre-set. I had two different units in the past, both over read my fuel consumption, and I never made any effort to play with the adjustment switches. My over read was in the range of 5-7% over 92 gallons.
They were both on a planing hull that went through the tank of fuel in less than 10 hours, so there seemed to be little point in getting closer to perfect. That was my experience with Floscan anyway, and having to adjust two of them for engine and return line seems even more complex...
They were both on a planing hull that went through the tank of fuel in less than 10 hours, so there seemed to be little point in getting closer to perfect. That was my experience with Floscan anyway, and having to adjust two of them for engine and return line seems even more complex...
As Ski noted above, Flowscans are notoriously inaccurate at low loads.
The Flowscan installation instructions tell how to calibrate them, but I don't know how you measure real world fuel burn to have a basis for calibration unless you have a small, calibrated day tank to measure fuel over a short term. And as Ski notes you can only calibrate at one point and I suspect that the Flowscan meter isn't totally linear.
I did see an interesting posting on another thread about an incredibly accurate flow meter- 0.25% (of full scale presumably). It was based on a positive displacement meter, unlike Flowscan which uses an electronic sensor. Each individual meter is factory calibrated to achieve that level of accuracy.
Using Ski's example above and assuming an absolute accuracy of 0.25% of 50 gph full scale flow which is .125 gph, then the worst that it could read would be 0.125 high on the supply and .125 low on the return or an error of 0.25 gph. That is still a significant error while measuring flow rates in the 1-2 gph range, but not bad at all at higher loads.
David
The Flowscan installation instructions tell how to calibrate them, but I don't know how you measure real world fuel burn to have a basis for calibration unless you have a small, calibrated day tank to measure fuel over a short term. And as Ski notes you can only calibrate at one point and I suspect that the Flowscan meter isn't totally linear.
I did see an interesting posting on another thread about an incredibly accurate flow meter- 0.25% (of full scale presumably). It was based on a positive displacement meter, unlike Flowscan which uses an electronic sensor. Each individual meter is factory calibrated to achieve that level of accuracy.
Using Ski's example above and assuming an absolute accuracy of 0.25% of 50 gph full scale flow which is .125 gph, then the worst that it could read would be 0.125 high on the supply and .125 low on the return or an error of 0.25 gph. That is still a significant error while measuring flow rates in the 1-2 gph range, but not bad at all at higher loads.
David
Nomad Willy
Guru
Baker wrote;
"Eric, it is pretty much equally disadvantageous for an SD boat to run near hull speed as it is for a FD boat."
Good input Baker,
Many SD hulls are hard to tell from planing hulls and almost as many are hard to tell from FD so there is a wide range of SD hulls having very significantly different performance re FD, SD and planing.
I think though if you jumped from one knot below HS on a GB 32 (SD) and noted the fuel burn increase (%) and did the same on my Willard the Willard (FD) would suffer the greatest increase. Or is that not so?
"Eric, it is pretty much equally disadvantageous for an SD boat to run near hull speed as it is for a FD boat."
Good input Baker,
Many SD hulls are hard to tell from planing hulls and almost as many are hard to tell from FD so there is a wide range of SD hulls having very significantly different performance re FD, SD and planing.
I think though if you jumped from one knot below HS on a GB 32 (SD) and noted the fuel burn increase (%) and did the same on my Willard the Willard (FD) would suffer the greatest increase. Or is that not so?
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Paul Swanson
Veteran Member
Paul Swanson wrote;
"Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased."
It is more efficient because the turbo pumps more air through the engine with less effort than the pistons do as a reciprocating air pump. And the CR is actually decreased to prevent over charging the combustion chamber w air creating too much pressure at compression.
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We are getting pretty technical here but I think a turbocharger produces more power per displacement (volumetric efficiency) for the reason you stated but the thermodynamic efficiency is about the same, except for the compression ratio thing. Turbocharging does raise the effective compression ratio since there is already an over pressure in the cylinder before the normal compression from the piston occurs. That is why turbocharged engines generally start with a lower compression ratio than normally aspirated engines. It avoids too much pressure. It is also why it is ill advised to simply bolt a turbocharger onto a NA engine.
"Turbocharging simply forces more fuel/air mixture in to the cylinders thereby producing more horsepower for a particular sized engine. It doesn’t have much effect on efficiency except that the effective compression ratio may be increased."
It is more efficient because the turbo pumps more air through the engine with less effort than the pistons do as a reciprocating air pump. And the CR is actually decreased to prevent over charging the combustion chamber w air creating too much pressure at compression.
