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Old 01-13-2015, 08:19 AM   #1
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Current/fuel calculations

We operate between 1600 and 2000 rpm, usually around 1700. We used to think
we could figure out the tidal currents on the ICW for optimal fuel use, but
are fooled often, so I have a formula: when going against the current I use
a higher rpm, thinking I'm going upstream for a shorter length of time that
way, using less fuel. When traveling with the current, I slow down,
believing I'm letting the current do some of the work instead of my
Cummins. I'm sure I've saved at least four tablespoons of diesel over the
years with this method.

So why not have an application that would incorporate the tide tables,
current, time of day, date, speed and place to figure out the optimal time
to travel from A to B? Of course this would just be a guesstimate, but a
worthwhile project for someone and maybe worth a few bucks,

I'm sure this has been done for boats that travel on the high seas and
really use some diesel, but why not for other boats?

Jeff Janacek on Adirondack
at Hidden Harbor in Brunswick, GA
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Old 01-13-2015, 08:53 AM   #2
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Good idea. A few reasons why it hasn't happened:
It would have to be specific for your engine, trans and prop.
the ICW currents, with so many channels and outlets is too complex for the few current measuring devices out there.
If you can afford the boat, you shouldn't worry about fuel consumption (just joking!)
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Old 01-13-2015, 09:00 AM   #3
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You post an interesting question that I wrested with but in a different way.

Traveling north on the ICW last summer there are several published trouble spots that got my attention. Calculating a departure time to be at these trouble spots at high tide was easy but trying to figure out the currents to make that arrival time good was difficult. I would be traveling at my normal 7.5 kts then 15 minutes later I would see 9 kts then 20 minutes later, 6 knots.
I had the ability to go fast so I felt that I could adjust speed as necessary to make good my arrival time at high tide.

But what I found out very quickly is there are so many inlets and so many advancing and retarding currents that they all just average out. I soon learned that if I calculated a departure time to arrive at high tide based on a 7.5 kt cruise, discarding the currents, that calculation worked pretty darn good.

Of course this only works with many advancing and retarding currents over a relatively short distance, say less than 75 miles.
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Old 01-13-2015, 09:22 AM   #4
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Coastal Explorer has such a feature, or at least in part. It's not going to calculate your fuel usage, but it will show your route timing taking currents into account.

Now I'm speaking a bit out of turn here, because I haven't used it myself. I've just followed the discussion on their support forum where people (me among them) were asking for it, and recently it was made available. They often have features that are available on a preview basis and you have to select an option to turn it on, knowing that it's still a bit experimental. But this discussion is a good prompt to go try it since I'm about to venture into the land of heavy currents and channels in the PNW.
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Old 01-13-2015, 12:26 PM   #5
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My strategy is to try to plan for long stretches where current is a factor, and ignore the short stretches. Lots of ICW jumps from inlet to inlet and current swaps somewhere in between. But long stretches you might be in a current for 20miles.

A case in point: Going from Georgetown SC back to Wilmington- Left just before peak of flood. Caught the favorable flood tide up the Waccamaw. Somewhere behind Myrtle Beach, started riding the ebb toward Little River Inlet. Then many hours later, caught the Cape Fear River in flood, got a boost there too. Those are three long stretches that really made a difference. Between Little River and Southport, it swapped back and forth several times.

Most other attempts at timing currents were abject failures. Currents lag tides, and the lag varies with geographical location, astronomical effects and even weather. I try to time a particular passage and end up being an hour or two off in when the swap occurs.

And yes, bucking a tide it is better to push a little harder, riding a tide better to pull back on the power. Taking an example to an absurd extreme: Say the current is 6kt and you have s 6kt boat. You will go nowhere against it, and can make 6kt with it with engine off.

I am fortunate that I can run 20kts if faced with an opposing current, get that crap overwith!!!

I doubt there is an app that could incorporate all that is needed to predict currents very well. I use Coastal Explorer and there is a feature there, but have not really tested it much.
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Old 01-13-2015, 12:35 PM   #6
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With a six-knot boat often fighting two- and three-knot headcurrents, I enjoy finding counter-currents.


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Old 01-13-2015, 12:43 PM   #7
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The longer you travel the less it will matter. Your going to gain and loose equally over time. If you have a schedule then there are certain times that are better for travel than others. One week may favor northbound during daylight hours. Next week will favor southbound. Best to prepare for all contingencies and deal with the cards as they fall. As ski pointed out sometimes it better to just stand there rather than do something.


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Old 01-13-2015, 02:13 PM   #8
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What the OP wanted was a program, app or feature on a chart plotter that would tell him when it would be worthwhile (fuel burn wise) to speed up in an adverse current and slow down in a favorable current. Much of what has been said so far is about predicting currents along the way. The more difficult chore is to manage them appropriately and do something about them.

