Detroit Diesel 71 series engines

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Lou_tribal

Lou_tribal

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Hello fellow TFers!
While reading another recent thread about engines I started to search info about Detroit Diesel 71 series engines. I am not owning any but reading history and information I was very interested about it (for my own curiosity/education).
I love old engines and I found really interesting the long history of the series and its architecture etc.
Would you have any story/info about these engines please chime in!

L
 
I talked to some of the old-timers on shrimp boats back when I was a kid about the way things were long before my time. To them the "new" Detroit diesels were a luxury. The old diesels were single cylinder, direct drive, and you started them rotating backward when you wanted reverse.
 
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The Detroit diesel was designed to be a light weight high performance diesel. While rather complex, it completely achieved its goal. Of course we are talking about a WWII engine.
 
Lou, I had 8v71 Detroit's. The 71 series, and it's cousins the 53, 92, 149 (and maybe other displacements) were designed in the 1930's. One elegant design element of these 2-cycle engines was commonality of parts. Many parts in a given series are the same regardless of the number of cylinders. Often criticized for leaking lube oil and being noisy, they are rugged and long-lived especially in lower output, naturally aspirated versions. There are thousands of these engines in workboats around the world. Parts are available and they are rebuildable. They just keep chugging.
 
Ken E. said:
Lou, I had 8v71 Detroit's. The 71 series, and it's cousins the 53, 92, 149 (and maybe other displacements) were designed in the 1930's. One elegant design element of these 2-cycle engines was commonality of parts. Many parts in a given series are the same regardless of the number of cylinders. Often criticized for leaking lube oil and being noisy, they are rugged and long-lived especially in lower output, naturally aspirated versions. There are thousands of these engines in workboats around the world. Parts are available and they are rebuildable. They just keep chugging.
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I read that the last 71 series engine was delivered in the 90s, this is a heck of a longevity for an engine that started to be built in the 30s!

L
 
The 6-71 is unique that it can be assembled to have either rotation ,

this can easily be changed later, and service items on either side, your choice.

Power can be adjusted with different sized injectors, and different timing , adjustable.

Power is 20-30 HP per cylinder for long life ,,75 HP for 1,000 hours sport fish.

Nice for boat builders. Not light (unless aluminum blocks for minesweepers) .

Great engineering for 1936 , killed by air police .World wide parts sources , will probably go another 50 years.
 
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I saw that the first inline serie was available in 1 to 8 cylinders, the 3-71 output was around 130hp, quite good for a 3 cylinder engine!
Was there any drawback about these 2 cycle beside the oil leak and noise mentioned above?
Also what was the cause for these known leaks?

L
 
Greetings,
Mr.L_t. I don't really know all that much about 71's other than my BIL had a pair of 425 HP's in his sport fish BUT his 6-71's were honkin' big engines. So I expect weight would be a drawback in certain vessels.
 
FF said:
Nice for boat builders. Not light (unless aluminum blocks for minesweepers)
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I had a pair of aluminums in my previous boat. Engines were 50 years old and I had just rebuilt them. The PO had cruised that boat from the Great Lakes to Aussieland and back - during the same time that Jacques Cousteau was amazing us with his exploits. Reading the 3 inch thick ship's log was like reading a novel by Jules Vern.

Marlinmike said:
Nothing sounds better then a DD starting up in the morning....
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Not sure I agree with that. I suffered some serious noise fatigue when out at sea for a week or more at a time. Perhaps a few thousand bucks worth of high tech sound insulation would have helped. Even performing engine room checks when underway did require putting on ear protection.
 
I did preference it to "starting in the morning"... I know a good number of people who go through that fatigue in down east boat with the big diesel in the wheel house and stack right near them and it didn't matter what brand of diesel, but I get your point for sure!

makobuilders said:
Not sure I agree with that. I suffered some serious noise fatigue when out at sea for a week or more at a time. Perhaps a few thousand bucks worth of high tech sound insulation would have helped. Even performing engine room checks when underway did require putting on ear protection.
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They leak oil out the access covers and plates that make them so inspectable and rebuildable. If you keep on top of things using modern sealants they can go a fairly long time between drips.......
 
I worked on steel hull fishing boats as a kid starting at age 12 (Pinhead, cleaned the decks in order to fish for free), and then eventually a deckhand through High School. Most of the boats had DD's. Thousands of hours on the water back then and never witnessed a major mechanical at sea.
 
I think I'm like most of the guys here who have previously owned Jimmies, with true sentimental feelings about these amazing pieces of machinery. Buying an existing boat with them would be fine and certainly in no way a negative. However I have seriously considered, and have decided, that I would not install them in a new build.
 
