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Old 07-06-2016, 04:19 PM   #1
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Detroit 671s

Hi everyone
I'm considering purchasing a 60 trawler that has a couple of 260HP Detroit 671s. Is anyone out there cruising with these lower Hp 671s that can give somewhat of an idea what the fuel consumptions is on these,
Thanks
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Old 07-06-2016, 05:06 PM   #2
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60 ft boat? At hull speed? about 1.5-2 miles per gallon. For better info, ask the question on BoatDiesel.com.
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Old 07-06-2016, 05:27 PM   #3
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Thanks for your input!
I always say that fuel consumption many times is a right arm issue. The further you push those throttles the more it will cost you. I estimate about 1.5 miles per gallon.
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Old 07-06-2016, 07:57 PM   #4
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I am guessing closer to 1 NM/gal at hull speed. Here is some math and a couple of rules of thumb:


A displacement hull trawler needs 1.5 hp per 1,000 lbs of displacement to reach hull speed. Assuming that the boat weighs 100,000 lbs then that is 150 hp, a reasonable number for a couple of DD 6-71s. Hull speed should be about 1.34*sqrt(55)= 10 kts. A NA DD should make about 15 hp per gph.


So you will burn about 10 gph at 10 kts or 1 NM/gal. If you drop your speed to about 8 kts you should easily hit 2 NM/gal.


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Old 07-07-2016, 07:33 AM   #5
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60 ft boat? At hull speed? about 1.5-2 miles per gallon. For better info, ask the question on BoatDiesel.com.
My 44 Viking has turbo'd 671s
Boat weighs 45K. At 7knt 800RPM
I consume just about 1.5 mpg of about 3.5 gph.
These are combined numbers for both engines.
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Old 07-07-2016, 08:19 PM   #6
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Po told me he gets about 1mpg
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Old 07-12-2016, 05:33 PM   #7
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My 44 Viking has turbo'd 671s
Boat weighs 45K. At 7knt 800RPM
I consume just about 1.5 mpg of about 3.5 gph.
These are combined numbers for both engines.
How do you like the 671s?
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Old 07-12-2016, 06:56 PM   #8
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I have early twin natural 671s in an 83' 80 ton hull. Running at 1800rpm I get 10 knots @ 8.5 gallons an hour. Fresh overhaul. 33x30 4 blade props. It's not the most economic speed, but I like 10 knots for dead reckoning in my oldtimer mind. Probably 1200-1400 would be a best economic speed.
I've been running engines like these all my life and probably get a better tune. I've never found a more reliable engine. I'll replace them when they take my starter buttons from my cold dead hands.
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Old 07-12-2016, 07:23 PM   #9
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I have early twin natural 671s in an 83' 80 ton hull. Running at 1800rpm I get 10 knots @ 8.5 gallons an hour. Fresh overhaul. 33x30 4 blade props. It's not the most economic speed, but I like 10 knots for dead reckoning in my oldtimer mind. Probably 1200-1400 would be a best economic speed.
I've been running engines like these all my life and probably get a better tune. I've never found a more reliable engine. I'll replace them when they take my starter buttons from my cold dead hands.
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Old 07-12-2016, 07:48 PM   #10
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I have early twin natural 671s in an 83' 80 ton hull. Running at 1800rpm I get 10 knots @ 8.5 gallons an hour. Fresh overhaul. 33x30 4 blade props. It's not the most economic speed, but I like 10 knots for dead reckoning in my oldtimer mind. Probably 1200-1400 would be a best economic speed.
I've been running engines like these all my life and probably get a better tune. I've never found a more reliable engine. I'll replace them when they take my starter buttons from my cold dead hands.
I could not agree more, my detroits will outlive me! Run my twin 671's at 1600 and get similar results in a 68' 65 ton hull, with 3.5:1 gears swinging 38x38 3 blade props. The engines are 1971 vintage 671 naturals, with 4 valve heads and n60 injectors.
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Old 07-13-2016, 06:09 AM   #11
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One nice thing about DD 6-71 is you can change the fuel burn by changing the injectors.

