Fuel Burn and Range (How far can you go)

ak-guy said:

Here is the range estimate from the builder of my Allweather. 112 gallon tank.


5 knots ~1 qt./hr. NMPG=~20 ~2000NM+
6 knots ~2 qts./hr. NMPG=~10 ~1000NM+
7 knots ~3-4 qts./hr. NMPG=~5 ~500NM+

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Love those boats!!

Fuel burn and range - slow and easy

Sanderling holds 525 gallons of diesel. In over 30K miles cruising in this boat and anchoring out or at a mooring about 80% of the days underway, we burn 1.8 gallons per hour at 6.5 knots, or 7.3 MPH. That includes running the genset whenever necessary to recharge batteries or provide heat/cooling. We have a propane stove/oven and a 1100 A battery bank with a Magnum inverter/charger for any cooking/coffee/hair drying/etc needs. Theoretically all of this combined gives us a range of 1900 statute miles with a 10% fuel reserve (50 gallons). We typically take on around 250 gallons every 1,000 statute miles.

Wifey B: On the boat we're on today, 270 nm at 35-36 knots with 10% reserve on tank of 845 gallons.

Same range at 15 knots. At 11 knots or so we increase range, but never run that speed.

Cost of time

The discussion on speed vs. range was very interesting. I went a step further and calculated the cost of time. The calculation gives the answer to the question, "What would it cost me to save an hour on this trip?" For example, we usually cruise at 1300 rpm, which give about 7.4 knots. If I increase the rpm, I'll get there quicker, but will burn more fuel. Turns out at $3/gal. it will cost me (approximately, of course) $8.42 for every hour I save. I'm not spending that much per hour, but that's the incremental cost for every incremental hour I save. Is my time worth $8.42? That's the question to answer. As the speed goes up the cost also goes up. At 1400 rpm it's about $10 an hour. At 2500 rpm it's about $30 an hour. Just to put it in perspective, the sailboat I sail sometimes gives about $5 an hour at 7 knots. My airplane figured out to $200 an hour at 165 knots. A car at 70mph calculates out at something like $20 an hour, depending on the mpg. Make sense?

I have never really bothered to measure it exactly, but my guesstimate on my twins is:

If I nursed it (1200 rpm) I burn 3gph and run ~ 5-6knots. around 1800-1900km, say 1800 with buffer.

At a comfortable cruise (1800 rpm) I burn 7gph giving me 9.5 knots. so 1300, say 1200 with buffer.

We left Jax with ~875 gallons in March, topped up with 150 gallons in Vero before crossing from Fort Pierce, and returned from six weeks moving around the Abacos without needing another fuel stop and with 184 gallons left in the tanks.

I have twins. And all this assumes current for and against is a wash.

Willard vega 36 sedan

If I run her at 1500 making 8 knts with 500 gallons I get about 2200 nm,could pull her back to 6knts and probably get close to 2800,runnin a 453n

500gal, 6.5kn = ~1500 miles

As an ex-delivery skipper, if I'd listened to owners estimates on fuel burn, I'd still be floating out of fuel 75 miles offshore.

Very, very few owners use their boats in a manner to accurately estimate fuel consumption, and very, very few have adequate instruments to measure. Especially against sea state.

Estimates in threads like these are well meaning. But are no better than "one that got away" fish stories.

Peter

In calm water, I figure the following for planning range. 165 nautical miles on plane at 17-18 kts or 350 at 6.5 kts. Against some seas I figure 140-150 on plane and closer to 300 at low speed. Numbers are all based on burning 300 of my 420 gallons of fuel, so 25 percent as reserve plus a little to account for a few gallons of unusable fuel in the tanks.

In five summers and about 17,000 nm of cruising mostly at 7-7.5 knots, we averaged about 3.7 nmpg, without trying to correct for some fuel being used by the genset. With 320 gallons of diesel tankage, our range with a 20% reserve would be about 950 nm. Well more than enough to go from Puget Sound to Ketchikan, which we have found convenient to do without fuel stops.

mvweebles said:

As an ex-delivery skipper, if I'd listened to owners estimates on fuel burn, I'd still be floating out of fuel 75 miles offshore.

Very, very few owners use their boats in a manner to accurately estimate fuel consumption, and very, very few have adequate instruments to measure. Especially against sea state.

Estimates in threads like these are well meaning. But are no better than "one that got away" fish stories.

Peter

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Peter, if the boat has the newer gen fuel monitoring systems, determining fuel burn and then comparing SOG against different sea states is fairly straight forward IMO.

Fletcher500 said:

Peter, if the boat has the newer gen fuel monitoring systems, determining fuel burn and then comparing SOG against different sea states is fairly straight forward IMO.

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Wifey B: But he's right about owners. I know other delivery captains who run into that all the time. If it's not monitored and recorded at various speeds then useless in that business. I've seen owners say their boat could make it but Captain knew otherwise and took 6 drums along and used them all.

RCook said:

In five summers and about 17,000 nm of cruising mostly at 7-7.5 knots, we averaged about 3.7 nmpg, without trying to correct for some fuel being used by the genset. With 320 gallons of diesel tankage, our range with a 20% reserve would be about 950 nm. Well more than enough to go from Puget Sound to Ketchikan, which we have found convenient to do without fuel stops.

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My experience would reflect almost exactly what Richard quoted (based on 4 cruising seasons). However, I have not gone from Puget Sound to Ketchikan without refuelling. 950 nm range with 20% reserve.

BandB said:

Wifey B: But he's right about owners. I know other delivery captains who run into that all the time. If it's not monitored and recorded at various speeds then useless in that business. I've seen owners say their boat could make it but Captain knew otherwise and took 6 drums along and used them all.

