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Old 12-02-2016, 05:40 PM   #1
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New Member-Question about Detroit Diesel

After 30 years of sailing, we sold our last sailboat, and decided to look at a Trawler. We've seen several 40-foot Trawlers that we like, and I'm trying to decide which to choose. One, an Ocean Alexander 40 has twin Detroit Diesel engines with 500hp total (8.2 liter). This seems almost excessive, as each engine is 550 cubic inches. My question is does anyone have any experience with these engines?
I've been told that the cruising speed is 8 kts. The displacement is 22,500lbs. I would think that these engines would barely be turning over at 8 kts, but I haven't heard from the broker the GPH while cruising. Does anyone have this information?
Thanks,
Jim
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Old 12-02-2016, 05:45 PM   #2
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Greetings,
Welcome aboard.
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Old 12-03-2016, 01:02 AM   #3
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Jim,

Welcome. Someone will probably be along shortly with some help, but in the interim check over at boatdiesel.com. The site might be particularly useful for you if you are likely to have questions about different engines as you are shopping. I have been a member for two years since I bought a diesel, and the education I have received has far exceeded the membership fee.

Good Luck

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Old 12-03-2016, 10:18 AM   #4
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Go to Yachtforums, lots of discussion on DDs.
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Old 12-03-2016, 11:06 AM   #5
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My last boat had 8v92 DD's, putting out 730 hp each, which is just under 1 hp per cubic inch, often regarded as the thresh hold to unreliability. At that power, heat dissipation became important, but that was about the only thing that could stop one from running. At lower horsepowers (and most DD's are not configured to put out anywhere close to 1 hp per CI).

Modern engines have gotten pretty good at squeezing out a lot of horsepower from small displacement, but only through computer control and significantly increased mechanical complexity. In a marine application, there is a lot to be said for the reliability that comes from purely mechanical control and simplicity. (A DD doesn't need electricity, except to get it started.) Especially if the boat travels to third-world locations where no one knows how to work on a modern engine, but everyone knows how to completely rebuild a Detroit.

On the other hand, DD's invariably leak a lot of oil, and smoke more and more as the hours grow.

It should also be noted that DD's are two stroke engines -- not a lot of moving parts. Very simplistic design, which contributes greatly to reliability and the ease of diagnosing and repairing problems.
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Old 12-03-2016, 11:12 AM   #6
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The DD 8.2 has had a bit of trouble over the years. As I recall it is related to the cooling system flow path getting air blocked which overheats the engine. There is a mod that anyone with an 8.2 should install that reduces or eliminates it. If you join boatdiesel, then look at the old posts in the DD 8.2 section and you will find a description of the problem and what to do about it. I also recall that the 250 hp DD 8.2s don't suffer much from this problem, particularly if run at low loads.

A 22,500 lb (light for that size, so probably an unloaded figure) 40' trawler is going to need about 60 hp and burn 3-4 gph to go 8 kts. As you note, those engines will be going slow, about 1,200 rpm to make that power.

Edit: The DD 8.2 is a 4 cycle V8. Not the same engine described in the previous post.

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Old 12-03-2016, 11:30 AM   #7
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Diesel's like to run hard so look for the lowest power you can find unless you have very deep pockets. Most boats are way overpowered if you are thinking of a cruising speed in the 8 knot range. We converted from sail to power a couple years ago and I just ran the numbers on our 1822 nm cruise from Detroit, MI to Saint Augustine, FL in the Gulf Star 43 we bought in September.

She is a sailboat hull so a bit more easily driven than most trawlers. Two 135 hp (in theory) Perkins engines and probably about 24,000 lbs displacement as currently loaded. Half of one of those engines would drive her at what would be considered hull speed for a sailboat. She can't begin to use the power except to make big waves and a large hole in the water. She'll mush along at 11 knots WOT but we can't afford it except to blow out the exhaust manifolds occasionally. We normally cruise at 1500 rpm which gives us about 8 knots in flat calm conditions and 7.8 most of the time. The engines seem happy at that rpm.

Our average speed over the 1822 nm was 7.0 knots according to the GPS. We burned 700 gallons of fuel over 270 hours of running time so 2.7 GPH. MPG was 2.6. This includes running a 7.5 kw generator most of the day and evening.

Why do they put such large engines in boats? A primary reason is that the large reduction gear boxes that would let the builders use a smaller engine tend to be expensive units intended for commercial vessels. They also often have a lot of shaft drop which raises the engines in the boat. Lots of horsepower is good for marketing so using the torque of big engines to drive props with more readily available and cheaper 2:1 reduction gears makes sense for the boat builders if not for the end user.
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Old 12-03-2016, 11:38 AM   #8
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I second the recommendation to invest $25 in BoatDiesel.com
You can ask questions and get answers from very knowledgeable people.

If I recall correctly from previous searches--Some internet research will show some people who have had good experiences with the 8.2L but I think more people would suggest it is not a highly sought after engine.
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Old 12-03-2016, 02:38 PM   #9
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The vessel in question was designed to be a planing vessel and needs that HP to satisfy the designers criteria. A similar sized and designed OA today would have the HP installed similar to Codger's 42 OA, 440 HP Yanmars.

If the OP wants a "true" OA 40' trawler with small engines they are not easy to find. Running an OA planing design vessel slow does not make it a trawler. It just makes it, well, slow and drastically over powered for the task.

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Old 12-03-2016, 02:48 PM   #10
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IMO is is a myth that "diesels like to be run hard"

Do some serious investigation of the 8.2 detroits. IIRC they don't have the reputation of the detroit two cycles
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Old 12-03-2016, 02:57 PM   #11
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I would think 8 knots would be high idle.
I've been running boats with DDs since the early 60s. Generator DDs before that. I have 671s in my current boat. 83', about 80 tons and run 1800rpm to get 10 knots @ 8.5 gallons/hour. Better mileage if I slow down.
No idea what model 8.2 liter is in US measure (and no desire to know). Maybe made after my time. But in many years of Detroit use, running, owning, rebuilding for myself and others, including combat, have found them to be one of the most reliable brands when properly run and cared for and giving reasonable mileage.
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Old 12-03-2016, 03:57 PM   #12
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Guys, read post #6 at the bottom again. A DD 8.2 is a totally different type of engine than the venerable 4-53, 6-71 and 8-92 two stroke engines that some are talking about in posts above. It is a four stroke V-8.

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Old 12-03-2016, 05:38 PM   #13
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Ditto the above suggestions to check out boatdiesel.com for Detroit info. The 8.2 Detroits have received a lot of bad press, although a friend has one and likes it. I had 8v71 Detroits which are 2-cycle engines .....8.2s are 4-cycle and a completely different engine. I liked mine but eventually repowered because of the oil leaks and deafening mechanical noise. If you can get Ron Sparks or 'Ski from NC' to come up on boatdiesel.com, you can learn a lot Detroit stuff from them.
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Old 12-03-2016, 10:33 PM   #14
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I had a 36' Sabre "trawler", about 25000 lbs with the DD 8.2 engines. We'd cruise about 1700(?) rpm and cruise ~8 knots and burn 4 GPH. We had the boat 4 years and the engines were reliable and economical. About 1900 hours on them when we sold the boat. We would periodically run up to 26-2800 rpm to make them work. It was scary watching the fuel gauge drop at that speed����
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Old 12-04-2016, 08:10 AM   #15
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We had a DD 8.2 on our '87 34' Mainship. Worked fine. Johnson & Towers (the marinizers) published a service bulletin, which we eventually discovered and performed. Something about slightly larger head bolts or some such, not a particularly big deal.

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