Reply
 
Thread Tools Search this Thread Display Modes
 
Old 12-09-2018, 02:37 PM   #1
Senior Member
 
MV Wanderlust's Avatar
 
City: Lakeland
Country: United States
Vessel Name: Wanderlust
Vessel Model: 1999 Jefferson Rivanna 52'
Join Date: Feb 2018
Posts: 480
DD 6v71 fuel burn rate?

Anyone have Detroit Diesel 6v71 with turbos? Were looking at a boat that has them and were trying to figure out fuel burn rate. We normally run 8 to 8.5 knots. Anyone else have those engines and run at those speeds?

Thanks.

John
__________________
Advertisement

MV Wanderlust is offline   Reply With Quote
Old 12-09-2018, 02:52 PM   #2
Guru
 
djmarchand's Avatar
 
City: Litchfield, Ct/Punta Gorda, Fl
Country: USA
Vessel Model: Atlas Pompano 23
Join Date: Aug 2012
Posts: 4,854
You can get fuel burn info from individuals, but I always find that suspect.



Another way is to search for a spec sheet and fuel curves for your engine. Look up the rpm on the prop fuel consumption curve (not the maximum power curve) that you run to reach the speed you want. That fuel burn will probably be more accurate than anything you get from individuals. People are ALWAYS optimistic about their fuel burn.


David
__________________

djmarchand is offline   Reply With Quote
Old 12-09-2018, 04:03 PM   #3
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: 5,237
BoatDiesel.com is an excellent resource. You need to know as much as you can what horsepower the motors are rated at, as well as prop and boat displacement/hull design and length to get increasingly accurate estimates. Does the prior owner have any logs?
__________________
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 12-09-2018, 04:53 PM   #4
Senior Member
 
alfamike's Avatar
 
City: Camarillo
Country: USA
Join Date: May 2015
Posts: 335
Fuel Burn - lots of different factors need consideration.

Fuel Burn questions are very individual & boat specific & are also very speed related. Many factors go into the number. Hull shape, displacement, prismatic coefficient, drag due to shaping, depth of the hull, hull design - like - planning hull vs semi displacement hull vs full displacement hull, all effect fuel burn.

You also need to know the waterline length of the boat that the engines are in as the relationship of the speed length ratio has more effect on fuel burn that you may realize.
Take the square root of the water line length multiplied by the Speed length ratio you choose to cruse at.: 1.0; 1.1; 1.2; 1.3; & top hull speed 1.34. to determine the S/L ratio cruising speed.
Please refer to Robert Beebe's book for that info in more detail.
--- the lower S/L ratio gives better fuel economy by a large % factor.

You say 8.0 to 8.5 knots - but if your at the Max hull speed for the water line length of your example boat at 8.0 knots you can have a huge increase in fuel burn just to go that 1/2 knot faster !

A S/L ratio economy example that is good at 1.2 S/L Ratio cruising speed for local scooting around cruising speed & 1.1 or 1.0 S/L would be good for long distance runs.

So be advised other considerations need to be put into the equation. For Instance, There is sometimes more under water drag with a twin engine boat than with a single. Twin engines combined fuel burn will often be more than a single engine vessel at same comparable specs.

However, if your just looking for a fuel burn number example for these engines, I had a DD 6 V 71 turbo on a 46 footer with a 39 foot water line length single & it burned about 5.2 GPH at 8 knots at 1800 RPM. The DD's are a two stroke design & in my experience they definitely are more thirsty than a regular 4 stroke engine of same displacement & power output.
I am guessing for comparison the same equivalent boat with 4 stroke would be around about 3.8 to 4.2 GPH. under same conditions.

I hope that info helps. There are many other factors as well, like are you towing your dink, etc.. I am sure there are more guys here on TF that can give their examples & input as well.

Good Luck.

