Twins are more efficient, got it?

[danderer]

No, I have no desire to ignite yet another debate here. I just found this interesting:

Fleming Yachts 58 (2017-) 2017 Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com

It is a 'review' of the Fleming 58 which goes to significant length to prove that 2x800 HP is more efficient than 1x175HP. See about 2 mins into the video.

 

[DHeckrotte]

Nope! Not gonna' buy a $2.9 million boat if their brochure contains typos. What else might they have missed? Unless a spiril stair is an Aussie thing which we Yanks cannot be expected to understand.

Truly interesting that a pair of idling behemoths are more fuel efficient than a single or pair sized for the job and running at some reasonable loading.

 

[JustBob]

Nope! Not gonna' buy a $2.9 million boat if their brochure contains typos. What else might they have missed? Unless a spiril stair is an Aussie thing which we Yanks cannot be expected to understand.

To be fair, I don't think this is a Fleming publication? It's boattest.com.

And, that's just the start of the typos. Ridiculous. "Waistcoating?" "staeroom?"

When I was in the market for a Fleming I reviewed all their materials extensively, don't believe I ever saw a typo.

 

[Insequent]

Well, I would lay the blame for the numerous typos at the feet of BoatTest rather than Fleming, but still compliment BoatTest for a comprehensive review.

My dream boat. I think the price is pretty good, but it is outta my reach right now!

My only suggestion would be to make it a hybrid: rather than putting hours on the two MAN's at slow speed, having electric generator/motors on each shaft would be great. Particularly given the high dealer service cost for the engines.

 

[GFC]

For as beautiful and seaworthy as the Flemings are, I don't care if they have a typo or two. Tony pays attention to the build, not the typist.


I'd love to have one of those 58's but, short of winning a lottery it's way beyond my horizons.

 

[what_barnacles]

Flemings are awesome machines.

 

[markpierce]

Next someone will claim that a 5000 h.p. engine at idle is more efficient than an 80.

 

[Simi 60]

Flemings are awesome machines.

They'd be even more awesome if they didn't need 800hpx2 and cost near $3mill.

Me, I'll stick with 315hp x 1 and near $3mill in the kitty for stripper poles and those who like to use them.

 

[twistedtree]

No mention what the other boat's length and beam are. That matters in the comparison. Also, fuel burn rates reported from electronic engines are often off by 10-15%.

If the same care went into their fuel burn comparison as went into their writing and proof reading, then I would completely discard the fuel comparison.

 

[danderer]

Silly me. I never even noticed there was a written review. I just saw the video and didn't scroll down the page further.

What I noted was how big a deal they made about the SE comparison and how cherry-picked the numbers appeared. Pick a faster-than-most-LR-trawlers-run number for the Fleming and make the SE match it. Then run the SE at WOT...???

And they had typos in the video as well.

 

[Pgitug]

Skipping the irrelevant typo issue, it is clear to me that when I run my Cummins 330hp between 1200 and 1300 RPM I get to cruise at 7.5 to 7.75 knots. My fuel consumption is minimal compared to running anything north of 1500 RPM. That said. Put one 800hp Engine in that boat and run it at 1300 RPM will be more efficient than two any day of any year. You will lose your ability for the top end speeds but save a Butt Load of annual fuel costs. IMHO. [emoji102][emoji41][emoji102]

 

[CaptTom]

Silly me. I never even noticed there was a written review. I just saw the video and didn't scroll down the page further.

Silly me. I never even noticed the video. I'm used to scrolling past that crap to read the text.

I must be a dinosaur. It's a good thing I'll be gone by the time reading and writing are totally obsolete.

 

[sunchaser]

Skipping the irrelevant typo issue, it is clear to me that when I run my Cummins 330hp between 1200 and 1300 RPM I get to cruise at 7.5 to 7.75 knots. My fuel consumption is minimal compared to running anything north of 1500 RPM. That said. Put one 800hp Engine in that boat and run it at 1300 RPM will be more efficient than two any day of any year. You will lose your ability for the top end speeds but save a Butt Load of annual fuel costs. IMHO. [emoji102][emoji41][emoji102]

For those lucky enough to own the F58 in question, one can cover most of the world as the F65 "Venture" has done. Just cut both engines back to achieve 8.5 knots or so. Saving a butt load of fuel is of little interest to most Fleming owners. If it were, they'd buy a Nordhavn. Different strokes.

 

[Seevee]

I can see where two is more efficient that one, but pretty much depends on what kind and size of engines are used.

