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Old 13-09-2013, 11:53   #1
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Detroit Diesel Question?

Hello all,

I can't believe that I am saying this, but I am seriously considering a motoryacht.

I have narrowed to 2 that we like. Both we have seen in person. Both boats have the same engine Detroit 12V71T1C. Does anyone know anything about these? Are they good? Can I run them slower to get better fuel economy? One of the boats is 70' and the other is 61'.

The 70' seemed to have more roll at the dock. The owner told me that the bottom was flat. I wasn't sure if all the movement was because it was located in a busy harbor with some wake or if the flat bottom was contributing to the roll?
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Old 13-09-2013, 12:16   #2
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re: Detroit Diesel Question?

The vessel I work on has several 12v71s, they run the generators, tunnel thruster etc... I have worked with many different DD over the years. They are generally reliable. They usually slobber oil, they are not too bad on fuel. I have a 471 DD in my sailing/fishing vessel that I am in the process of swapping out for an Isuzu, not because of any mechanical issue other than the above stated. I hate having oil leaking out of my main engine, and I really don't like the high pitched whine that is generated by the blowers on the 71 series engines. On the plus side, they will generally run, they are less expensive to rebuild. No electronic failure will stop the engine, as long as you have fuel, oil, and water, she will go. I am not aware of a flat bottom type vessel rolling anymore than a round bottom. I have seen plenty of round bottom vessels with rolling chocks installed. Most of the diesel engineers and mechanics that I have discussed the whole RPM for longevity of the engine versus fuel economy, come in at about 1500 rpm as the optimum setting. The size of your injectors will also determine your fuel economy. N65 injectors are generally your best bet. Before I get flamed for offering this opinion, one of the things you must do while running at 1500 rpm is occasionally run the engines up to 22150/2000 for some time to keep her blown out. Also you want to make sure that your thermostat is at a level to keep your engine temp. up 185* F to 195*F so that the working parts are at their correct size. These are only generalized observations from an operator, not a mechanic.
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Old 13-09-2013, 12:31   #3
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re: Detroit Diesel Question?

Quote:
Originally Posted by GalaxyGirl View Post
Hello all,

I can't believe that I am saying this, but I am seriously considering a motoryacht.

I have narrowed to 2 that we like. Both we have seen in person. Both boats have the same engine Detroit 12V71T1C. Does anyone know anything about these? Are they good? Can I run them slower to get better fuel economy? One of the boats is 70' and the other is 61'.

The 70' seemed to have more roll at the dock. The owner told me that the bottom was flat. I wasn't sure if all the movement was because it was located in a busy harbor with some wake or if the flat bottom was contributing to the roll?
This is a better question to be asked on Trawler Forum.

With out knowing some detail about the both boats and how you are going to use/run the boat is hard to answer. DD 71 in general are great reliable engines in the right application and use.
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Old 13-09-2013, 12:35   #4
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re: Detroit Diesel Question?

Never been around 71s, but the 8V92s I looked after were the loudest things ever. I am not speaking about exhaust noise. It's the noise eminating from the engine itself.

Very high pitched (2 stroke).

They don't call them Screaming Jimmys for nothing.

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Old 13-09-2013, 12:57   #5
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The Detroit engines are dead simple reliable, but the sound and slobber kills them for me in a vessel to be used for pleasure. The sound sets me on edge, no amount of sound insulation has been able to stop it from my ears. I've crossed the gulf on a Broward motor yacht with these and could hear the blowers the whole way. The engine room was exceptionally well insulated.
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Old 13-09-2013, 13:44   #6
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re: Detroit Diesel Question?

One big reason for the so called slobber is that using a high detergent oil will create this issue in DD 2 strokes, use a low det. oil and it usually eliminates the issue, N 65 injectors in a tubo engine may not be a great idea, DD's also use the fuel as a coolant for the pistons and using too small an injector can be detrimental. otherwise they are a great dependable engine, always, I repeat, always use a certified DD mechanic for any serious engine work or analysis.
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Old 13-09-2013, 14:47   #7
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re: Detroit Diesel Question?

What is slobber?
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Old 13-09-2013, 15:07   #8
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re: Detroit Diesel Question?

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Originally Posted by GalaxyGirl View Post
What is slobber?
it's oil, leaks and mist in the air while running.

If one of them is the Hatteras in Hingham she looks like a well kept ole girl, 85, in good shape and well within your stated budget which leaves room for the unexpected. Staterooms are well suited to your young family.
good luck and don't get discouraged; stay positive. Things happen for a reason. It's a Karma thing...
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Old 13-09-2013, 15:20   #9
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re: Detroit Diesel Question?

Quote:
Originally Posted by GalaxyGirl View Post
Hello all,

I can't believe that I am saying this, but I am seriously considering a motoryacht.

I have narrowed to 2 that we like. Both we have seen in person. Both boats have the same engine Detroit 12V71T1C. Does anyone know anything about these? Are they good? Can I run them slower to get better fuel economy? One of the boats is 70' and the other is 61'.

The 70' seemed to have more roll at the dock. The owner told me that the bottom was flat. I wasn't sure if all the movement was because it was located in a busy harbor with some wake or if the flat bottom was contributing to the roll?
A 70' motor yacht with Detroit Diesels will be 30 years old at least. You should be sure you have a well researched budget for repairing and maintaining such a beast, also running it.

