Tuesday, May 12, 2020

May 2020: Back in FMY - Fixing Stab Overheat

Foreword:  We and the boat are back home in Fort Myers and we’re still being diligent about social distancing and hygiene practices.  Hopefully that’s true for all y’all, too.  But here in the USA, with most states proceeding with “re-opening” without much regard for science, we’re also preparing for a long slog with multiple setbacks looking very likely.  If you want a historical analog, go study the Spanish Flu outbreak of 1918…along with how & why it continued into 1920.  Mark Twain reportedly once said “History doesn’t repeat itself, but it often rhymes.”  It seems we’re destined to prove him right again.  But there is good news….have discovered if you keep a glass of scotch in each hand then you can’t touch your face.  That’s good hygiene.

This entry is all about a system repair and nothing else.  So unless you are a boat geek, or bored enough to seek almost any distraction, you’ll want to skip it.

Even though we’d had a very successful maintenance stop with Yacht Tech in Palm Beach, we still had ample boat projects to keep us busy on Ghost Rider.  But the priority problem-to-solve was the stabilizer oil overheating condition we had encountered at the midpoint of our journey back to Fort Myers.  After a few days at the dock we now had a much cooler engine room, allowing for a relatively relaxed and comfortable approach to troubleshooting…and the luxury of temporarily disabling the main engine without consequence.
The Red Arrow Points to the Main Engine's Heat Exchanger on the Port Side of the Maine Engine.  Tucked Under That, the Blue Arrow Points Out the Gear (Transmission) Oil Cooler.  The Yellow Arrows Point at the Pencil Zincs on the Heat
Exchanger.  And then the Green Arrow Indicates the Down Elbow....Just Underneath that a Small 5/8" Diameter Hose
Branches Off to the Cooling Circuit that Routes Over to the Stabilizer Oil Tank.
The design of the cooling circuit for our stabilizer oil tank is relatively simple. Ghost Rider is an exception compared to most Nordhavns….rather than being keel-cooled with a dry exhaust stack, our Nordy’s main engine is cooled via a raw (sea) water heat exchanger and uses a traditional wet exhaust.  That arrangement provides for a secondary cooling circuit that branches off at the bottom of the main engine’s heat exchanger and is then routed over to the stabilizer oil tank reservoir for additional cooling duties there.  After exiting the stab oil tank, that auxiliary cooling circuit empties into the sea via its own dedicated discharge thru-hull.  Except now we could not detect any outflow coming from that thru-hull opening….meaning no cooling flow.
The Stabilizer Oil Tank is Located in the Port Side Forward Alcove of the Engine Room.  The Yellow
Arrow Points to the Input Cooling Line that Branches Off the Bottom of the Main Engine's Heat Exchanger.
The Green Arrow Points to the Output Line that Goes to the Thru-hull Discharge.
So there is a minor flaw in that design:  while the heat exchanger itself is quite beefy, by necessity it utilizes two sacrificial pencil zincs (anodes) to prevent galvanic corrosion inherent with that salty seawater flow…and as those zincs decay (as intended) the shedding debris will descend to the bottom of the heat exchanger – right where that stabilizer cooling loop begins.  That loop uses much smaller diameter hosing and piping (5/8” ID), and occasionally is prone to clogging with that decaying zinc detritus.  When that occurs the result is hot stabilizer oil….rather than the normal 155F temps we were seeing nearly 200F on the leg to Marathon following the overtemp warning light.
Flushing the Line Going from the Heat Exchanger to the Stabilizer Oil Cooling Loop.  The Red Arrow Points to One of the Two Pencil Zinc Ports We Used for the High Pressure Flush Out.  The Yellow Arrow Indicates the Low Point of the Stab Cooling Circuit Where We Diverted its Feed Hose Into a Drain Bucket.
Rick decided to start the investigation where the stabilizer cooling loop originates at the bottom of the main engine’s heat exchanger.  After removing the two zinc plugs (they were due for replacement anyway) and disconnecting a segment of the hose feeding the start of the stab cooling circuit, he stuck a high pressure hose into the zinc ports for a flushing out.  While that netted some debris it certainly was not enough to cause a blockage or overheat issue.
After Disconnecting the Stab Oil Tank's Cooling Hose from the Thru-hull Discharge (Yellow Arrow) and Augering Out
a Significant Zinc Debris Blockage, We Observed a Good Cooling Flow from the Hose End (Green Arrow.)
After reassembling the main engine side of the loop, the next step was to check at the opposite end of the stabilizer cooling circuit, where it exited the boat via the thru-hull fitting.  It was a likely suspect since there was a 90 degree elbow where the outflow hose clamped to the thru-hull fitting – and in there Rick found enough stray zinc debris to form a very effective dam.  A stiff wire coat hanger server as a suitable plumbing auger, and in the end that blockage turned out to be the only culprit.  After clearing the clog and starting the main engine we observed good cooling flow exiting from the thru-hull discharge.
The Yellow Arrow Points Out the Resulting Normal Stab Cooling Flow from the Thru-hull.
The question now was whether such an event could be prevented in the future.  The zinc decay itself is a critically important feature that protects expensive diesel engine parts from dissolving through galvanic corrosion; it would not be wise to change that portion. One option was to design an entirely separate cooling loop for the stabilizer oil tank, which would entail a new intake thru-hull as well as a new electric water pump (and then sealing off the existing feed from the heat exchanger.)  But another path was just to insert a sea-strainer device into the existing cooling circuit to trap zinc debris, with periodic inspection and cleaning.  The latter sounded a whole lot easier and cheaper, at least as a first try.  So that’s the approach we’ve taken for now.  Rick installed a standard Jabsco PumpGuard strainer at the circuit’s low point, close to where it branches off the main engine’s heat exchanger.
The New Sea Strainer Inserted into the Stabilizer Cooling Circuit at its Low Point.  It's Positioned Just Under the Front End
of the Main Engine Where It's Readily Visible and Accessible for Cleaning When Necessary.
It was a relatively simple modification, but it needs to be sea-trialed to determine whether it still allows for an adequate flow rate for cooling the stabilizer oil tank.  A better long-term solution might also call for an inline shut-off valve just upstream of the strainer to allow cleaning of the strainer basket without having to first shut down the engine and drain the heat exchanger (that beast holds a LOT of sea water.)
Just What is a Heat Exchanger?  The Schematic Above is Generic, but is a Good Illustration.  It's Basically Like a Car's Radiator, but Instead of Outside Air Cooling the Antifreeze that Circulates Through the Diesel Engine, it Takes in Seawater to Perform the Same Function.  On Ghost Rider a Portion of the Seawater is Expelled Through the Wet Exhaust, but Another Portion is Routed to the Stabilizer Oil Tank to Perform Cooling Duties There.

1 comment:

  1. Terri and I are being really careful; Shiptshop and InstaCart gives us our Publix fix. Swiping cards at gas pumps, drive-up windows at the bank and parking lot mailboxes at the Post Office keeps us isolated. When I walk the docks at sundown as per my custom, I have to be wary of people sneaking up from behind and getting too close. It's astounding to both of us how many are not paying enough attention to this.

    On the good side, I keep hearing a good dose of Vitaman C thru UV rays helps vanquish it. For the first time in 50 years I have an excuse to get a tan.

    Congrats on the Stab fix. Good Catch!