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

60 ft boat? At hull speed? about 1.5-2 miles per gallon. For better info, ask the question on BoatDiesel.com.

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.

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.


David

caltexflanc said:

60 ft boat? At hull speed? about 1.5-2 miles per gallon. For better info, ask the question on BoatDiesel.com.

Click to expand...

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.

Po told me he gets about 1mpg

mbevins said:

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.

Click to expand...

How do you like the 671s?

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.

Lepke said:

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.

Click to expand...

Lepke said:

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.

Click to expand...

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.

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.

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 ?

Rustybarge said:

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 ?

Click to expand...

Go to "boatdiesel" website, they the specs for most all of DD configrations.

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.

makobuilders said:

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.

Click to expand...

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.

I think DD usually publishes engine weight with gears, which is around 2600-2700lb. Without a gear probably 2300.

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.

FF said:

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.

Click to expand...

I like your reasoning; every engineering choice is a compromise of some sort ....

What sort of weight are the 2-71 and 3-71?

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.

Pgitug said:

You gotta love DD's and their distinctive sound.

Click to expand...

I've heard it described as three Harleys starting up at the same time......

Being two stokes, a six cyl fires six times per rev. A four stroke like a Lehman fires three times per rev. That's part of the reason for the distinctive exhaust note. Much higher frequency than a four stroke given the same rpm. That and the blower is super loud, and even with a silencer a good bit of noise is from the blower.

And 12-71 fires 12 times per rev!!

Basic design dreamed up by Alex Winton, a Cleveland motor car builder. GM bought him out in the 30's when they started to sense the promise of his designs. Same design used in EMD locomotive engines, many still in service today.

Take a DD apart and it is amazing how complex the block casting is. Not just a chunk of iron with holes drilled in it like a four stroke. Humbling that they could do that like 40yrs after the Diesel was invented. And they are still in service 120yrs later.

It won't be long and someone running a Detroit is going to look at the caledar and realize there engine is 100yrs old!!

Sorry for the hijack.
I've just come to the conclusion that I have to replace the return line on one of my 6-71s. It's 5/8" blue hose. Original from Viking. (Circa 1987) Anybody know what it is?

This and the "best trawler engine" thread are really good reads.

Ski;

Ski in NC; said:

Humbling that they could do that like 40yrs after the Diesel was invented. And they are still in service 120yrs later.

It won't be long and someone running a Detroit is going to look at the caledar and realize there engine is 100yrs old!!

Click to expand...

I came across this tale last night;
The Detroit Diesel - the iconic American high speed two stroke diesel engine

And by the way, I know you would get tired of me but I sure would like to have you as my neighbour. I'm like a an old mop with everything you write.

And Oscar;

oscar said:

I've heard it described as three Harleys starting up at the same time......

Click to expand...

An absolute fallacy. You can't get 3 HDs to start at the same time.

an absolute fallacy. You can't get 3 hds to start at the same time.

Click to expand...

lol

Detroits leak enough that if you run out of fuel you can get home on bilge oil.

bayview said:

Detroits leak enough that if you run out of fuel you can get home on bilge oil.

Click to expand...

I hear this all the time but reality is mine don't leak at all.
Save and except the calamity caused last year when the yard decided I didn't really need that second crankcase vent and removed it.

mbevins said:

Sorry for the hijack.
I've just come to the conclusion that I have to replace the return line on one of my 6-71s. It's 5/8" blue hose. Original from Viking. (Circa 1987) Anybody know what it is?

Click to expand...

I had the same lines on my boat and they held up wonderfully for years. However, when I had the boat surveyed I was told that I needed to replace them with metal lines (copper?). Perhaps that is the USCG requirement? You may wish to check because this would be a good time to upgrade.

mbevins said:

I hear this all the time but reality is mine don't leak at all. Save and except the calamity caused last year when the yard decided I didn't really need that second crankcase vent and removed it.

Click to expand...

I never understood what people meant by DDs leaking. I placed a small tin can under each crankcase breather and they never filled to the brim. The 6 inspection ports on the block dripped a bit but I wiped them down after each run. No big deal. BTW, the "factory" gaskets under those port covers are horrible. I've always thought that replacing them with something more spongy/compressible would eliminate those leaks 100%.

Build a Detroit carefully, and they won't leak. Well, maybe a drip here and there. The leakers are the ones put together sloppily, mostly due to poor cleaning of the mating surfaces and seal areas.

The thing is, if any other engine was built sloppily, it too would leak IF IT RAN, which it probably won't for long. The DD will run!!!

The problem as Ski notes is two fold , some skill IS required to work on a DD ,
\
but many factory gaskets stank, and worst of all the sealants DD chose were lousy.

MY DD 6-71 is a factory fresh 1950 era "war reserve" engine taken from the storage can in Y2K or so that has never yet been apart for repairs , so is tight.

Having lost many contracts to the euros DD cleaned up its act ,

but now that they have been taken over by the Germans , good sealants and good gaskets are normal.