-----------------------------------------------------------------------------------
We are getting pretty technical here but I think a turbocharger produces more power per displacement (volumetric efficiency) for the reason you stated but the thermodynamic efficiency is about the same, except for the compression ratio thing. Turbocharging does raise the effective compression ratio since there is already an over pressure in the cylinder before the normal compression from the piston occurs. That is why turbocharged engines generally start with a lower compression ratio than normally aspirated engines. It avoids too much pressure. It is also why it is ill advised to simply bolt a turbocharger onto a NA engine.
CmdrDick
Veteran Member
The turbo isn't getting enough exhaust flow at low rpm to help with the power. That is why many new engines have two turbochargers, one with a wheel for the low rpm range and a second designed for the higher rpm's. Under 1500 rpm, most older turbo engines get little help. A boost gage will give you a better idea about what a turbocharger is doing.
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It is called a "sequential turbo" and is not generally found in the boats we are talking about here. It is fairly common in cars. We operate our cars in a wide range of RPMs and power bands. So to avoid turbo lag and then a rush of power, they put the turbos in sequence...to give a more linear throttle response. Boats we generally set the power and are done with it. I guess all I am saying is sequential turbos don't really help in our application. You will see them in very high powered marine diesels....the "MTU range". I am not that familiar with the newest marine diesels so maybe they have small marine diesels with sequential turbos.....
FF
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"but I don't know how you measure real world fuel burn to have a basis for calibration unless you have a small, calibrated day tank to measure fuel over a short term"
Many cruisers have a "normal" cruise RPM range (ours is 1000 to 1500, but most time its 1200).
A tiny tank is not required , just fill the tank and when its refilled adjust the fuek burn reading to the actual gallons on the tank fill.
Nothing to it and the set becomes more accurate after a few fills.
With a tiny 200G fuel cap , after the loop we could estimate the next fill with in a gallon or two.
The biggest inaccuracy is probably from NOT filling the tank to full to the top because of the sheen police should a tea spoon be vented.
Many cruisers have a "normal" cruise RPM range (ours is 1000 to 1500, but most time its 1200).
A tiny tank is not required , just fill the tank and when its refilled adjust the fuek burn reading to the actual gallons on the tank fill.
Nothing to it and the set becomes more accurate after a few fills.
With a tiny 200G fuel cap , after the loop we could estimate the next fill with in a gallon or two.
The biggest inaccuracy is probably from NOT filling the tank to full to the top because of the sheen police should a tea spoon be vented.
Nomad Willy
Guru
FF,
I'm suprised at you.
That would represent all engine running and there's a lot of running that is not at cruise speed. And like you many don't cruise at the same rpm much of the time.
Makes it look like your boat's really efficient though .. and many like that. That's why you get so many unbelieveably low fuel burn numbers .. among others.
Actually the small tank would deliver if done right. Small enough and big enough (one or two gallons?) and plumbed to be switchable .. in the system .. out. Just a lot of trouble to rig up. Way more accurate than observing the #of gallons put in the fuel tank and the reading of the hour meter.
By the way it would be nice to know who you were quoting in your opening comment above. I'd like to see who said that and the other relavant info related.
I'm suprised at you.
That would represent all engine running and there's a lot of running that is not at cruise speed. And like you many don't cruise at the same rpm much of the time.
Makes it look like your boat's really efficient though .. and many like that. That's why you get so many unbelieveably low fuel burn numbers .. among others.
Actually the small tank would deliver if done right. Small enough and big enough (one or two gallons?) and plumbed to be switchable .. in the system .. out. Just a lot of trouble to rig up. Way more accurate than observing the #of gallons put in the fuel tank and the reading of the hour meter.
By the way it would be nice to know who you were quoting in your opening comment above. I'd like to see who said that and the other relavant info related.
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Britannia
Wannabe
For me the only real interest in getting an accurate fuel burn rate is for ocean crossing. That's when it will really matter how accurately I can measure fuel consumption.
With 1200 gal tanks I can't realistically check at fill up - plus I'm not looking for an average - I want to know what the burn rate is at various RPMs in different sea states.
What I plan to do is install a small day tank or even temporarily plumb in a small gravity feed plastic tank. Then use that to measure the accuracy of my Flo Scan (or Maretron equivalent if I buy one). Once I have confidence in the electronic measurement system I can use that to monitor fuel consumption in various ocean conditions so I can get the best possible idea of what my real world consumption will be.
