Gas mileage / hours - Best Engine?

[External Combustion]

Re: Gas mileage / hours - Best Engine?

I have enjoyed reading the debate between QC and Silvertip on this thread and others. I think it is a lot like the chicken and egg controversy.

Needless to say I don't think I will ever see either one of them adrift with no fuel. If I do, I will gladly throw in another log and pull them home. In my launch I have to run blind because there are no fuel flow guages for firewood. I know it averages 40 pounds per hour at 7 1/2 miles per hour and that is just about a cubic foot of wood. I have a 240 mile range when the bunkers are full. Wind and loading does not seem to be factors of concern and I can get to shore and refuel with a chainsaw almost anywhere.

My MFG Gypsie is another story! The MPG varies wildly as to the wind, loading and even where I buy the gas.

I can't remember where I read it first or heard it first but, "1/3 out, 1/3 back 1/3 reserve on fuel." After some use with carefully kept records you should be able to cruise where you want to with confidence.

Thanks guys for the well reasoned advice you give here. I have reccommended this website to several beginning boaters as I know they will get potentially life saving advice here.

 

[45Auto]

Re: Gas mileage / hours - Best Engine?

You can look at the units involved with the data terms and figure out which is relevant if you?re talking about efficiency. Fuel consumption and distance are the two COMPONENTS of efficiency.

Flow Rate is defined as Gallons per Hour ? this tells you how much energy is going INTO your engine. Notice there are no units for engine OUTPUT, which is how much work is usefully done by the fuel in driving the boat through the water.

Speed is defined as Miles per Hour ? this tells you how much energy is coming OUT of your engine and being put into the water by the propeller, thereby driving your boat forward. More energy put into the water makes your boat go faster. Notice that there are no units for how much energy is going INTO the engine to turn the propeller.

NOTE: Some people need to read the next sentence VERY SLOWLY so they understand it!

Efficiency is a measure of how well your engine converts the energy INPUT (fuel flow) to energy OUTPUT (horsepower driving the propeller ? in other words MPH).

Flow rate (gallons/hour) is useless without speed. Flow rate is only a measure of instantaneous fuel consumption with no measure of how much energy from the fuel is being applied to moving the boat. Can you figure my boats efficiency if I tell you it is using 10 gallons per hour? Will I make it to my destination 200 miles away if I?m burning 10 gallons per hour and have a 40 gallon tank? No way to tell unless you know the speed.

Speed (miles per hour) is likewise useless without flow rate. Speed can be looked at as an instantaneous measure of how hard your engine is working to move your boat. If it can do the same amount of work with less fuel it would be more EFFICIENT. But you can?t figure that out without knowing how much fuel it?s using. Can you figure my boats efficiency if I tell you it is going 40 miles per hour? Will I make it to my destination 200 miles away if I?m going 40 MPH and have a 40 gallon tank? No way to tell unless you know the fuel flow rate.

As you can see from the above Flow Rate and Speed are both components of efficiency. Neither one of them is sufficient by itself. So how do we combine them to measure efficiency?

Speed divided by Flow Rate will give you efficiency. Obviously more speed from the same flow rate will give you a higher number and is more efficient. Likewise, if you can go the SAME speed but with a LOWER flow rate you will also get a higher number for efficiency.

For example, 40 MPH (miles per hour) divided by 10 GPH (gallons per hour) gives an EFFICIENCY of 4 MPG (miles per gallon). If you can go 50 MPH on the same 10 GPH then your efficiency is 5 MPG. Likewise, if you can go 40 MPH on 8 GPH then your efficiency is also 5 MPG.

As you can see from the above paragraph, knowing you have a flow rate of 10 GPH would not be enough to tell if you could make a 200 mile trip on a 40 gallon tank. The boat capable of 50 MPH at 10 GPH would just make it (5 MPG times 40 gallons would give you 200 miles), the one capable of 40 MPH at 10 GPH would leave you 40 miles short of where you wanted to be (4 MPG times 40 gallons is only 160 miles)!

Obviously the same thing applies to speed. Knowing you have a 40 gallon tank and whether your boat will go 40 or 50 MPH is useless in telling you if you can make a 200 mile trip or not unless you also know flow rate.

The COMBINATION of Flow Rate and Speed is Miles Per Gallon, which will allow you to easily calculate how long of a trip your boat is capable of if you know your fuel capacity.

 

[JCF350]

Re: Gas mileage / hours - Best Engine?

A fuel flow meter with cumulative total will let you know how much fuel you've burned. Also the most efficient throttle settings.

As for distance you have to use the 1/3 out 1/3 back 1/3 reserve. Only time (experience), tide and wind will let you know how far you can go.

