Tied Up in Knots, Part 2

In Tied Up In Knots, Part 1, you read about our abrupt loss of 1/3 of the cruising speed of the yacht, for no apparent reason, and our expensive – ultimately fruitless – 10-month Odyssey to locate and correct the problem(s). If you haven't read that post yet, take a moment, and then come back.

We pick up the tale after I inherited Clark Hurley, a most excellent Volvo mechanic introduced to me by the Southern California Carver dealer. Clark went on a speed trial (the first of a half-dozen or so over the next year) and confirmed the lack of performance.

Clark's area of focus, however, was the turbochargers. These big diesels depend upon vast quantities of high pressure air being shoved down their throats in order to develop their rated power and RPM. On our diesels, a very sophisticated electronic control system manages the turbocharger pressure (i.e., the waste-gate control) and the fuel injection. Everything has to be precisely in balance and in spec. And if you read Part 1, you will recall that a mere 50 RPM difference results in a 1-knot change in speed.

Clark quickly discovered that one of the two turbochargers was not delivering the expected manifold pressure at cruising RPMs. And because of the sophistication of the control system, if one turbocharger is "low", then opposite engine automatically retards itself as well. Here's how that happens:

At full throttle, if one turbo is extremely low, its engine develops much less than rated power, so the boat runs slower. Since the boat is running slower, the props are turning at less than the calculated RPM for the throttle setting. In response, the good engine increases its turbocharger boost pressure in an attempt to regain the lost RPM. But within seconds, the good engine exceeds maximum allowable boost pressure, says "Oh Dear!" and dials the power way back – perhaps to about half-throttle equivalent. So, even though the good engine is being commanded to produce full power, it actually produces as little or less power than the bad engine with the low boost pressure. The result: a half-dozen knots in speed – gone in 60 seconds. And it stays that way.

Now, right after Clark discovered the low boost, the suspect turbocharger froze, failed, died, gasped its last breath. This failure took Clark down a several month rathole (considering waiting for parts, repair windows, and availability of various parties for speed trials), as he believed (I disagreed) that the impending failure – perhaps a deteriorating bearing – accounted for the low boost pressure. Sadly, after he got the new turbo installed, the boost was still low.

The next suspect was the waste gate. If you are not familiar with turbocharging, the waste gate is the valve that moves to set the boost pressure to a target value by diverting part of the turbocharger's pressurized air output away from the intake manifold. If the waste gate sticks, the boost pressure becomes uncontrollable by the engine's electronics (or mechanics). It turned out our waste gate was acting up, so it got replaced. That also had no impact. But many more had weeks elapsed.

Now, it gets interesting. Part of the boost pressure regulation system is an electronic sensor that lives at the end of a long, narrow tube (about the diameter of a soda straw) that is taken off the pressure side of the turbo. Since there is moisture in ocean air (imagine that!), and since the turbocharger is just a big compressor, water gets compressed out of the air, and some of it enters this sensor tube. This is salt-laden air, mind you, hence salt-laden water. Well, that saltwater gets forced down the tube and collects at the electronic sensor. Sizzle, sizzle, phizz... The sensor stops working correctly, and the house of turbocards tumbles. Result: inappropriate boost pressures.

This system was actually designed by engineers! Volvo engineers. From that nexus of Swedish technology. Made me stick my engineering degree under a mattress.

Well, I guess Volvo got enough complaints as the field reports started rolling in, so they came up with a fix. They drilled a tiny, catwhisker-sized "weep" hole in a brass hex drain plug that lives at the sensor end of that tube. The hole is not so big that it affects the pressure inside (hence the pressure readings taken by the sensor) but is large enough to allow all moisture to blast out under the high boost pressures. Result: a continuously dry (and suddenly reliable) sensor. Of course, the fact that there was a drain plug in that location to begin with was highly suspicious.

After eliminating and replacing all the truly expensive parts of the the system, Clark finally got to that sensor. Upon removing the old-style drain plug, saltwater flowed right out. The electronic sensor, of course, was severly corroded, beyond dead. Clark shook his head, and told me the whole weep-hole saga. He then ordered a new sensor, and two of the new-style, drilled drain plugs. I think the plugs were about $1.00 each.

When this system went back together, Clark was able to correctly calibrate the boost pressures on both sides, and suddenly, we had our RPMs back, in fact, too many. Seems that now those flatter-pitch props I talked about in Part 1 were overkill.

Not wanting to remove and pound our props back to their original profile, particularly if some future expert told me to reduce the pitch again for some reason, I prevailed upon Carver to arrange for me to purchase a pair of props at dealer cost. Perhaps to help put to bed what was now about a 20-month ordeal, and a lot of letters and emails to read from me, and maybe because it was partially on their advice that I flattened the props to begin with, they came through. And with the original props – and a working turbocharger pressure sensor – the boat attained a cruising speed of 18.5 to 19 knots.

And while I still wanted to know where the rest went, with one exception, I stopped chasing the speed problem. Instead, I had the boat weighed at the next haulout. Against a factory nominal weight of 47,500 lbs, our boat weighed 52,000 lbs. This weight – which included the new, improved davit system dragging down the stern (see The Pickle Fork, October 1, 2004) – probably accounted for the difference. I did note, however, that I had run out of travel on the trim tabs, and so in December, 2006 replaced the 66% span trim tabs with 100% span, which helped (see photo).

As I wind down here, however, I want you to understand the irony of all of this. In the end, it was a tiny hole drilled in a 1/4-inch brass hex plug that, had it been present originally, would have saved nearly two years of frustrating investigations, hundreds of man hours of my time and that of several mechanics, and a breathtaking amount of money. If you have a turbocharged diesel or two in your yacht, ask your mechanic about whether or not there is anything equivalent to the system on the Volvo-Penta TAMD74P-A. And if you don't have that little plug with the weep hole, install it immediately. It will cost you just a few dollars. But oh, the potential savings!