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Old 10-28-2019, 05:40 AM   #1
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Calculating fuel needed

Say I want to go 3,000nm @ 7kt/hour @ 2 gal./hr. w/o refueling. I'm math challenged, but I figure 3,000/7 = 429 hrs. 2 gal/hr x 429 hrs = 858 gals. I would round this up to 1,000 gals. needed. Is this calculation correct? I suppose there are other factors to consider, so maybe 1,500 gals. would be preferable.
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Old 10-28-2019, 06:17 AM   #2
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Old 10-28-2019, 06:21 AM   #3
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This is one of the "other factors" I didn't consider.
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Old 10-28-2019, 06:26 AM   #4
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Say I want to go 3,000nm @ 7kt/hour @ 2 gal./hr. w/o refueling. I'm math challenged, but I figure 3,000/7 = 429 hrs. 2 gal/hr x 429 hrs = 858 gals. I would round this up to 1,000 gals. needed. Is this calculation correct? I suppose there are other factors to consider, so maybe 1,500 gals. would be preferable.
2 gal per hr is notba hard number as it varies under engine loading and speed varies also.

Sure the math is a rough number....but I would never travel 3000 miles in a boat with no refueling possible without some hard numbers squired over time.

Doing it cold....the nearly double number would be closer to my requirement.

The more accurate you can measure hourly rate and total consumed the faster you can believe some hard numbers.
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Old 10-28-2019, 06:58 AM   #5
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IF you can make that speed OVER THE GROUND subject to wind and current so the hours AND the gph are correct your math is good. 15% reserve is not a lot though. I'd be happier with 25-30%.

Furthermore you will need to do some "point of go-no go" calculations. IE you proceed to a point where either the numbers jive and you press on, or you return with the numbers as they are. This calculation will be updated hour by hour as you progress. In fact most missions will have several of these decision points.

In other words, don't paint yourself in a corner. The fact that you come here and reduce a complex question to an over simplified one is of some concern as to your experience level versus the level required to do such a mission.
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Old 10-28-2019, 10:35 AM   #6
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IF you can make that speed OVER THE GROUND subject to wind and current so the hours AND the gph are correct your math is good. 15% reserve is not a lot though. I'd be happier with 25-30%.

Furthermore you will need to do some "point of go-no go" calculations. IE you proceed to a point where either the numbers jive and you press on, or you return with the numbers as they are. This calculation will be updated hour by hour as you progress. In fact most missions will have several of these decision points.

In other words, don't paint yourself in a corner. The fact that you come here and reduce a complex question to an over simplified one is of some concern as to your experience level versus the level required to do such a mission.
+1 One of the first things I make sure to do with the boat is accurately measure fuel burn -vs- distance -vs- time to start amassing knowledge of how the boat (and I) perform over distance. There are two figures for fuel consumption, like Oscar and psneeld point out, the important one is not how many gallons per hour the engines CAN burn, but the final equation of current/wind/weather and your own use of the boat systems and how much the boat DOES burn.

I've measured my fuel burn at 6.5 knot speed settings at just a shade under 2 gph.

BUT. But I rarely see 6.5 kts at that throttle setting due to current. I try to plan so I see that or more than that and sometimes do, but also do not like to get up at 4 am to make the flood. Any cruise that has you running all day in the PNW where I live has you hitting at least one ebb and flood cycle so it will tend to even out at best.

For a trip, my equation of time/speed/distance/fuel burn would look something like:

3000 miles (+distance to jog in weather + distance spent chasing course + distance spent avoiding ships or obstacles + distance spent seeking shelter not in chart plan) / 7 kts (+ current in my favor - current against me + wind in my favor - wind against me + speed slow down for weather or obstacles) * 2 gph (+ any time spent at higher RPM due to anything + generator fuel burn + diesel heater fuel burn)

For my local inshore cruising the above seems to be the factors that influence me the most - albeit the wind influence tends to affect more WHEN I decide to move or whether I intentionally slow own or jog into slop rather than purely slowing me down pushing on the boat.

