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Old 04-25-2013, 04:31 AM   #1
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Fuel use per hour

I am watching the current costs of trawlers and was wondering the diesel fuel usage per hour a cruising speed 8-9 knots on a 40-43 footer. What I have read is 2-3 gallon per hour all the way up to 9 gallons per hour. This has created confusion for me. Thank you for any knowledge you can bestow on me. Everett
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Old 04-25-2013, 06:09 AM   #2
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The confusion is 8-9K is way to fast for economical cruising in a 40 -43 ft boat.

Slow to 6 or perhaps 6.5K and yes under 2-3 gph is possible .

The problem is raising the speed 50%, 6K to 9K can easily 3X or more the fuel burn.

When folks say trawlers are efficient at SLOW speeds there not kidding.

For economy the SQ RT of the LWL is about the place to travel.

"Hull speed" is for sail boats in a blow with FREE energy.

A "trawler" can go that fast , but at an immense fuel cost.
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Old 04-25-2013, 10:28 AM   #3
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Lots of variables. I have twin diesel 6 cyl engines that have a displacement of about 337 cu in. Another trawler my size that is newer and a go fast might have 6 cyl 400 plus cu in turbo charged and 100 plus more horsepower. Put both of them at 8 kts and mine will be burning 2.25 gph each engine for over 2 kmpg and the other won't be 1.5 kmpg.

Hull design another variable. A Nordhavn is a true displacement hull and a Grand Banks a semi-displacement. Add stabilizers and more fuel burn. The displacement hull will only go hull speed and a semi-displacement can plane with enough power.
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Old 04-25-2013, 11:20 AM   #4
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And a lot of variation results from sloppy calculations. Just using hour meter numbers and engine time ignoring all the time spent at less or way less than cruising throttle.

Nine gph is probably MUCH closer to what you'd encounter but as FF says slow way down and enjoy much better economy.

Extremes:
A 32' GB w one Lehman at 6 knots = 2gph
A 42 GB w two Lehmans at 9 knots = 8gph or 9gph.

The above is just guesses but probably not far off.
At full speed a Lehman will burn 6gph at 2500rpm propped correctly.

So Everett you can see that not only will burn rates vary a lot opinions on burn rates will vary quite a bit too.
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Old 04-25-2013, 11:29 AM   #5
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My GB 42 burns 2.25 GPH each engine @ 1700 RPM which gives me on calm seas 9 kts ph according to my GPS. BUT, I have Dagenham Ford engines, not the Dorset which burn more.

I have a 5500 mile cruise to back up those numbers.

1700 rpm is 8.5 in mild seas and I have seen 7 in a head wind of over 65 mph.
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Old 04-25-2013, 11:58 AM   #6
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I have no flow meter installed but after years of operation I am quite confident in these numbers and they seem to agree with the engine manual (all taken in ideal sea condtitions & minimal wind) I have no generator and normally cruise @ 1300

RPM KNOTS GPH MPG S/L
1000 5.4 .75 7.2 .94
1100 5.6 .8 7.0 .98
1200 5.8 .95 6.1 1.0
1300 6.3 1.1 5.7 1.1
1400 6.5 1.4 4.6 1.14
1500 6.8 1.6 4.25 1.2
1600 7.1 2. 3.75 1.24
1700 7.3 2.25 3.25 1.28
1800 7.5 2.75 2.8 1.3
1900 7.7 3. 2.5 1.34
2000 7.9 3.5 2.25 1.38
 
 
 
 
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Old 04-25-2013, 12:18 PM   #7
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Incredible numbers.
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Old 04-25-2013, 12:24 PM   #8
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Not incredible at all Cap. That's what a full displacement hull does.

My 8 ton 30' Willard burns 1gph but I run at 2300rpm w a max of 3000. That's working it harder than most here on the forum. About 55% engine load.

Brooksie burns just about exactly the same as I do at a bit over 6 knots. His longer water line length makes up for his greater disp and wetted surface. If you want a heavy boat that has some economy FD is the way to go. Ever seen a planing barge?

