Sherwood P1730x Raw Water Pump at 3,200 Hours
I love the Cummins B-Series engines for good reliable operation and affordable parts when you need them. Its a mature design with most of the bugs worked out. However, there is an exception. The raw water pumps are fairly service intensive. During the first 1200 hours of the boat's life, I needed to change two raw water pumps and numerous impellers. These beasts really don't seem to match the overall design and build quality of the B-Series line. In fact, they have been troublesome enough over the years that Tony Athens' of Seaboard Marine has decided to produce a P1730x drop in replacement pump that aims to address the longevity issues. This new pump design is available from Seaboard Marine (www.sbmar.com).
What's particularly strange is that after the starboard engine went through two pumps in 1,200 hours, in the over 2,000 hours since then neither the engine has experienced a pump failure. The port side pump has never been replaced in the entire 3,200 hour life of the boat. That raises two questions: 1) why is the raw water system now operating so reliably when it was initially so bad and others report similar poor experience and 2) what is the condition of the raw water pump that has been in use for over 3,200 hours?
Looking at the first question, why has the raw water system operated so reliably for the last 2,000 hours, I can only speculate. Conceivably the pumps have been through design changes and the new pump designs are notably better. This is possible although the port side pump is original equipment and the same age or older than the two that failed on the starboard side. This explanation seems unlikely. Another possibility is that the water conditions in which we operate may have changed. Perhaps we have lower sand and sediment levels in the water over the last 5 years as compared with the first two. This too seems unlikely in that we moor in the same place, use the boat in the same places, and there have been no reports of conditions changing nor is it likely they would over such a wide area.
The explanation I lean towards is changes in engine operating conditions. During the first 12 months of operation, we ran the Cummins 270Bs at 2,300 to 2,350 RPM most of the time. We then removed 1" of pitch and dropped down to running 2150 RPM at the high end and frequently running at hull speed. We've since removed an additional inch of pitch from the props due to increasing weight that comes from just having more stuff on board. We now run in the 2150 to 2200 RPM range at cruise. Much more than half of the time, we're down around 1,200 RPM or less. Clearly these reduced engine speeds and loads have been good for the engines as evidenced by the 3,200 trouble-free hours they have delivered thus far. But, why would decreased load help the raw water pumps? Really it's not clear why it would make much of a difference and other factors may be at play. But, reduced engine speed and load is my best guess as to why the pumps have been so reliable over the last 5 years.
The second question is what prompted this post. We have 3,200 hours on our port side Sherwood P1700x raw water pump. What's its condition? From having looked hard at these pumps when changing them in the past, I'm skeptical that I want this old pump still running at 4,000 hours so I decided to pull it yesterday and inspect it for wear and overall condition.
As I mentioned in an earlier post, these pumps are actually fairly easy to access if you first remove the engine mount that shrouds the pump in most installations. I explain the details of the pump change on this page: Changing Raw Water Pump. The general process, is to put a small hydraulic jack on a piece of wood resting in the bilge, jack the engine up less than 1/16" and remove the mount. At this point, changing the pump is fairly easy ignoring the fact that I'm jammed in between the engine and the fuel tank in a dark but fortunately fairly clean cave.
Pulling the pump proved interesting. Pictured below is the entire pump with impeller, cover, and key. At this distance, the pump looks fine. Some wear on the cover but, generally, the pump and impeller look to be in fairly good condition.
In this picture of the inside bore of the pump, we can see heavy wear.
Looking above and of the left, you can see the section of the pump bore that actually does the pumping, the cam is worn fairly seriously. This pump will be operating at a fraction of its rated output. Fortunately, we run these engines at around 150 HP a side and those same water pumps are used on Cummins C-Series engines producing 450 HP and beyond. The pump even in its current degraded condition, operating far below it's rated output, is still more than adequate for the engines under these conditions. However, this level of wear brings up an important point. If you run your engines at or beyond their rated output (see Diesel Engine Overload: http://www.mvdirona.com/DieselEngineOverload/Default.htm) and not all parts are 100%, it can and frequently does lead to quick and expensive failure.
Looking closer at the pump bore above, you'll see that it has actually worn the cam down to the point where the bolt that attaches it inside the pump bore is actually now being worn by the impeller. The small silver circle in the middle of the cam is the polished attachment bolt.
What I though was particularly interesting is the impeller was loose on the shaft. When the pump came off, the impeller could be moved 10 degrees without moving the shaft. Looking at the picture above on the right, you can see that the impeller key has worn into an hour glass shape. The shaft no longer has vertical edges at the keyway. The keyway in the shaft has worn into a mildly V-shaped channel.
I could replace the worn cam cost effectively but the keyway wear makes the pump effectively junk. It's just not cost effective to rebuild it and I'm much more comfortable going with a new pump when wear is this far along. Dirona now has a new pump installed and running fine and earlier today I put a new impeller on the starboard side.
The lesson I get out of this is check pumps carefully for keyway and cam wear when changing impellers. Generally I'm happy and surprised to have this pump last as long as it did. If I always got 3,000 hours from a raw water pump, I would very happy indeed.
Last updated: 2007.11.17
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