Alfa Mike
alfamike is offline   Reply With Quote
Old 12-09-2018, 05:21 PM   #5
Guru
 
tiltrider1's Avatar
 
City: Seattle
Country: USA
Vessel Name: AZZURRA
Vessel Model: Ocean Alexander 54
Join Date: Aug 2017
Posts: 1,522
As stated by others, there is a lack of information for making an educated response. If you can live with a gallon a mile at displacement speeds you won’t have a problem with a 671. If you need to make 2 miles to the gallon you probably won’t be happy with a 671.
tiltrider1 is offline   Reply With Quote
Old 12-09-2018, 06:03 PM   #6
Senior Member
 
alfamike's Avatar
 
City: Camarillo
Country: USA
Join Date: May 2015
Posts: 335
agree with "tiltrider1 "

I would agree with " tiltrider1 " above.


Those engines are a thirsty bunch with the two stroke design.

Alfa - Mike
alfamike is offline   Reply With Quote
Old 12-09-2018, 06:30 PM   #7
Guru
 
djmarchand's Avatar
 
City: Litchfield, Ct/Punta Gorda, Fl
Country: USA
Vessel Model: Atlas Pompano 23
Join Date: Aug 2012
Posts: 4,854
Quote:
Originally Posted by alfamike View Post
I would agree with " tiltrider1 " above.


Those engines are a thirsty bunch with the two stroke design.

Alfa - Mike

You guys make it sound like the DD drinks diesel. It is actually as efficient as similar older diesels like the Cummins 855. The older NA engines make about 16 hp per gph. The newer turbocharged and intercooled engines make about 18 hp per gph of fuel. Maybe not as good as a modern Cummins, but not bad.


David
djmarchand is offline   Reply With Quote
Old 12-09-2018, 06:42 PM   #8
Guru
 
tiltrider1's Avatar
 
City: Seattle
Country: USA
Vessel Name: AZZURRA
Vessel Model: Ocean Alexander 54
Join Date: Aug 2017
Posts: 1,522
DD are great engines. They were extremely economical by 1930’s standards. They became a tad thirsty by 1980’s standards. Unfortunately they are very thirsty when compared to 2000’s standards.

I would not consider them thirsty if I was comparing them to Cummins 555 or CAT 3208.
tiltrider1 is offline   Reply With Quote
Old 12-09-2018, 07:07 PM   #9
Enigma
 
RT Firefly's Avatar
 
City: Slicker?
Country: Bumpkin?
Join Date: Oct 2007
Posts: 12,761
Greetings,
Mr. JE. It has been said many times that fuel is one of the lesser costs of boating. I would be much more concerned about the actual condition of the DD than consumption. Are there maintenance records? Being a turbo, has the snot been run out of it? Do you like the rest of the boat?
My $.02...
__________________
RTF
RT Firefly is offline   Reply With Quote
Old 12-09-2018, 08:43 PM   #10
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,359
A DD is close to the same efficiency of a modern four stroke. Maybe 16hp/gph compared to 18-20hp/gph as Dave posted. But your burn rate all depends on the boat. OP, post details of the boat. Size, weight, beam etc.
Ski in NC is offline   Reply With Quote
Old 12-09-2018, 08:54 PM   #11
Senior Member
 
MV Wanderlust's Avatar
 
City: Lakeland
Country: United States
Vessel Name: Wanderlust
Vessel Model: 1999 Jefferson Rivanna 52'
Join Date: Feb 2018
Posts: 480
A lot of great information already. I guess I should have posted these details at the beginning.
It’s a 54’ Ocean Alexander. Dry weight is 52,000. It holds 800 gallons of fuel and 400 of water so I’m figuring roughly 60,000 lbs, wet. The beam is 15.6. It has fin-type stabilizers.

If it matters, the 6v71s were in-frame rebuilt about 1900 hours ago.
MV Wanderlust is offline   Reply With Quote
Old 12-09-2018, 09:00 PM   #12
Senior Member
 
MV Wanderlust's Avatar
 
City: Lakeland
Country: United States
Vessel Name: Wanderlust
Vessel Model: 1999 Jefferson Rivanna 52'
Join Date: Feb 2018
Posts: 480
The current owner runs it at trawler speeds. I checked the engines over carefully (not surveyor carefully) and saw no evidence of exhaust leaks. Some green corrosion around the impeller cover plates but nothing surprising. The only oil leak was a little seepage from a sensor. The turbos themselves aren’t leaking any oil or discolored.