I think a 40' willard actually requires less power than my W30 at 6 or 7 knots.
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Old 04-25-2013, 12:36 PM   #9
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I weigh 38,000 pounds max and probably 36,500 pounds realistically. I am probably high on my burn numbers. I get 2.5 nautical MPG at 8 to 9 kts and that's real good for a twin screw 42' boat.

My range is 1100 miles.

We had a very nice Willard in our marina many years ago and I haven't seen many since. Like I reported in another thread, I knew Willard Buchanon's grandson. They lived in San Pedro, Ca the next city from my marina. He made great boats.
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Old 04-25-2013, 01:27 PM   #10
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I weigh 38,000 pounds max and probably 36,500 pounds realistically. I am probably high on my burn numbers. I get 2.5 nautical MPG at 8 to 9 kts and that's real good for a twin screw 42' boat.

My range is 1100 miles.

We had a very nice Willard in our marina many years ago and I haven't seen many since. Like I reported in another thread, I knew Willard Buchanon's grandson. They lived in San Pedro, Ca the next city from my marina. He made great boats.
With only 50 SHP installed, I couldn't burn more than 3 GPH if I tried (50/17=2.94) and if I did have more HP and tried to press beyond hull speed, it would take much more power than a semi-planing trawler. This hull is a troller not a trawler. As you can see from the chart only 12-13 hp are required to drive it to S/L 1 or 1.1 even heavy as it is.
I always tell prospective boat owners: "first you must decide how much of a rush you are in"

Tell me about Willard Buchanon, I only know that the original Willard, the 36, was named Vega and designed by Wm. Garden. I never knew there was a Mr. Willard
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Old 04-25-2013, 01:29 PM   #11
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On my sea trial of Letitia (39' LWL) yesterday, I saw 7.1kts SOG (S/L 1.136) over 2 directions on a closed course (the ICW at high tide in Chesapeake VA). The digital tachs told me the Perkins 6.354s were turning between 1176 and 1194 rpms. The boatdiesel calculator tells me I'd need 38.8 SAE hp for a 32,000 pound displacement hull under those conditions, with a total burn of just under 2 gph (3.55+ mpg). Seems about right to me. Brooksies numbers are a shade better, but he is running a single engine, with a lot less tophamper.

I decided on glass this time around. If I still wanted wood, this is a prime example of making the best use of a gallon of fuel...

1962 Wiley PASSAGEMAKER TRAWLER Power Boat For Sale - www.yachtworld.com

Even though she's overpowered with 140 hp
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Old 04-25-2013, 01:49 PM   #12
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Yes indeed you can see her transom is almost entirely out of the water.
Classic full disp hull.

She is overpowered as you say. A JD 80 would be perfect.
Most boats are overpowered. Willard got a few things wrong but the 30 and 36 came w the right amount of power when new. The W30 had 36hp and was perfect.

Thanks for posting thurman.
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Old 04-25-2013, 02:16 PM   #13
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Actually 90% of the SQ RTx waterline is the max end of efficient cruising.
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Old 04-25-2013, 02:29 PM   #14
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Actually 90% of the SQ RTx waterline is the max end of efficient cruising.
That's what I've heard too
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Old 04-25-2013, 02:42 PM   #15
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Actually 90% of the SQ RTx waterline is the max end of efficient cruising.
The hull shape determines the magic ratio. A look at the mono hull America's Cup design or Volvo Ocean Series sail boats will find many that can easily exceed 1.34. The best tank testing I am aware of that can be found on the net is that done by Dashew where 90% of 1.34 is easily exceeded. Or look at Cats and Tris for their limitations if any.

The word "efficient" is all in the eye of the guy paying the bills. Some modern down east boats are marvelously efficient at around 20 knots, especially those done by Doug Zurn.
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Old 04-25-2013, 02:45 PM   #16
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Tell me about Willard Buchanan, I only know that the original Willard, the 36, was named Vega and designed by Wm. Garden. I never knew there was a Mr. Willard
All I know, which isn't much, is his name was Willard Buchanan and his grandson and I worked together for a short time. He told me he used to go to his grandfather's house and he would build boats in his backyard. He said he was making boats up to his death even after selling the boat company.

