RPM's And Fuel Used Per Mile

After looking at the fuel usage figures I am backing off my throttle.

I normally cruise at 7 knots, 3.5 NMPG, 2 GPH. Had to kick it up a bit at the Alligator river bridge as the tender was holding it for us. 8 knots was 4+ GPH, <2 NMPG. Not worth the extra fuel for normal cruising.

Ted

We cruise at 2700 rpm and let the speed fall wherever it may due to currents and/or wind, with an average speed of somewhere around 7 knots. Over the course of a year it averages out to be a bit over 1.25 gallons per hour for the 100hp Yanmar 4JH2-UTE the previous owner put in.

Next year I'm thinking of dialing it back to 2600 rpm to knock down the bow wave a bit and to see how much of an effect it has on fuel consumption over the course of a year. Hoping it'll settle out to 1gph for relatively effortless fuel/distance/time calculations...

Even though we boat twelve months a year, fuel burn is our least expense. We spend far more on slip fees than we do on fuel burn.

Murray,
You're only burning a quart an hour more than we are. Probably a bit more efficient engine and less weight. Rpm isn't very meaningful unless you state your rated power rpm AND what rpm you get at WOT. We cruise ay 6.15 knots at 2300 but top out at 3000 .... rated rpm. I'm at 50% load. You're probably at about 30%. The 4JH series is a wonderful engine .. so I've heard many times.

Most engines and engine combinations follow a similar pattern, regardless of type boat. Almost all displacement and semi-displacement do.

Now, as a comparison, take most higher speed boats, 40 knots or so WOT and it's a much different pattern. For instance, MAN 800's and 1360's on these boats show very little difference from 1250 to 2000 rpm. They do show higher nmpg at 1000 and a big drop-off at 2300.

Most outboards I've looked at have what to diesel inboard owners appears to be a very strange curve. Often they hit a low point (low nmpg) around 2000 rpm and then improve up to 3000 rpm or so where they get their best mileage. Then they drop off gradually to about 4500 or 5000 rpm but still better than the low point around 2000 rpm. However, it's really much like a bell curve. Then above 5000 rpm they may or may not drop further and more rapidly.

By coincidence I just finished a 93 day cruise yesterday and looked at the figures.
Trip distance - 2035 nm
Fuel - 1532 USG
Economy - 1.33 nmpg
Engine hours - 263
Average speed overall - 7.7 kn
Whales hit - 0, but a very near miss 2 days ago!

ancora said:

Even though we boat twelve months a year, fuel burn is our least expense. We spend far more on slip fees than we do on fuel burn.

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This is very similar to our motor home. Fuel is our smallest expense, but it is frequently the first thing someone asks about.
At gas stations, there have been many times when folks will say "Wow...that's a beautiful rig!...but what do you get for mileage?" I frequently respond with "Oh...around 22-24mpg", just to watch their expressions...then explain that we get about 7-8mpg and are very content with that.

Rmp are about, speed and consumption smartcraft from cummins computer.
Boat Nordic Tug 37, cummins qsb 5.9 380hp, twindisc mg506sc1 2.41:1


19knots 20,0 gal/ hours 3050 rmp
15 knots 13,4 gal / hours 2550 rmp
9,0 knots 4,2 gal / hoursabout1700 rmp
8,5knots 2,9 gal/hours about 1500 rmp
8 knots 2,1 gal /hours about1400 rmp
7,5knots 1,4 gal/ hours about1300 rmp
7,0knots 1,23al/ hours about 1200 rmp
6,5knots 1,1 gal /hours

Everybody figures it differently ...

So I pay almost no attention to fuel burn numbers on the internet or via " dock talk.

I normally run at 2,000 rpms for about 7 knots. Fuel burn there is 0.7 gph. If I slow down to 5.5 knots (1550 rpms) I burn about 0.25 gph. Wide open throttle is 3,100 rpms. The boat will do 9 knots at WOT on 2.25 gph. I averaged about 0.4 gph this summer because my wife prefers the quieter ride at 1,550 rpms.