Richard
With 1200 gal tanks I can't realistically check at fill up - plus I'm not looking for an average - I want to know what the burn rate is at various RPMs in different sea states.
What I plan to do is install a small day tank or even temporarily plumb in a small gravity feed plastic tank. Then use that to measure the accuracy of my Flo Scan (or Maretron equivalent if I buy one). Once I have confidence in the electronic measurement system I can use that to monitor fuel consumption in various ocean conditions so I can get the best possible idea of what my real world consumption will be.
Richard
Ski in NC
Technical Guru
One technique is to have a day tank that is kept solid with fuel, with engine both drawing from and returning to that tank. Then put the flowmeter in the makeup line to that tank. Then only one instrument required and accuracy should be quite good.
FF
Guru
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"For me the only real interest in getting an accurate fuel burn rate is for ocean crossing."
Was on Block Island one holiday weekend when a charter fish boat went for fuel.
He knew very well how much fuel the boat would take , the dock pump read far higher .
After a short discussion with the dockmastrer it was decided there would be no cost for the fillup.
All knowledge is power.
Was on Block Island one holiday weekend when a charter fish boat went for fuel.
He knew very well how much fuel the boat would take , the dock pump read far higher .
After a short discussion with the dockmastrer it was decided there would be no cost for the fillup.
All knowledge is power.
Dougcole
Guru
dhays
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I see two reasons for wanting to know fuel burn at various RPMs. One is simply being able to calculate how much fuel is left. On my boat, I burn less than 1 g/h and have a 40 gal tank. So I try to make sure I fill up as I approach 40 hours on the clock. I normally only need to fill up twice a year. (400 hours on the engine in the 5+ years I have owned her). The fuel gauge on the boat is laughably inaccurate and their is no sight tube. I was really sweating it this winter when I realized that I had run 38 hours since the last fill up and was 1 1/2 hours away from fuel. If I had either an accurate gauge or site tube, I really wouldn't care about fuel burn rates in this regard since I am never going to have to calculate max range.
Second reason is to figure out the most efficient speed to run at. At what point does the fuel burn vs boat speed really diverge from linear? I like to know that because I am inherently cheap. For this, I don't need accurate absolute numbers only consistent relative numbers.
So unless you are making long passages and need to know your range, I guess I don't see the point of knowing exactly what the fuel burn is.
Second reason is to figure out the most efficient speed to run at. At what point does the fuel burn vs boat speed really diverge from linear? I like to know that because I am inherently cheap. For this, I don't need accurate absolute numbers only consistent relative numbers.
So unless you are making long passages and need to know your range, I guess I don't see the point of knowing exactly what the fuel burn is.
FF
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On a normal displacement hull the SQ RT of the underway LWL times .9 to 1.15 on a light really sweet hull is your LRC fuel burn.
Convert K to MPH if you normally use lubber miles to navigate , the norm inshore.
Very light boats do better bit an extra 2240 lbs (a ton) only requires 2-3 hp more.
At 16 hp/gallon , 2 hp is pretty cheap.
Convert K to MPH if you normally use lubber miles to navigate , the norm inshore.
Very light boats do better bit an extra 2240 lbs (a ton) only requires 2-3 hp more.
At 16 hp/gallon , 2 hp is pretty cheap.
kulas44
Guru
For ocean crossing, knowing your immediate fuel burn is most important. Limited fuel capacity makes it important. Average GPH is not going to let you know that you need to slow down 2 knots in current conditions to make it across the pond with a decent reserve. My trawler has about 1000 gallons capacity now, and I plan to add another 500. Still, thats only 1500 gallons, Without accurate fuel management and knowing the burn rate "now" a 3000 mile crossing would be risky. I like the day tank arrangement for simplicity and accuracy. I have two 25 gallon tanks that gravity feed to the mains. They are fed from the belly tanks (125 gallons each) by gear rotor pumps. The belly tanks are fed by the bulk tanks. The day tanks have adjustable float switches that turn on and off the transfer pumps at preset levels. 2 gallons is the norm. A pump counter keeps track of the cycles. Check the time and reset the counter. In a few hours you have a very accurate GPH estimate.
Britannia
Wannabe
My point exactly. Ocean currents and sea state my mean a considerable reduction in RPM in order to make the crossing with available fuel & reserve. Knowing the burn rate accurately becomes important if the passage is near the limit of your range.
Richard
sdowney717
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So the fuel dock was cheating stealing people's money everyone who filled up? Free fuel for the charter boat the price to keep it a secret?
