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Old 03-23-2023, 05:07 PM   #1
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Big HP vs Small HP

So I have done a lot of reading on the trawler fuel use verse HP question. The articles have said that by far the biggest difference in fuel use/economy is speed. They say that if you run at displacement speed or less that the size of the engine doesn't matter all that much. So if motoring along at 7 kn on dual 135 HP engines you will use about the same at if you motored along at 7 kn on dual 350 HP engines.

Now part of me says that makes perfect sense as the required power is the required power and all that changes is the efficient operating point of the engines.

But the other part of me says ............BS bigger engines use more fuel PERIOD!

So what have the kind boaters here on TF with trawlers with big engines have to say on the matter?
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Old 03-23-2023, 05:11 PM   #2
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My Bayliner 4788 has twin 350 Cummins diesels and gets 1.75NMPG average at 7.5 knots.

Lets hear what others with similar size boats get, and then we will know.
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Old 03-23-2023, 05:22 PM   #3
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What affects how much fuel two different engines burn at the same hp is the displacement and to a lessor extent, modern CR injection vs mechanical injection.

A low hp moderate displacement diesel like the Lehman 120 will burn almost the same fuel as a Yanmar 370 at 50 hp for both because both have the similar displacement.

The Yanmar can make 3 times the hp at wot as the Lehman because of its high output turbocharger but at 50 hp, the Yanmar’s turbo charger isn’t making any boost. It might as well not be there so it acts the same as the Lehman.

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Old 03-23-2023, 05:28 PM   #4
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Modern diesels are pretty consistent in specific fuel consumption over a wide range. X amount of HP requires X / constant for fuel. The constant is around 20 hp/gal/hr. There is some fixed overhead just from getting the engine to turn, but it is small at any reasonable cruising speed, even a slow cruising speed. So the practical difference between a big and small or one or two engines is not great. A bigger consideration is the extra maintenance in two, and the perceived extra reliability of two.
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Old 03-23-2023, 05:31 PM   #5
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My Cherubini gets 3.5 MPG (nautical miles per gallon at 7 knots with a single John Deere 4045 132 HP engine.

With new electronically controlled diesels, many have fairly accurate fuel consumption displays built in. My warmed up out of gear idle with the house bank fully charged is .3 to .4 GPH. When I bought the boat it had a Cummins 6CTA 450 HP engine. Fuel consumption was under 2 MPG. Some of the reasons included a mechanical injected engine that when running 7 knots was way out of its efficiency zone (peak torque area). It was also twice the liter displacement, requiring more fuel just to idle.

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Old 03-23-2023, 05:57 PM   #6
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43 ft waterline; 7Kn gets about 2.5 plus NMPG. QSB/380HP. I am in the open ocean mostly, not protected waters, therefore constant swell and current so numbers can vary.

I spec’d a 380HP Cummins for the build. Figured it would be nice to get the extra speed on occasion but I rarely use it and often shoot for 7.5Kn where the boat is happiest. However, running down swell and increasing power to match up better with swell speeds at 8 to 9 Knots is a bonus and makes for a more comfortable ride. As well as throttling up for a few minutes to dodge a ferry or whatever.

In retrospect, the base 250HP engine that comes with the Helmsman 43 would also have been fine.

You appear to have some strong opinions on subjects based on your previous posts, which is fine. Sometimes there are no right or wrong answers, just different use cases for everyone and hence different approaches whether its power, hull design, etc.
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Old 03-23-2023, 07:44 PM   #7
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Our 2001 Nordic Tug 37 has an SD hull, waterline of about 39 feet, beam 12'11", and weighs about 28,000 lb loaded. Engine is a 330hp Cummins 6BTA diesel turning a 28" x 24 prop.

At 1300 RPM, traveling at 7.25 knots, we average 3.7-3.9 nmpg. I have no idea how much, but some of that fuel is used by the generator and diesel heater.

We could cruise considerably faster, but just one additional knot drops fuel economy significantly, by maybe 1 nmpg. We could do 12 knots all day (and 16 knots at WOT - 2600 RPM), but we've done 12 knots only for short periods, so don't know fuel economy at that speed. I'd guess maybe 1.5 nmpg or less.
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Old 03-23-2023, 08:35 PM   #8
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Bigger engines will use slightly more fuel to make the same power at a slow cruising speed. But diesels don't lose much efficiency at light load, so it's not a big difference. Other differences in efficiency from one boat to another are often just as significant, if not more so.
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Old 03-24-2023, 12:14 AM   #9
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'Bigger' is a nebulous term. You'll see 6l marine engines ranging from 120 to 400 HP rated output. They're all going to use approximately the same amount of fuel to run at 40 HP/2 gph.

