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Author Topic:   Average Fuel Economy: Proper Weighting Factors
jimh posted 09-06-2007 10:59 PM ET (US)   Profile for jimh   Send Email to jimh  
Please use this discussion for follow-up questions or comments about my article Average Fuel Mileage: Proper Weighting Factors which appears in the REFERENCE section at

http://continuouswave.com/whaler/reference/averageMPG.html

The articles demonstrates the proper method for estimating the average fuel economy of a boat which is operated at various speeds with different rates of fuel consumption.

pglein posted 09-07-2007 06:48 PM ET (US)     Profile for pglein  Send Email to pglein     
I think it's interesting. But what I do is simply assume that I will burn at my cruise rate the entire time and carry enough fuel for that amount.

Better to error on the safe side, I think.

Tonym posted 09-09-2007 10:56 PM ET (US)     Profile for Tonym  Send Email to Tonym     
Jimh,
This is a particularly timely article for me. I just returned from a 7 day vacation in Ensenada, Baja California with my 19 foot raider. I have a Honda 130 but do to excessive weight I had to be very frugal with my cruising speed. I found that at 20 mph cruise I burn more or less 10 Gallons per hour in other words 2 MPG. At displacement speeds ( 5-6 mph) I get 4-5 mpg. In order to fish offshore I needed to leave at midnight and maintain hull speed in order to reach the seamounts where we fish off shore and still have a safe supply to get back to the harbor. Thanks for the article. Tonym
jimh posted 09-10-2007 08:30 PM ET (US)     Profile for jimh  Send Email to jimh     
The behavior I am describing here may be an example of Simpson's Paradox.

Cf.: http://en.wikipedia.org/wiki/Simpson's_paradox

Here is another hypothetical example which illustrates the point:

Consider two boats and motors, which we'll call A and B, that produce these fuel economy figures at two different speeds:

Boat A:
6-MPG at 5-MPH
3-MPG at 25-MPH

Boat B:
12-MPG at 5-MPH
2.5-MPG at 25-MPH

If we use these boats 60% of the time at 5-MPH and 40% of the time at 25-MPH--which I think is a fairly representative pattern of normal use--which one will get the higher average fuel economy? My suspicion is that most people will look at the big difference in fuel economy at 5-MPH, where boat B gets TWICE the fuel economy as Boat A, and, figuring since we are spending most of our time at that speed, Boat B will have the best average fuel economy, overcoming a small difference (0.5-MPG) at the other speed. Let's find out.

Boat A Analysis:

6 hours at 5 MPH = 30 miles
5 MPH / 6 MPG = 5/6 GPH for 6 hours = 5-Gallons

4 hours at 25 MPH = 100 miles
25 MPH / 3 MPG = 25/3 GPH for 4 hours = 33.3-Gallons

TOTAL MILES = 130
TOTAL GALLONS = 38.3
AVERAGE MPG = 3.4 (3.394)

Boat B Analysis:

6 hours at 5 MPH = 30 miles
5 MPH / 12 MPG = 5/12 GPH for 6 HOURS = 2.5-Gallons

4 hours at 25 MPH = 100 miles
25 MPG / 2.5 MPG = 10 GPH for 4 hours = 40 gallons

TOTAL MILES = 130
TOTAL GALLONS = 42.4
AVERAGE MPG = 3.1 (3.058)

The results are somewhat surprising. Even though Boat A only gets half the gas mileage at slow speed that Boat B does, it gets slightly better mileage at high speed. Because so much more fuel is burned at high speed running, Boat A ends up with the better overall fuel economy.

elaelap posted 09-11-2007 02:53 AM ET (US)     Profile for elaelap  Send Email to elaelap     
Using Occam's razor to slice up the paradoxes and complicated math, I agree with pglein. For my "Real Boat" calculations, I'm much more interested in fuel usage per my 'normal' day than some sort of artificial, theoretical MPH or GPH. Now what's a 'normal day' for me and for perhaps the majority of my advanced amateur salmon fishermen/women colleagues boating in the Pacific north of San Francisco? Well, the cruising distances will vary greatly for most of us from day to day. One week the word gets out that the fish are biting enthusiastically fifteen miles away from the harbor in a certain location, and a fleet of recreational, commercial and charter/sportfishing boats will gather there, sometimes day after day for a week or more. The next hot bite might come right off the harbor, no more than a couple of miles from the ramp. I think it would be a fairly simple task to log every mile covered at cruising speed on every trip during a season and then come up with an average daily distance to and from the trolling locations. (For simplicity I'm leaving out the multiple cruising days, when one runs to a location, trolls there for a couple of hours, and then moves, sometimes many miles, to try another spot...running from place to place in response to VHF reports is referred to as chasing 'radio fish', by the way, Jim). Anyway, those extra cruising distances could easily be logged and averaged in as well.

Thus, let's say that during an average season I travel in my "Real Boat" at cruising speeds about twenty-four miles per day, on average, to, from, and in between trolling/drifting locations. This takes me between one and two (and very rarely on rough days almost three) hours, counting our harbor's half mile 'no wake' zone leaving and returning, depending upon weather and sea conditions. The rest of my day--five, six, sometimes seven hours--is spent at trolling speeds or drifting (mooching) with my motor on. (I'm obviously leaving out the reality of the situation...the off-subject complication caused by the fact that on many days I troll/drift with my kicker and don't use my main motor at all except for running.)

