Reply
 
Thread Tools Display Modes
 
Old 04-26-2015, 02:20 PM   #21
Guru
 
O C Diver's Avatar
 
City: Fort Myers, FL... Summers in Crisfield, MD
Country: USA
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 4,803
Quote:
Originally Posted by eyschulman View Post
I was not interested in opening the whole can of worms just the fuel burn issue. There are good reasons for a single and good reasons for twins and hopefully the builder and designer matches the propulsion to the boat and visa versa. Tony Fleming and Fleming yachts has done some long term thinking and testing relative to the twin verses single motor and fuel burn and range were included in their decision to stay with twin motors. I come across owners of the typical Trawler type like the Helmsman multiple Asian build's including GB etc. who just assume their single engine boat is more fuel efficient than my big motor twins and it just isn't so even when the boats are close in size and weight meaning with good design you can have twins and good fuel burn.
The two take aways from your post:

With good design you can have twins that are likely to have as good fuel consumption as a single. Problem is most boat designers don't view fuel efficiency with twins as an important an issue as with a single. The KK 39 would be a great example. Hard to find 2 small diesels that will give you the fuel efficiency of that one 4 cylinder JD.

2nd take away: One never truly knows why someone like Fleming stays with twins. He may conclude that his average customer does little more than check the oil and needs the redundancy of twins for when the neglected fuel filter clogs. From some of the stories posted on TF about previous owners lack of basic maintenance, it would be easy to conclude that many owners lack the preventive maintenance discipline to own/ operate a single.

Ted
__________________
Advertisement

__________________
Blog: mvslowhand.com
I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush.....
"Slow Hand" by The Pointer Sisters
O C Diver is online now   Reply With Quote
Old 04-26-2015, 02:29 PM   #22
Guru
 
City: NC
Country: US
Join Date: Jan 2014
Posts: 635
To throw another variable in the comparison is the affect EPA/MARPOL regulations on engine fuel burn. An older engine might be more efficient than a newer one.

The Tier II engine put in the boat we want, produces 121 HP with 6.7 GPH. The new Tier III engine is 125 HP at 8 GPH. At cruise speeds and trying to keep a proper load on the engine, it looks like one will burn about 1/2 GPH MORE on the Tier III engine vs the old Tier II engine. Instead of 2 GPH you will need 2.5 GPH which is a pretty big increase in fuel burn.

JD has published different GPH values for the Tier III engine as either 7.7 or 8.0. They do seem to do some rounding on their power curve for the Tier III engine vs their other engines but even if one uses 7.7 GPH for 125 HP that is still 1 GPH MORE than the 121 HP Tier II engine. If a gallon of diesel creates 18-20 HP where is the HP in the Tier III engine? Not that the extra HP is needed but if one is burning the fuel.....

Later,
Dan
__________________

dannc is offline   Reply With Quote
Old 04-26-2015, 02:47 PM   #23
Guru
 
City: Carefree, Arizona
Country: usa
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 6,355
Quote:
Originally Posted by O C Diver View Post
One never truly knows why someone like Fleming stays with twins. He may conclude that his average customer does little more than check the oil and needs the redundancy of twins for when the neglected fuel filter clogs.
Ted
Good one Ted, that should get some twin devotees spinning. As well as buyers of Nordhavns, 99%+ whom opt for the get home or twin option.

Having spent some time talking directly with Tony Fleming, hard to see where he feels this way either. I have been on several Flemings including Tony's and can only say owner maintenance appears top notch with Fleming's Porsche like resale values indicating the same.

Not to mention that Flemings are designed and expected to cruise easily at 16 knots or throttle back to 8 - 9 knots and trawl along without the engines gumming up. Don't know of a single diesel that would FIT in a 55 that could do that. Let alone allow the F55 or F58 to sell in the market place.
sunchaser is offline   Reply With Quote
Old 04-26-2015, 03:10 PM   #24
Guru
 
Join Date: Aug 2012
Posts: 741
[QUOTE=caltexflanc;328128]Who? I have never seen an actual test where there was a significant difference at the same speed. A few guys on the Hatteras owners Forum tried it. I once had the need to do this test myself on a chartered Mainship 430 with Floscans over the course of 120 miles when one engine wouldn't start. I don't have the stats anymore, but it was basically a push, at some speeds slightly better with two.