I don't think he will see such a program any time soon. Several reasons:

1. Most people don't care. They set their throttle and stick to it.

2. Different boats and engines have different fuel consumption curves.

3. As others have said there are many places along the ICW where you would be changing your speed every 15 minutes to optimize the current. In some places it changes every 10 miles as you approach or leave an inlet.

And finally you can do it yourself fairly easily- the brain is a much better processor of that kind of problem. Look at your speed through the water vs GPS speed over land. You have to first calibrate these closely. Every time you see about a knot difference, then speed up or slow down a little. If the current difference increases then increase or decrease the speed some more.

Do the calculations yourself to see how much to speed up or slow down and how much fuel you might save. After a while it will become intuitively automatic I suspect.

I have done this once or twice but never routinely.

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Old 01-13-2015, 03:25 PM   #9
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Twistedtree,

I worked with that feature in Coastal Explorer last summer and it seemed to be pretty good in predicting accurate arrival times at waypoints and the final destination. However it only dealt in predictions and not the actual tidal current encountered. It would tell you when to leave to have the least time in route to a location. If least time translates into less fuel used based on a strict GPH calculation it sort of does it. However the basic question of whether to speed up going into a tide or slowing down with a tide, is almost purely situational where we boat in southeast Alaska. If I am running with a tide, my speed is dictated by how far it is to the best place to be when the tide turns or do I really want to be out here when the tide turns and is going against the wind. I think the OP is asking a question that will take us into the realm of artificial intelligence. At a minimum it will be a great mathematics exercise for a high school student.

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Old 01-15-2015, 07:58 AM   #10
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On old piston aircraft a headwind would see a 10% POWER INCREASE , A TAIL WIND A 10% reduction for long distance over water.

Bumping up the rpm 200 or so and pulling back the same would be EZ on the ICW where day markers are like a picket fence so watching the current is possible.

A GPS might help in figuring the extra RPM.

But I think doing nothing but enjoying the scenery is probably worth the tea cup you might save in a seasonal transit.
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Old 01-15-2015, 08:14 AM   #11
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Tidal currents aren't "known" for every inch of the ICW and even if they were predicted for every inch, environmental conditions can vary them significantly enough to throw off any calculations based on predictions alone.

The same wind conditions are causing a slight surface current and possibly affecting your speed with windage (very noticeable on my boat)....further reducing computer program accuracy.

The best method I can think of to use is an accurate fuel burn gauge with GPS input to give instantaneous GPM.....figure your best and adjust throttle to keep it close....if you care that much to ultimately be guessing on a small percentage of fuel use.

Overall you may develop some averages for into or following currents, and after thousands of miles you may find your best fuel burn for a reasonable speed....but I will bet that most of us are in that range anyhow just from listening to others and "feeling" our own boats.
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Old 01-15-2015, 02:05 PM   #12
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Quote:
Originally Posted by jjanacek View Post



So why not have an application that would incorporate the tide tables,

current, time of day, date, speed and place to figure out the optimal time

to travel from A to B?



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Jeff,

from a pure mathematical point of view we need to know what your criteria for "optimum" is. And the answer to that question isn't simple e.g. if it's "fuel consumption to be a minimum" you should stop your engine if going with the tide - regardless how long it will take to your destination...

From a practical point of view: We avoid if ever possible to run against the tide (on the Elbe), it can be so frustrating if you are traveling with a full displacement hull...



best regards / med venlig hilsen
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Old 01-15-2015, 02:46 PM   #13
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I agree with Psneeld. You need a very accurate instant fuel burn flow meter, such as a FloScan, to complete the equation. Once you know your GPH fuel burn at various power settings, the rest is simple math that can be charted in a table for quick reference on the boat. If you have the electronics to take the FF input and compute NMPG based upon GPS SOG, you'd always have an instant digital readout. But we don't have that, so we can make a chart. You don't need an app for that.

Let's take a hypothetical boat that runs at 7.0 Kts at 1700 RPM. Here is a chart of our boat, the Canardly Stayafloat.



The engine RPM/Fuel Flow values are fixed values that do not change for our trusty boat. Set the throttle to 1700 RPM and you'll always burn 1.8 GPH. Our still water performance is represented with the green cells. 1500RPM/1.5 GPH gives us an even 6 kts at 4.0 NMPG, but we find that too slow. When we bump the power to 1700RPM/1.8 GPH, we get 7 Kts at 3.9 NMPG. Bump the power further to 1900 RPM and we get 8 Kts and 3.6 NMPG but that's too inefficient for our taste so we settle for a standard power in this boat of 1700 RPM.

Each of these chart NMPG values is simply the SOG in Kts divided by the GPH for that given engine RPM. As we cruise into the current, our speeds slow and our NMPG values shift to the left. As we cruise with the current, our SOG increases for any given engine RPM/FF pair and the NMPG readings shift to the right on the chart. The yellow and pink boxes represent a 2 kt current working against us (yellow) and pushing with us (pink).