Using anything but the 1/3 rule is just plain TOOPID! Which is why the Coasties will rack you hard for running out fuel.

 

[Silvertip]

Re: Gas mileage / hours - Best Engine?

The discussion here is not about efficiency -- it's about QC's assertion that flow rate is of no importance but MPG is. MPG is fuel use computed over distance which you pointed out. What you didn't point out is that it is an "average". It doesn't matter what speed you are going or how efficient or inefficient the hull/engine combination is -- until, and only until you determine how much fuel you actually used on a trip can you determine MPG and even then it will be an average. To calculate it instantaneously you need flow rate data. What people do with the data they gather and how they gather it is of importance to some and no or little importance to others. I happen to find flow rate of great value for my boating and QC apparently feels MPG has more significance to him. Hooray -- if everyone thought alike this would be one helluva boring world we live in.

 

[45Auto]

Re: Gas mileage / hours - Best Engine?

MPG does NOT have to be an average. As a measurement, it is no different than flow rate. If you have a fuel flow meter you can see instantaneous flow rate. If you have a speedometer, you can see instantaneous speed. If you have both, you can see instantaneous MPG (or calculate it if your electronics aren't tied together). This allows you to find the "sweet spot" for your motor. Either measurement by itself will NOT allow you to do this.

If you don't have an instantaneous flow meter you compute fuel flow the old fashioned way. You run the engine for a given amount of time then divide the amount of fuel used by the time the engine ran. No different than going a given distance then dividing by time to get speed. In both cases you get an average. Only difference between doing it like this and using modern instruments is that the time interval is much smaller with the electronics, so the value you see is much closer to the actual real-time number.

 

[HONKER1]

Re: Gas mileage / hours - Best Engine?

WOW....This is one of the best threads I've sat and read through. It was like sitting in a room of very experienced gentleman, with my newbee mouth shut, and letting the knowledge pour in my ear.
Bye the way Silvertip...Wife heard me say "I need one of those", and she hid the checkbook. Take it from someone that had the outboard coughing as I come back to the boatlaunch, it would not be fun to paddle my boat home.

 

[Silvertip]

Re: Gas mileage / hours - Best Engine?

45 -- you are preaching to the choir and are missing the point of this discussion which is the "value or lack of value of a flow monitor". Of course you can get instant MPG if you know flow rate. Your example of fuel calculation without a flow meter ("old fashioned way") requires that you actually make a trip. That's all well and good but does you no good on the next trip because the results are not the same due to operating conditions. You will not get the same MPG with four in the boat that you would with one or two. You will not get the same flow rate at full throttle that you will at 1/2 throttle. A flow monitor tells you right now what's happening so you can make educated decisions during the trip. QC doesn't seem to feel flow rate is important -- but without it you cannot obtain INSTANT MPG or flow rate. So without one, how do you propose that calculation be made during a trip? You can on a car because you have an odometer and if the car has a fuel computer it also provides instant and average MPG. Fuel monitors provide one more tool that can work to your advantage. If you don't have one, the boat won't sink, and you may never run out of gas either. But you will not know what the real flow rate is for your boat at any throttle setting, load, and water condition and all fuel consumption discussions you have must be followed with the word "about". I published my fuel flow numbers some time ago -- what are yours at 500 rpm increments from idle in gear to 5000 RPM? As you ponder that, how far would you have to run at each rpm to consume enough fuel to get any sort of meaningful consumption numbers? A flow monitor does it in a heartbeat (actually updates at any interval you elect to set the monitor for. Mine calculates on 15 seconds worth of flow). For those of you who feel the need, I did a quick search and the Northstar 210 is available for as little as $117.84. iBoats didn't pop it up on a search.

 

[45Auto]

Re: Gas mileage / hours - Best Engine?

Not sure what your point is ST.

Everyone seems to agree that a flowmeter is a good thing. You seem to be missing the point that a flowmeter by itself gives you no more useful data than a speedometer by itself.

Given either one by itself all you have is some nice numbers to look at. The combination of a flowmeter and a speedometer or odometer (GPS) is what allows you to make useful decisions on fuel management.

Knowing you are flowing 10 gallons/hour is useless if you don't know how fast you are going so you can compute how much range you have. If you have 10 gallons of gas on board, your destination is 30 miles away, and you're flowing 10 GPH, will you make it???? No way to tell if you don't know your speed. You will make it if you are going at least 30 MPH at that 10 GPH flow rate. You won't if you are going any slower at that flow rate.

From Silvertip:

I published my fuel flow numbers some time ago -- what are yours at 500 rpm increments from idle in gear to 5000 RPM? As you ponder that, how far would you have to run at each rpm to consume enough fuel to get any sort of meaningful consumption numbers? A flow monitor does it in a heartbeat (actually updates at any interval you elect to set the monitor for. Mine calculates on 15 seconds worth of flow).