[QUOTE]

Depends what you consider significant. I did a comparison several years ago....semi-planing, 34,000 pound hull with twin turbo 250's. I used manufacturer prop charts versus flo-scan or actual fuel burn for the comparison. At 8.4 knots, free wheeling prop, the single engine operation showed 10-15% improvement over the twin. No surprise that the engines see efficiency gains when operated a little farther into the turbo boost area. I believe TIMJET produced similar results. NA engines would likely tell a slightly different story.
Underway is offline   Reply With Quote
Old 04-26-2015, 03:11 PM   #25
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,879
Quote:
Originally Posted by dannc View Post
To throw another variable in the comparison is the affect EPA/MARPOL regulations on engine fuel burn. An older engine might be more efficient than a newer one.

The Tier II engine put in the boat we want, produces 121 HP with 6.7 GPH. The new Tier III engine is 125 HP at 8 GPH. At cruise speeds and trying to keep a proper load on the engine, it looks like one will burn about 1/2 GPH MORE on the Tier III engine vs the old Tier II engine. Instead of 2 GPH you will need 2.5 GPH which is a pretty big increase in fuel burn.

JD has published different GPH values for the Tier III engine as either 7.7 or 8.0. They do seem to do some rounding on their power curve for the Tier III engine vs their other engines but even if one uses 7.7 GPH for 125 HP that is still 1 GPH MORE than the 121 HP Tier II engine. If a gallon of diesel creates 18-20 HP where is the HP in the Tier III engine? Not that the extra HP is needed but if one is burning the fuel.....

Later,
Dan
As I understand it, to meet EPA regs for NOx, new engines run slightly retarded injection timing. Downside is lower efficiency. I've gone through lots of burn rate data for various engines and the new common rail stuff is generally similar or slightly worse BSFC. Nowhere have I seen a significant improvement with CR tech.

Also, not all manufacturers are very good at publishing dyno data. Cat and Cummins do, others just some coarse charts.
Ski in NC is online now   Reply With Quote
Old 04-26-2015, 03:51 PM   #26
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,855
Depends what you consider significant. I did a comparison several years ago....semi-planing, 34,000 pound hull with twin turbo 250's. I used manufacturer prop charts versus flo-scan or actual fuel burn for the comparison. At 8.4 knots, free wheeling prop, the single engine operation showed 10-15% improvement over the twin. No surprise that the engines see efficiency gains when operated a little farther into the turbo boost area. I believe TIMJET produced similar results. NA engines would likely tell a slightly different story.

Thanks didn't recall who until you posted and mentioned Timjet...I too think he might have posted some results showing single engine economy over running both.

There just so many variables with what boat at what speed requiring what HP, etc..etc... that I have often posted just that..."it depends" but always leaning toward the premise a boat designed for a given speed and engine accordingly...the losses have to be greater for twins if everything else is or nearly perfect. So pure chance favors some boats run at certain speeds on one versus two just might get significant fuel savings...but you have to live with the parameters that give that efficiency.
psneeld is online now   Reply With Quote
Old 04-26-2015, 06:00 PM   #27
Guru
 
caltexflanc's Avatar
 
City: North Carolina for now
Country: USA
Vessel Name: Small Incentive
Vessel Model: Boston Whaler 130 Sport
Join Date: Aug 2011
Posts: 3,783
[QUOTE=skidgear;328160][QUOTE=caltexflanc;328128]Who? I have never seen an actual test where there was a significant difference at the same speed. A few guys on the Hatteras owners Forum tried it. I once had the need to do this test myself on a chartered Mainship 430 with Floscans over the course of 120 miles when one engine wouldn't start. I don't have the stats anymore, but it was basically a push, at some speeds slightly better with two.

Quote:

Depends what you consider significant. I did a comparison several years ago....semi-planing, 34,000 pound hull with twin turbo 250's. I used manufacturer prop charts versus flo-scan or actual fuel burn for the comparison. At 8.4 knots, free wheeling prop, the single engine operation showed 10-15% improvement over the twin. No surprise that the engines see efficiency gains when operated a little farther into the turbo boost area. I believe TIMJET produced similar results. NA engines would likely tell a slightly different story.
Improvement how? In fuel efficiency? My results, and those of others was a push overall in actual consumption. with the twins, as you got closer to hull speed, actually outperforming (Cat 3116 in this case). Think of it this way, it essentially takes a certain amount of HP to drive the boat a given speed.And a certain amount of fuel to generate that horsepower. In addition, the single engine operation on a set of twins now requires rudder to maintain course, causing the single to have to "crab" the boat forward. This is likely what caused the twins to outperform at higher speeds.