I've put > 1000 hrs at less than 2 gph on three different engines - 210 HP, 250 HP, and 135 HP. Any differences in economy between the engines was negligible compared to differences in speed or boats, or even props. There are lots of factors in achieving good mpg.
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Old 03-24-2023, 05:20 AM   #10
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Originally Posted by Jeff F View Post
'Bigger' is a nebulous term. You'll see 6l marine engines ranging from 120 to 400 HP rated output. They're all going to use approximately the same amount of fuel to run at 40 HP/2 gph.

I've put > 1000 hrs at less than 2 gph on three different engines - 210 HP, 250 HP, and 135 HP. Any differences in economy between the engines was negligible compared to differences in speed or boats, or even props. There are lots of factors in achieving good mpg.
No, that's not really correct.

While it's generally true that you get roughly 20 HP for 1 gallon of fuel, the cost (in fuel) of the engine to over coming the resistance of running doesn't get magically recovered when the engine starts working under load.

Secondly, you can expect engines within the same family to have roughly the same resistance of running loads. A Cummins 6B series engine is roughly the same engine from the naturally aspired 135 HP to the turbo charged after cooled 370 HP. But to say the resistance of running loads is the same when engine size grows in cylinder displacement (liter size) and the number of cylinders increase, is factually incorrect.

A 135 HP engine producing 60 HP is going to be more efficient (less fuel consumption) than a 500 HP engine if it has 3 times the cylinder displacement (liter size), producing 60 HP. One engine is running near its peak torque and the other is running at a quite low RPM. One only has to look at the trajectory of modern diesels to see more HP at higher RPM for the same cylinder displacement as a means to improve efficiency and reduce emissions.

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Old 03-24-2023, 05:47 AM   #11
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I just helped a friend move his 52 foot power cat 50 nms from yard to his home dock. Has modern 435hp Cummins. Will do 20 kts but we kept it at 9.5 kts. 1 nmpg according to the onboard sensors (burned 45 gals for 50 nms over 5.5 hrs). 9.5 kts is pretty fast - but on that boat, 7.5 kts seems dang slow. Fuel economy would probably go to 1.5 nms/gal. Probably never know- my friend is a go-fast sorta guy.

In Naval architect terms, there is what is called "prismatic coeeficient." It's a factor that accounts for cross sectional hull displacement. Higher PC means the hull design is intended for higher speeds, lower is more displacement. A displacement hull is trapped by itself - my round-bottom full-keel boat wouldn't do 9-kts if it had a pair of Pratt & Whitney's bolted to her deck. But a semi-displacement hull can do displacement speeds.

Another non-engine factor is rudder size. A small rudder at low speed is probably going to move around a lot more, have to do more work, and create a bit more drag.

When the Willard 36 was in production from 1961 through 1970, a 6-cyl was an option that many folks ordered. I'll have to ping the owners group on fuel economy but I suspect those larger engines pay a 10%-15% penalty in economy with no payback whatsoever.

Finally I'll point out that claimed fuel economy claims likely need some explanation and context. 100 engine hours running the ICW over a 2-3 week period with slow spots, idling, maneuvering into marinas, etc will burn significantly less fuel than if the same boat made a 100-hr / 750nm run offshore run over 4-days

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Old 03-24-2023, 06:29 AM   #12
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One more example I just thought of. I once delivered a Sunseeker 55-ish footer from Seattle area to San Francisco. The owner, an early dot-com millionaire was aboard. He wanted to fast and we did. Boat easily did 25 kts but has a range of under 200 nms with 700g of diesel aboard. We made it to Gray's Harbor, just south of Flattery, for our second stop for fuel. It was April and fuel dock was closed for two days so we hung out which sucked. From there, we did 7.5 kts and made it all the way to SF on a single tank of fuel, approx 1 nm/gal. Definitely a tortoise and hare story. BTW - with exception of windows needing to be re-bedded, the wide transom Sunseeker was a decent sea boat, at least headed south.

Bottom line is there are many factors to fuel economy, but speed is by far the biggest. At least 80%

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Old 03-24-2023, 06:32 AM   #13
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This is intended to echo comments above, but just throw something else out there.

The Cummins QSB series is one popular engine in use today. One can get a QSB with hp ranging from 250 to 550. Same engine block. I have been told the thing that differentiates the lowest hp to the mid range hp is the governor that limits top end RPM. Which would represent no change at lower RPM. Somewhere above the mid range, there are some component differences, but I don't recall what those are, but its still the same block.