Our 2 week cruise this summer showed average speed made good (I didn't compute this - it was vacation) of around 5 1/2 kts, and our total average fuel burn was 2.41 gph with the accessories contributing. Quite a difference from the "manufacturers suggested"!

Depending on how I look at it, my 400 gal total fuel capacity could get me 1300 miles (2 gph @ 6.5 kts) or 900 miles (2.41 gph @ 5.5 kts) based on my real world factoring. leaving myself a 25% reserve (300 gal / 2.41 gph x 5.5 kts) eats that down to 684 miles, roughly half of what my initial thumbnail of 1300 miles would have been...

As we use NWD more, I will continue to track our fuel use and speed made good as a whole, so our trip planning will be more spot on.

(now, when the solar is in place and I can stop running the genset so much, how far is Ketchikan from Anacortes again?!
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Old 10-28-2019, 11:14 AM   #7
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Not to disagree with any of the above, but to point out that on an ocean crossing (presumably what is being considered for 3,000nm), fuel efficiency should be better than the average experienced during "normal" cruising because the engine should be running at its optimal speed continuously. The OP does need to figure in at least one stop for changing oil on this guesstimated 500 hour crossing. Also, whether there will be additional fuel for the generator, though there may not be much need to run it. If we are not talking about an ocean crossing, then give the tankage it should not be hard to fill-up en route.
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Old 10-28-2019, 02:15 PM   #8
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Go to Alaska instead of Hawaii, more fueling stations along the way.
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Old 10-28-2019, 03:29 PM   #9
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Quote:
Originally Posted by ddw36 View Post
Say I want to go 3,000nm @ 7kt/hour @ 2 gal./hr. w/o refueling. I'm math challenged, but I figure 3,000/7 = 429 hrs. 2 gal/hr x 429 hrs = 858 gals. I would round this up to 1,000 gals. needed. Is this calculation correct? I suppose there are other factors to consider, so maybe 1,500 gals. would be preferable.
Another consideration is having to turn back, reroute, and/or having reserve for unexpected issues or unfavorable conditions.

One third of your fuel going out, a third for coming home, and third for safety/reserve for what you don't expect is a general rule many espouse.
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Old 10-28-2019, 03:58 PM   #10
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Some issues to consider regarding fuel consumption:

1) There is a difference between Tank Capacity and Usable Capacity. The fuel system will start drawing air before the tank is actually empty.

2) Speed can be increased or decreased based on tide, current, wind. As a result times and fuel consumption are impacted.

3) Distance and travel time can be impacted by tide and current. A 20 kt wind on the beam will have you crabbing diagonally to your destination. So will large waves. It is very possible for the distance traveled over water to be greater than the distance measured on a chart or traveled over land, once drift is factored in.

4) Are measurement devices (capacity and consumption) accurate?
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Old 10-29-2019, 11:17 AM   #11
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Quote:
Originally Posted by ddw36 View Post
Say I want to go 3,000nm @ 7kt/hour @ 2 gal./hr. w/o refueling. I'm math challenged, but I figure 3,000/7 = 429 hrs. 2 gal/hr x 429 hrs = 858 gals. I would round this up to 1,000 gals. needed. Is this calculation correct? I suppose there are other factors to consider, so maybe 1,500 gals. would be preferable.
What boat carries 3000 g and gets 2 gph at 7 knots? None that I know of.
Any that can carry 3000 g will consume far more at 7 knots. Conversely, any boat that can achieve 2 gph at 7 knots will be smaller and lighter than any that can carry 3000 g.
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Old 10-29-2019, 05:25 PM   #12
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What boat carries 3000 g and gets 2 gph at 7 knots? None that I know of.
Any that can carry 3000 g will consume far more at 7 knots. Conversely, any boat that can achieve 2 gph at 7 knots will be smaller and lighter than any that can carry 3000 g.
I agree. However, I think the OP said 3,000nm. Nordhavn 40 and 41 carry around 920 Gal. and at 6 kts report a fuel consumption of around 2GPH. This won't allow for the 3,000nm range though.