I'd say hull form , distance between the props and the prop configuration itself are the biggest differentials, and why a variety of different tests yield different results. Some transmissions do not allow freewheeling, but require the shaft to be locked with the engine off, so there is another possible factor.
So I would never generalize that one out of two engines is better than two.
__________________
George

"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"
caltexflanc is offline   Reply With Quote
Old 04-26-2015, 06:26 PM   #28
Guru
 
Join Date: Apr 2014
Posts: 1,291
Quote:
Originally Posted by manyboats View Post
The efficiency of the engine itself running in the boat and the way the engine is matched to the power requirement of power for the boat and that in turn matched to the engine speed that produces the best efficiency for that engine. The difference between a single and a twin isn't worth talking about if both are run at 60 to 75% load.

The main reason singles are considered more efficient that Twins is that Twins are almost always equipped w twice the power. So while cruising at any economical speed the twin engines will be very underloaded. A twin running at the same load as a single will be just as efficient as a single except for the consideration of the drag of the keel and exposed running gear of the twin. Probably fly stuff.

So load the twin the same as the single and there won't be enough difference to talk about.
Re; the overloading issue- That certainly was the case with old iron. The new common rail full electronic engines are wizards at self correcting and adjusting the fuel burn relative to the demands of the drive system. This was developed mainly for environmental reasons so the engines will burn clean and anybody who now has new iron and compares it to what they used to have becomes very aware of the situation no smoke no slick from start up and any rpm short of a bad overloaded prop. A side benefit of this fueling system is that big diesel engines can be run for long periods at lower rpm and loads with no harm and many big yachts have been crossing oceans at low load on larger engines to get long range including NH-KK and similar boats. So large motors single or twin that put out lets say 120HP or 2X60HP will burn close to the same fuel. There is some extra burn on the twin when idling and maybe from the twin running gear but not always the larger keel usually associated with the single also has its drag. I am not tiring to say twins are better than single or vise versa just pointing out that people who want twins can also have good fuel burn if they pick the boat for that criteria. The assumption that a single engine boat is more efficient than a twin just does not hold water as a generalization. I suspect but have no proof that even in an identical boat with modern motors there would probably not be enough fuel burn difference to use that criteria to pick one over the other.
eyschulman is offline   Reply With Quote
Old 04-26-2015, 06:41 PM   #29
Guru
 
twistedtree's Avatar
 
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 3,174
Physics is everywhere, and you can't escape it.

Back in elementary school (or maybe high school?) you might remember significant digits. The digit in the 1000s position matters much more than the digits in the 100s, 10s, and 1s position. 10 times as much to be precise.

With fuel consumption we all seem to become blind to significant digits. The factors influencing fuel consumption are just like significant digits - some matter much more than others. Speed is the most significant digit. Next is boat length and displacement, but the two are closely linked and somewhat cancel either other out. You can't build a longer boat to reduce fuel consumption without also increasing displacement which increases fuel consumption. Next are things like singles vs twins, how you prop your boat, and other such things.
__________________
www.MVTanglewood.com
twistedtree is offline   Reply With Quote
Old 04-26-2015, 08:35 PM   #30
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,701
Our Diesel engines are heat engines and any heat lost is lost efficiency. So if you have a big engine doing little work putting out low levels of power it's burdened buy the huge amount of heat loss through big cylinders, pistons and combustion chambers. You don't run your engine coolant at 120 degrees because there's too much heat loss.

So yes twistedtree,
Physics, physics is everywhere, and you can't escape it."
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 04-26-2015, 08:51 PM   #31
Guru
 
Join Date: Aug 2012
Posts: 741
[QUOTE=caltexflanc;328194][QUOTE=skidgear;328160]
Quote:
Originally Posted by caltexflanc View Post
Who? I have never seen an actual test where there was a significant difference at the same speed. A few guys on the Hatteras owners Forum tried it. I once had the need to do this test myself on a chartered Mainship 430 with Floscans over the course of 120 miles when one engine wouldn't start. I don't have the stats anymore, but it was basically a push, at some speeds slightly better with two.



Improvement how? In fuel efficiency? My results, and those of others was a push overall in actual consumption. with the twins, as you got closer to hull speed, actually outperforming (Cat 3116 in this case). Think of it this way, it essentially takes a certain amount of HP to drive the boat a given speed.And a certain amount of fuel to generate that horsepower. In addition, the single engine operation on a set of twins now requires rudder to maintain course, causing the single to have to "crab" the boat forward. This is likely what caused the twins to outperform at higher speeds.