I'll publish my flow numbers this weekend if I get a chance while I'm pondering them. I'll do them in a graph at 1 RPM increments if you'd like. Didn't realize anybody wanted to see them so bad. My flowmeter (built into the engine computer - Mercury Smartcraft) takes it's information directly from the actual fuel pressure and the time the fuel injectors on the engine are open so it does it much quicker than a heartbeat. Not sure what the actual cycle time on the microprocesser is, but the engine turning 4500 RPM means it's cycling at least 75 times per second. So at a top speed of about 70 MPH (about 105 feet/second) I would say that the distance I would have to run would be about 1.5 feet (1/75 of a second at 70 MPH) to get meaningful consumption numbers. At slower speeds the distance would be less.

 

[arboldt]

Re: Gas mileage / hours - Best Engine?

Now, now.

Both MPG and GPH can be important, depending on what I'm doing.

When I'm just cruising around our small lake or taking my granddaughter water skiing, gallons per hour can be roughly converted to fun per gallon for those activities.

When I'm on a trip on Michigan's Inland Waterway, miles per gallon can be very important. In this case, the measure could be either fun per hour OR fun per mile -- or maybe both.

Flow rate, speed, miles per gallon, and all the other metrics vary in importance depending on what I'm doing.

 

[Silvertip]

Re: Gas mileage / hours - Best Engine?

45 -- Did I ever imply that it is the "only" instrument that is of any value? Don't think so. You are now injecting an unreasonable twist to the discussion -- that being no speedometer. Other than some entry level boats, nearly every boat manufactured in the last 20 years comes with a speedometer. If it doesn't work or you don't have one, the flow monitor obviously limits what you can obtain from its display. you can say the same if you don't have a gas gauge as you can't determine fuel remaining or used unless you can see the level. Even then the flow monitor is of value since it knows how much fuel you've used and can tell how much remains. If you don't have a tach a speedometer, or a gps or a locator with speed and distance capability, one of the last devices you should be looking at is a flow monitor. Be reasonable here. I personally don't care if one uses or does not use a flow monitor. But I do contend people would operate their boats differently and understand its performance much better if they did. Looking forward to your numbers -- especially those at 1 rpm increments I posted mine as part of an in-depth study comparing three other identical boats so people could see some real life fuel consumption data. I didn't post them as a challenge to anyone nor is this a challenge to you. If you do, be up front about how you obtained them.

 

[45Auto]

Re: Gas mileage / hours - Best Engine?

Sorry if I took your posts wrong Silvertip. Maybe I'm just confused about what you mean. I'm just having a hard time figuring out why miles per gallon (MPG) is useless and flow rate is good.

in post #9 you say:

Since boats don't have odometers, miles per gallon is a relatively useless term. Gallons per hour is more meaningful

then in post #30 you say:

You are now injecting an unreasonable twist to the discussion -- that being no speedometer. Other than some entry level boats, nearly every boat manufactured in the last 20 years comes with a speedometer.

Until that last post I was figuring that you were thinking no speedometer in the boat. Otherwise how could MPG be "relatively useless"? If you know MPG and speed then flow rate is already inherently included in the MPG data. Since flow rate (gallons per hour - GPH) is a component of miles per gallon (MPG), if I know my speed and MPG then flow rate is easily derived. For example, if I'm going 30 MPH and getting 3 MPG, I am obviously using 10 GPH.

I'll post my fuel consumption if I get a chance this weekend. Kid's car broke a timing belt so will probably be working on that. Only fuel consumption data point I remember off-hand is that the "sweet spot" is 28 MPH at 2200 RPM. That gets me 3.7 MPG (from the Smartcraft tach/GPS speedo that ties into the engine computer). Obviously I can back that out (or read it off the screen) and figure out that it's using 7.6 GPH. However, it's easier for me to use that “useless” MPG data and figure out that if I want to make it to somewhere 40 miles away I better have more than 10 gallons of gas on board!!

 

[QC]

Re: Gas mileage / hours - Best Engine?

QC doesn't seem to feel flow rate is important

Ridiculous . . .

Well this is fun, glad I could help . . . Just to clarify, I NEVER said flow rate was useless. I've been in the engine biz my entire 30 year career, and I guarantee I understand fuel flow rates. Jeez. What I was reacting to is Silvertip's belief, quoted above by me and 45 Auto, that MPG was useless. My assertion, was then, and still is, that Flow Rate gives you MPG, it is what you need, what you use, and what is beneficial. Without taking the flow and using speed, you got nuttin' but an indication of how bigga bucket you need to feed the animal over time. This is NOT relevant to determining range. I AGREE, that flow rate along with speed will give you accurate data to determine your fuel use at that time, date, condition, etc. etc. etc. . . . BUT ULTIMATELY YOU USE MPG TO FIGURE OUT WHAT THE FLYIN' FLIP YOU'RE GONNA DO!!!! What is so hard to undasthtan?

