When the studs crack, the seal between the exhaust manifold and turbo is compromised, and because both the manifold and turbo are cooled by engine coolant, it all starts leaking out. Sooner or later this completely disables the engine because without enough coolant, it will overheat. Fortunately both times it happened we were on our way to port, and made it in without overheating.
Below you can see the exterior evidence of the failure. The yellow stuff is coolant. You can also see the vertical brace installed last year to help take the weight of the exhaust off the turbo. It may have helped, but clearly wasn't a complete solution.
|Leaking coolant due to turbo stud failures|
In the picture below, once the turbo has been removed you can see the two upper studs have parted. The one on the left broke leaving a good stub to grab onto to remove the stud. But the one on the right broke off about 1/2 thread inside the manifold, so I had to drill it and use a bolt extractor to get it out. You can also see the coolant passages. The center opening is the exhaust passage, and the oblong openings above and below are the coolant passages.
|Cracked turbo mounting studs|
Since the exact same thing happened again despite installation of a support brace after the first occurrence, further investigation was clearly warranted. One common factor is that leading up to both failures, we were running the boat pretty hard - on the order of 2000 RPM out of a max of 2200. This is absolutely fine - the engine is rated for 24x7x365 operation at 2100 RPM, and up to full 2200 RPM 16 out of every 24 hrs. - but it's different from our normal cruising at 1600-1900 RPM.
To make a long story short, the picture below will show what's happening. The tall structure circled in red is rigidly attached to the turbo. Because it's rigidly attached, when the engine vibrates, the exhaust structure has to vibrate with it. But the structure has a very long lever arm, so requires lots of force to make it move along with the engine. It's like holding a baseball bat and trying to shake it around. If you hold it in the middle where it's center of mass is located, it's not real hard to shake it back and forth. But if you hold it from the handle end, now the center of mass is out maybe 2' from your hand and it's much harder to shake it back and forth. That's what's happening with this exhaust structure, except instead of weighing 2 lbs like a baseball bat, this structure weights over 30 lbs. Trying to move it in time with the engine vibrations simply overwhelms the turbo mount and breaks the studs.
|Excessively tall and heavy exhaust riser|
After talking with a number of people, I decided to pursue a two part solution. The first, and most important part, was to install flex piping in the vertical exhaust section. Doing so would reduce the mass directly attached to the turbo, and decouple any engine vibrations from the rest of the exhaust structure. The second part was to add a new set of spring hangers over the big flange to carry the weight of the exhaust over the turbo. Ideally the exhaust structure should mate with the turbo elbow, but exert no pressure on it in any direction. It should just float above it.
I probably could have ordered parts and gotten all this done in Port McNeill, but we didn't want to lose any more time than necessary, so I decided to haul everything down to Seattle in a rental car, get the work done there where I knew the people and places, then come back and install it. So I removed all the insulation blankets, that exhaust riser, and the turbo. Got a rental car, and drove for 10 hrs including a ferry crossing and boarder crossing to get back to Seattle. I had lined up Hatton Marine to do the exhaust modification, and they started working on it before I even left Port McNeill. They also pulled and ordered all the parts that I would need for reassembly. And I lined up Ballard Insulation to modify and/or make new insulation blankets as needed to fit around the modified exhaust, and around the new mounting brackets for the hangers.
The next morning I was at Hatton at 7:00, dropped off the exhaust structure, picked up the parts, then went to visit some friends while I waited. Hatton finished early in the afternoon, and I hauled everything over to Ballard Insulation for them to work on. By 3:00, everything was done and I was back in the car on my way north with plans to stay the night in Nanaimo, and continue on to Port McNeill in the AM.
Below is the modified exhaust riser with attached mounting brackets to connect to the hangers. And note the buckets of coolant. It's about 12-13 gallons that has to be drained out to do this job.
|Modified exhaust with flex section and new hanger brackets|
After arriving back in Port McNeill, I spent the next two days putting everything back together again. First, the turbo goes back on along with the oil deliver and return pipes, the compressed air pipe, coolant return, and the vent pipe. Oh, and don't forget new mounting studs and nuts.
Then the turbo elbow gets installed. It's easier to attach the elbow to the turbo and ensure a good fit, then align the rest of the exhaust to just float over and kiss the square flange that you see below.
|Exhaust elbow attached to turbo|
And fast forward to installed hangers and fitted exhaust section. Once everything is in place, the spring hangers can be adjusted and the various joints rotated to get everything perfectly aligned, and with no weight on the turbo flange.
|Modified exhaust installed, with flex section and spring hangers|
Here's a video overview of the modification, also showing how the mating flange should float freely over the turbo elbow.
One obvious question is why, after building 60 of these boats, am I now having this failure? I think the two pictures below say it all. The first picture is of a friends earlier vintage N60. His boat, and pretty much the entire N55/N60 fleet were built with the Deere 6081 engine. The engine and turbo stand taller, with a resulting shorter exhaust riser.
Compare that to the next picture of my boat with the newer Deere 6090, introduced to meet EPA Tier II regulations. The engine sits noticeably lower, resulting in a significantly taller exhaust riser. This larger structure apparently is just enough to overwhelm the turbo, where the older arrangement was fine.
|Shorter riser on Deere 6081 engine|
|Taller riser on Deere 6090 engine|
So that was our major repair last spring. But to be sure you don't go away thinking that I do nothing other than fix things on this boat, I have come up with a new benchmark for measuring a boat's condition. You judge based on how quickly you run to the hardware store or marine supply store when you arrive in port. The faster you run, and the more you buy, the more trouble you are having with your boat.
I'm very pleased to report that after this repair, and for the rest of the summer, I did not make a single purchase at a hardware store or marine supply store. Based on that, I'd say the boat is running pretty darn well.