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Old 12-04-2017, 09:57 PM   #1
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Large Engines vs Fuel Economy

Hello y'all once again. Still shopping for a 40'-45' trawler or motoryacht, and now I'm trying to figure out what size engines we should look for.

A number of people have told me that since diesels are meant to run hard, it's best to get a boat with the smallest engines that will deliver the desired cruising speeds: because a smaller engine running near capacity will be the most fuel efficient and won't build up carbon deposits. They say larger engines are going to use lots more fuel even at slow speeds.

Other people have told me that in boats with similar hull shapes and displacements, larger engines running at lower RPM's will get just as good fuel economy as smaller ones running at higher RPM's (to deliver the same speed), and that winding the larger engines up periodically for short stretches will blow out the carbon without killing your fuel economy. They recommend buying a boat with larger engines.

We started out looking at boats with no more than 150HP engines because we expect to cruise at no more than 6-8 knots. But I would love to have some extra speed in reserve if I can have it and still get good fuel economy at lower speeds.

If you have a cruising yacht with engines of 175HP or larger, I would be very interested in any information you can share regarding how fast you typically cruise, what RPM you typically run at, what kind of fuel economy you achieve at that RPM, and particularly what kind of fuel economy you achieve if you do cruise at 6-8 knots.

Thanks for taking the time!
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Old 12-04-2017, 10:39 PM   #2
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Quote:
Originally Posted by ScottH View Post

If you have a cruising yacht with engines of 175HP or larger, I would be very interested in any information you can share regarding how fast you typically cruise, what RPM you typically run at, what kind of fuel economy you achieve at that RPM, and particularly what kind of fuel economy you achieve if you do cruise at 6-8 knots.

Thanks for taking the time!
Wifey B: Boat we're on at the moment, normally cruise at 23 knots or so, using 92 gph. Could cruise at 8 knots using 8 gph. 12 knots is 22 gph. 2x1500 hp.
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Old 12-04-2017, 10:48 PM   #3
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Find a boat you like. Get the engines checked out and if OK start cruising.

Running higher HP engines at low rpm, for newer engines in particular, is fine. The difference in fuel economy between small engines at say 80% load and larger HP engines loafing along (same hull) will be a rounding error in your annual boating costs.
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Old 12-04-2017, 11:08 PM   #4
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Agree with Brian. I’ve got 420 HP at 2600 RPM and spend most of our time at 1200-1400 using about 50 HP and around 2 GPH at 7 knots.

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Old 12-04-2017, 11:18 PM   #5
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This same exact question was thoroughly discussed a week or so ago here: How much engine do I really need?

I presented an analysis of the fuel economy of a Lehman 120 making 50 hp (what it takes to push a 35-40' trawler to 7-8 kts) and a Cummins 6BTA 370 hp engine making the same hp but with the capability of three times more than the Lehman can do. The analysis looked at the manufacturer's fuel burn curves.

The conclusion was they burned the same fuel to make the same hp (not surprisingly).

The corollary is, there is no downside to a high output engine other than a little extra maintenance (after coolers mostly) that can give you more hp when you need it.

So get the engine that works for you.

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Old 12-05-2017, 12:29 AM   #6
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Here is a diesel engine fuel map. At any given RPM your fuel burn per HP can vary by 50% depending on loading. Don't let anyone sell ypou more engine than you need to make the speed you desire or can afford to fuel.
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Old 12-05-2017, 01:16 AM   #7
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I moved up from 145 hp engines to 200s. I gained 1 nmph, from 7 to 8. I gained economy by about 10% right away, despite going faster, plus another 5% when I re-propped to lower rpm from 2700 to 2000. All good.
I have a further 1700 hrs since those changes, no glazing.
My mechanic, a Volvo expert, recommended the engine swap and the prop changes, also predicted the economy improvement.

Get the larger engines, run them at a lower speed. They will thank you for it by lasting longer.
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Old 12-05-2017, 01:59 AM   #8
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Hi,

Here's a table rmp, speed kn, consumption liters per hour, and a theoretical range of 10% reserve. You can see how economical 380hp / 5.9 l diesel is low rmp. When looking at Cummins qsb FUEL CONSUMPTION - PROP CURVE it consumes evenly the g / kwh of the full rmp range.

Min NT 37 in full load, water, fuel, 5 people, food and equipment for far cruise. With less weight I can get 1.5 kn more speed and consumption a bit different.

I've got the answer to the question Cummin eternity US, can I run my Cummin qsb 5.9 Low rmp for long periods and their clear answer yes, if rmp more than 800rmp all very well without any problems. Less than 800rmp is not good for max 20min exaples when the machine is cold.