Love the layout and so does the wife. Could totally see ourselves living on it after some refurbishment.
MV Wanderlust is offline   Reply With Quote
Old 12-09-2018, 10:25 PM   #13
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,359
Should run pretty well at 8-8.5kts. Engines should be pretty happy there, 1200-1400rpm doing the wild guess thing. Another wild guess would be about 6-8gph total.
Ski in NC is offline   Reply With Quote
Old 12-09-2018, 10:51 PM   #14
Senior Member
 
Boat's Avatar
 
City: SchoolHouse Branch
Country: USA
Join Date: Jul 2018
Posts: 468
A rough rule of thumb as taught years ago for the 71 series DD was one gallon per hour per cylinder while making power. Take it as a very rough estimate.
Boat is offline   Reply With Quote
Old 12-10-2018, 12:41 AM   #15
Guru
 
tiltrider1's Avatar
 
City: Seattle
Country: USA
Vessel Name: AZZURRA
Vessel Model: Ocean Alexander 54
Join Date: Aug 2017
Posts: 1,522
I know OA 54’s very well. The boat won’t go any faster than 12 knots, fuel consumption at 12 knots is like 2 gallons a mile. At 10 knots you will burn .95 gallons per mile. At 8 kts you will burn .8 gallons per mile. Normally I don’t like high HP DD671’s but with that boat the fuel curve is so severe that I doubt they were operated Hard.
tiltrider1 is offline   Reply With Quote
Old 12-10-2018, 05:21 AM   #16
Senior Member
 
MV Wanderlust's Avatar
 
City: Lakeland
Country: United States
Vessel Name: Wanderlust
Vessel Model: 1999 Jefferson Rivanna 52'
Join Date: Feb 2018
Posts: 480
Quote:
Originally Posted by tiltrider1 View Post
I know OA 54s very well. The boat wont go any faster than 12 knots, fuel consumption at 12 knots is like 2 gallons a mile. At 10 knots you will burn .95 gallons per mile. At 8 kts you will burn .8 gallons per mile. Normally I dont like high HP DD671s but with that boat the fuel curve is so severe that I doubt they were operated Hard.
I guess you would know them very well.

That's exactly the first hand information we were looking for. Thanks!
MV Wanderlust is offline   Reply With Quote
Old 12-10-2018, 05:22 AM   #17
Senior Member
 
MV Wanderlust's Avatar
 
City: Lakeland
Country: United States
Vessel Name: Wanderlust
Vessel Model: 1999 Jefferson Rivanna 52'
Join Date: Feb 2018
Posts: 480
Quote:
Originally Posted by Ski in NC View Post
Should run pretty well at 8-8.5kts. Engines should be pretty happy there, 1200-1400rpm doing the wild guess thing. Another wild guess would be about 6-8gph total.
That's right in line with what Tiltrider indicated. Perfect. Thanks!
MV Wanderlust is offline   Reply With Quote
Old 12-10-2018, 06:28 AM   #18
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 19,761
Our ex Navy Utility is quite light perhaps 10-12 tons and at 50 Ft is almost all waterline 49ft?

This relates to few boats because of the light weight , but at SL of 1 (7k ,8mph statute) we burn under 3 GPH .Measured over the loop , 6000miles.

On the flowscan 11K is 10GPH , but we have never operated there , only ran up to 1800 for a look.

The weight and LWL of the boat should give a ballpark fuel burn.

On some installations a simple injector change can increase the fuel efficiency .

If the engine was built for max power and you run at 1200-1500 as we do , smaller injectors 55-70 will be an improvement over the stock 135's.
__________________

FF is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 06:10 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012
×