I could argue that pushing any hull requires xxx hp to go xxx speed, and if it's supplied by a single or twins there isn't a huge difference.

However, overall, the single is often a tad better with fuel.

As for the Fleming, gorgeous boat, but I'm not a blue water boater, and for any travel to another country long distance, I'm taking a plane. I'm just a coastal cruiser so smaller is better.

But if I got a big boat, I'd probably look at a small Burger, made in the US over the Fleming.

 

[Mako]

But if I got a big boat, I'd probably look at a small Burger, made in the US over the Fleming.

I used to own a small Burger of 65ft. It was an older steel model, not aluminum, and I was quite surprised at all the failures in its design, inaccessible places, etc., which lead to corrosion problems. Also noticed similar problems in other steel Burgers that I viewed. So if you're serious then stick with the aluminum models.

Or buy Chinese - they make great boats at much better prices.

 

[diver dave]

It's hard to believe the fuel flow is even measured accurately.
3 ways I know of doing this.
1. fuel flow sensor. and all this great news would have been presented by a sensor working its low end of range. "full scale" vs accuracy of readings??
2. Measurement of injector ON time, pulses per second, and fuel rail pressure. Again, is this system calibrated? Its asking the engine manufacturer to tell you its fuel flow, not a independant 3rd party. Hmmmm.
3. Tank usage. There was no time for the magazine to work this technique, arguably the most accurate.

At a 10,000 foot view, there are some "windage" losses in huge engines that are unrecoverable. Two large 8 cyl. running 1200 rpm, I would not expect to beat a single 6 running 1600. And add the shaft losses (even submerged in oil), and blade friction addition of 4 more blades. Hmmm, smells like bad fish to me.
And the big clue is all the spealing misteaks two!

 

[Ski in NC]

Yea, sloppy to compare twins loafing at 1000 rpm to a little engine tweaked out at 2100. And the hulls are apples and oranges. And the burn rate of the MAN's could be a good bit off down at that low power setting. And the little engine boat burn numbers could also be off.

No doubt that there is more efficiency loss from two engines and two drivelines churning vs a single. If both are optimized, the single will be more efficient.

Also, if you operate any modern diesel in the middle of it's operating envelope, the differences in efficiency are very small. They all burn within 10% of each other for the hp they create. Different boats however, require very different amounts of hp to get same speeds.

Another quibble: The MAN's "have a dry turbo with a water jacket for cooling for better performance and efficiency." Ummm, that's a wet turbo. And dry turbos are more efficient than wet, but plenty risky in boats.

 

[Nomad Willy]

You-all are all scream’in and fly in around in the trees like monkeys at someone suggesting a twin be more efficient. There isn’t enough difference to talk about and that difference could go either way. Singles being “more efficient” is more religion that fact or science.

This “single is more efficient” is part religion and part a result of guys forgetting that most twins in our neck of the woods have twice the engine displacement and power. If apples and apples were ever used as a base it would be a twin and a single w the same power. But since that’s a rareity guys slip into think’in singles burn less fuel. When it’s the installed power and has almost nothing to do w singles or twins.

 

[JustBob]

I'm going with TRIPLE's!

When I'm boating with my single inclined friends, I'll just use the center engine.

People who think in pairs? Just the outer two.

Someone goes by in a speedboat? All three, maxed out. "Eat my trawler wake!"

My boat, "Menage à Trois," settles all arguments!

(and yes, that is the correct spelling for threesome!)

 

[Nomad Willy]

Bob,
Make a lot os sense.
However ..... the props will only be right for one configuration. Even w variable pitch props.

Chris Craft offered their 62’ Motor Yacht w three CC flat head 6cyl engines of 160hp. Very big engines for an old gasoline 6. Lots of cast iron.

Probably a marketing tool to offer the boat w in-house engines. Don’t know if there was a diesel option.

 

[diver dave]

These discussions make me think of a wreck dive i've done numerous times off Boca Raton, the Hydro Atlantic. German ocean dredge setup with 4 diesels, driving 2 shafts and a two story engine room. Great place to do these 1 vs 2 vs 3 vs 4 experiments. before it sank, of course.

 

[AusCan]

Chris Craft offered their 62’ Motor Yacht w three CC flat head 6cyl engines of 160hp. Very big engines for an old gasoline 6. Lots of cast iron.

I attended a Chris Craft convention at Maple Bay BC about 20 years ago. There was one boat with triple Chrysler 426 hemi's. I'm not sure if they were original, but it must have been one efficient boat.