The boat is 32'5.5" LOA, 32' LWL, 9,500 pounds displacement. The engine is a Volvo-Penta D2-40 (40 hp). I carry 40 gallons of fuel so my 7 knot range is about 360 miles assuming a 10% reserve. At 5.5 knots my range is about 790 miles.

TDunn,
What is the displacement of that Volvo? My Mitsu is 37hp from 107 cu. in. ... 78mm X 92mm.

My boat's WOT is 2150rpm's and it's running at 30.1kts and burning 65gph. Ouch.


Our normal cocktail cruise speed is at 950rpm's at 9.5-10kts. At planing speed we're at 1750rpm's (80% of WOT) and running between 19-22kts.


I haven't actually calculated fuel burn at either cruise speed but at cocktail speed I've rough guessed at ~1nmpg. At planing speed it's a bit less than that.


I've been told and read that the most efficient speed is the boat's "hull speed". That number is entirely dependent on the length of the waterline on the hull. Here's the formula:


Hull Speed = 1.34 * [square root of LWL]


If your LWL is 36', the square root of that would be 6 and 6 times 1.34 would mean your hull speed is 8.04kts. My LWL is about 52' so my hull speed is 9.66kts.

The boat we're looping on:

RPM Knots Gal/Hr NM/Gal Range 90% Decibels
-900 10 9 1.11 925 62
1200 12 21 0.57 476 64
1400 15 33 0.45 378 66
1600 19 46 0.41 344 69
1800 24 64 0.38 312 70
1950 26 79 0.33 274 71
2150 29 99 0.29 244 74
2300 31 109 0.28 237 75
2400 32 117 0.27 228 76

Cruise is 27-28 knots. (70-80% load).

And best fuel economy is generally slightly below calculated hull speed. We could do ok at knots. Did ok coming down the Illinois and Mississippi, but we were so happy to be able to run at speed again.

Nomad Willy said:

Everybody figures it differently ...

So I pay almost no attention to fuel burn numbers on the internet or via " dock talk.

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I've found with both aircraft and boats pretty much everyone with an identical model claims to go faster than I do and burn less fuel.

Guess I have a knack for buying bad examples of products.

Nomad Willy said:

TDunn,
What is the displacement of that Volvo? My Mitsu is 37hp from 107 cu. in. ... 78mm X 92mm.

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It is 92 cu. in. and the bore/stroke is 77/81 in mm. Actual rated hp is 39.6 at 3200. Shaft hp is rated at 38 at 3,200.

The more car, truck and boat fuel I burn the happier I am. It means we're going somewhere and doing something. On our boat fuel burn is only relevant for range considerations and the ability to wait for low priced fuel to appear over the horizon.

All BS aside, our engines are quite happy at 1650 to 1800 RPM where fuel burn is about 4 - 5 gph combined. If I wanted bragging rights for low fuel use I'd own a 30' sailboat and motor along at 5.5 knots,

sunchaser said:

The more car, truck and boat fuel I burn the happier I am. It means we're going somewhere and doing something. On our boat fuel burn is only relevant for range considerations and the ability to wait for low priced fuel to appear over the horizon.

All BS aside, our engines are quite happy at 1650 to 1800 RPM where fuel burn is about 4 - 5 gph combined. If I wanted bragging rights for low fuel use I'd own a 30' sailboat and motor along at 5.5 knots,

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edit...

SC, you should of said "and only use the sails".

I friend with a fantastic 45' sailboat like to brag how little fuel they use a season.. but he wastes a ton of time sitting waiting for wind while we fire up the noisemaker and travel.. there is a reason we switched from sail.
HOLLYWOOD

GFC said:

...........I've been told and read that the most efficient speed is the boat's "hull speed". That number is entirely dependent on the length of the waterline on the hull. Here's the formula:


Hull Speed = 1.34 * [square root of LWL].

Click to expand...

I posted that once and got a bunch of crap about it. One thing for certain though, in a displacement or semi displacement boat, your fuel economy goes down very quickly above hull speed.