They still make the 471, probably for outside the US. I don't know of a more reliable engine, even in very old age. 671 blocks were made in cast iron, aluminum and stainless steel (minesweepers). When I was a fisherman, late 70s, the inline 671 was still the most common engine. 8v and 12v were in bigger boats. 371, 471 and 53s in smaller boats. The inline 71s were made in a 1 cylinder version. Usually DC generators. The 1-71 will run about a 10kw AC generator. Ships usually had a 71 emergency generator. Lots of boats had WWII surplus engines, some never overhauled.
In the navy during Vietnam, all the patrol boats had DDs. There must have been thousands in the USN.
Thanks for this post. I love talking about these engines.
My first boat in 1961 had twin 671s and my current boat has nearly identical engines. The oil leak problem isn't nearly as bad as most people think. Most of the leaks come from the rear main seal. Older engines have an old style rope seal.
You can run the natural 671s 20,000 hours at their maximum rated rpm. I've done it.
 
During WWII, if you needed more hp, they would bolt on more 200 hp 6-71's connected at the crankshaft IN TANDEM :thumb:


Example 151' LCI(G) specs that my dad served on:


Propulsion two sets of 4 GM diesels, 4 per shaft, BHP 1,600, twin variable pitch propellers
 
High Wire said:
During WWII, if you needed more hp, they would bolt on more 200 hp 6-71's connected at the crankshaft IN TANDEM :thumb:


Example 151' LCI(G) specs that my dad served on:


Propulsion two sets of 4 GM diesels, 4 per shaft, BHP 1,600, twin variable pitch propellers
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Or a quad installation, and on a single screw. USS Cape, MSI2.

“Keeping the Salish Sea Mine Free since ‘93.”

For sale here:
https://www.yachtworld.com/boats/1958/Bellingham-Shipyard-Minesweeper-2862249/#.W6qGsIFlChA

Given enough money, what an awesome project that would be.
 
High Wire said:
During WWII, if you needed more hp, they would bolt on more 200 hp 6-71's connected at the crankshaft IN TANDEM :thumb:


Example 151' LCI(G) specs that my dad served on:


Propulsion two sets of 4 GM diesels, 4 per shaft, BHP 1,600, twin variable pitch propellers
Click to expand...

Indeed, I just read the owner manual for the 3/4/6-71 and they were describing the connected twins or quad engines, really amazing!

L
 
The DD series has the advantage of tremendous torque because of its 2 cycle design. I’ve run them for 50 years on our family’s oyster luggers in 3,4,6, & 8 cylinder 71 series configurations. IMHO no modern 4 cycle Diesel engine can compare to the throttle response, the acceleration (torque rise) of the DD. The last lugger we built with a Detroit had an 8-71 NA in 1986. That engine is now 32 years old with over 30,000 hours without major overhaul nor major breakdown. During oyster harvesting which accounts for about 80% of the engine hours the engine runs at around 800 rpm. Although I’ve heard so much about how bad low speed operation is on diesels, we never experienced that on the workboats around here with the Detroits.
 
Ive got twin 6-71’s, one has about 100 hours since a frame in rebuild and the other has about 150 hours, great efficient engines, I run them about 1400 and get 1.6 mile per gallon between the two, pushing my 65’ Boat about 9mph
 
A quad setup on a tug I ran, only 2 of the engines had starters. The other 2 were started by engaging their clutch after a starter equipped engine was running. When not towing and full power wasn't needed, I ran on 2 engines.
I have only seen 671s in dual or quad setups. These were made before the turbos and 12v71, 92, and 149 series. The 16 cylinder engines are 2 8v blocks bolted together.
Not much fun to work on quad setups. The ports are supposed to have the carbon buildup cleaned every 1000 hours. On quad setups, you have to take the blower off because the air box covers on the other side aren't accessible. You have to work thru the ports on the blower side to clean the opposite side ports.
 

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Lepke said:
A quad setup on a tug I ran, only 2 of the engines had starters. The other 2 were started by engaging their clutch after a starter equipped engine was running. When not towing and full power wasn't needed, I ran on 2 engines.
I have only seen 671s in dual or quad setups. These were made before the turbos and 12v71, 92, and 149 series. The 16 cylinder engines are 2 8v blocks bolted together.
Not much fun to work on quad setups. The ports are supposed to have the carbon buildup cleaned every 1000 hours. On quad setups, you have to take the blower off because the air box covers on the other side aren't accessible. You have to work thru the ports on the blower side to clean the opposite side ports.
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This is a massive beast!

L
 
There was an 80ft torpedo retrieval vessel that I nearly purchased years ago. It had two sets of twin 6-71’s to one tranny, but I recall they were end to end, not side by side.

Anyway by my standards, 4 Engine’s are 3 too many [emoji846]
 
One of the main reasons they leaked oil is due to the pressurized air box--charged by the blower, this area around the cylinders was pressurized to several/5 or 6 psig, so when the piston came down on the power stroke and uncovered the intake ports, that air pushed exhaust out the open valves. The piston moved up, covered the intake ports, and compressed the air for the next power stroke.

there are hand hole covers on the air box. In older engines, if those gaskets get brittle or leak and are not replaced, they will weep oil. There is also an airbox drain that is usually directed to what we called a "slobber box" intended to capture those minor drips. Mine were gallon paint cans filled with bronze wool.