The 3,000nm range is an aggressive one as is the 7 kt speed if you also have a low GPH rate in the mix.
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Old 10-29-2019, 06:21 PM   #13
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Many thanks to all w/ invaluable insights for a novice. Apparently, more goes into this than sheer numbers. I do think the 7kts speed is too high. 5kts/hr is probably more realistic & even 4. I am thinking of a San Francisco > Hawaii passage which is about 2300nm w/o a chance of refueling. Thus, the high amt. of tankage needed, and I would not mess w/ bladders.



I do know of a couple of steel hulled Diesel Duck 462s at different times made a west to east voyage from the Phillipines to the U.S., refueling in Hawaii & making it to the U.S. ok. So, the 3,000nm is based on that.


An earlier post suggested I could just punch in some numbers on a Navionics program & easily get the info I need. However, I still believe manual backups, i.e., paper charts, DR & sextant skills would be desirable. Hate to think my question was purely academic. Again, thanks to all for valuable input.
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Old 10-29-2019, 06:51 PM   #14
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Get a copy of “Voyaging Under Power” by Robert Beebe. He discusses this topic in depth.
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Old 10-29-2019, 06:56 PM   #15
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There are quite a few trawlers that have traveled from Hawaii to Seattle. This is not an impossible mission. However one can not assume that it takes the same fuel to go both directions. Since no boat type was mentioned I can’t comment on whether 7kts at 2gph is realistic or not. Others have mentioned that winds and currents have an effect. Wave and swell size also has an effect. One needs to have a good plan that starts with good consumption data and takes into account prevailing winds, current and weather. Now the big question is when do you make the go/no go decision. The 1/2 way point is to late.
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Old 10-29-2019, 07:01 PM   #16
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I believe you did the math correctly, but the result doesn't pass the smell test. 3.5 miles per gallon is too good to be true on any non-sailing boat that is going to cross an ocean.
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Old 10-29-2019, 09:18 PM   #17
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Isn't there a safety factor built into the speed ? For example, if you are halfway there and have used more fuel than expected, you could slow down to gain efficiency ?? Surely the miles per gallon at 5 knots is better than at 7 knots.
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Old 10-29-2019, 09:28 PM   #18
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I believe you did the math correctly, but the result doesn't pass the smell test. 3.5 miles per gallon is too good to be true on any non-sailing boat that is going to cross an ocean.
I agree! Wind fwd of the beam and seas will complicate those predictions
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Old 10-29-2019, 09:55 PM   #19
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Nordhavn 40 and 41 carry around 920 Gal. and at 6 kts report a fuel consumption of around 2GPH. This won't allow for the 3,000nm range though.

The 3,000nm range is an aggressive one as is the 7 kt speed if you also have a low GPH rate in the mix.
We burn 1.8 GPH @ 6 knots for a range of 3066NM. Two off-the-shelf bladders in the cockpit of 75 gallons each will bring our range to 3566NM.

Interestingly, Cape Flattery or San Diego are roughly 2300NM from Oahu. I am comfortable with that.
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Old 10-29-2019, 10:03 PM   #20
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Isn't there a safety factor built into the speed ? For example, if you are halfway there and have used more fuel than expected, you could slow down to gain efficiency ?? Surely the miles per gallon at 5 knots is better than at 7 knots.

It'll depend on the situation. If you're fighting a head current or a strong headwind, slowing down will often make things worse. Doing 7 kts against a 3 kt current means 4 kts made good. Slow to 5 kts and you're only making 2 kts now. So by slowing, you've cut your effective speed in half. So unless slowing that 2 kts cuts your fuel consumption to less than half, it's worse, not better. Depending on the fuel consumption vs speed curve, with a head current it can end up being more efficient to speed up.
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