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Old 07-17-2014, 01:20 PM   #41
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A verifiable real world test for cruising prop selection benefits can be done several ways, here is one:

--Prop vessel so it can achieve full rated RPM without incident
--Measure and tabulate fuel burn and vessel speed vs 100 RPM increments
--Repeat with larger cruising prop
--Compare the results
--Do this in flat water with no current and at 180 degree reversal to compensate for wind and latent currents
--Insure tachometers, GPS and fuel measuring equipment are accurate


This testing method requires money, patience and good measuring equipment. Without such the differences are relegated to arm waving and citing fuel burn charts from the manufacturer as proof one way or the other.
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Old 07-17-2014, 05:14 PM   #42
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Originally Posted by Delfin View Post
Yes, peak torque looks to be right at 1375 or so on this engine. As per my question to Sunchaser, isn't a bit of load from minor overpropping potentially beneficial in that it provides a more "reasonable load"?
The simple answer is yes. Don't know if you can obtain the Hp, torque, and fuel consumption graphs for your engine (maybe Boatdiesel). I would look at the graphs for you cruise RPM range and see what HP and fuel consumption the graphs show. I would want to be well below the maximum, but probably at or above 1/3 rd for adequate loading and engine heat generation. This could be comparable to a generator in that you certainly don't want to run it under loaded for extended periods of time for the same reasons. That's how I see it anyway.

Ted
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Old 07-17-2014, 06:11 PM   #43
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Quote:
Originally Posted by sunchaser View Post
A verifiable real world test for cruising prop selection benefits can be done several ways, here is one:

--Prop vessel so it can achieve full rated RPM without incident
--Measure and tabulate fuel burn and vessel speed vs 100 RPM increments
--Repeat with larger cruising prop
--Compare the results
--Do this in flat water with no current and at 180 degree reversal to compensate for wind and latent currents
--Insure tachometers, GPS and fuel measuring equipment are accurate


This testing method requires money, patience and good measuring equipment. Without such the differences are relegated to arm waving and citing fuel burn charts from the manufacturer as proof one way or the other.
The process you describe is very similar to the results turned into one graph in the article...thus my post....

Someone actually did it and came up with results that made Calder make the opening statement.
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Old 07-17-2014, 08:01 PM   #44
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Originally Posted by psneeld View Post
The process you describe is very similar to the results turned into one graph in the article...thus my post....

Someone actually did it and came up with results that made Calder make the opening statement.
It would seem Calder is citing the very good report done by Norwegian O Gulbrandsen in 2012. The report points to bigger props turning slower, increasing the gear ratio and derating the engine by the use of lower RPM fuel stops to prevent damage.

The report is well worth reading and spends much time on hull design and saving fuel by easing back on the throttles. The report also cites the need to install smaller engines to best take advantage of slower speeds. Way to go Coot.

Now if only I were doing a new build and could do it perfect, kinda like Steve Dashew has already done in his FPBs. Calder and Dashew are two very smart cookies, with Dashew putting his money on the line.
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Old 07-18-2014, 12:15 AM   #45
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Originally Posted by O C Diver View Post
The simple answer is yes. Don't know if you can obtain the Hp, torque, and fuel consumption graphs for your engine (maybe Boatdiesel). I would look at the graphs for you cruise RPM range and see what HP and fuel consumption the graphs show. I would want to be well below the maximum, but probably at or above 1/3 rd for adequate loading and engine heat generation. This could be comparable to a generator in that you certainly don't want to run it under loaded for extended periods of time for the same reasons. That's how I see it anyway.

Ted
Thanks Ted. Can't really hit the 1/3rd target since my hull drives pretty well at very low horsepower levels. Think big sailboat hull design. However, to get 9+ knots hull speed I need a lot of horsepower because I am pushing a lot of water at those speeds. However, I am told that the 3306 is good for a trawler because it can run at low loads for a very long time. Be that as it may, thanks again for helping me with your advice.
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