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Old 17-07-2017, 07:23   #1
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Fuel consumption - real world numbers

I did a little number crunching this morning on fuel consumption and costs and was so stunned by the results that I decided to share.

We have a 42' 1986 Chris Craft aft cabin. She weighs in at 36,000 lbs and has been repowered with 330 hp Cummins 6BTA diesels.

In May, we took her from near Tampa, FL to the Florida Keys for a two week trip. During that trip, there were stretches near bridges, marinas, and no-wake zones where we were at minimum wake speeds but most of the time we ran her at cruising speed of 2300 rpm, which works out to about 19 mph on plane. The trip was 501.6 miles long. Some of it was Intercoastal waterway and some was open Gulf waters. We used 693 gallons of fuel for a cost of 1925.89. This worked out to 14.6 gallons per hour or about .7 miles per gallon. Overall, we were pretty pleased.

Then we got home and started looking at the log book entries. By the time we motored in and out of anchorages or marinas, passed through no wake zones, etc, we were averaging about 9.9 mph for the whole trip. Really???

After discussing with a couple of friends, we decided to do a test. We filled her up, did 86 miles in two day trips, and then filled her up again. During the day trips, we ran the engines at "cruising" speed of 2300 rpm for about five minutes to blow out the turbos but otherwise kept the speed at about 1400 rpm, which yielded 10 mph with some trim tabs engaged. When we refilled, we took in 43 gallons. This worked out to 4.9 gallons per hour or 1.58 mpg! The two day cruises were less noisy and almost... serene. We still got to our destinations without a significant delay and still had a great time.

So, we took the slower burn rate and recalculated the recent Keys trip. If we had used the same approach as the two test trips, we would have used well under half the fuel and would have saved nearly $1100!!! We were pretty shocked.

Your mileage may vary.

Still Learning John
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Old 17-07-2017, 07:39   #2
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Re: Fuel consumption - real world numbers

Are you talking statute or nautical miles and statute miles per hour or knots?

Most of us think in terms of Nm and knots when working out fuel consumption

But yes, running at displacement speed (8-9 knots) will only use a fraction of the fuel you use pushing at twice that speed.
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Old 17-07-2017, 08:01   #3
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Re: Fuel consumption - real world numbers

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Originally Posted by StuM View Post
Are you talking statute or nautical miles and statute miles per hour or knots?

Most of us think in terms of Nm and knots when working out fuel consumption

But yes, running at displacement speed (8-9 knots) will only use a fraction of the fuel you use pushing at twice that speed.
I can think and discuss in nautical and knots or statute miles but prefer to use statute miles.

I've seen lots of theoretical burn rate articles and discussions. The real world, hard numbers were fascinating to me. Just wanted to share.
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Old 17-07-2017, 08:16   #4
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Re: Fuel consumption - real world numbers

Yes that's interesting. The difference not too surprising though I dont think. a 19 mph plane is pushing a lot of water/wake I imagine? Planing vs trawler speed. ut it does sound you are doing pretty good MPG in the scheme of things.
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Old 17-07-2017, 08:25   #5
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Re: Fuel consumption - real world numbers

The same for us on a lower consuption rate. If I cruise at 7 knots or 2600-2700rpm our Yanmar 100hp burns around 3gph. If I take my time and go 5 knots at 1800, it burns about 1gph. I do revv the engine once an hour five times to plow out the soot.

It's a big difference, especially when talking about a long run.
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Old 17-07-2017, 08:49   #6
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Re: Fuel consumption - real world numbers

Its nice to know that you can get there in a hurry if you want to, but there is another mode which is way more economical. We did the Great Loop in a GB 42 at 1400 rpm and had no problems with rings/turbos--averaged about 2 mpg with genset included.
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Old 17-07-2017, 09:05   #7
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Re: Fuel consumption - real world numbers

This is not radically different from sailboats that max out at hull speeds of 7-8 knots. The last 1.5-2 knots are fantastically expensive compared to the first five or six. At five knots in flat water, I can motor a 16 ton sailboat (steel hull) with a 60 hp naturally aspirated 2433 cc diesel on about .75 gal/hour. A fraction of your burn, but it's also a sailboat!

Most diesel makes have a fuel map where you can find the sweet spot for fuel economy. How that translates to speed is a function of drag, prop pitch and sea state.
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Old 17-07-2017, 09:27   #8
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Re: Fuel consumption - real world numbers

The higher the RPM, the more horsepower the engines produce, AND, the more fuel they use. There is no free lunch, power costs you fuel. That is just the engine.

As others have mentioned, the drag increases with speed, displacement is preferable to being on a plane... you therefore also pay additionally for speed.