If we leave the power fixed at 1700 RPM while cruising into varying currents, it's simple to see that our efficiency (NMPG) drops into the current and increases when moving with the current. Assuming a 2 kt current, we go from 7 Kts/3.9 NMPG in calm water to 5 Kts/2.8 NMPG in 2 Kts of water on the bow (yellow cell at 1700 RPM) and 9 Kts/5.0 NMPG with 2 kts of current pushing us along (Pink cell at 1700 RPM).

These are the numbers we have to beat to make all this power jockeying worthwhile.

So let's look at varying the power to maintain fixed speeds in varying currents. The numbers below were just plucked off the chart shown above and organized for clarity.



I can see that on this boat as I increase my engine RPM while travelling into the 2 Kt current, my efficiency drops from my 1700 RPM efficiency of 2.8 NMPG. So with this boat, you wouldn't be helping your efficiency by increasing power into the current, but, of course, you'll get there a little sooner.

When moving with the current, I can see incremental improvement in efficiency as I reduce the power. My NMPG increases to 6.0 if I want to go as slow as 6 kts at near idle power.

So with this Canardly Stayafloat, I'd be tempted to just leave the power at my normal cruise setting of 1700 RPM and accept the varying speeds. But every boat/engine/prop combination is going to have its own 'pocket' in which it likes to run. Of course, YMMV!
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Old 01-15-2015, 02:48 PM   #14
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Quote:
Originally Posted by psneeld View Post
Tidal currents aren't "known" for every inch of the ICW and even if they were predicted for every inch, environmental conditions can vary them significantly enough to throw off any calculations based on predictions alone.

The same wind conditions are causing a slight surface current and possibly affecting your speed with windage (very noticeable on my boat)....further reducing computer program accuracy.

The best method I can think of to use is an accurate fuel burn gauge with GPS input to give instantaneous GPM.....figure your best and adjust throttle to keep it close....if you care that much to ultimately be guessing on a small percentage of fuel use.

Overall you may develop some averages for into or following currents, and after thousands of miles you may find your best fuel burn for a reasonable speed....but I will bet that most of us are in that range anyhow just from listening to others and "feeling" our own boats.

You are David are spot on, but there is one more fly in the ointment.

Even if all the currents were known for every mile of the ICW, the currents vary greatly from inside and outside the curve of every curve. And then what makes it ever more interesting, is that while the current is fastest on the outside of the curve and therefore the water is deeper, each boat handles it differently depending on its draft and how it reacts to the bottom.
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Old 01-15-2015, 03:32 PM   #15
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Even if all the currents were known for every mile of the ICW, the currents vary greatly from inside and outside the curve of every curve. And then what makes it ever more interesting, is that while the current is fastest on the outside of the curve and therefore the water is deeper, each boat handles it differently depending on its draft and how it reacts to the bottom.
My NMPG goes to crap if my boat rubs it's bottom on the channel bottom!!
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Old 01-15-2015, 03:40 PM   #16
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You are David are spot on, but there is one more fly in the ointment.

Even if all the currents were known for every mile of the ICW, the currents vary greatly from inside and outside the curve of every curve. And then what makes it ever more interesting, is that while the current is fastest on the outside of the curve and therefore the water is deeper, each boat handles it differently depending on its draft and how it reacts to the bottom.
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...
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Old 01-15-2015, 07:14 PM   #17
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Actually, you can make it all work for you, some, if you are in no particular hurry to get to a particular place at a particular time. The simplest method is time your arrival at the inlet intersection for slack before the current that will push you up the other side. At the very least, the water will be calmer. (Almost by default, but surprisingly not always, you will have some push on the way too the inlet depending on configuration. Also ebbs tend to be stronger than floods). I used to make a game of it, just for sport in nothing else, though there are plenty of inlets, even at crossings, where you do not want to there at max ebb.
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Old 01-15-2015, 09:26 PM   #18
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I just built a mathematical model (read simple Excel spreadsheet) based on fuel consumption of my boat, and here is what I learned. 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.

So Jeff's hypothesis would seem to be true if you are going into a current that is more than 1/2 your SOG in calm water. Of course I may be wrong, but I generally try to avoid 3 knot tides anyway.

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Old 01-16-2015, 01:19 AM   #19
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Tom

I have found the currents in your area very difficult to predict. No matter how much I try to time things to gain time, I get flummoxed. Four years ago a group doing a predcited log cruise from Seattle found the same to true. I'll look you up this summer and get a tutorial.
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Old 01-16-2015, 01:26 AM   #20
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Sunchaser,

Do that. We'll be somewhere between the Canadian border and Cape Spencer. Shoot me an email when you get in the area. We haven't laid out our itinerary yet but plan to get started toward the end of April. The only thing definite is we will be back in Wrangell for the 4th of July. It's the best fireworks display in Alaska.

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