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!

ScottH said:

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!

Click to expand...

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.

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.

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.

Tom

This same exact question was thoroughly discussed a week or so ago here: http://www.trawlerforum.com/forums/s3/how-much-engine-do-i-really-need-35652.html

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.

David

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.

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.

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



NBs

BandB said:

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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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

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

ScottH said:

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.

Click to expand...

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

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.

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

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."

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.

Alabama Boater said:

[FONT=Arial Do NOT believe all you read about running higher hp and lower output.[/FONT]

Click to expand...

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.

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

North Baltic sea said:

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

Click to expand...

Wifey B: Boat we're on today is 85'.

ScottH said:

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

Click to expand...

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.

tadhana said:

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."

Click to expand...

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------
From: @outlook.com
To: powermaster@cummins.com
Cc:
Subject: Low rmp vs life time qsb 5.9 marine

North Baltic sea said:

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------
From: @outlook.com
To: powermaster@cummins.com
Cc:
Subject: Low rmp vs life time qsb 5.9 marine

Click to expand...

North Baltic

You cheated, you got the straight scoop from the engine builders and designers. BTW, this is pretty much the same thing Tony Athens has been saying for years.

Brooksie said:

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.

Click to expand...

Mercedes diesel car? This shows, very much like a gas car, that you want to cruise at a low engine rpm. This cuts windage losses in the motor. In a multi-speed vehicle, this is easily done. Not so easy in a boat with fixed prop pitch and gearing.
There is some merit in these curves, but, not a great chart set for the boater who is looking for simple answers.

ie; PROP CURVE.

ScottH said:

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

Click to expand...

My experience is with small diesels on sailboats and my current Cummins QSB 5.9L 380hp engine in my 43’ North Pacific.

I was very concerned about fuel economy and was asking the very same questions you are now. Here are my relatively ignorant thoughts based on listening to folks more knowledgeable than I and my own experience over the last 1 1/2 years of operation.

1. In our boats, throttle position is the single most important factor in fuel economy. Every hull is different, but there is a point where adding speed really drives up the fuel burn.

2. Larger engines will generally be less efficient than small engines. The difference is relatively small all things being equal. However, all things are NEVER equal. So the difference is more theoretical than practical.

3. Fuel cost for most of us is a very small percentage of our overall boating expense.

4. All boats are different. My North Pacific 43 has the 380hp version of the 5.9L QSB. The standard engine that was being put into those boats at the time was the 330hp version of the engine. The original buyer wanted more power. I don’t think I am being helped or hurt by that extra 50hp as it is the same block.

5. While Cummins doesn’t care if your engines are run for long periods at a low rpm, most experienced Cummins mechanics that I have spoken to or read, do recommend a short period of running at a higher rpm after hours of low rpm running. This is a very vague recommendation. I typically run at 1450 rpm. I have been bringing the rpm up to 1800+ for the last 15 minutes before I enter my harbor. It then takes me 20 minutes of running at a very low rpm through my no wake zone harbor to get to my slip so there is plenty of time for the engine to cool down. Does it help? No idea.

As others have said, if you are buying used pick the boat you want, not the engine. You are looking at buying a boat for what it can do for you. For most of us, any engine would do as long as it was put in the right boat.

Our 2002 Nordic Tug 37 has a Cummins 6BTA 5.9 M3, 330 hp. Max RPM is 2800. We bought it in 2016 with 1485 hours on the engine, and have added 1600 hours in the last two summers.

We cruise typically at 1200-1400 RPM, sometimes at 1100, sometimes 1500 or 1600. We very ocasionally run her up to 15-16 knots (2600-2800 RPM) just to see how she's doing (always fine so far), but not often.

Last summer traveling 4,400 NM mostly at 7-7.5 knots (1300-1400 RPM), we averaged 3.83 nautical miles per gallon.

sunchaser said:

North Baltic

You cheated, you got the straight scoop from the engine builders and designers. BTW, this is pretty much the same thing Tony Athens has been saying for years.

Click to expand...

So often we get information and experiences from different time periods and they're just wrong. I believe in two sources which are basically the same. The manual that came with an item and the manufacturer. Engines have changed dramatically over the years.

We got back to the marina from dinner last night and mentioned where we ate, Bern's Steakhouse. A man then said, "when I went there they were so busy and the service was really slow." My wife asked when that was. He said, 'It's been a while, about 20 years ago." I just shook my head as we walked away. Who cares what the service was like one time 20 years ago? Yet, we see that all the time in engine and other equipment discussions.

Alabama Boater said:

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.

Click to expand...

AB,
So at 9mph it takes 70hp to push your boat. That could fly.
When you say “temps” are you talking about oil temp? Coolant temp is important but fly stuff compared to oil temp. Oil temp is a good indication of what the temp of the engine parts the oil has just been in contact with.

I agree all is not to be belived but neither is all to be ignored. And your low fuel in your case is largely due to the fact that you’re boat is light. And I’ll bet your speed is 8 or even 7 mph .. not 9. But that’s just a guess.

The BMEP or fuel map already posted is the key to understanding.

By staying in the bullseye the boat engine is as efficient as it can be.

Even in the bullseye there is a range of acceptable RPM.

The lower the RPM at cruise , usually, the better life is on board.

BandB wrote,
“Who cares what the service was like one time 20 years ago? Yet, we see that all the time in engine and other equipment discussions.”

I know you don’t care about anything that happened over a year ago thinking we were just Neanderthals then and didn’t know anything. Well whatever they knew back then that was true is still true. Now. You say “Yet, we see that all the time in engine and other equipment discussions.” Of course you see it all the time because it’s relevant .... now. What is is, always.

1) Fuel is going to ba s mall part of your expenses. Trying to gain a few gph or mpg, while it could add up over the years, is really just chasing a unicorn.

2) Good marine diesels, with good upkeep, will last tens of thousands of hours. Likely, longer than any other boat system. Just find a well-maintained one and go enjoy boating.

Don't over-think it. You're splitting hairs IMHO.

diver dave said:

Mercedes diesel car? This shows, very much like a gas car, that you want to cruise at a low engine rpm. This cuts windage losses in the motor. In a multi-speed vehicle, this is easily done. Not so easy in a boat with fixed prop pitch and gearing.
There is some merit in these curves, but, not a great chart set for the boater who is looking for simple answers.

ie; PROP CURVE.

Click to expand...

Yes, diesel car map. Marine engine manufacturers don't seem to provide these maps that I can find. Only charts showing what they want you to see like loading with "perfect" propeller to reach their "perfect" RPM say 2800. Yet they will take the same engine and run it continuously at 1800 as a generator. If you have a map for a small marine diesel engine, I would like to have a copy.