Large Engines vs Fuel Economy

Codger2 · Dec 6, 2017

Nidza said:
About the fun part, as everybody agrees, only correct way to exactly know your load, whichever engine, is to measure fuel consumption, although not always very practical way.

If one really wants to nail down their fuel consumption plus a lot of other fuel related info (ie: time to go. distance to go, miles /gal, etc,) I recommend the Maretron FFM100 Fuel Monitoring System. It's not cheap but it gives you everything you want to know at a glance! No charts, no searching or comparing engines, just accurate data in real time. The photo below displays what the Raymarine es128 can show but that's only a small part of what the Maretron DSM100 (needed for the FFM100) can tell you. As I said, if you really want to know (not guess or compare charts) get a real time fuel management system and be done with it! I know this might be interpreted as "an elitist ($$$) approach to the fuel management problem but I'm 76 years old and tired of "guessing about such matters!

BandB · Dec 6, 2017

Codger2 said:
As I said, if you really want to know (not guess or compare charts) get a real time fuel management system and be done with it!

I agree completely. Floscan and Maretron are the leaders.

Expensive, perhaps, but not so sure. By actually knowing your consumption and mileage per gallon at all speeds, not just thinking you know, you might actually pay for the system. Plus by knowing accurate fuel on hand can perhaps plan purchases better for savings.

ranger58sb · Dec 6, 2017

Codger2 said:
If one really wants to nail down their fuel consumption plus a lot of other fuel related info (ie: time to go. distance to go, miles /gal, etc,) I recommend the Maretron FFM100 Fuel Monitoring System. It's not cheap but it gives you everything you want to know at a glance! No charts, no searching or comparing engines, just accurate data in real time. The photo below displays what the Raymarine es128 can show but that's only a small part of what the Maretron DSM100 (needed for the FFM100) can tell you. As I said, if you really want to know (not guess or compare charts) get a real time fuel management system and be done with it! I know this might be interpreted as "an elitist ($$$) approach to the fuel management problem but I'm 76 years old and tired of "guessing about such matters!

Can something like that show the most efficient hp/load/fuel flow/speed presented as something beyond just current flow rate/GPH, etc.?

Ski's calculations go a step beyond the "right now" factoids like GPH, etc...

Just knowing I'm burning 10.4 GPH at 2000 RPMs, for example, doesn't really tell me about efficiency...

-Chris

Codger2 · Dec 6, 2017

ranger42c said:
Can something like that show the most efficient hp/load/fuel flow/speed presented as something beyond just current flow rate/GPH, etc.?

Ski's calculations go a step beyond the "right now" factoids like GPH, etc...

Just knowing I'm burning 10.4 GPH at 2000 RPMs, for example, doesn't really tell me about efficiency...

-Chris

I'm not going to go into everything this system can do & I take a lot of what Ski posts to heart. I can say, however, that any fuel questions one might have are answered with this system. Not horse power just fuel!

Here's an example of what the company posts on their web site.

Nomad Willy · Dec 6, 2017

If people really cared about fuel consumption they’d just buy a smaller, slower, lighter and less powerful boat. People buy/drive hybrid cars because they want to be seen as being economical/conserving not because they want to or expect to save money. I drive a Toyota Avalon that is a rather large car. I’m a rather poor man here on TF and buying the fuel for the big Avalon is absolutely no problem. And I drive a lot. If I needed to spend less on fuel I’d simply buy a smaller car. But if I wanted to be seen as eco/conservative I’d just get an Avalon that said “hybrid” on the car where everybody can see it.

Boats are no different.

diver dave · Dec 6, 2017

Codger2 said:
I'm not going to go into everything this system can do & I take a lot of what Ski posts to heart. I can say, however, that any fuel questions one might have are answered with this system. Not horse power just fuel!

Here's an example of what the company posts on their web site.

not a lot different from the 2017 Impala I just rented...

These are great to find out about what you already bought. The better questions are how to optimize a solution for a boat. Basically, a PC run database of hulls, engines available, etc.