1 litres is 0,26 US gal

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Old 12-05-2017, 05:43 AM   #9
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Quote:
Originally Posted by BandB View Post
Wifey B: Boat we're on at the moment, normally cruise at 23 knots or so, using 92 gph. Could cruise at 8 knots using 8 gph. 12 knots is 22 gph. 2x1500 hp.
Hi, You can be happy cruising US. If you live in Finland, your diesel would pay about $ 25 / nm at your normal speed, thankfully you have more friendly prices.

What sitze your boat is?

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Old 12-05-2017, 07:46 AM   #10
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Engine manufacturer's consumption charts are relatively meaningless for fuel burn as consumption can vary hugely for a given RPM based on load. My engine burns less than half of the manufacturer's projected fuel consumption at 7 knots / 1500 RPM. Repowering from 450 HP to 135 HP about halfed my fuel consumption, but then I only need 40 HP to go 7 knots.

Ted
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Old 12-05-2017, 08:08 AM   #11
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Quote:
Originally Posted by ScottH View Post
If you have a cruising yacht with engines of 175HP or larger, I would be very interested in any information you can share regarding how fast you typically cruise, what RPM you typically run at, what kind of fuel economy you achieve at that RPM, and particularly what kind of fuel economy you achieve if you do cruise at 6-8 knots.

We have twin Cummins 450Cs, planing hull, approx 8 kt max theoretical displacement speed.

We often cruise at slow speeds, at least when the hull form and sea states permit.

Approx 6.3 kts is approx 800 RPMs average, and nominal fuel consumption per engine is approx 1.1 GPH. Times two, approx 2.2 GPH total, approx 2.9 NMPG.

Approx 7.8 kts is approx 1000 RPMs average, and nominal fuel consumption per engine is approx 1.9 GPH. Times two, approx 3.8 GPH total, approx 1.9 NMPG.

Approx 8.6 kts is approx 1200 RPMs average, and nominal fuel consumption per engine is approx 2.9 GPH. Times two, approx 5.8 GPH total, approx 1.5 NMPG.

We usually run at about 1200 RPMs, or at least as close to between 1200-1225 as I can manually sync both engines, to keep our engine temps up where they ought to be. That can be affected by raw water temps, so in full summer we can run slightly slower to get the same temps. And then we usually run up the engines periodically for a few minutes, as at the end of a slow run.

Our current guess-timate for our ongoing trip, almost 500 miles so far is about 6.8 GPH and 1.2 NMPG total --even including about 120 NM at planing speeds arount 18-20 kts. It's a guess, though, because I won't actually FILL the tanks to get a positive total consumption figure until the end of all this.

-Chris
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Old 12-05-2017, 08:37 AM   #12
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Engines are heat sinks.

The more (cyl wall/combustion chamber)area the more heat is lost and the less efficiency.

Brooksie’s post #6 nails it IMO. But there are variables like electronic control and turbochargers. But if you take two engines where the only variable is the engine size the small engine wins every time from what I can see.

But I think one can come very close to the truth using the specific consumption of a specific engine and applying it to a resistance curve of one’s boat. And comparing big engines to small engines the load will obviously be very different.

This question is very simple for FD boats but gets considerably more complicated for SD unless you choose the power for a specific speed of a SD boat. But most SD boat operators (here anyway) seem to want high speed capability. But that adds thousands of pounds of weight to the boat that you’re dragging alond almost all the time. It’s a choice.

Sometimes I go to extremes in comparing to answer questions like this. I may ask what if I put a Perkins 6-356 in my little Willard? Would I gain efficiency and burn less fuel? How-bout a 500 ci in Cat? I dont think so. But if I put a 100hp turbo Yanmar maybe so. But that’s a higher tech different type of engine. Apples to apples the little engine wins.
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Old 12-05-2017, 09:02 AM   #13
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As you can see from all of the replies the fuel burn has much more to do with the exact speed you choose to cruise at rather than the engines you are using at the time. Even small changes in speed (1 knot) will make very large changes in fuel use over water.
With a few very small liberties to make the numbers very easy to remember this was the fuel burn on our 47' boat at 35,000 #'s plus with twin 310 hp engines.