Repeat after me: I love, need, want, flow rate info. I love, need, want, flow rate info. I love, need, want, flow rate info. I love, need, want, flow rate info. I love, need, want, flow rate info. . . . and when I get it, I "instantaneously" (or otherwise) use it to figure out MPG, or Nautical Miles per Liter, or Nautical Miles per Imperial Gallon, or inches per milliliter so that I can determine how far I can go on a 1/3 tank, a 1/4 tank, a quart bottle, a full tank, whatever, so that I am sure that I will actually get to where I think I want to go . . . Holy Flip!!!

 

[45Auto]

Re: Gas mileage / hours - Best Engine?

Yep, what QC said ^^^^^^^^^^!!!!

LMAO!!!

 

[BillP]

Re: Gas mileage / hours - Best Engine?

I'm with 45Auto on this one...efficiency is a major player and it's still pretty much the chicken and egg depending on who is at the helm. Mpg is more practical than fuel burn for me though. The distance I can go on useful fuel capacity is paramount to all other info. Fuel capacity is a known value no matter what variable data point (gph or mpg) is desired. Boats are no different than cars. How many prefer gph instead of mpg? Both change with speed, loading, rpms, etc and both require calculations with each other to know how far or long you can run.

I run a boat with the Yamaha fuel mgt system and if I could have only ONE data point to look at it would be mpg. Here's why: Tweaking only trim tabs and motor trim without touching the throttles result in a spread (instant mpg) of 1.5 mpg (2.5 to 4) with same fuel burn. Which in turn gives me 42 more miles or 1.5 hrs longer fuel burn per 80 gal fills. Using ONLY fuel burn (WITHOUT time/speed/distance) as a benchmark could/might/maybe calculate 42 less miles and 1.5 less hrs running time. The same results can be done by using gph and calculating time/speed/distance. But why do most want to know fuel burn? I say efficiency (mpg). Chicken or egg?

bp

 

[dcg9381]

Re: Gas mileage / hours - What is your prediction?

Re: Gas mileage / hours - What is your prediction?

rule of thumb is 12.5 gallons at wide open throttle. it varies on the hull, proper set up, and tuning of the motor. impossible question to answer. one savings is to got to wot, then throttle back to 7/8 throttle.

I don't run 2-strokes, but the "efficient" setting for a 7.4L motor and my boat is less than 50% throttle, 30 knots, and about 3000-3100 rpm.

You'll need to find the right "cruise" spot for your boat and motor.

 

[QC]

Re: Gas mileage / hours - What is your prediction?

Re: Gas mileage / hours - What is your prediction?

Boat efficiency is more abut the hull than the engine. Obviously the combo matters, but that same 7.4 if in a car would suck major fuel if run at 50% throttle at cruise. Depends on the boat etc., and this is when you need a fuel flow meter. Don't make me bring out the wide open is the most efficient point for an Otto cycle engine stuff. Just don't. Silvertip really hates that one . . .

 

[Locke]

Re: Gas mileage / hours - Best Engine?

Geo2008

So did you get your question answered?

Locke

 

[Hitech]

Re: Gas mileage / hours - Best Engine?

Whether MPG or MPH is more important depends on what you are doing. When I'm boating MPG would be meaningless. I have NO idea how far (or how fast for that matter) I'm going, I'm just going around in circles . When water sking, pulling a tube and playing around on a lake MPG doesn't mean much. Now, when traveling somewhere, esp. if it is a "long" trip with no fuel along the way, MPG is way more important.

Regardless of the technicalities, what you are doing make the most difference.

 

[Hitech]

Re: Gas mileage / hours - Best Engine?

Knowing you are flowing 10 gallons/hour is useless if you don't know how fast you are going so you can compute how much range you have.

Sure it is. I don't know, nor do I care, how far I can go. I'm going in circles anyway (boating on a lake). The only thing I need to know is how long can I stay out before I need to get fuel. Even if I want to know if I have enough fuel to get somewhere on the lake, I know how long it takes me to get there, but I have no idea how far it is.

Then again, on a "small" lake, you're in trouble if you have to calculate whether you can get to the dock with the fuel you have. That means you are "runnning on fumes"...

 

[QC]

Re: Gas mileage / hours - Best Engine?

I just got it!!!! Check it out . . . We really, really need solar boats.