I had a pair of 8V53NAs on a 1969 38 Hatteras DCMY that we owned some 25 years ago. they were basically bullet proof. I ran the snot out of them, often on the pins, for hours at at time. At 285 hp each, they did a fair job of moving the boat, cruise was 17 knots, WOT was close to 20 knots. Had to keep the bottom clean, though.

Only complaint was they were very loud, and the blower made a high pitched whine that was quite loud in the saloon, not so bad at the helm of that boat.

And, as already noted, they sound so good when started and idling at the dock, whap/whap/whap/whap/whap...a steady, rhythmic staccato that is unmistakably DD.

Even after nearly 30 years, with zero significant maintenance, and unknown hours, they started instantly, even in cold weather. Within one to two revolutions. Sold the boat in 2000 or 2001 when the kids got to high school and had better things to do.

I love the Cummins in my current boat, but I will always have a soft spot in my heart for those old two stroke DDs.
 
I love this thread. I have never owned one, but have looked to purchas a few. I always found something I thought was a better purchase but deep down I still want a Fiberglass hulled 47' Chris Craft with twin DDs Saw one once but was not, at that time, ready for it. Watching those engines turn so slowly that you could count the RPM visually was an awesome experience.
 
jimisbell said:
I love this thread. I have never owned one, but have looked to purchas a few. I always found something I thought was a better purchase but deep down I still want a Fiberglass hulled 47' Chris Craft with twin DDs Saw one once but was not, at that time, ready for it. Watching those engines turn so slowly that you could count the RPM visually was an awesome experience.
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Huh? DDs don't turn that slowly that revs can be counted. We have 4 8v71's aboard the tug I work on. Gen sets run for 24 hrs then shut down 24 hours (swapped). for years... Usually rebuild at 30,000 hrs or so. Tow motor gets the worst,hardest engine service imaginable. Start up,don't warm up and full throttle for a few seconds to a half hour. Shut down.
 
"DDs don't turn that slowly that revs can be counted."


Our 6-71 will idle at 300RPM and not die when shifted , 32x32 prop.
 
sawdustdad said:
One of the main reasons they leaked oil is due to the pressurized air box--charged by the blower, this area around the cylinders was pressurized to several/5 or 6 psig, so when the piston came down on the power stroke and uncovered the intake ports, that air pushed exhaust out the open valves. The piston moved up, covered the intake ports, and compressed the air for the next power stroke.

there are hand hole covers on the air box. In older engines, if those gaskets get brittle or leak and are not replaced, they will weep oil. There is also an airbox drain that is usually directed to what we called a "slobber box" intended to capture those minor drips. Mine were gallon paint cans filled with bronze wool.

I had a pair of 8V53NAs on a 1969 38 Hatteras DCMY that we owned some 25 years ago. they were basically bullet proof. I ran the snot out of them, often on the pins, for hours at at time. At 285 hp each, they did a fair job of moving the boat, cruise was 17 knots, WOT was close to 20 knots. Had to keep the bottom clean, though.

Only complaint was they were very loud, and the blower made a high pitched whine that was quite loud in the saloon, not so bad at the helm of that boat.

And, as already noted, they sound so good when started and idling at the dock, whap/whap/whap/whap/whap...a steady, rhythmic staccato that is unmistakably DD.

Even after nearly 30 years, with zero significant maintenance, and unknown hours, they started instantly, even in cold weather. Within one to two revolutions. Sold the boat in 2000 or 2001 when the kids got to high school and had better things to do.

I love the Cummins in my current boat, but I will always have a soft spot in my heart for those old two stroke DDs.
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Most of the oil on Detroits came out as mist from the vents as you mentioned. One thing that made a huge improvement in keeping my 8v-71 clean from “leaking” oil was a crankcase air-oil separator/filter like Racor sells. The crankcase vent and air box drains are routed to the filter and from there back into the air intake. From the bottom of the filter I had the oil drain into a five gallon can. After about 1000 hrs I have around 5 gallons of dirty oil in the can that would have been in the engine room.
 
Various places I worked in the 70s, 80s and 90s kept very close track of fuel consumption on the various sizes of DDs. On gensets and carefully monitored haul trucks the fuel consumption was easily 20% (or more) higher with the two strokes. Phase out was cost related. Much more to it than the air police as FF frequently cites.

In an underground mine the 2 strokes' worse emissions and higher fuel burn required a proportionate higher and more expensive ventilation system. For high daily hour use engines, shutting them down and replacing with four strokes was an easy decision.

In low hour boats, fuel costs can hardly justify a swap out. So wipe up the leaks, enjoy the roar and keep the myth alive.
 
I think I remember the rule of thumb for the 71 series was 1gal/hr/cylinder while making power. We didn't need all the power of our 16v71 engines but they could use 32gal/hr between them. I could be totally backward on that too.


The hardest I remember running them was holding position with a strong current/wind under a crane on an oil platform. It was an awesome sound.
 
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