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Old 05-21-2018, 06:36 PM   #1
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Question GPH efficiency?

We were finally able to cruise our Chris Craft Commander twice this month. Our first trip was about 50nm round trip, and the other was around 70-75nm round trip. The first trip we faired pretty well there and back with a tank of gas, but the second trip we coasted into our home dock on fumes on the starboard tank.

After the second trip, my partner decided that we needed to sell it and look into something else a little later on. $500 in fuel this month was just too much to comprehend I guess. So I listed it, and if it sold we've been talking about going the trawler route. (I've always wanted a trawler anyway.)

However, after starting a few fuel efficiency conversations in various places and forums, the general consensus is that fuel efficiency ends up working out the same either way. Whether you're burning 2gph and taking a 12 hour trip, or you're burning 11gph and that same trip is only 2 hours.

So what I'd like to know is what kind of boat do you have, what engine(s) are in it, and what your fuel efficiency is at whatever your cruise speed is?

Thanks!
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Old 05-21-2018, 06:40 PM   #2
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The ratios don't work.
My boat--single engine-we cruise at about 7 kts at 2.5-3 gph.


You won't get a similar sized boat to go 21 kts at 7.5 to 9 gph
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Old 05-21-2018, 07:36 PM   #3
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Twin Lugger LP668T - 174hp.

Manufacturers figures:

1600 RPM ..................................... US gph (lph) 2.8 (10.5)
1800 RPM ..................................... US gph (lph) 3.7 (14.1)
2000 RPM ..................................... US gph (lph) 5.0 (19.0)
2200 RPM ..................................... US gph (lph) 6.6 (25)
2400 RPM ..................................... US gph (lph) 8.7 (33)

I get ~ 9 knots at 1850, ~7 at 1500. My numbers are close to that though not identical, in fact slightly better.
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Old 05-21-2018, 07:36 PM   #4
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we cruise out President 41 with twin SP225 Lehmans at 10 knots and burn about 7 to 8 gallons per hour. We could slow down to 8 knots and burn less but we like the little higher speed.
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Old 05-21-2018, 07:55 PM   #5
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SWIFT TRAWLER 44
For the most part - MPH = MPG

Sweet spot is between 17 and 20 MPH. Burn Rate 17 GPH and 20 GPH respectfully
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Old 05-21-2018, 08:01 PM   #6
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Agree with your numbers don't work. My boat at ~7 knots = 3 nautical MPG. Same boat powered with an engine big enough to plane around 12 knots gets maybe 1.5 NMPG. Bottom line going faster takes more horsepower which is exponentially more fuel per mile.
Monk 36
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Normal cruise 6.8 knots, 1.8 gph
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Old 05-21-2018, 08:12 PM   #7
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around 4-5 mpg on my Mainship 34 single diesel @ hull speed(6-7knots)..

Same boat can run 20 knots with ~300hp burning about 20gph..

4-5x more efficient at hull speed.
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Old 05-21-2018, 08:20 PM   #8
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My 31’ Mainship with twin 454’s gets roughly 1 MPG at 7 kts. If you were to run her at 20 kts you’d be looking at at fuel flow similar to a DC3.
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Old 05-21-2018, 08:31 PM   #9
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My 45' boat came with a 450 HP Cummins. It would do 14 knots at 21 GPH (1.5 gallons per mile). I re powered with a 135 HP John Deere. It will cruise 6 knots at 1.2 GPH (5 MPG), 7 knots at 2 GPH (3.5 MPG), 8 knots at 3.7 GPH (2.2 MPG). I cruise at 7 knots. Around 8 years ago, diesel spiked to $5 per gallon. Part of my rational for 7 knots 2GPH was that I can't control the cost and don't want the price, whether it's $2.50 or $10 a gallon, to prevent me from cruising.

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Old 05-21-2018, 08:35 PM   #10
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Quote:
Originally Posted by menzies View Post
Twin Lugger LP668T - 174hp.

Manufacturers figures:

1600 RPM ..................................... US gph (lph) 2.8 (10.5)
1800 RPM ..................................... US gph (lph) 3.7 (14.1)
2000 RPM ..................................... US gph (lph) 5.0 (19.0)
2200 RPM ..................................... US gph (lph) 6.6 (25)
2400 RPM ..................................... US gph (lph) 8.7 (33)

I get ~ 9 knots at 1850, ~7 at 1500. My numbers are close to that though not identical, in fact slightly better.
Per engine?
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Old 05-21-2018, 08:41 PM   #11
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I burn 3 litres (0.8 US gallons) per hour @ 1800 rpm averaging 6 knots
That works out to 0.13 gallons per mile in imperial measurements. or 7.5 mpg

If I want to save fuel I can raise the sails and reduce the rpm.