DD 2 strokes need to be loaded to about 60% of the rated HP, so if as is common the engines are too powerful, when you get the injectors rebuilt just specify the tips that will work better for your boat..

Older DD dealers may still have the engine graphs , so look for the Cruise rpm of your DD configuration , see what HP is available , and chose a lower HP that will be over 60% at cruise.

Our Launch went from 135 to 60 injectors with no problem.

Injectors are rated in CC per THOUSAND injections.

One problem is IF the engine is early and uses early H injectors the later N series must NOT be installed.

Poor starting will result.
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Old 07-13-2016, 06:22 AM   #12
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Some very interesting stuff about DD's, and all the positive anecdotal evidence supporting their robust nature.

Can't find any weight data on the various DD x-71 engines; anyone got a link ?
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Old 07-14-2016, 05:49 AM   #13
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Some very interesting stuff about DD's, and all the positive anecdotal evidence supporting their robust nature.

Can't find any weight data on the various DD x-71 engines; anyone got a link ?
Go to "boatdiesel" website, they the specs for most all of DD configrations.
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Old 07-14-2016, 06:27 AM   #14
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I have been holding onto this chart for a long time as a guide. The surprise is that it indicates about 18.5 hp/gal-hr, which is an excellent efficiency.
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Old 07-14-2016, 07:29 AM   #15
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I have been holding onto this chart for a long time as a guide. The surprise is that it indicates about 18.5 hp/gal-hr, which is an excellent efficiency.
That is an impressive efficiency for such an old design, partly due I think to the low revving nature of the engine. Depending on a scavenger blower to clear the cylinders of exhaust gasses with no inlet valves must be a very slow method of getting a fresh air charge into the cylinders.

..but the engine weighs a hefty 1250kg for 175hp continuous output which is more than double a modern high speed diesel which would make it only suitable for heavy weight FD hulls.

I love the idea of all the shared components of the different sizes and cylinder layouts of the - 71 series DD's.
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Old 07-14-2016, 08:26 AM   #16
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I think DD usually publishes engine weight with gears, which is around 2600-2700lb. Without a gear probably 2300.
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Old 07-14-2016, 03:07 PM   #17
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1500lbs for a lightweight power source with gear , 3000 lbs for a real heavy duty motor .

At 3 hp per ton (2240) ,to move, that 1500lb difference would cost about 2 hp of extra fuel burn,per hour.

At 16 to now! 18 HP per gallon of fuel 2 hp is worth the price based only on parts availability.Both now and in 50 years.

By simply selecting a 2-71 or 3-71 the weight would be cut dramatically ,

if 40-60hp or 60-90hp would cruise the boat.
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Old 07-15-2016, 04:10 AM   #18
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1500lbs for a lightweight power source with gear , 3000 lbs for a real heavy duty motor .

At 3 hp per ton (2240) ,to move, that 1500lb difference would cost about 2 hp of extra fuel burn,per hour.

At 16 to now! 18 HP per gallon of fuel 2 hp is worth the price based only on parts availability.Both now and in 50 years.

By simply selecting a 2-71 or 3-71 the weight would be cut dramatically ,

if 40-60hp or 60-90hp would cruise the boat.
I like your reasoning; every engineering choice is a compromise of some sort ....

What sort of weight are the 2-71 and 3-71?
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Old 07-15-2016, 07:12 AM   #19
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We had 671's for our two generators. But the twin mains were DD V12's. After sitting in the cold Christmas vacation one of the V12's were started. The fuel rack was worked on by a mechanic wanna be tug pilot. The cold V12 started with WOT. Ran for about five minutes before getting shut down. No damage. That was over ten years ago. Unit still running strong. You gotta love DD's and their distinctive sound.
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Old 07-15-2016, 10:03 AM   #20
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You gotta love DD's and their distinctive sound.
I've heard it described as three Harleys starting up at the same time......
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