Boat and houses are very different things. A million dollar house can be a realistic prospect in some cases, for people without a huge income. The house -- in the right neighborhood and economic time -- will increase in value which will wipe out a lot of maintenance expenses. An old former millionaire's boat, however, will require at least 10x the maintenance of a house of the same value, and depreciates relentlessly, so there is no way to later pay the expense out of appreciation, re-mortgaging, etc. A boat, especially a large motor yacht, is a very complex device, an order of magnitude more complex than a house, with dozens of systems which are time-limited and requiring regular replacement, and costing thousands or tens of thousands each. And all the time you're doing that, the boat is going down in value despite the vast sums of money you are spending on it. Large motor yachts, especially, large old motor yachts, are for people with tons and tons of money just to burn, who just don't mind. Is that you? It would be a terrible fallacy to think that -- well, I can afford a million dollar house (or a $5 million house), so what's the problem with having a half million dollar boat? The answer is -- these are totally different things!!

A word to the wise!
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Old 13-09-2013, 16:02   #10
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re: Detroit Diesel Question?

Dockhead.... GG is pretty aware of the difference in maintaining a house and a yacht and the depreciation aspect and so on. She wants to raise her family on a boat and so she's doing her due diligence and climbing the steep learning curve. She'll get there and her eyes are wide open. Be supportive! She came to this board for support and not negativism and discouragement. She's a big girl and she knows what she's up against. And seeing who is running the boats out there I am sure she will do just fine.
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Old 13-09-2013, 16:18   #11
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re: Detroit Diesel Question?

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Originally Posted by Sandero View Post
Dockhead.... GG is pretty aware of the difference in maintaining a house and a yacht and the depreciation aspect and so on. She wants to raise her family on a boat and so she's doing her due diligence and climbing the steep learning curve. She'll get there and her eyes are wide open. Be supportive! She came to this board for support and not negativism and discouragement. She's a big girl and she knows what she's up against. And seeing who is running the boats out there I am sure she will do just fine.
Let's hope so.

The problem in this case is not running the boat, but paying to keep it running. Very many people don't quite imagine what it costs -- not even at the level of an order of magnitude. Therefore, taking some care and running some numbers is a really good thing before comitting to something like this.
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Old 13-09-2013, 17:09   #12
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Let's hope so.

The problem in this case is not running the boat, but paying to keep it running. Very many people don't quite imagine what it costs -- not even at the level of an order of magnitude. Therefore, taking some care and running some numbers is a really good thing before comitting to something like this.
Couldn't agree more, with either of your posts. And since when is pointing out the operating expense being somehow negative? I can afford to obtain far far more boat than I wish to feed. Large motor yachts, more so than large trawlers, feed on a steady diet of cash.

I know personally more than one owner who "thought" they knew this truth find out 3-9 months later how little they "actually" knew. Btw every one of them climbed the ranks of ownership the typical way most of us have. One of them now has twice his purchase price tied up in a nice older Hatt.
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Old 13-09-2013, 18:10   #13
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re: Detroit Diesel Question?

I sold Hatteras for years and am very familiar with their models.


Feel free to PM me if you have specific questions.


The vintage Hatteras MY’s are getting a little long in the tooth but they started out well engineered and well built. Almost every MY hull 60-75 and those that have been stretched came out of the same 18’2 mould. The trick is to find one that has not been painted/wall paperd out in an attempt to “modernize” it.


Generally speaking and this is very general, the 71 series motors were more reliable than the 92 series motors. This makes sense when you consider that higher hp uses more fuel and thus more wear.



A lot also has to do with how they were run. Take two MY’s each with 2,500 hours. One was run at 10/12 knots and the other at 16/18. Both have the same hours but the latter will have more wear.
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Old 13-09-2013, 18:11   #14
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re: Detroit Diesel Question?

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Couldn't agree more, with either of your posts. And since when is pointing out the operating expense being somehow negative? I can afford to obtain far far more boat than I wish to feed. Large motor yachts, more so than large trawlers, feed on a steady diet of cash.

I know personally more than one owner who "thought" they knew this truth find out 3-9 months later how little they "actually" knew. Btw every one of them climbed the ranks of ownership the typical way most of us have. One of them now has twice his purchase price tied up in a nice older Hatt.
This is perfectly normal point to make. I am sure GG is aware of a maintenance and operating budget though not the specifics yet because she doesn't have the boat nor even the survey. I am sure she will set aside a money for refit and maintenance. Will it be enough? Is it ever?
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Old 13-09-2013, 20:06   #15
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re: Detroit Diesel Question?

Long time lurker, First post

12v71 where some of the best iron Detroit Ever poured.
They are, however, a dry block with a sleeve, which requires a series of steps be followed when the engine is rebuilt, in order for the liners to not have hot spots. Surprisingly few people seem to want to go through all the steps, I guess feeling that they know more than the people who designed the engine.
A fairly unknown weak point is the spring pack and the splined shaft which drives the blowers and that usually only fails in applications with lots of throttle changes (think in a truck). However, I would still carry the parts in my spares collection. Otherwise, very very simple and very reliable. Fuel wise they are thirsty compared with anything which is 4 stroke and way less efficient than any of the modern (electronic) engines. But you don't need a laptop to work out what is wrong with them. Parts are mostly available although some things can be hard to find as they don't make new ones anymore and most of what is left is in boats.
92 series Detroit I would suggest staying away from - they where not the best thing Detroit ever built.
Slobber- detroits under light load or at idle tend to pump oil up past the piston rings and it comes out the ports in the lower portion of the liner and then the constant pressure inside the air box blows the oil out the air box drains. Many people rig something up to drain this oil back into the pan. I would not do that as there is too much junk in that oil to ever want it in the oil pan. Also, gaskets tend to leak and there are a number of bolts that tend to leak oil if the bolt is not installed properly. But, with work they can be made quite tight in terms of oil leaks.
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