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Old 12-21-2018, 09:48 PM   #81
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Amazing stuff is going on with tugs and ships utilizing constant speed Diesel engines and variable pitch props, not unlike the airplane engine mentioned by Delfin. Hate to see us get too hung up on the nuances of predicting over loading with so many direct measurement or newer engine algorithms available.

Another reason is that as diesels get really big, they tend to have lower max RPM, which also means a narrower RPM range. Without the RPM range, you need need to change the shape of the prop load curve to get use of the power range.
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Old 12-22-2018, 06:56 AM   #82
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That seems like the critical qualification. Just going off my own experience, I took 1.5" of pitch out, and now cruise 100 rpm higher, but at the same EGT, speed, and fuel consumption as before. It doesn't seem like there are any markers of higher specific load before detuning, but am I missing something?
Carl, I can't see that you're missing anything, taking into account your engine's (mine too) vintage. With today's engines having very precise exhaust temperature, fuel and air "measurement", the relevant BSFC numbers Ski brings up can be neatly optimized for various prop and RPM setups.

TT's new build with very nice engine monitoring and prop tuning will yield some great data for this discussion.

Thirty years ago a place where I worked installed four very large Cat diesel gensets. Even that far back, the number of gensets online were computer optimized for engine load, fuel burn and power draw as user demand fluctuated. This is not new technology, just always getting better. One big reason being worldwide emissions' dictates.
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Old 12-22-2018, 10:07 AM   #83
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“Note that some Yanmars have the EGT port upstream of the turbo turbine, so due to expansion and pressure drop across the turbine those will read higher than other engines, where the probe is post turbine.”

The Yanmar 4lh has a plug right at the rear of the manifold. The end of the probe is basically at the exhaust valve outlet. So yes before the turbo right at the source of max temp other than in the cylinder itself.
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Old 12-22-2018, 11:26 AM   #84
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Re:


Consider this example. I run my semi-displacement hull up on 'plane'. It takes a LOT of throttle to get me over the hump and up on top of the water. Once I'm on plane, even a very slight reduction in throttle can cause me to 'fall off' of plane, and now I'm plowing water!


In this very common case,
This is a description of a boat that is under powered, not over or under propped. I gather you think it is common for builders of semi displacement boats to provide only enough power to get up on 'plane', and no more?

I don't think that is true, but if it was, since it takes more power to get up on plane than to stay on plane, the scenario you describe appears to defy the laws of physics.
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Old 12-23-2018, 07:08 AM   #85
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Many boats will pick up speed after getting out of the water , but usually not semi displacement boats as they frequently are not up on a true plane , as say a sport fish with 2x the installed power.

To plane the boat needs to be up on top of the bow wave , and have the boat hull run at about 4 deg or so.

If the bow doesn't come all the way down , your still in SD and need every bit of power you have to stagger along bow high.
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Old 12-23-2018, 11:10 AM   #86
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Many boats will pick up speed after getting out of the water , but usually not semi displacement boats as they frequently are not up on a true plane , as say a sport fish with 2x the installed power.

To plane the boat needs to be up on top of the bow wave , and have the boat hull run at about 4 deg or so.

If the bow doesn't come all the way down , your still in SD and need every bit of power you have to stagger along bow high.
That's true FF. I wasn't sure what the word 'plane' meant when you put it in parentheses in reference to semi-displacement boats. My Albin 28 was semi-displacement, and it cruised at 16 knots with a top speed of around 24. It wouldn't plane, but there was a spot where when the speed got to 16 knots after throttling if not to WOT, but close to it for a few seconds, then you would throttle back and go all day. No one toodles around in those boats at WOT because of noise, fuel economy, comfort, and I suspect other semi-displacement designs would be used in a similar manner.
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