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Old 08-07-2015, 02:12 PM   #1
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HP Power Loss due to high E.R. temp

Morning Gentleman
Has any one come across a downloadable excel spreadsheet for calc HP loss as engine room temp rises?. Found some webpages but I need to take a excel sheet with me. My boat is in the Caribbean were the avg ER temp is 85F when cool. However it was climbing to 120F when underway. I have since refitted a new ER ventilation system which changes the complete ER air every minute, with ducted air right to the engine intake areas. Sadly I have not been able to sea trial these improvements, only alongside trials

Provisional calcs revealed my 30yr old FL 120's were losing 11% to the hot 120F air aspiration. So take in transmission, age, and other loss's. I was losing some performance.

But I would like to accurately quantify these loss's when I go back to the boat after the hurricane season at the end of September. So if anyone has come across a spreadsheet I would appreciate the info
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Old 08-07-2015, 02:57 PM   #2
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Until the intake air gets so hot that the engine reaches its smoke point you won't lose any power, you will just burn more fuel.

The governor will inject more fuel to compensate for the diminishing BMEP caused by reduced air density until it is effectively overloaded at the governed rpm. At that point it will start smoking black because the air fuel ratio reaches the smoke point. As long as you can maintain rpm you have not lost any power, you are just paying more to obtain it.
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Old 08-07-2015, 03:03 PM   #3
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If power loss is directly proportionate to the reduction in oxygen molecules (seems like it should be in the worst case), it is easy to calculate using Boyle's law (P=VT). By my calcs, going from 80F to 120F would give a 6.9% reduction. If you PM your email address to me, I will forward a very simple spreadsheet.

I suspect this overstates the loss, however, as there is probably an abundance of oxygen, especially in a turbo'd engine.
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Old 08-07-2015, 04:58 PM   #4
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hot fuel also causes a power loss.
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Old 08-07-2015, 05:39 PM   #5
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Unless running at or very near full power, or if making black smoke, neither the hot air nor hot fuel will affect boat performance.
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Old 08-07-2015, 06:34 PM   #6
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RickB and Ski are absolutely correct. The other consideration though is efficiency, but I suspect that it is too complicated to calculate and too small to measure.


Thermodynamic theory says that an Otto cycle engine will be more efficient the cooler the intake air is. But there are other considerations. With the engine room cooler the engine block will loose heat and lost heat is lost efficiency. Also for short term cruising, maybe less than an hour, the heat required to heat the block up and the increased drag due to cold oil is a consideration in efficiency.


All in all I suspect that you will never see the difference in fuel economy with an engine room temp of 80F vs 120F.


The only practical reason to worry about engine room temperatures is that cooler is better for the engine- EGTs, valve temps, etc.


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Old 08-07-2015, 08:06 PM   #7
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This leads us back to the over propping question. Sure you can compensate for power loss by upping fuel but we do hear about boats set up in northern areas that cant reach proper rated RPM when moved to FL so IMO there is noticeable power loss.
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Old 08-08-2015, 08:23 AM   #8
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I used to run a Bertram sportfish with twin Cat 3412's. The engine room was a comfortable temp when running as the growlers sucked in lots of outside air. It got like an oven when the engines were switched off and the heat soaked out of them. I imagine the engine spaces in our trawlers work similarly.

If air is ducted directly to the intakes, why is temperatue elsewhere in the engine room a concern?
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Old 08-08-2015, 08:42 AM   #9
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Quote:
Originally Posted by bayview View Post
This leads us back to the over propping question. Sure you can compensate for power loss by upping fuel but we do hear about boats set up in northern areas that cant reach proper rated RPM when moved to FL so IMO there is noticeable power loss.
Most likely the engines in those boats were turbocharged / aftercooled and had little reserve power available.

Inlet temperature on a turbocharged engine can be critical because of all the downstream impacts related to high air and water temperatures.

If you have a naturally aspirated engine operating at the upper right hand corner of the envelope in a clean hull and increase the inlet air temperature it will also begin to lose power and show signs of overload.

When the hull gets dirty it will show signs of overload. If it is a twin making a sharp turn, the inboard engine will show signs of overload.

All of the above are why reserve power is or can be so important to any engine installation. Something to keep in mind for the minimalist crowd.
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Old 08-08-2015, 09:35 AM   #10
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Someone smarter than me could probably calculate the volume of air the engine is using. My guess is that it's a significant percentage of the volume of air in the ER over a very short period. In other words, does the air really have time to warm up that much on it's path to the air intake?

I suspect Mike is correct that it's only after shutting down or when idling that the ER air has time to heat up a lot.
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Old 10-28-2016, 11:05 AM   #11
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Just an update.... Well just did a quick check of miles covered and what I've ran through the floscans, and my 120hp 30 year old lehmans are doing .801gall per mile (both). In a previous post I mentioned some problems with high engine room temps and efficiency. We installed high capacity exhaust fans changing the e.r. air every min and blowers pushing outside 85F cooler air directly at the engine intakes. Ball park we are running at 20% less fuel at the same RPM and 3/4 knot faster. We've dropped the E.R temp by a good 25 degrees F. All the rest of the Engine room components, batteries, a/c pumps, water pumps are performing better. When I started this project to increase my efficency I found very little info generally. However I did find some helpful sites. Which showed some possible gains, wereas the engines temps have remained stabil the ability of the engines to breathe cooler denser air and get more bang for your buck has been very positive.

Previously when I got the boat E.R Temps could hit 130F now we cruise at 1800 (7.3kts) with E.R. temp around 103F..
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Old 10-28-2016, 11:13 AM   #12
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Hello Martin J

"Ball aprk we are running at 20% less fuel at the same RPM and 3/4 knot faster."

If you are running 3/4 knot faster at the same rpm it has nothing to do with engine room temps.
Taking that 3/4 knot increase at the same rpm observation further you will see less fuel burned per distance if you have increased speed at the same engine rpm.
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