I'd say hull form , distance between the props and the prop configuration itself are the biggest differentials, and why a variety of different tests yield different results. Some transmissions do not allow freewheeling, but require the shaft to be locked with the engine off, so there is another possible factor.
So I would never generalize that one out of two engines is better than two.
You said you'd never seen a test where there was a significant difference in efficiency at the same speed. I cited two tests that were documented in considerable detail here on T-F that netted in the range of 10-15% (in fuel efficiency). While that's not a large improvement....it's not always a "push". Obviously, the results depend on many factors (add rudder size/efficiency and engine configuration to your list).
Underway is offline   Reply With Quote
Old 04-26-2015, 09:33 PM   #32
Guru
 
City: Hotel, CA
Country: Fried
Join Date: Sep 2011
Posts: 8,328
Quote:
Originally Posted by eyschulman View Post
I was not interested in opening the whole can of worms just the fuel burn issue.

Oh yes you where.
__________________
Craig

It's easier to fool people than to convince them that they've been fooled - Mark Twain
CPseudonym is offline   Reply With Quote
Old 04-27-2015, 12:26 AM   #33
Art
Guru
 
Art's Avatar
 
City: SF Bay Area
Country: USA
Vessel Model: Tollycraft 34' Tri Cabin
Join Date: Feb 2011
Posts: 7,979
Quote:
Originally Posted by CPseudonym View Post
Oh yes you where.
Art is offline   Reply With Quote
Old 04-27-2015, 01:17 AM   #34
Guru
 
Join Date: Apr 2014
Posts: 1,291
Quote:
Originally Posted by eyschulman View Post
I am reopening a old can of worms. the issue of the fuel burn of a single motor Vs twins. Theoretically on the same boat the single should be more efficient and the motor size closest to the HP needed should be the most efficient. What I have noticed over and over again is that on a practical real world basis this is not always the case in any significant measure particularly if similar but not identical boats are compared. I see many circumstances where twins even large twins do as well as single engine boats. As an example that is not in any way unique we can compare the fuel burn published for the Helmsman 43 at approx. 35,000 lb disp. against my boat at 48 ft and approx. 35,000lb disp. The helmsman has a single 370 HP engine and my boat has twin 330 Hp engines. The Helmsman at 8.3K burns 4.9 G/Hr and at 9.5K burns 8.4 G/Hr at 10.5K the burn was 16.3 G/Hr. Compared to my boat we find 8.3K at 4.4 G/Hr and 9.6K at 7 G/Hr and 10.3K at 11 G/Hr. There are realistic reasons against twins but for me the fuel efficiency argument does not fly. I am of the school that believes the boat and prop or props will demand x amount of power for y amount of speed and the x from one or two engines is pretty much in the same ball park in real world terms and whatever drag and lose from two engines is not significant to speak against twins.
Note first two sentences above. While I do own a boat with twins I also own two with singles and in the past have cruised and lived aboard a full displacement trawler with single motor with a very fuel efficient drive train. Actually the boats I have owned numbering over 20 have been single to twin in a ratio of 10/1. So I appreciate both sides of the twin single arguments. What I have pointed out is that you can have a boat with twins and still have good fuel burn numbers even better than some if not many of the boats thought of on this site as trawlers and therefore fuel efficient. There are many factors that influence a boats fuel burn and what I point out is that the single twin option is not the major factor. Many point out that twins have more drag and I wonder if that big unusually wide keel and rudder that has to offset prop side thrust on a single is drag free. The extra weight of twins will effect fuel burn but all in all on a practical basis I agree with Tony Fleming the fuel burn difference if it exists is not enough to discourage the choice of twins.
eyschulman is offline   Reply With Quote
Old 04-27-2015, 06:38 AM   #35
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,855
Parasitic/skin drag drops way off the slower you go...thus the whole argument starts to fall apart unless specific speeds...in fact everything concerned....are addressed.


Even hull shape starts to become less important the slower you go...like the post in another thread that sowed the effort to move a planning hull at very slow speeds isn't much different than a full displacement.


So wasted motion, wasted friction, prop inefficiencies, etc etc...CAN, but not NECESSARILY can add up to make equal footing (efficiency) very difficult.


Flemmings wouldn't be as popular as a 8-9 knot (only) boat...and the price puts it out of the "worried about fuel costs crowd" unless they are cruising around the world every year.
psneeld is online now   Reply With Quote
Old 04-27-2015, 08:03 AM   #36
Art
Guru
 
Art's Avatar
 
City: SF Bay Area
Country: USA
Vessel Model: Tollycraft 34' Tri Cabin
Join Date: Feb 2011
Posts: 7,979
Quote:
Originally Posted by psneeld View Post
Parasitic/skin drag drops way off the slower you go...thus the whole argument starts to fall apart unless specific speeds...in fact everything concerned....are addressed.