We cruise at 2K RPM which is 7 knots through the water. Hull speed calculates as 6.83 knots but 2K RPM is a convenient number.

We normally have a significant tidal current most of the time so speed over ground (SOG) varies quite a bit. At 2K RPM, I have seen as low as 3.8 knots and as high as 12.4 knots.

Any reference to MPG is a guess at best. You can calculate it for a trip or series of trips but you can't count on it to get from point A to point B because of the current.

We cruise mostly at 1250-1400 RPM, ~7 knots over water, or sometimes a bit more. Over the long haul (4,500nm this summer on the Inside Passage) we could count on an average of 3.4-3.5 nmpg or a bit more, despite lots of current one way or the other much of the time.

Coastal Explorer tracks miles traveled, and I divide by 3.4 to estimate fuel used. Gives me a pretty good idea how much fuel I have left in my 324 gal tanks. Correlates well with the tank watch, sight tubes, and how much it takes to re-fill.

This summer we were probably less efficient than we might have been due to worn out bottom paint, and a good bit of greenery we were hauling around. With fresh bottom paint, I hope to do a bit better next summer.

Richard

Now that you are racking up miles in the lap of luxury, what do you see as an annual summer fuel burn vs previous years at available higher speeds?

Hi Sunchaser,

Lap of luxury indeed!

Dream Catcher's fuel efficiency (3.5 nmpg at 7 knots) seems nearly the same as New Moon's (4 nmpg at 6 knots). We very rarely traveled on plane with New Moon the last many years.

OTOH, we easily do more miles in Dream Catcher, in a longer cruise, than we did most years in New Moon. I haven't yet added up the gallons used this summer, but I figure it's pretty close to 4500nm/3.5nmpg, or 1286 gallons. Last year New Moon used only 543 gallons, traveling 2100 nm.

I'm guessing that with a clean bottom DC will be as good as New Moon, even though traveling faster. Like that big, slow-turning prop!

What a strange new experience it is to leave the boat 900 miles from home.

My 11 year old son did a science fair project on theoretical hull speed. I actually learned something about my boat when he did. I always thought that 1100rpm was the best engine speed for efficiency but when we calculated it out that was not the case.

I can choose to travel at approximately 6.7kts. on average and get a range of 521nm or I can speed up to 8.9kts. and get the same range and be there quicker.

RPM FUEL/hr GEN.FUEL SPEED 1 SPEED 2 SPEED 3 AVG. SPEED NMPG TIME/100 NM TOTAL Fuel burn
1100 3.5 gal 1.0 gal 7.0 kts 6.3 kts 6.9 kts 6.7 kts 1.5 14.9 hr 67
1400 5.0 gal 1.0 gal 9.0 kts 8.8 kts 9.0 kts 8.9 kts 1.5 11.2 hr 67
1600 7.0 gal 1.0 gal 10.1 kts 10.0 kts 10.0 kts 10.0 kts 1.3 10.0 hr 80
1800 9.8 gal 1.0 gal 10.9 kts 10.7 kts 11.0 kts 10.9 kts 1.0 9.2 hr 99

k9medic - Is that a gas or diesel powered boat?
I have never seen a boat that length that can get near the efficiency at 9 knots.

GFC wrote;
"I've been told and read that the most efficient speed is the boat's "hull speed". That number is entirely dependent on the length of the waterline on the hull. Here's the formula"

Not so.

Hull speed (HS) is a relationship between wave lengths and hull lengths. It's a numerical thing. It's just a point on the speed scale whereas the length of the hull matches the length of the wave a boat is making. No magic at all.

And not particularly the place to be. FD boats are approximately best off one knot below HS. That's not even close to HS. Many SD boats are best off burn wise about 1/2 a knot below HS .. a bit faster than FD boats. Some SD boats that are closer to planing hulls may run best at hull speed. That's a guess and planing hulls will probably be best off at a higher speed. There's so many variables a bunch of boaters w different boats selecting one best speed to go is nonsense.