 

[ancora]

The Fleming is a solid glass hull with no coring accordin' to the article...that has to be a plus.

 

[Nomad Willy]

I attended a Chris Cnraft convention at Maple Bay BC about 20 years ago. There was one boat with triple Chrysler 426 hemi's. I'm not sure if they were original, but it must have been one efficient boat.

AusCan,
So smooth and quiet it surely must have been a pleasure to cruise.
But I can’t see efficient. What would be more inefficient for a big CC cruiser? The hemi engines were prone to detonation because there was so little turbulence it the symmetrical and compact combustion chamber. Compression ratios were very limited compared to other engines of the day. Efficiency? ... probably a tad above a Buick straight eight w a c/r of less than 7-1.

 

[Sabre602]

Hello all, I'm new here and you may discover that I'm sort of a science geek.

The "efficiency" being described is a bit of a red herring. What is being discussed, engine efficiency or vessel efficiency? Engine efficiency is simply a ratio of fuel energy to work, i.e., how much work can you do with the same liter/gallon of fuel? In general, fewer engine cylinders equal higher efficiency due to increased frictional losses as the number of cylinders is increased. My single-cylinder 650 cc motorcycle engine will always operate more efficiently than a multi-cylinder engine of the same displacement, with decreasing efficiency as the number of cylinders is increased.

Duplicating the single engine in a twin-engine configuration doubles not only internal friction losses (cylinder walls, rod bearings, valve trains, etc) but also adds new losses in a second gearbox, shaft and prop.

Owners, operators and builders of commercial cargo ships are motivated by one thing and one thing only: optimizing profit. These ships use single engines, enormous great beasts, with a relatively small number of cylinders.

Performance? Sure, add another engine or two. But if you're really looking at engine efficiency...the work obtained from each tank of fuel...it will decrease as the fun factor increases.

As far as loping along at low revs versus operating at higher RPMs, this is complicated by different engineering factors in modern diesel engines. In general, higher revs result in higher friction losses...but not always. In general, higher revs means a more favorable effective compression ratio and therefore increased efficiency...but not always. Direct injection versus indirect, variable turbocharging, electronic injection and/or valve timing...it just depends upon the design of the engine, whether it's designed to be operated at higher or lower revs.

Now if we're talking about vessel efficiency, effectively miles per gallon, the comparison of number of engines is just silly because hull shape and length are left out of the equation.

As was astutely pointed out by a previous poster, it's really apples-to-oranges to compare a multi-engine, semi-displacement vessel with a single-engine, pure displacement vessel designed to travel much more slowly. They're designed for vastly different performance and for vastly different owners.

Oh, and the single versus twin thing? Which is more efficient? What does the math reveal?

Who cares? The one for me is the one that gives me the biggest grin. That's why I chose a single-engine vessel...not because of percentages and ratios, but because it's the one I LIKE.

 

[Wxx3]

Hello all, I'm new here and you may discover that I'm sort of a science geek.

...

Sabre, a very thoughtful, articulated and scientific response.

Thanks.

I don't have the patience to explain as well as you, as all I was going to say was:

No, it's not a matter of opinion, but simply science and physics.

 

[markpierce]

Cruise ships typically have about five large diesel engines to generate electricity, no doubt not all are continuously used. Must be interesting to balance the electrical load as they spend much of their time anchored or docked.

 

[Nomad Willy]

Sabre602 wrote;
“Duplicating the single engine in a twin-engine configuration doubles not only internal friction losses (cylinder walls, rod bearings, valve trains, etc) but also adds new losses in a second gearbox, shaft and prop.”

Why would there be added losses? One would assume trying to approach apples and apples an appropriately smaller gear, shaft and prop. Seawater pumps, cyl walls, rod bearings valve trains ect ect would all be 1/2 the drag of the bigger single engine. So both should have very close to the same % of loss per hp.

But I’ve never seen or heard of a comparison that involved 2 boats and engine combinations that would yield an actual apples to apples comparison.

Thanks for your very excellent post #25. Hope you become a long time poster and member.

 

[markpierce]

How many angels can dance on the head of a pin?

 

[Lepke]

I commercial fished a 55' boat for salmon and tuna off most of the northern west coast. I had twins, uncommon, as most people had singles, but most engines were Detroits. I got similar mileage or better than folks with similar sized boats. After I sold the boat, the new owner converted to a single, from 2 Detroits to a Cummings and got slower speeds at higher fuel use. Not a scientific study, but the same hull.