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Old 12-07-2017, 06:25 PM   #41
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See, just like my old MG. If you could pour the garage floor into the engine, you would have a full oil change -
“If a British car ain’t got oil under it, it ain’t got oil in it.”
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Old 12-07-2017, 09:41 PM   #42
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My newer rebuilt jimmy doesn’t leak a drop of oil after the rebild
SSHHHHH! You'll upset all the re-tellers of the urban (or should I say dockside) legends!
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Old 12-08-2017, 07:41 AM   #43
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If you assemble a Detroit correctly (especially the later models) they don't leak. Or leak no more than other brands. Many of them out there running over clean bilges.
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Old 12-08-2017, 09:34 AM   #44
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If you assemble a Detroit correctly (especially the later models) they don't leak. Or leak no more than other brands. Many of them out there running over clean bilges.
Years ago I worked for a company that had a very strong Union. We went through a dozen or so fleet engine rebuilds per year. Traditionally these engines including several 6-71s were sent out for rebuilds. The union cried foul on contracting out language so we agreed to have the simplest (so we thought) engines, 6-71s of course, totally rebuilt in house.

The DD premature failures mounted up. With very good records we tracked each and every component on rebuilt engines whether done in house or not. The 6-71s were for various reasons failing when rebuilt in house at an alarming rate vs those that were rebuilt outside at a DD reman center.

This alarming rate would likely not be noticed in a low hour marine setup but it sure showed up in a high hour underground mine utilizing tight record keeping.

Eventually all our various sized 2 stroke DDs were tossed due to higher emissions and heat issues vs their 4 stroke diesel competitors. These issues were critical for underground operations.

We placated the union by placing our rebuild specialists elsewhere - from their cushy white glove surface shop back underground. Yes the DDs were quick and easy to rebuild. No they did not stand the test of time in tough high hour applications. Equipment downtime was mission failure. Money was lost and production targets not met. Hardly an issue though for an old Hatt or CC.
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Old 12-08-2017, 10:32 AM   #45
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My goal would be efficient displacement cruising , inshore or offshore.

As the 71 series is happy with 20-30 Hp per cylinder the boat would be propped to cruise at SL at 1200-1500 RPM on the 3-71.

It would be rather simple to run the 6 -71 to hull speed , which should require no prop change , just the normal doubling of the fuel burn.

Depending on how heavy the vessel was would determine if just larger injectors or both engines would ever get her up on a plane.

Light weight is more expensive to construct , so I think twin 6+3 would mostly be done for illegal "No Wake Zones.

" The 6-71s were for various reasons failing when rebuilt in house at an alarming rate vs those that were rebuilt outside at a DD reman center."

This was common in the bus industry too , in house overhauls usually failed at a much higher rate than a pro shop..

This seems to be common where some boatyard "rebuilds" an engine.

More costly but shipping it off to the pros is the safe course.
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Old 12-08-2017, 11:02 AM   #46
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Great thing about DDs is that they leak so much lube oil that you can get home running on bilge oil if the fuel runs out.
Very true. So much of a chance of a runaway that some mechanics are scared of them. I once had a mechanic on board my boat with 6-71's in Golden Isles, GA who was fearful but fixed my broken exhaust connection anyway. Even though my engine's had jerry rigged air seps and the engine room was clean he didn't stay down there very long because he had once experienced a runaway.
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Old 12-08-2017, 11:26 AM   #47
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In w the new out w the old ......

Are we through bashing one of the best engines ever built?
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Old 12-08-2017, 01:59 PM   #48
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My goal would be efficient displacement cruising , inshore or offshore.
Somehow you just don't get it. Diesels produce a certain amount of power per gallon of fuel regardless of brand.

Two strokes produce a little less than four strokes.

If you want more look up brake specific horsepower. BSFC
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Old 12-08-2017, 03:42 PM   #49
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Boats are not efficient regardless of design.

At certain speed some hulls and less weight makes them use less fuel but within a category there is not much difference and they all are inefficient compared to other means of transport.

We would all be more informed if fuel use was described in term of nautical miles per gallon at a certain speed than gallons per hour.
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Old 12-09-2017, 06:09 AM   #50
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There are 2 types of "runaways" that could happen on a DD 2 stroke.

The first is a stuck injector , the engine was not properly serviced before storage and then sat 2,3,10 years .

The injectors stick and the engine speed is not under throttle control, so a board is used to cut off the air .Rebuilt injectors are about $30 each in what ever fuel flow is desired.

This allows HP produced to be selected at various RPM.