Speed Length Ratio is a much more meaningful thing to talk about.

smitty477 said:

k9medic - Is that a gas or diesel powered boat?
I have never seen a boat that length that can get near the efficiency at 9 knots.

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This is diesel. The fuel flow numbers came from the CAT manual which shows flow at a given RPM. The variable is what speed can be obtained. Having run this boat for the past 2.5 years it truly does have that type of efficiency.

From my son's research

"On October 9, 2016, we went to XXXXXXX, FL and I started the experiment by driving a dinghy alongside the Sea Ray boat to observe the bow wave. My Dad was driving the big boat. We drove a measured distance of one nautical mile on the boat, while the tide was going out and made several passes to obtain an average speed.
First, we ran the engines at 1100rpm. The speeds we recorded were 7.0kts, 6.3kts, and 6.9kts. The average speed was 6.7kts at 1100rpm. Next, we ran the engines at 1400 rpm. The speeds were 9.0kts, 8.8kts, and 9.0kts. The average speed was 8.9kts at 1400 rpm. We then ran the engines at 1600rpm. The speeds were 10.1kts, 10.0kts, 10.0kts and the average speed was 10.0kts at 1600 rpm. We increased the engine speed to 1800rpm. At this engine speed, the boat is really plowing through the water trying to get on plane. The speeds at 1800rpm were 10.9kts, 10.7kts, and 11.0kts with an average speed of 10.9kts. The measured distance we used for this was one nautical mile going down and then back up a channel that leads into XXXXXXXXX, FL."

From the Cat manual? How does that relate to the load on your boat?
Being propped 50 rpm different can make a very significant difference. Too many varibles.
Not apples/apples.

Nomad Willy said:

From the Cat manual? How does that relate to the load on your boat?
Being propped 50 rpm different can make a very significant difference. Too many varibles.
Not apples/apples.

Click to expand...

It is an apples to apples comparison provided you remember that the RPM is the constant as it's a fixed/constant rate fuel flow at that given RPM and the variable in this experiment is speed.

It does not matter to the engine what the speed of the boat is going. If the boat weighed 80,000lbs then it might only be able to achieve 3kts. at a given RPM.

If you started both engines and ran them at 1100rpm for an hour with no load you would have still burned 3.5 gallons. The CAT 3116's engines are in everything from boats to construction equipment. A CAT powered front end loader burns fuel at the same rate.

Last year when I went to the Abacos (GTC) I ran slow on the way over and at cruise speed on the way back. According to my log the trip over was at 1100rpm and I burned a total of 135 gallons of fuel. The trip over included an overnight stop at Great Sale Cay which accounts for the extra fuel burned but that comes out to 1.5 nmpg.

The return trip was at 2400rpm and I topped off in Stuart with 225 gallons. which is 0.7nmpg.


As another, simple example, I fly for a living. At a particular power setting it does not matter if my aircraft is at gross weight or nearly empty. I still burn the same amount of fuel.

K9,
With all due respect how do you expect to burn the same amount of fuel with varying loads?

I worked in a powerhouse at a mine w a 1500hp engine that turned 327rpm. When they turned on the high pressure water pumps on the dredge there was an instantaneous 30% (maybe even 40%) increase in load. Sounded like a car going into passing gear but it was all noise. The rpm hardly changed at all. A 90" X about 5" thick flywheel was to thank for the nearly constant rpm. The large increase in load accounts for the instantaneous and large increase in noise. The noise of course was almost directly proportional to the power produced. And the power produced is almost directly proportional to the fuel consumed. Generally speaking you get X amount of power for X amount of fuel. You will notice I didn't say anything about rpm. Rpm is putting an engine in a position to make power. Making power only requires that you add the appropriate amount of fuel and air.

The operative word in this discussion is load.

Great dialog... Here is the fuel burn chart from CAT.

http://oa.upm.es/14340/2/Documentacion/7_C_Maquinas/CatPropulsion_153kw.pdf