For my new build I spec'ed a 380. I did so knowing that many never run it often at WOT or the highest continuous output. That was just a choice from the school of "better to have it and not need it, than need it and not have it". The reason to stop at 380 is that Helmsman advises that above there one needs trim tabs, and arguably could benefit from them if one intends to run at the high end regularly. But that is about the specific hull, not an engine characteristic. To get into trim tabs for speeds I don't expect to use often just means more to buy, break, and maintain, and so that's a natural limit for this hull. A limit that others choose to push through with tabs. 380 and 250 appear to be the most popular choices for the boat. From everything I have seen there is zero difference in fuel burn at a 7 knot speed between a 380 and 250.
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Old 03-24-2023, 06:52 AM   #14
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So if motoring along at 7 kn on dual 135 HP engines you will use about the same at if you motored along at 7 kn on dual 350 HP engines.
So does TF users agree the answer is the bigger engines are going to use 10-15% more fuel?????
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Old 03-24-2023, 06:55 AM   #15
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Quote:
Originally Posted by Don L View Post
The articles have said that by far the biggest difference in fuel use/economy is speed. They say that if you run at displacement speed or less that the size of the engine doesn't matter all that much.

But the other part of me says ............BS bigger engines use more fuel PERIOD!

So what have the kind boaters here on TF with trawlers with big engines have to say on the matter?

We have twin 900-hp common rail diesels in our "not a trawler." Our maximum theoretical hull speed speed (planing hull) is probably somewhere around 9.5 kts, although that's a bit squishy since I don't really have a perfect measurement of our waterline length.

Our best fuel consumption rate will likely fall between 7.0 to 9.5 kts.

It varies with wind and current, but 7 kts is often just above our idle speed (600 RPM) at less than 1 GPH per engine. Sometimes wind or current causes me to increase RPM enough to reach 9.5 kts at 4 GPM/engine.

I'd say our average, taking wind, current, and whatever actual "trawler" speeds I'm making is somewhere in the 2-3 GPH/engine area.

Not as parsimonious as many here with real trawlers, but then again we can also run at near 30 kts should the need arise... and that's an option that works for us.

Previous 42 with twin mechanical 450s did better (of course), probably averaging more like 2 GPH/engine most of the time.... with a 22 kt high cruise option.

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Old 03-24-2023, 07:30 AM   #16
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Suspect there is a difference between best mileage and average mileage over a season. I'm amazed at planing or semi displacement boats that are getting 3.5 nmpg!

Our Bayliner 3888 has twin Hino 6.4l naturally aspirated diesels. When we did the loop we averaged 6.6 knots over 792 engine hours burning 2960 us gallons. We travelled 5237 nautical miles for a total average of 1.769 nautical miles per gallon. Thats over the full year with currents, against currents, idling, occasionally over 8.5 knots etc.

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Old 03-24-2023, 07:50 AM   #17
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Suspect there is a difference between best mileage and average mileage over a season. I'm amazed at planing or semi displacement boats that are getting 3.5 nmpg!

Our Bayliner 3888 has twin Hino 6.4l naturally aspirated diesels. When we did the loop we averaged 6.6 knots over 792 engine hours burning 2960 us gallons. We travelled 5237 nautical miles for a total average of 1.769 nautical miles per gallon. Thats over the full year with currents, against currents, idling, occasionally over 8.5 knots etc.

James

The planing and faster SD hulls are less efficient at low speeds in terms of hull shape, but that's at least partly offset by them typically being lighter weight than the slowest SD shapes and the FD boats.
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Old 03-24-2023, 07:52 AM   #18
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By the way, concerning the QSB series, I have seen it said, but cannot vouch, that its easy to upgrade a 250 to a 380 after its installed. A tech comes aboard and downloads different code into the governor board, and cuts you a bill. It would interesting to know if that's true.
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Old 03-24-2023, 08:04 AM   #19
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The planing and faster SD hulls are less efficient at low speeds in terms of hull shape, but that's at least partly offset by them typically being lighter weight than the slowest SD shapes and the FD boats.
That must be true, the examples given above show a 37/39' semi displacement with 330 hp, a 43' displacement with 130 hp, and a 58' planing hull with twin 900 hp all getting 3.5 nmpg at 7 knots. I'm guessing all are electronically controlled common rail engines. Seems to disprove the original posters theory that bigger engines must use more fuel at the same speed.

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Old 03-24-2023, 08:09 AM   #20
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That must be true, the examples given above show a 37/39' semi displacement with 330 hp, a 43' displacement with 130 hp, and a 58' planing hull with twin 900 hp all getting 3.5 nmpg at 7 knots. I'm guessing all are electronically controlled common rail engines. Seems to disprove the original posters theory that bigger engines must use more fuel at the same speed.

James

Exactly. Now if you put bigger engines in the same boat (especially if they're heavier), you'd likely see some decrease in economy.
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