A real runaway comes when the engine is still operating 5000, 10,000 hours after it should have been overhauled.

These are the engines that have so much blowby they spray all over the engine room. That is their telltale to purchase a cylinder kit.

Eventually the rings and cylinders are so bad the amount of lube oil left on the cylinder walls will operate the engine as fuel.

This is a genuine runaway , it too can be stopped by cutting off the air , but folks claim the engine will go to almost 4,000RPM and eventually hand grenade as it eats all the lube oil..

Beware if your DD requires a few gallons of oil every hour.
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Old 12-09-2017, 10:59 AM   #51
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If you assemble a Detroit correctly (especially the later models) they don't leak. Or leak no more than other brands. Many of them out there running over clean bilges.

My 6-71s were 1940’s genset engines from a minesweeper. Aluminum blocks. I never knew how many hours were on the rebuild but the Boat logs were nearly three decades old and did not indicate rebuilding. Leaks were nothing that a tissue couldn’t wipe up after each run. My bilges were painted white and always clean - no diapers needed. Little cups under the breather tubes and I installed a lot of new gaskets.

Really wonderful engines but the noise was horrible. I was very jealous of the guys with Cummins under their feet - so quiet 🤫
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Old 12-09-2017, 11:16 AM   #52
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Really wonderful engines but the noise was horrible. I was very jealous of the guys with Cummins under their feet - so quiet ��
The 6-71 TIBs in my 1989 Californian MY had 10" exhausts with no mufflers. Just straight tubes from the engine room to the stern.

I kept it under the shed on A dock in the Galveston Yacht Basin backed in to a solid concrete bulkhead for the car parking area. You could hear those jimmys starting in the next county.

I ran the boat at trawler speeds so the exhaust opening was above the water. You could hear it coming in the next county. Up on plane the noise got better but it was still loud.

Although it was a great boat, noise was one of the main reasons I sold it and bought a boat with a Cummins. It was so quiet that guests thought only the gen set was running.
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Old 12-09-2017, 11:35 AM   #53
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Boats are not efficient regardless of design.

At certain speed some hulls and less weight makes them use less fuel but within a category there is not much difference and they all are inefficient compared to other means of transport.

We would all be more informed if fuel use was described in term of nautical miles per gallon at a certain speed than gallons per hour.
“Not much difference” ....
Well everything is relative. I evaluate the power of a boat by the power/displacement ratio. Many of us compute or evaluate how much power a boat has or should have by the hp per ton of displacement. Weight is (for all practical purposes) directly proportional power required. So if you shed 10% of a boats weight you’ll need 10% less power to drive her the same speed. Your dinghy planes w one person but not w two. Ect ect ....

Can’t resist .. weight matters.
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Old 12-09-2017, 11:42 AM   #54
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I did mention weight
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Old 12-09-2017, 01:08 PM   #55
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The 6-71 TIBs in my 1989 Californian MY had 10" exhausts with no mufflers. Just straight tubes from the engine room to the stern.

I kept it under the shed on A dock in the Galveston Yacht Basin backed in to a solid concrete bulkhead for the car parking area. You could hear those jimmys starting in the next county.

I ran the boat at trawler speeds so the exhaust opening was above the water. You could hear it coming in the next county. Up on plane the noise got better but it was still loud.

Although it was a great boat, noise was one of the main reasons I sold it and bought a boat with a Cummins. It was so quiet that guests thought only the gen set was running.
Did the Cummins have mufflers? My boat with 8v92tti's did, so seemed no nosier externally than other diesels. Inside the ER's? Ear muff time. In the lower helm and salon above the ERs? Normal conversation.
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Old 12-09-2017, 02:12 PM   #56
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Great thread, lots of good observations. I have spent a lifetime of operating boats with 271, 371, 471, 671 generators and 671N, 871N, 1271TI and a few 1671's for propulsion. My current boat a 47 Bertram M/Y that has 871TI's. At a young age I began rebuilding the 671's from the boats in the family business. I said all of this to openly admit that I am a bit biased because I am comfortable with the 71's.
If money were not a factor would I buy a new boat with DD? No way! But as stated in an earlier post the DD provided a pretty good aft cabin M/Y for around a 50K investment. I never run above 1000 to 1200 rpm (displacement speed) and obtain about 1 mpg. Over the 1500 miles traveled this year.

Yes they are old, they smoke when cold and have a few oil leaks. But, when I rebuild over the next couple years it will be cheap and yes 99% of the oil leaks will stop with the rebuild. After market parts are available everywhere. Just compare the cost of a cylinder kit for a Cat to a DD and you understand why when on a budget I rebuild the DD rather than repower.