At trolling/drifting 'speeds'--from zero MPH to four MPH--the amount of gasoline I use in my four stroke EFI motor is almost impossible to calculate without a very accurate flowmeter of some kind, which I don't have. I watch the fuel gauge carefully, and truly don't notice it move at all at those low revs while the motor's hour meter ticks merrily away. And the 'mileage' I cover while trolling is sometimes experienced as a series of large or small circles around the same general location, while at other times it significantly decreases the return cruising distance--when I decide toward the end of the day to troll for several hours in a direct line back toward the harbor.

At the end of every trip I refuel our boat, so I get a fairly accurate read on that day's total consumption of fuel. From the number of gallons consumed I'm able to make a rough estimate about my cruising mileage, which does vary greatly depending upon weather conditions. I just discount a very approximate couple of gallons, maybe a little more on a very long day, for trolling use, and divide my estimated cruising miles by the amount of fuel used minus the trolling 'discount'
...all very rough, which emphasizes the point I'm trying to make. In MY reality, in my "Real Boat", it's the GPD number--the gallons per day--which has any interest and relevance for me. Your interesting paradox, where better and better fuel usage at slow speeds actually seems to decrease overall fuel economy at least in MPG terms, is just that, a quirk, a paradox which while interesting philosophically has no practical application, at least in my boating life. Still, great fun and a much better way to pass the time than watching the 49s barely squeak out a win against hapless Arizona...

Tony

jimh posted 09-11-2007 03:06 AM ET (US)     Profile for jimh  Send Email to jimh     
Once you have a real boat and a real motor, it is not hard to find the average fuel economy. You just record the total miles and total gallons.

The analysis I have offered here is more useful if one is trying to decide on what motor to purchase. Some motors excel at fuel economy at certain engine speeds, while other motors perform differently. One ought to give special attention and consideration to the fuel economy of a motor at the speed range where it will be burning the most fuel because this will have the greatest weight in the overall average fuel economy.

Peter posted 09-11-2007 07:33 AM ET (US)     Profile for Peter  Send Email to Peter     
You might consider using the ICOMIA standard duty cycle to compare hourly fuel consumption between two different sized motors on the same boat or two different types of motors on the same boat.

Let's use a hypothetical 18 Outrage with 115 and 150 HP motors. We'll use Tony's favorite 4-stroke Yamaha F115 and an F150 to compare. We'll assume that the F115 can push the 18 Outrage to a maximum of 40 MPH and the F150 to 45 MPH.


Peter posted 09-11-2007 07:59 AM ET (US)     Profile for Peter  Send Email to Peter     
....Pushed the submit button too soon...

I did a quick calculation based on analogous boats, Edgewater 188 CCs, and came up with an ICOMIA duty cycle number of about 2.93 GPH for the F115 and 4.0 GPH for the F150. Now the average speed for the Edgewater 188 CC with the F150 under the ICOMIA duty cycle is actually higher so average MPG is actually much closer than the 1 GPH difference for each operating hour thus for that extra 1 GPH, the F150 powered boat traveled farther.

Based on the Yamaha performance reports, at a reasonable 26 to 27 MPH cruise speed, the two motors consume gas at about the same rate but the F150 is running at a much more relaxed 3500 RPM rather than a racy 4000 RPM needed by the F115 to maintain the same speed.

elaelap posted 09-11-2007 07:57 PM ET (US)     Profile for elaelap  Send Email to elaelap     
I did love and very much miss my sweet classic Outrage 18 with an F115 Yamaha, Peter. I posted three computer printouts (from three dealer 'tune-ups') showing my use of that motor during the couple of years--and 760 hours--I enjoyed that combination. I can't remember exactly, but I think that well over 85% of the hours on the motor were at less than 4000 rpm, and only something like .5% were wide open. I think about two-thirds of the hours were at trolling speeds. I don't get to cruise outside the harbor here at much over 25 mph on most days, and often I'm very happy making 22 or 23 mph, especially if I'm heading into the stuff. Anyway, that motor worked great for me, but a 150 would be fine as well, and probably much better in many applications (long distance cruising in calmer seas, boating often with three or four adult guests, pulling waterskis, wakeboards, etc).

Vaya con dios, Cetaceous ;-)

Tony

Davey1000 posted 09-03-2011 12:08 PM ET (US)     Profile for Davey1000  Send Email to Davey1000     
Planing boats are very thirsty. If one looks at the difference in power requirements for displacement hulls and planing hulls the figures are mind boggling. A forty foot canal boat (narrowboat) weighing several tons can travel at 4mph on about 10HP and as the engine will be a diesel the mpg will be good. The 100HP Mercury outboard (2 cycle) that I had on a ski boat was rated to drink 11 US gallons per hour at WOT. In actual practice it used about four imperial gallons per hour when used for towing skiers. A friend got so much into ski-ing that he clubbed together with another person and they used a Winner Wildcat ski boat with an inboard V8 engine running on LPG! I'm not sure whether that was 100% legal but as it was on a private lake on private land the taxman couldn't say much! A days skiing and more out of a cylinder of Calor gas was a bargain!
jimh posted 09-04-2011 10:53 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for your memoir of your boating and your friends boat building. I am having a bit of trouble understanding how that relates to the proper weighting factors for estimating overall fuel economy based on engine use, but I will give it some careful thought.

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