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Old 09-11-2020, 07:27 AM   #1
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Trawler wanna-be: What are fuel burn rates

Hi, just starting our search for our next boat, and coastal cruising is our highest priority (not a marina condo!). We have seen numerous motor yachts and trawlers with big V-8 gas engines and various sizes of diesels, and were wondering, what are typical trawler speeds and fuel burn rates?

Would you folks share what boat you have, typical cruise speeds, and fuel use in mpg?

Thanks,
Allan
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Old 09-11-2020, 07:30 AM   #2
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6 - 8 kts, generally from 2 to 6 gph depending on boat. Anything over 2mpg is considered pretty good!
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Old 09-11-2020, 07:34 AM   #3
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My 45' boat isn't a true displacement hull. With a 135 HP engine, it burns 2 GPH at 7 knots ( 3.5 nautical miles per gallon). 8 knots moves the fuel burn to between 3.5 and 3.7 GPH, so not worth it, IMO.

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Old 09-11-2020, 07:48 AM   #4
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Note that the previous two responses are for diesels but also note that the OP also asked about gassers. A gasser will burn at least 33% more fuel and even more if a big engine is lightly loaded. Gassers do very poorly lightly loaded, burning as much as twice the fuel as a moderately loaded diesel.

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Old 09-11-2020, 08:45 AM   #5
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Quote:
Originally Posted by Awsmits View Post
Hi, just starting our search for our next boat, and coastal cruising is our highest priority (not a marina condo!). We have seen numerous motor yachts and trawlers with big V-8 gas engines and various sizes of diesels, and were wondering, what are typical trawler speeds and fuel burn rates?

Would you folks share what boat you have, typical cruise speeds, and fuel use in mpg?

Our not-a-trawler (see avatar) with twin 450C Cummins diesels... gave us about 7-9 kts at about 1000-1200 RPM at about 4 GPH total at about 1-2 NMPG... depending on actual speeds versus RPMs versus wind, tides, current, etc. Usually closer to about 1.2 NMPG average at those speeds, I'd guess.

Note "about" is all over that.

And of course all that changed -- usually to about 26-ish GPH -- whenever we'd get up on plane.

-Chris
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Old 09-11-2020, 08:58 AM   #6
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With big gassers like my boat, don't count on good mileage at low speed. As an example, I get between 0.5 - 0.6 nmpg on plane and in the 1 - 1.3 nmpg range at 6.5 kts (depending on wind / waves, etc.). Gassers lose a ton of efficiency at light load, so the fuel burn difference between fast and slow gets a lot smaller. Other than burning a bunch of fuel the gassers have no complaints about being run slowly though.

I ran the numbers for a pair of modern diesels in my boat and by my best estimates, I'd expect about 0.9 nmpg on plane and around 3 nmpg at 6.5 kts. Basically, the gassers burn 40 - 50% more on plane, but more than double what the diesels would at low speed / light load.

That said, if the gassers are cheaper enough to buy and maintain, it could take a long time to make up the cost difference in fuel savings with diesels. I figure for my boat it would take about 3000 hours of running (50/50 fast vs slow) to make up the cost of a diesel repower (and that's not accounting for any change in maintenance costs).
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Old 09-11-2020, 08:59 AM   #7
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Some credible numbers:

1. Willard 36 from San Diego to Hawaii. 330 gallons for 2300 nms. Around 0.9 gph, or 6.5-7.0 mpg. He had decent sail-plan so that may be optimistic. Perkins 4/236

2. Willard 36 from San Francisco to Ensenda. 500 nms. 6.6 kts and burned around 90-gallons or around 1.2 gph, 5.6 mpg. Also with a Perkins 4.236

3. 52-foot Horizon Power Cat (2014, so newer boat with Cummins' of around 500hp each). From the electronic fuel monitors aboard the Cummins' electronic controls (appears to be about 10% optimistic judging from refill-calculations). At 18-20-kts/65% load, she burns 38-40 gph total for both engines (pretty efficient for a boat this size at this speed). At Trawler Speed, she burns around 12-14 gph at 10-12 kts; drops to 7 gph at 8.5 kts.

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Old 09-11-2020, 09:43 AM   #8
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An example of my "real world" experiences with a Nordic Tug 37 (40 feet LOA) using a Cummins 6BTA 330 HP diesel engine. Semi displacement hull, capable of 15+ knots when wide open.

We operate the engine at between 1200-1400 RPM (wide open is 2800) cruising at between 6.5 and 8.0 knots (a good average I have used for calculating travel time to reach rapids etc. is 7 knots SOG). Using fuel refill numbers, tank fuel guages, and sight guages over the past 4 years (and approx. 500 hours), we have used almost exactly 2 gallons per hour. Due to the fact that I run my generator so little (about 25 hours per season) I have not factored that usage out of the above figures, so I would actually be a bit less than 2 gph for the engine (1.9-1.95 gph?).
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Old 09-11-2020, 11:39 AM   #9
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My experience in a 37 Nordic Tug is just like firehoser's, averaging 3.75 nmpg over about 16,000 nm.