Because at displacement speed I do not need the current HP, when rebuilding I will remove the turbo and after coolers, use N style pistons over the current turbo piston and step down to 45 or 55 injectors. This will now produce an 871N that will run long past my lifetime at a fraction of repower cost. I've seen similar 1200 rpm generator packages run 24-7 for 3-4 years and then only need basic service.

Love the 4 stroke John Deere, Cat's and Cummins but financially not a sound $ investment in an old boat.
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Old 12-09-2017, 03:15 PM   #57
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Great thread, lots of good observations. I have spent a lifetime of operating boats with 271, 371, 471, 671 generators and 671N, 871N, 1271TI and a few 1671's for propulsion. My current boat a 47 Bertram M/Y that has 871TI's. At a young age I began rebuilding the 671's from the boats in the family business. I said all of this to openly admit that I am a bit biased because I am comfortable with the 71's.
If money were not a factor would I buy a new boat with DD? No way! But as stated in an earlier post the DD provided a pretty good aft cabin M/Y for around a 50K investment. I never run above 1000 to 1200 rpm (displacement speed) and obtain about 1 mpg. Over the 1500 miles traveled this year.

Yes they are old, they smoke when cold and have a few oil leaks. But, when I rebuild over the next couple years it will be cheap and yes 99% of the oil leaks will stop with the rebuild. After market parts are available everywhere. Just compare the cost of a cylinder kit for a Cat to a DD and you understand why when on a budget I rebuild the DD rather than repower.

Because at displacement speed I do not need the current HP, when rebuilding I will remove the turbo and after coolers, use N style pistons over the current turbo piston and step down to 45 or 55 injectors. This will now produce an 871N that will run long past my lifetime at a fraction of repower cost. I've seen similar 1200 rpm generator packages run 24-7 for 3-4 years and then only need basic service.

Love the 4 stroke John Deere, Cat's and Cummins but financially not a sound $ investment in an old boat.
On the 871's, if you want to derate you can leave the turbos on so you don't have to reroute the exhaust. You can take off the intercooler, or leave it in place and disconnect and reroute sw so that side is dry. Turbos and the intercooler do knock down some of the exhaust and blower noise and will only have a tiny effect on efficiency.

Like the idea of going to N kits to get the 19:1 (think that is it) comp ratio. N45 is tiny for a 71. Nat 71 usually has N70, so maybe go with N55. Go too small and injection duration gets long and that has no benefit.

Maybe reshim the gov to max at 1800, but really no need to mess with that

Set up like that they will run for ever.

Detroits are like a box of Legos. Can build just about anything you want.
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Old 12-09-2017, 04:24 PM   #58
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Good thought Ski on leaving the turbo/aftercoolers. My goal was to eliminate anything that must be maintained which was not needed or take up space in the engine room. Trying to make a bit of space around the engines to ease access to the engine room corners on an old boat that always seem to have something to fix or replace. One rebuilt turbo is expensive, but I will consider your suggestion.

And yes the reason for the N piston was the compression ratio. It would seem that the engine would not work well without the turbo at low RPM if I stuck with the turbo piston/compression.

Injectors: When I bought the boat it had 70's in one engine and as I recall 90's in the other. They were of unknown age and leaked fuel out when stopped as evidenced by the fuel residue from the exhaust. Changed both engines to N65 with a bit of improvement. Did not run boat much with old injectors but I am sure fuel consumption is improved because the unburned fuel now does not now go out the exhaust in the water or in the form of smoke.
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Old 12-09-2017, 04:39 PM   #59
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Did the Cummins have mufflers?
That was on my first Nordic Tug 42. It was a single 5.9 ltr. 330 hp with a muffler and underwater exhaust. At idle I had to look at the tach to see if it was running.

My current Nordic Tug 42 has a single QSC 8.3 ltr. 540hp with a muffler but an outlet above the water line. Although I can hear it running at all rpms it is still pretty quiet.
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Old 12-09-2017, 05:45 PM   #60
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That was on my first Nordic Tug 42. It was a single 5.9 ltr. 330 hp with a muffler and underwater exhaust. At idle I had to look at the tach to see if it was running.

My current Nordic Tug 42 has a single QSC 8.3 ltr. 540hp with a muffler but an outlet above the water line. Although I can hear it running at all rpms it is still pretty quiet.
OK got it. You were comparing an unmuffled Detroit to a muffled Cummins, correct?
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