Diesel Exhaust Gas Temperature - Page 2 - SailNet Community
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post #11 of 16 Old 09-11-2009
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Zanshin, if you're looking for a temp gauge on a permanent mount, I think EGT on manifold will be the fastest way to warn you against overheat. VOLVO sells an exhaust mounted sensor to measure the temp of raw water mixed to gas discharge, and claims this way, you can get warnings earlier than the fresh coolant temperature. Otherwise, you can simply use a precise poin-and-shoot temp sensor and create your own table rpm/speed/temp, then add a 10-15% safety factor.

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post #12 of 16 Old 09-12-2009 Thread Starter
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[QUOTE=jrd22;521656]...I'm not familiar with the Gori... but you can't have increased speed through the water at the same rpm without a change of pitch or diameter of the wheelQUOTE]
Correct, the Gori prop in overdrive mode has a different pitch.


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post #13 of 16 Old 09-12-2009
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Zan- thanks, I'll have to read up on the Gori prop, but on the surface it would seem that in overdrive you are no longer using the correct pitch and therefore putting a dangerous load on the engine. When does Gori advise using the overdrive mode, and is there a warning against prolonged use or % of maximum rpm to run? Seems like a very strange concept to incorporate into a prop, it's hard enough to dial in the correct pitch in order to achieve WOT, why would you want to change it?

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post #14 of 16 Old 09-13-2009 Thread Starter
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jrd - Yes, Gori does warn against overdriving the engine with the overdrive mode. It is designed for and particularly useful when motorsailing, using the added pitch to let the engine work at lower RPMs for a higher resultant speed. I am looking into a way at trying to get the most out of that mode without overworking the engine or other systems, the only time I used was on one passage to Antigua when I was delayed a couple of hours because of a lobster pot that got caught on the boat. Even with radar, I didn't want to go into English harbour at night and used the overdrive mode so that I would make it in time. The engine alarm caught me just as I was heading in, so I ended up heaving to and researching the cause and by the time I had it figured out it was dark (so I went into Falmouth harbour instead, where I had already anchored several times). Thus I decided to see about ensuring that I could not overheat my engine inadvertantly again and the EGT seemed the most direct solution.
The Gori propellor has 2 pitches, the "overdrive" being engaged by putting the boat into reverse until the boat makes no headway or slight aft headway, then shifting through neutral to forward. This ensures that the blades remain in their "reverse" mode, which has a stronger pitch than forward mode. By switching while the boat is moving the blades don't go into neutral, feathering mode as they are held in place by the boat motion and centripetal force keeps them extended. A very nifty and elegant solution and the overdrive mode really does make a difference to fuel consumption when motorsailing (and when not overheating the system).


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post #15 of 16 Old 09-13-2009
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Hmmm, interesting. So in fact you are turning a properly pitched prop into an over pitched prop in order to get more speed at a lower rpm. Over pitched is over pitched, doesn't matter what rpm you are turning, it's hard on your engine (that's why it overheated). The EGT will definitely give you advanced warning of overloading, and eventual overheating, of the engine. In trucks that I have run with EGT (generally referred to as pyrometers, or "pyros") the EGT will start to climb, rapidly, when you start to climb a hill if you don't downshift. I would think that by monitoring the EGT on the boat you might be able to gradually raise the rpm and see when the temp starts to spike (the actual temp will depend on the location of the sensor). If you can locate the probe in a spot that does not have cooling water mixed with the exhaust I think you will get a much better indication of actual EGT. Good luck, be careful, seems like you're walking a fine line between fuel savings and engine damage.

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post #16 of 16 Old 09-14-2009 Thread Starter
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Thanks for all the advice - I've ordered an EGT gauge and meter and will install it (once the hurricane season is over and I can move aboard again).


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