Even hull shape starts to become less important the slower you go...like the post in another thread that sowed the effort to move a planning hull at very slow speeds isn't much different than a full displacement.


So wasted motion, wasted friction, prop inefficiencies, etc etc...CAN, but not NECESSARILY can add up to make equal footing (efficiency) very difficult.


Flemmings wouldn't be as popular as a 8-9 knot (only) boat...and the price puts it out of the "worried about fuel costs crowd" unless they are cruising around the world every year.
Well put ps

As our twin 34' Tolly planning hull does near 3 nmpg at 5.5 to 6 knots on one engine and about 2.5 nmpg at 7 knots on two engines (7.58 knots is math calced "hull speed")... that suffices for me regarding efficient cruising.

When I want/need to get some place due to time schedule I can cruise her at 16 to 17 knots (or even a bit above) on full plane with approx 1 nmpg consumption. Also, Tolly's hard chine, quick return hull shape makes for considerable comfort in seas as well as wakes while running or at anchor. We appreciate its hull design's hard return much more than high-degree rolling back and forth of a round bottom. Also, with ability for nearly instant higher speed adjustments our Tolly's nimble ability to deal with tall wakes of oncoming or passing big-fast boats is greatly increased compared to displacement hulls that do not enable so much immediate hull position alterations on tall wake waves.

For a self-contained, capable, and very reasonable purchase price as well as a low maintenance cost boat… it is my and my wife’s considered opinions that Tollycraft 34' tri cabin fits the (our) bill!

Happy Boating Daze! - Art
Art is offline   Reply With Quote
Old 04-27-2015, 10:15 AM   #37
Guru
 
City: Carefree, Arizona
Country: usa
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 6,355
Art

How do measure gallons consumed?
sunchaser is offline   Reply With Quote
Old 04-27-2015, 10:53 AM   #38
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,701
I know where you're going Tom and probably like most I suspect. Looking at the Hobbs and taking note of fuel purchaseses in gallons. I think most everybody does that thinking their non-cruising time is inconsequential. But it's usually not ... IMO.

But also in Art's case his boat is light (comparatively) and has a rather narrow beam at the WL. So his boat is somewhat ideal for what he is doing.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 04-27-2015, 11:27 AM   #39
Senior Member
 
cafesport's Avatar
 
City: Miami
Country: USA
Join Date: Jul 2013
Posts: 492
For me the conventional wisdom on the whole 1 vs 2 engine passage maker was driven by Beebe. After I crossed to the dark side from sailboats he became my reference and consequently I wanted no part of any twin engine boat. Well now having owned one for 2 1/2 years, and having done the same trips on a single engine version of my boat I've concluded that 2 engines are a luxury and I'm willing to pay for that. For me the small difference in fuel efficiency with regards to maximum range between fuelings are more than offset by the advantages of twin keels twin rudders twin hydraulic pumps and propulsion redundancy. Last month when a dock master in the Bahamas guided us to a slip that was a little shallower than normal with the spring tides we ran aground 20' into the slip. As the tide rose we attached lines and pulled her in and then secured her for 2 days. No worries about heeling over for four hours each low tide cycle.


Via iPhone.
__________________
Via iOS.
cafesport is offline   Reply With Quote
Old 04-27-2015, 12:13 PM   #40
Guru
 
City: NC
Country: US
Join Date: Jan 2014
Posts: 635
Quote:
Originally Posted by Ski in NC View Post
As I understand it, to meet EPA regs for NOx, new engines run slightly retarded injection timing. Downside is lower efficiency. I've gone through lots of burn rate data for various engines and the new common rail stuff is generally similar or slightly worse BSFC. Nowhere have I seen a significant improvement with CR tech.

Also, not all manufacturers are very good at publishing dyno data. Cat and Cummins do, others just some coarse charts.
What is odd about the new JD Tier III 4045 spec sheet is that it is not in the same format as previous versions. I have spec sheets for a couple of the older engines and they are consistent in format. The Tier III is the odd ball, in more ways that one. In the grand scheme of things, loosing the precision is not really that important, just slightly frustrating/questionable.

Seeing a new engine that is less fuel efficient as the older engine is frustrating. I wonder if these engines can be tweaked to improve fuel burn after installation....

Later,
Dan
__________________

dannc is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 08:11 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012