I think his company was in Costa Mesa, CA or close to that city and he helped design boats as well. I don't think he had formal training. Mike is the grandson and he was living in San Pedro, CA.

Here is a story about Willard.

http://www.heritech.com/seawitch/wilbo.htm

New Page 1

Wilmington Boat Works

I just learned a lot more about him myself.

I used to haul out at the Wilmington Boat Works. The owner recently was a Serbian named Dinko and he was notorious. He either liked you or he hated you. No inbetween. I sent a few people there that went on the hate list. They were launched quick and he wouldn't do work on their boats again.
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Old 04-25-2013, 02:47 PM   #17
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Sunchaser,
You know better We're talking about under power. Yes some hulls are fairly efficient at high speeds but they are more (mpg) at or very near the 90% rule.
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Old 04-25-2013, 02:51 PM   #18
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Originally Posted by everett.arndt2 View Post
I am watching the current costs of trawlers and was wondering the diesel fuel usage per hour a cruising speed 8-9 knots on a 40-43 footer.
Everett--- As a single point of reference our 30,000 pound GB36 with two 120hp diesels burns a total of just over 5 gallons per hour at 1650 rpm for a cruising speed of 8 knots. So a bit over 2.5 gph per engine.

But as others have said, fuel use figures are affected by a number of variables. For example the last batch of GB42s that were made in the 2000s have as stock engines two Cat diesels of 400-plus hp each. The fuel use figures in the magazine ads for these boats listed two figures, one for slow cruise and one for fast cruise.

The slow cruise figure was about 7 gph for a cruise speed of about 9 knots.

However the GB hull (the basic hull design that was used from 1964 to the late 2000s) is semi-planing and so with enough power can easliy sustain a cruise speed of 15 knots or more.

So that same late model, Cat powered GB42 that burns about 7 gph to go 9 knots has a fast cruise fuel burn of about 23 gph which moves the boat along at 14 or 15 knots. So a bit over three times the fuel consumption to go less than twice the speed.

While the hull of our boat is capable of cruise speeds in the lower teens we don't have the power to do it. If we follow through with our idea of repowering our boat with a pair of 200 to 250 hp engines, we should be able to have a sustained cruise speed of 12 knots or more. However, our fuel burn will be considerably higher than the 5+ gph we have now for our 8 knots cruise speed.

Everything in boating is a trade-off. Given the same type of boat, if you want to maximize fuel economy, you go slower. If you want more speed, you have to be willing to pay for a lot more fuel.
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Old 04-25-2013, 03:31 PM   #19
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44' Ocean Alexander, 39.5 lwl, 14' beam, semi-planing twin, 30,000 pounds....

1500 rpm 8.4kts 4.0 gph
1600 8.9 4.5 gph (normal slow cruise to keep turbos on boost)
1700 9.2 5.0
1800 9.5 5.6
1900 9.8 6.4
2000 10.6 7.2
2100 11.2 8.2
2200 11.8 9.2
2300 12.4 10.4 (Solidly "on plane" such as it is)
2400 13 11.6
2500 14 13.2
2600 15 14.8
2700 15.9 16.4
2700-3000 I don't go there

25-30% improvement in fuel burn @ slow cruise with one engine stopped/propeller free-wheeling.
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Old 04-25-2013, 04:02 PM   #20
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Originally Posted by skidgear View Post
44' Ocean Alexander, 39.5 lwl, 14' beam, semi-planing twin, 30,000 pounds....

1500 rpm 8.4kts 4.0 gph
1600 8.9 4.5 gph (normal slow cruise to keep turbos on boost)
1700 9.2 5.0
1800 9.5 5.6
1900 9.8 6.4
2000 10.6 7.2
2100 11.2 8.2
2200 11.8 9.2
2300 12.4 10.4 (Solidly "on plane" such as it is)
2400 13 11.6
2500 14 13.2
2600 15 14.8
2700 15.9 16.4
2700-3000 I don't go there

25-30% improvement in fuel burn @ slow cruise with one engine stopped/propeller free-wheeling.
What motors are you.running?
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