To Repitch or not to Repitch?

I got a question earlier this year that essentially asked: I can’t quite reach full rated RPM under load but I’m only 50 to 100 RPM low in my Bayliner 4788. I’m considering playing it safe and repitching my props but my dealer recommends that I not bother until next season.  Is it OK to wait until next year since I’m close to correct and don’t run the boat hard for long periods of time?

 

My response:

 

When giving other people advice, I’m conservative.  Having spent 6 or 7 years servicing cars professionally, I know just how upset a customer can get when you say “it’ll be OK” and it ends up not being.  The safe answer is to remove 1” of pitch.

 

However, you aren’t nearly in as bad shape as many 4788s.  Since you clearly care and have a good strong set of engines to start with, invest up front in great instrumentation.  Buy boost gauges, pyrometers, and digital tachs.  Boost gauges and pyrometers provide valuable engine load information to help avoid overload (http://www.mvdirona.com/TechnicalArticles/DieselEngineOverload/Default.htm). The standard Faria tachs tell you when the engines are running but not much more—get good digital tachometers (http://www.mvdirona.com/TechnicalArticles/DigitalTachometer.htm).  Also get the fuel curves for your engine from the local distributor or the Cummins marine support team (wave.master@cummins.com).  From the fuel curves sheet you’ll see exhaust gas temp at full rated RPM.  It’ll be around 850F.  My general rule is to not cross that line although many argue this is unnecessarily conservative.  Some engines have acceptable load levels that produce exhaust temperatures above those at rated RPM. I chose to avoid this condition entirely.

 

The right answer is to do both: 1) get the instruments I recommend above and 2) re-pitch right away.  However, if you are careful, don’t run hard, and watch the instruments, you’ll probably be fine running with the current pitch.  The pyro’s will tell you for sure. 

 

I needed to take out a second inch of pitch in mine, but since I’m both careful and cheap, I didn’t want to re-pitch the second time right away. Instead I did three things: 1) ran light at lower RPM, 2) watched the pyros and didn’t ever go over the max rated temp (I prefer it 50F under), and 3) read the fuel burn.  From fuel burn you’ll know the HP you’re consuming at cruise.  With your engines, multiply gallons/hour/engine * 19 and you’ll find how much HP you’re asking for at cruise.  The constant 19 is the horsepower produced per gallon per hour and it’s very constant across all high speed diesels.  Newer common rail engines are closer to 20 HP/gal/hour but these numbers are remarkably stable across all manufacturers.  I was introduced to this approach by Tony Athens (http://www.sbmar.com/Articles.cfm).  Ensuring the HP you are using is always less than the manufacturer performance curves at that RPM will ensure that you are not overloaded.

 

In your case, I lean slightly towards re-pitch now. That way you can get to know the boat with everything running correctly.

 

James Hamilton

jrh@mvdirona.com

http://www.mvdirona.com/

 

 


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