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Old 04-04-2019, 09:06 AM   #21
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I appreciate all the sound advice.
I will have an oil analysis done and a mechanical inspection including a compression test prior to purchase. I didnt know that these engines are 2 stroke. That concerns me some
Many of us have steered away from DDs for valid reasons. Namely, there are better options available once into newer boats. By the late 1990s DD's two stroke were replaced with Series 60s or other brands due to fuel costs and environmental reasons.

That said, if you are pursuing a pre 1990 vessel there are so many DDs around still that your vessel engine choices can be limited. Therefore follow Ski's and other's advice on mechanically checking out the 4-71s. To replace those engines with something newer would be way too costly.
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Old 04-05-2019, 05:53 AM   #22
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Depending on the oil pan a single 6-71 can hold 8 gallons , do multiple old 5 gal pails are required to lift the weight.
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Old 04-08-2019, 01:29 PM   #23
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Oil samples are a must. I trust these guys and recommend them highly:

http://www.blackstone-labs.com/?sess...scwvq%29%29%2F
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Old 04-08-2019, 03:45 PM   #24
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Originally Posted by sunchaser View Post
Many of us have steered away from DDs for valid reasons. Namely, there are better options available once into newer boats. By the late 1990s DD's two stroke were replaced with Series 60s or other brands due to fuel costs and environmental reasons.

That said, if you are pursuing a pre 1990 vessel there are so many DDs around still that your vessel engine choices can be limited. Therefore follow Ski's and other's advice on mechanically checking out the 4-71s. To replace those engines with something newer would be way too costly.
Not quoting you to disagree specifically, just expanding on the idea.

Most of the "problems" associated with DDs happened in the 80s when there was a market for faster sportfish but not an engine that was ideal for the task. TIA Detroits putting out more than double the hp of their naturally aspirated cousins used a LOT of fuel and didn't last long.

The EPA effectively killed 2 strokes, and that included the Detroit 71 series.

For trawler uses, I'd put a natural Detroit running 1200rpm up against any 4 stroke out there, for both fuel economy and longevity, with no hesitation.

Go to full cruise, and absolutely Detroits get thirsty. At hull speed they are surprisingly economical. And every shade tree mechanic on the planet is familiar with them.

They were introduced in the late 1930s. They powered landing craft on D-Day. The US Navy uses the 71 series to this day in generator applications. A Detroit that's running before the atomic apocalypse will still be running afterwards. They are not the quietest or the cleanest of engines, but they are closer to bulletproof than anything developed before or since.

Also, its a common old fisherman's tale that nothing raises fish like Detroits. We've found it to be true...
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Old 04-08-2019, 04:39 PM   #25
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I have twin DD 671s in an 83', 80 ton boat. I cruise at 10 knots and burn 8.5 gph. At 7 knots it's about 4 gph. Natural DDs in the right hull are economic. And I haven't found a more reliable engine for marine use. No injector pump, no bleeding issues, no electronics.
If the people with high speed boats would run their turbo Detroits at 25 knots instead of 30, they'd probably double the life between overhauls.
And my mains were built in 1947 and rebuilt once, by me. The engine and everything on it was designed to be rebuilt in the field. Any good mechanic with a service manual can rebuilt these engines. It does take some experience or very careful reading to set the governor and injectors.
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Old 04-08-2019, 06:22 PM   #26
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Hello Dale. I’ve DD 6-71’s on my 97 mainship. PO (original) did work for Mobile did strongly recommend that I stay with the mobile delvack 40 that he had always used.
I didn’t want to change or mix oils so I have stayed with it. Advanced auto carries it or available online.
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Old 04-10-2019, 12:12 AM   #27
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DDs

Great engines been around them for 50 yrs, trucks, heavy equipment , boats you name it, have a friend that has a dive boat with 2 V8-71s. talk about a racket. BUT there are 'antiques' that slobber oil if you run then slow or half speed have to run them wide open so all the excess oil goes out the exhaust if you don't believe me run a couple fingers around the exhaust pipe when you stop. if you like to get your hands dirty they are the engine for you, do yourself a favor an get your self a nice little boat with a John Deere you will be much happier. RHR
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Old 04-10-2019, 08:48 AM   #28
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The US Navy uses the 71 series to this day in generator applications. .
I'm not so sure this is the case on new builds, say since 1998 or so. The government has been taken to task for allowing their various fleets to languish in the old emissions' eras while forcing new guidelines on the rest of us. Even doing replacements with old tech engines! OMG
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Old 04-10-2019, 09:16 AM   #29
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I think it’s pretty well established that there are a lot DD lovers and there is a lot to love about these engines. I find the gas guzzling description is only fair if you compare them to modern engines.

What is important is the old rule of thumb, one hp per cubic inch max. Everyone loves a DD 6-71 at 220hp. At 400 hp it’s a short lived hot rodded power plant. The OP was looking at some 300 hp 4-71’s. That’s a DD I have some concerns with.
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Old 04-11-2019, 06:39 AM   #30
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For long term , use trawler or prime (24/7/365) generator the simple rule is 20-30hp pr cylinder on the 71 Natural series. .

At 1200 to even 1800 RPM they take a long time to wear out.
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