30 foot boat; full displacement hull, 7 tons
Vetus (Mitsubishi) 42 hp diesel
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Old 05-21-2018, 09:05 PM   #12
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Per engine?
Yes. Per engine.

Going slowly between the islands in the Abacos at 1500 I was seeing (rough guess) 1.6 - 1.7 MPG. No current and minimal wind effect. So each engine was burning ~ 2 GPH.

2GPH X 2 = 4GPH. ~ 7 knots = ~1.75MPG.

I don't have any fuel scanners nor am I going to install any. I just don't want to be concerned about a gallon here or a mile there. I want to enjoy the experience.
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Old 05-21-2018, 09:31 PM   #13
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Yes. Per engine.
Phew, had me worried there for a minute.

We worked our economy out by marking the sight glass/tube at 500 litre intervals during fill so not highly accurate but close enough.
A mate has a longer, beamier and heavier boat with the same engine AND he has floscan or similar fitted.
His numbers aren't far from ours at the same rpm.

We get around 2 US mpg doing 8. knots @ 1250rpm and a bit better again at 7. knots @ 1150 rpm

That's on 60ft and 75 US ton of boat with a single 14 litre Cummins nta855m 325hp no electronics engine.

Your numbers seem pretty close to ours.
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Old 05-21-2018, 09:38 PM   #14
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I've got a planing hull boat, so I can choose to go slow and save fuel or go fast and burn it up. It has a lot of miles on it and I keep careful fuel dipstick readings and the numbers are very consistent:

7.7kts at 950rpm, 1.9gph, 4nm/gal

19kts at 1900rpm, 11gph, 1.7nm/gal

Takes about 2.5 times the fuel to cover the same distance fast. On other boats I have set up or tested, that factor is pretty typical.

38' cold molded, about 14k lb wet, single Cummins 450C.
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Old 05-21-2018, 10:00 PM   #15
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Fuel economy

My Lien Hwa 47 with twin Lehmans 375s got 1 gal per nm at 10 knots total. My brother's Offshore 48 with twin Cat 3208s got 1 gal per nm at 10 -12 knots.
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Old 05-21-2018, 10:00 PM   #16
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The best fuel economy I have ever seen on a planning hull was 2 nmpg on a 24’ searay. Sailboats can easily average 10 nmpg with out sails. Sub 30’ displacement/semi displacement hulls often get 6 nmpg at 5-6 kts. From 40-50’ fuel consumption drops to 4 nmpg but speed is 6+ kts. Over 50’ fuel consumption drops to 2 nmpg and speed moves up to 7 kts.

This is an avg for avg boats. Some semi displacement hulls can’t make these numbers and some full displacement hulls are optimized to beat these numbers.

I find most people running at 10kts are burning 1 gpnm. I think this is a cost/time comfort zone. I run at 1800rpms with a pair of cat 3208’s at 10kts and burn 10.5 gph. Out in big swell & chop my economy has dropped to 1.5 gpnm. I can drop to 1200 rpms and get 2 nmpg but my trip will take almost twice as long and Cat does not recommend running the engines below 1500 rpms.
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Old 05-21-2018, 10:02 PM   #17
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Originally Posted by toocoys View Post

After the second trip, my partner decided that we needed to sell it and look into something else a little later on. $500 in fuel this month was just too much to comprehend I guess. So I listed it, and if it sold we've been talking about going the trawler route. (I've always wanted a trawler anyway.)

!
Don't even think about another boat with that partner unless you've done some serious budgeting together. If $500 in fuel is going to lead someone to sell a boat, I can't imagine what a serious expense will do. Forget fuel data until you resolve that.

Boating is Expensive or Very Expensive. Not a low cost endeavor.
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Old 05-21-2018, 10:09 PM   #18
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Quote:
Originally Posted by BandB View Post
Don't even think about another boat with that partner unless you've done some serious budgeting together. If $500 in fuel is going to lead someone to sell a boat, I can't imagine what a serious expense will do. Forget fuel data until you resolve that.

Boating is Expensive or Very Expensive. Not a low cost endeavor.
Well said
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Old 05-21-2018, 10:17 PM   #19
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Here's my numbers. 31'er with a single 200 diesel and displaces about 12,500 lbs (approx).

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Old 05-21-2018, 10:47 PM   #20
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Quote:
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Don't even think about another boat with that partner unless you've done some serious budgeting together. If $500 in fuel is going to lead someone to sell a boat, I can't imagine what a serious expense will do. Forget fuel data until you resolve that.

Boating is Expensive or Very Expensive. Not a low cost endeavor.
If it cost me $500 a month in fuel then I'd be making some changes, as well.

Boating does not have to be expensive.
Boating can be whatever budget you want it to be, as long as you are realistic about it. Just don't buy a mega-cruiser when you have a row boat budget.
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