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A lot of us definitely don't have great data. I have pretty good numbers for fuel burn on plane, but not so great at low speed. So I stick to conservative numbers for the low speed burn.

For range, I rely on an accurate fuel flow maretron instrument to measure usage, which feeds data to the Garmins for dynamic range calculations. I compare SOG to my speed through the water to keep an eye on the effects of current. 600 gallons of diesel...

Spinner said:

For range, I rely on an accurate fuel flow maretron instrument to measure usage, which feeds data to the Garmins for dynamic range calculations. I compare SOG to my speed through the water to keep an eye on the effects of current. 600 gallons of diesel...

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Sue, have you ever ran a full tank of fuel to prove your equipment is telling the Truth?

I make a habit of filling my tanks to the same point each time and computing the actual fuel used against my hour meters and my RPM log. Over the last 5 years I have been able to develop a data base that accurately predicts my range based on rpm setting.

Hi Dave, I fill up the tanks to full and then double check my fuel taken on against the numbers recorded by my instruments. It’s pretty accurate as far as I can tell. I wish I had a separate way to measure fuel burned by the generator and the furnace. Having that data would enable me to fine tune my projected range. That fuel line (it’s the same one) comes off before the fuel flow sensors.... I just didn’t want to put in 3 more of them, and figure out how to get the data to display

Fletcher500 said:

Peter, if the boat has the newer gen fuel monitoring systems, determining fuel burn and then comparing SOG against different sea states is fairly straight forward IMO.

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Understood - and I've been on several boats newer boats now with full engine instrumentation. The ones I've been on (most recently, 2014 Cummins') fuel burn numbers still don't track on refill - the electronic totalizer calculation is always optimistic, and often by a lot. I suspect these controls are less accurate at low flow volumes. And there is thr difficulty of filling large, often awkwardly placed tanks with crossovers that sometimes vary by up to 10% of full capacity. Plus most owners do day hops with in/out in protected water, then extrapolate this to some sort of range with implication they could get to Hawaii. Not with me aboard.

I have no quarrel with folks using their gph metrics, though those too are often wildly optimistic - I have a hunch that a poll of FL120 owners self-reporting fuel burn would likely vary by +/-40%. Extrapolating these to range is deceiving as it implies distance potential. It's a gold-painted brick buried in the backyard. It feels good but has minimal real value.

Electronic controls are a good guess, but to my thinking, the only reliable data is packing the tanks, doing a very long run, then repacking the tanks. Calculate the gph from there. Even that I wonder about - in 1987 a W36 went from San Diego to Hawaii and reportedly burned 335 gals of diesel. It's a credible report as that particular model carried 300 gals and owner states in a letter to Willard Marine that he carried 3 bbls on deck. But that works out to 0.9 gph at 6-kts. That's low for a Perkins 4.236 so either he had a favorable current (likely did the run in sepr/oct, known period of good weather), or had a push from his sail plan. A better question is how did the boat return to the west coast, where she currently cruises. still a mystery to me, but I can almost guarantee, it wasn't at 0.9 gph.

Peter

"I've seen owners say their boat could make it but Captain knew otherwise and took 6 drums along and used them all."

Owners seldom operate at the throttle settings of a delivery crew.

Range in table is calculated using a 10% reserve. In reality, I would probably never plan a trip with less than 15% reserve, though I've successfully run it down below 10% before...but in calm conditions and not far from fuel docks.

Numbers are based on cruising in diesel-only mode, calm weather and calm seas, interceptors on at "higher" speeds (i.e. above ~7 knots). 2 people and 1 cat on board. Loaded with gear for a 2 month trip. Half-full fuel tanks and half-full water tanks. Also, the numbers include light charging to the Hybrid battery to power the inverter, which was providing 220v for computers, 12v battery charger & refrigerator.

Depending on weather and conditions, electric mode would provide an additional 15-30 NM at 3.0-4.5 kt speeds. In perfectly calm, sunny weather, we can go 2.2 knots (I know, yawn...) on solar alone for most of the day (days can be looonnnggg during Scandinavian summers). It's nice to know this is a possibility for, perhaps, a get-home situation if the diesel fails. We don't cruise this way, however. Would fall asleep...

We generally travel at 7.0 knots. So, range at that speed is a whisker under 500nm

fill fuel tanks to the top.
Run 200 NM with gens running.
refill fuel tanks, and add 10% to actual fuel burn.
then run your numbers.

My boat burns about 1gph at 6-6.5 knots, or a little less if I have some decent wind. My tank is only 225 gallons however, so with a 10% reserved that’s about 1300km range I guess.

Generator burns about .5 gallons however, and I like to run it underway if it’s hot, so maybe more like 850-900km range.

It has a little 4cy 110 HP yanmar 4JH4-HTE engine.

Simi 60 said:

In theory 2 x 3500 litres (1850 gal) should give 3700 NM range @ a bit over 8 knots at 1250 rpm burning under 4 gph.
We usually do a bit over 7 knots @ 1150 rpm on passage so range should increase marginally.

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A few years on and a few measured* 2000 litre /528 gallon runs across varied conditions and we have decided 15 lph covers us.
That includes the times we are pushing weather running at 1300, cruising at 1150 and running genset.
Reality is its a bit less than that

Good for 3500nm

* We have sight tubes, electrical tie at current fill, add 1000/side and slide second electrical tie to that spot.
Use boat, note results, the end.

8500litres @ 15lph (~4 USgal) @8kn is about 4700nm.

That's a bit faster and louder than I like so 6kn Is over 10,000nm. I'm never in a hurry.

About 10% of that is generators that are getting the flick in favour of solar.

Not too bad for a big 'ol girl.

Theoretically... at 7 knots, burning 1.6 gph, with no reserve... I can go 1,750 miles.

I have no intention of ever testing that.