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Old 01-16-2015, 07:23 AM   #21
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The higher fuel burn at higher RPM in adverse current will make the duration shorter.

So the extra GPH must be viewed with the shorter run time to find weather the total fuel consumption at higher speed is worthwhile.
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Old 01-16-2015, 09:23 AM   #22
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Like I mentioned previously, it all seems to wash if you travel long enough. Many times you won't even have to travel for the duration of a complete tide cycle. By example the area where our boat is currently kept, incoming tides create currents which run north in one section of the bay and south in another section. After roughly six hours they reverse. This happens over an 8 mile stretch of water, so it's possible to have fair, foul or split currents for the whole stretch (1 hour) and this is not unique. We saw it last summer on many stretches of the icw along the east coast.We have electronic diesels which accurately report and record all fuel use over an n2k network tied into a sat compass and navnet (think instantaneous nmpg data ie the holy grail of info,apparently). We also have a laptop running software which shares this data over the net. So what do with all this info? Ignore it mostly, as we run the boat by ear and feel. Now there are certainly some areas that are notorious for fast tidal currents and for those we look to time things in our favor as best we can, but even then there have been more than enough surprises. Then there are those areas where there are no tide stations and you need to make your own tables, sorry no app for that. Over the last 2 years this instantaneous nmpg number has spanned such a large range it has proven itself to be pretty worthless (hear that Oliver? Your dad understands) and we simply take our total mileage travelled and divide it by our total fuel use. That number encompasses all types and strengths of weather and currents. Since for us, the trip IS the destination, we just try to enjoy it, no matter what it is costing for any particular mile. One more thing, we have found that on our boat, a foul bottom costs way more than any foul current has.


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Old 01-16-2015, 10:55 AM   #23
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Quote:
Originally Posted by psneeld View Post
That's why I said every inch of the ICW....me and the Cape Fear river are arch rivals when I am headed north...there are a couple other rivers too that I tend to zig zag if I'm bucking an ebb...
I also discovered this past year that when steering by hand, many times, I can let go of the wheel and Dauntless will stay in the deeper channel, even as the river meanders.

I discovered this the hard way as i realized I was over controlling and forcing her into too shallow waters, even though the markers indicated a wider channel.
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Old 01-16-2015, 02:30 PM   #24
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I have recalculated our rpm-speed-fuel curves for the case "running against current", i.e. subtracted different figures (0-7 knots) for the current from the speed curve to get the SOG and calculated the nmpg (please remark that I have converted the data to your US units :-) based on the SOG and the fuel consumption at given rpm according to our files:
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(continuous lines indicating SOG, dash dotted lines nmpg)

Starting at 3 knots head current there seems to be an optimum rpm for our boat giving the best range out of a certain amount of fuel. This optimum rpm is shifting to higher figures the higher the current flows against us. The red dash-dot-dotted line is indicating those points of maximized range.

However - as nice such diagrams are looking to engineers like me I am afraid that this particular one is of very limited practical relevance.
Up to now we have been able to avoid currents above 5 knots against us. And we never run our Lehmans continuously below 1300 rpm as it would give the best milage below 5 knots of current.
In most cases we just continued our journey against a current with the throttle setting we have had before...


best regards / med venlig hilsen
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Old 01-16-2015, 06:47 PM   #25
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Well, it looks like 3 members and engineers, tpbrady, waddenkruiser and me, ran our own independent analyses and came to the same conclusions regarding fuel saving practices. I like how tpbrady summed it up in post #18.

"As long as the current/tide or whatever you are running into is less than 1/2 than the SOG in calm water at a cruise rpm then going faster only increases total fuel used over any distance. Once the current exceeds more than 1/2 the SOG in calm water you can increase RPM to increase SOG up until you start trying to climb your bow wave and you will burn less fuel over any distance.

When running with the tide or current, slowing down always reduces the fuel used over any distance. Speeding up always increases it. "


Now if we could only get some reliable current prediction models for all waters! Our models out west seem to be as error prone as yours' on the east coast.
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Old 01-16-2015, 09:27 PM   #26
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Al,

I was a forestry major who spent the last 43 years in telecommunications. You might say I am unencumbered by formal training in what I do.

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Old 01-17-2015, 10:35 PM   #27
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The OP question has some bearing on a problem I've thought about concerning the currents effect on fuel use. On the pool of the Mississippi River where I boat the difference in sog going with the current vs against is 5 mph. I've checked it several times when out for a Sunday cruise, never touch the throttles just turn the boat 180. I want to check my gph over a long distance cruise against the current this year. Over the last 5 years the average is 3.1 gph including gen time, but this is all 50/50 with and against the current. I've thought about taking a cruise to Memphis and with the lack of fuel stops having a known burn rate for 300 or so miles against the current will help with planning. If I remember right it's 265 miles from Memphis to the 1st fuel stop going upriver, I hold 304 gallons and think I should be able to make it, my average mpg over 5 years is 1.8, in case I have any doubt I have a 75 gallon bladder.
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Old 01-18-2015, 01:38 PM   #28
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Ron,
I don't know if I've got you right but you can calculate the mpg for the upriver tour from your average mpg going up and down the river (with the same throttle i.e. same velocity through the water):
MPGup = 0.5 x (1 + SOGup / SOGdown) x MPGavg
Don't know your typical SOGs. For SOGup = 5 knots and SOGdown = 10 knots your average MPGavg = 1.8 mpg will reduce to MPGup = 1.35 mpg.


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Old 01-18-2015, 05:35 PM   #29
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Thank you very much, I was hopeful that I could get 1 to 1.2 mpg against the current.
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