If you were motoring at 3-5 knots with one engine, you likely would also save on fuel mileage. Your choice of conditions.
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Old 17-07-2017, 09:45   #9
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Re: Fuel consumption - real world numbers

We also sail, which keeps the fuel bills mighty low.

Two, 2 week sailing vacations in the BVI..,,,

We used 7 to 7.2 gallons of fuel for each of the two weeks. Most of that charging up the batteries every day ( 1400 rpm). We motored a few times for short passages ( 2300 rpm). Fuel guage was still on Full at the end of both bareboat charters. We topped off the fuel tanks before returning the boat to Conch Charters.

All that being said.....we have been doing this sailing for many, many decades, our bodies are getting old , well sort of, and feel we still have a couple of years of sailing left in us...I am 77. However, we are both very fit, and stay that way, but the joints are creaking a bit. They get louder when I am wearing my hearing aids.

But, I am also interested in fuel bills ( general ides ) for , oh maybe 35 foot motor vessels, that are comfortable for us entering geezerdom. Twin screw, and able to cruise 15 knots or so.

I really like the sleek looking super fasts, like the sea rays, but those are totally out of our range.



For distance covered, a power boat from newport to catalina ( 26 miles ) would be about one or two beer miles......a sailing vessel is a case of beer and quart of rum. Actually, we save our partying until we are on a mooring, anchored or in a slip. But, that is a pretty good analogy.

Keep the fun light illuminated
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Old 17-07-2017, 10:36   #10
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Re: Fuel consumption - real world numbers

What is really astonishing is when you crunch the numbers on range.
I have the numbers on the boat on our last trip where I ran reduced power motorsailing a lot, and the numbers are astonishing to me.
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Old 17-07-2017, 10:59   #11
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Re: Fuel consumption - real world numbers

Quote:
Originally Posted by JohnEasley View Post

Still Learning John
Hi John,

Great you like numbers because it's in the physics and math.

Us sailing types know the physics of Hull Speed. It's the same maths as with your motor boat.

Hull speed = square root of LWL x 1.34

So work out the hull speed of your boat.
If you are BELOW hull speed you use little fuel. Above it (before you are on the plane) you use logarithmicly MORE fuel.
When you are on the plane you use less, but still more than below hull speed.

So, work out the hull speed and keep below that. When you want to go fast get up on the plane quickly and ease back till you just maintain the plane.

Do not stay in the speed range between hull and planing speed.

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I'd tgat doesn't help just tell the boss to give you more $$$
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Old 17-07-2017, 11:35   #12
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Re: Fuel consumption - real world numbers

Quote:
Originally Posted by MarkJ View Post
Hull speed = square root of LWL x 1.34
More numbers to play with!
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Old 17-07-2017, 12:29   #13
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Re: Fuel consumption - real world numbers

Well, I've "Looped" several times in trawler type vessels and found myself passed once or twice daily by the " Go Fasters". Usually would end up in same area at late afternoon. They would brag about being there 2-3 hours earlier then me. At some point I would approach them with the subject of fuel usage, and they would be amazed at my much lesser burn rate. I would generally travel at 7-8 knts and they would run between 15-20 knts. My savings was $1000's of dollars difference by running in a sensible manner.

it's about the adventure, not the trip!
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Old 17-07-2017, 14:27   #14
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Re: Fuel consumption - real world numbers

If you really want to find the "sweet spot", install fuel flow meters. I have always done the poor mans way by installing the less expensive meters on the inlet and return line, then just doing the math myself on the difference for the burn rate. No mater what theoretical speed/rpm that supposed to run, seems each boat has it's own sweet spot of harmonics and burn rate that when you do the math on the water (daily log), it really helps to keep the temptation to throw the throttles forward the next day.
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Old 17-07-2017, 14:41   #15
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Fuel consumption - real world numbers

I have installed fuel flow meters on a 45' Sportfisherman years ago, with a Diesel engine you need two per engine, you need to subtract return fuel .
I expected that our most fuel efficient speed would be that speed that she could just barely maintain plane.
Boy was I wrong, most economical speed was one engine idling, the fuel consumption rose from there, went nearly vertical in the area of climbing onto plane, but there was not a big drop on achieving plane, and from there it continued to climb, at no time was there ever an RPM that showed a drop in consumption when graphed over speed except right where she was climbing onto plane, again that surprised me.
I expected a sweet spot, one RPM range that was best for fuel consumption, but there was none.
I did this both on a 36' Sportfisher with twin big block gas motors and the Diesel powered 45' boat, almost exactly the same graph.
Off plane and going slow speed is exceedingly annoying, you know you can go much faster, so you want to, plus if there is any sea way you can be rolling your guts out and she doesn't steer very well, go fast boats have tiny rudders compared to say a sailboat
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