Diesel advantage to me is not just cost, but also range. We easily go from Puget Sound to Ketchikan, some 700 nm, without refueling.
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Old 09-11-2020, 02:08 PM   #10
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My 64' 100,000 pound boat with twin 660hp Cummins engines averages a little better than 1 nautical mile per gallon (and that is with the genset running). I rarely use most of that horsepower, instead cruising between 8.5 and 10 knots. At 10 knots, I am a little less than 1 nmpg, and 8.5 in good conditions I am a little better than 1.33 nmpg, both of which are better economy than my buddy's 38' with twin 540 hp Cummins, primarily because at that speed his hull is comming on plane -- very inefficient.
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Old 09-11-2020, 02:14 PM   #11
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My 64' 100,000 pound boat with twin 660hp Cummins engines averages a little better than 1 nautical mile per gallon (and that is with the genset running). I rarely use most of that horsepower, instead cruising between 8.5 and 10 knots. At 10 knots, I am a little less than 1 nmpg, and 8.5 in good conditions I am a little better than 1.33 nmpg, both of which are better economy than my buddy's 38' with twin 540 hp Cummins, primarily because at that speed his hull is comming on plane -- very inefficient.

That's a very good point. Longer boats will go faster before efficiency falls off a cliff. Generally the cliff starts around a knot below hull speed. For my boat (38', 33.5' waterline), hull speed is 7.75 kts. Somewhere around 6.5 - 6.7 is the start of the efficiency cliff. By 7 kts, I'm already making significantly more wake and taking a good bit more power / fuel to push the boat. Slowing down to 6 kts is even better, but the difference from 6 to 6.5 is a lot smaller than 6.5 to 7. So I basically treat 6.5 kts as slow cruise (with the ability to push for 7 if needed). And if I need faster than that, it's time to get on plane and do 16+.
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Old 09-12-2020, 12:55 PM   #12
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Our CHB 41 trawler with twin Lehman 120s did about 2nmpg at 8kts over about 200 nautical miles this summer so far - about what I expected from researching before I bought.
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Old 09-12-2020, 01:37 PM   #13
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My American Tug 34 with a Cummins QSB 380hp.... 1000rpm equals about 6.5knt .... 1gph..... / 1200rpm about 7.4knts ... 1.4gph ..../ 1400rpm about 8knts --- 2.1gph. It is my opinion, the cliff is 1200rpm.

Of course this is under ideal conditions.
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Old 09-12-2020, 01:55 PM   #14
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Two more long distance runs where I was pretty confident of fuel burn.

2000 Willard 40 with JD4045T. Long Beach CA to La Paz, around 1000 nms. 7.2 kt average and under 1.5gph.

N57 from Dana Point to Ft Lauderdale. 4500 nms in 500 engine hours (9 kts) and burned 3000 gallons diesel for the trip (6 gph). I forget the engine model. Either a Lugger or JD straight 6 with about 325hp.

Peter
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Old 09-12-2020, 02:01 PM   #15
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Our 60ft 65 tonne (143,000 lb) trawler with a single 350 HP 855 Cummins consistently gets close enough to 2 litres/nm @ 1150rpm doing 7.5 knots so 15 litres/hour (3.5 gallons)
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Old 09-12-2020, 06:02 PM   #16
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0.75gph
7.2 nautical miles per hour
9 nautical miles per gallon

Benford 38 Fantail (avatar), displacement hull
55hp Mitsubishi
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Old 09-12-2020, 06:43 PM   #17
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38' with single Cummins 6cta 450hp. 7.7kt at 950rpm, 1.8gph. Right about 4nmpg.

Can also do 20kts at 1950rpm, 11gph. About 1.8nmpg.

I chose either of those two speeds. Depending on whether we are enjoying the ride or trying to get somewhere.
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Old 09-12-2020, 06:48 PM   #18
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36 Foot Albin, cruising speed 7 knots, about 4 mpg.

More important than fuel economy though is engine life. My Ford Lehman should run a minimum of 10,000 hours, maybe 15,000 before it needs anything.

A gasser is "tired" at 1,000 hours, ready for a rebuild at 1,500 and almost always junk by 2,000 hours.

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Old 09-12-2020, 07:20 PM   #19
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You're confusing recreational trawlers with boats that can readily exceed hull speed. Most so-called but mislabeled trawlers are motorboats that have overnight accommodations. Most all such motorboats have several times the horsepower of a trawler.
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Old 09-12-2020, 08:13 PM   #20
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Quote:
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A gasser is "tired" at 1,000 hours, ready for a rebuild at 1,500 and almost always junk by 2,000 hours.
Only if it's run very hard, or neglected. One of mine is darn close to 1600 hours and checks out very healthy. Compression is good, runs every bit as well as the other one that's newer (was killed due to external factors, not worn out). Hardly uses any oil and oil pressure at hot idle is still several times the minimum spec, so I'd say it's got lots more life left in it.
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