Codger2 · Dec 6, 2017

One must go to the Maretron site to get a full understanding of the systems's capabilities. The DSM150 fuel computer is shown below (bottom left) and is capable of showing more data than the typical skipper needs to know. Upper right in the photo is the Raymarine eS128 displaying a very limited amount of data.

diver dave · Dec 6, 2017

Codger; so, here is a question. I'm in a SD trawler running into a 4kt current. Will it tell me what rpm to run to make best nm/G? I'm sure it is ultimately possible even with a dumb FloScan, but is this task easier with the DSM?

Woodland Hills · Dec 6, 2017

ScottH said:
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!

IMHO you are going about this backwards: unless you can afford new construction or a re-power, you find the boat with the layout, the equipment and the condition/price that works for you. And then you learn to live with the engines and make the best of them.......
Unless the whole point of boating for you is the motor, shouldn't other things take priority? Like the motion and carrying capacity of the vessel or the specific layout that works best for you and where and how you cruise , etc. Once you nail that down, you will find that there is a limited choice of makes and models on the market that suit and an even more limited choice of engines. Unless you can afford to have a custom boat built or to re-power an existing one you have limited options in any given layout.

MVDarlin · Dec 6, 2017

Fuel consumption, mysterious, arcane art. I have 2 120Hp Ford Lehmans that burn 4.5gph combined at 80% power 8.5 knots. Best thing I did was change from a 3 blade to 5 blade prop with more pitch and 1inch size increase. Increased speed 1/2 knot and significant vibration reduction. However, harmonics at 50% power are a nuisance - ringing sound.

JLD · Dec 6, 2017

I going to throw out there for big engines, maybe fuel economy is only part of the equation.

I've found a boat model that I would like to have, a 2004-2007 Silverton 34 Convertible. Twin 5.7's were standard on the boat, but they also came with the options of twin 6.0's, 8.1's, and Yanmar 315's. Here's prices I've gleaned from sale sites:

5.7's and 6.0's - 120k
8.1's - 125k - 130k (5k -10k premium)
Yanmar 315's - 150k+ (30k+ premium)

Folks always say that for a SD hull, the more torque the quicker the boat gets on plane. Thus, that would favor the 8.1's or diesels. But are they really worth the extra $$$ ? (likely would depend on how one planned to use the boat).

This gets me into thinking, 'what is the true cost of ownership' for the engines.

Bigger engine, higher upfront cost (this is pretty much a given).

Bigger engine, greater or less fuel economy? Here are some numbers from owners on the Silverton forum with cruise of about 20 knots:

5.7's - about 0.8 mpg at cruise
6.0's - about 0.9 mpg at cruise
8.1's - about 0.75 mpg at cruise
Yanmar's - about 1.2 mpg at cruise

So, it would seem that maybe yes/ maybe no.

But, what about engine maintenance costs? I would think that the bigger engines would last longer since they run at lower rpms. But, how much longer? What are the cost to rebuild sets of each engine? Would have to think the small-blocks would be cheaper than the large-block or diesels? How much so? So many decisions!

Jim

Simi 60 · Dec 6, 2017

Nomad Willy said:
. But if I wanted to be seen as eco/conservative I’d just get an Avalon that said “hybrid” on the car where everybody can see it.

If appearance is the only concern simply spend $5 on a badge.
Instant greenie.

BandB · Dec 6, 2017

They complete systems will tell you all that's asked for here. You'll know the various options of speeds and loads. You learn what many here think they know by other means and that is exactly where the savings come and how much they are. I was in a discussion the other day with a very experienced captain talking about running at 80% load. I showed him the data on a specific boat that we cruise at 68-70% load. On that specific boat between 70% and 80% we gain one knot at a cost of 25 gallons per hour of fuel. On that specific engine combination on that specific boat, there's a nice smooth curve to 70% and then from there to 80% a big jump and a much more aggressive curve on to 100%. 70% to WOT throttle is only the difference from 24 knots to 27 knots but 45 gallons per hour.