6 knots - 3.5 nmpg
8 knots - 2 nmpg
10 knots - 1 nmpg
16 knots - 0.95 nmpg
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Old 12-05-2017, 09:06 AM   #14
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Here are the fuel burn graphs for our Helmsman 38 with a Cummins QSB 5.9 230 HO. This is a tier 3 electronically managed engine. The fuel consumption data is from the "Vessel View" display. The boat speed is speed through the water not SOG, so current is not a factor in the MPG. To reiterate what previous commenters have said, according to Cummins, these common rail Tier 3 engines do not require a periodic "blow out."
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Old 12-05-2017, 09:10 AM   #15
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My higher hp experience

I have a 37 express cruiser twin 435 hp that is use mostly in the 8-9 mph range (1000 rpm) but is capable of 35+. This is not an ideal hull design for slow speed operation but tracks true and is exceptionally easy to maneuver at all speeds. The average fuel consumption, per engine, is 1.6 gph when at 9mph. I have a flow scan system and have tank dip tubes for manual verification so no speculation on consumption. I track fuel usage for each engine separately. I have no problem bringing engine temps to 180 degrees but do clean out the turbos about every 3 hrs +/- @ 2400 for about a 10 minute run up. My last longer trip was 360 miles and travel about 1500 2000 miles per year. Do NOT believe all you read about running higher hp and lower output.
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Old 12-05-2017, 09:40 AM   #16
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[FONT=Arial Do NOT believe all you read about running higher hp and lower output.[/FONT]


Also, for planing design boats the worst thing you can have is too small of engines that constantly are running plus 75 % load with a recreational duty engine.
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Old 12-05-2017, 10:18 AM   #17
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Hi Everyone,

Your replies have been a HUGE help. They have completely changed my thinking about large vs small and have opened up a number of boats to consideration that I had originally passed over due to engine size
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Old 12-05-2017, 10:21 AM   #18
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Hi, You can be happy cruising US. If you live in Finland, your diesel would pay about $ 25 / nm at your normal speed, thankfully you have more friendly prices.

What sitze your boat is?

NBs
Wifey B: Boat we're on today is 85'.
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Old 12-05-2017, 10:26 AM   #19
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Hi Everyone,

Your replies have been a HUGE help. They have completely changed my thinking about large vs small and have opened up a number of boats to consideration that I had originally passed over due to engine size
You'll see two very distinct views here.

One group that is the full displacement, go slow, get the least engine that will do the job, minimalists when it comes to that aspect.

The other group is semi-displacement or semi-planing or even planing in nature and they like having the ability to run faster but can still run slow economically. Most run slower the majority of the time but there are a very small number of us who normally cruise faster.
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Old 12-05-2017, 10:30 AM   #20
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Here are the fuel burn graphs for our Helmsman 38 with a Cummins QSB 5.9 230 HO. This is a tier 3 electronically managed engine. The fuel consumption data is from the "Vessel View" display. The boat speed is speed through the water not SOG, so current is not a factor in the MPG. To reiterate what previous commenters have said, according to Cummins, these common rail Tier 3 engines do not require a periodic "blow out."
You're right on these Cummins qsb tier 2 or 3 machines do not have to burn clean anything on the machine. This letter exchanged regarding the long-time use of low rmp and low load that I asked Cummins US.

My question for Cummins ...

Hi,

I have a Cummins 5.9 qsb marine engine 2009 and it reaches max rmp 3065 which is perfect my Nordic Tug 37.

I have read a lot of conflicting opinions on the engine to run at low rmp a long time, because the machine may damage the carbon and etc. Some say it's ok to run this type engine at low rmp if the coolant remains in the correct slot.

The time i run my engine is most often 850-1300 rpm since the boat is most economical in this rmp area and temperature is ok. Cummins runs fine and does not smoke any, exhaust pipe mouth environment does not show any black carbon.

What is the manufacturer's view of low load low rmp almost always, whether it is ok or damaging the engine?

It would be great to have an expert answer, all the web instead of rumors.

Best regards
Xxxx xxxxx
Finland


And Cummins answers...

Xxxx,
This is fine for our engines. It is not suggested to Idle (650-750rpm) for long periods. Generally speaking, you can idle for about 20 minutes or so at this range and be okay. If you plan to idle longer than 20 minutes or so it is suggested that you ramp up your RPM to about 800-1000. Working the engine under a light load/rpm is fine. This is not uncommon for some our engines (like Generators which normally work at 1800 RPM or less) and will not cause any undue harm.

Thank you for contacting Cummins.
Xxxxx xxxxx

Customer Care Representative
If we have misunderstood the information in your communication or you have additional questions please feel free to respond to our support staff by e-mailing support.engineparts@cummins.com or 1-866-CUMMINS (286-6467). If you are located outside of North America, you can reach us at 1-615-871-5500.
Visit us at www.cummins.com or www.cumminsengines.com
-------Original Message------
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Cc:
Subject: Low rmp vs life time qsb 5.9 marine
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