Every boat I've gathered data on has had some unique little quirk of it's own. One fast boat that doesn't gain anything in nmpg by slowing from 2000 RPM to 1250 RPM. 1250 RPM is 15 knots, 2000 RPM is 36 knots and through that entire range no change in nmpg. To most people that's very counter-intuitive as we're just conditioned to think that faster is going to take more gallons per mile or be less fuel efficient.

Nomad Willy · Dec 6, 2017

MVDarlin said:
Fuel consumption, mysterious, arcane art. I have 2 120Hp Ford Lehmans that burn 4.5gph combined at 80% power 8.5 knots. Best thing I did was change from a 3 blade to 5 blade prop with more pitch and 1inch size increase. Increased speed 1/2 knot and significant vibration reduction. However, harmonics at 50% power are a nuisance - ringing sound.

80% power would be 8 or 9gph.
You probably mean 80% throttle.

dhays · Dec 6, 2017

Woodland Hills said:
IMHO you are going about this backwards: unless you can afford new construction or a re-power, you find the boat with the layout, the equipment and the condition/price that works for you. And then you learn to live with the engines and make the best of them.......
Unless them.

^^^ this.

Northern Spy · Dec 6, 2017

Simi 60 said:
If appearance is the only concern simply spend $5 on a badge.
Instant greenie.

I like this idea.

I think I'll get a Toyota hybrid badge for my old diesel LandCruiser.

koliver · Dec 6, 2017

B n B "Every boat I've gathered data on has had some unique little quirk of it's own. One fast boat that doesn't gain anything in nmpg by slowing from 2000 RPM to 1250 RPM. 1250 RPM is 15 knots, 2000 RPM is 36 knots and through that entire range no change in nmpg. To most people that's very counter-intuitive as we're just conditioned to think that faster is going to take more gallons per mile or be less fuel efficient."

I have always believed that the fuel burn is directly related to the size of the stern wave.
There are many boats that make the tallest wave before getting fully onto a plane, so should have their worst nmpg at that pre-planing speed and once fully up on the step, should see an improvement in nmpg, and a leveling out of nmpg no matter the top speed. Your example comes as no surprise.

BruceK · Dec 7, 2017

BandB said:
..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? ...

He does, it`s why he never went back, even after 20 years.

ScottH · Dec 8, 2017

This is great to know. Certainly opens up my options.

ScottH · Dec 8, 2017

Engine size has been a concern because we don't want to wind up with a boat that that we love in every other respect but that we can't afford to cruise any distance in because of fuel costs. If small engines were the only way to get good fuel economy, then we'd be forced to limit ourselves to boats that have them.

ScottH · Dec 8, 2017

Well, my interest in fuel economy on my boat has nothing to do with appearance, it has to do with my desire to cruise vs the depth of my pockets. Sure don't want a 'dock queen' that I can't afford to travel far in.

Simi 60 · Dec 8, 2017

ScottH said:
Sure don't want a 'dock queen' that I can't afford to travel far in.

Don't park at a dock.
By doing that we have about $15,000 a year to spend on fuel and maintenance.

North Baltic sea · Dec 9, 2017

ScottH said:
Engine size has been a concern because we don't want to wind up with a boat that that we love in every other respect but that we can't afford to cruise any distance in because of fuel costs. If small engines were the only way to get good fuel economy, then we'd be forced to limit ourselves to boats that have them.

Hi,

I know this ship, it's an incredible economy of fuel consumption. engines is 14 liters / cylinder and has five rows of 70 liters displacemet and about 500 hp. He's 83 feet tall and weighs 135,000kg. Cruises at 8.5kn at 6.5 gal / h or 1.3nm / gal. I think the big engine can be economical, this engine (Wärtsilä 514T) is from 1966, that is not the latest technology.

Vinces (Turku, Finland)&

NBs

Large Engines vs Fuel Economy

Guru

Attachments

Guru

Guru

Guru

Attachments

Guru

Guru

Guru

Attachments

Guru

Guru

Senior Member

Guru

Guru

Guru

Guru

Guru

Guru

Guru

Moderator Emeritus

Veteran Member

Veteran Member

Veteran Member

Guru

Guru

Similar threads