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Old 11-07-2013, 12:14 PM   #41
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Not "hung up" really. Just using it as a starting point to get a feel for best rate. That doesn't mean lowest GPH or hull speed. It's a lot like trying to get your arms around Best Rate versus Best Angle as a pilot.

For me... best rate is about 8-8.5kts. I have two plants running, pushing the hull just up to a speed where diminished returns start showing. That gets me there at a rate which optimizes full economy and time and wear and tear.

That number truly is different for every boat. I have the fuel curves for my engines and I have speeds versus RPMs so it's an easy calculation and model to build.
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Old 11-07-2013, 12:20 PM   #42
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Absolutely! Guys are still hung up on the "hull speed" thing.
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Old 11-08-2013, 06:49 AM   #43
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Originally Posted by SomeSailor View Post
Not "hung up" really. Just using it as a starting point to get a feel for best rate. That doesn't mean lowest GPH or hull speed. It's a lot like trying to get your arms around Best Rate versus Best Angle as a pilot.

For me... best rate is about 8-8.5kts. I have two plants running, pushing the hull just up to a speed where diminished returns start showing. That gets me there at a rate which optimizes full economy and time and wear and tear.

That number truly is different for every boat. I have the fuel curves for my engines and I have speeds versus RPMs so it's an easy calculation and model to build.

Yep, starting point.

Coincidentally I just made up a similar model of an imaginary trip from here to Norfolk, 140 NM, and using the published engine prop curves for fuel consumption info:

2400rpm 22kts 35gph 223gals 0.63nmpg $890($4/gal)
2200rpm 20kts 27gph 190gals 0.74nmpg $762($4/gal)
1200rpm 08kts 06gph 105gals 1.33nmpg $420($4/gal)
1000rpm 07kts 04gph 080gals 1.75nmpg $320($4/gal)
0800rpm 06kts 02gph 056gals 2.50nmpg $224($4/gal)

Our LWL is approx. 38' so theoretical max displacement hull speed is approx. 8.26 kts. Idle is 600rpm. From the table, slower is still "better" in terms of fuel cost.

Time efficiency? perhaps that's another thing... since fuel savings might be eaten up by marina costs unless we anchor out

And our hull shape isn't always comfortable on the Chesapeake in all sea states... so we sometimes couldn't travel at 6 kts without getting beaten up...

-Chris
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Old 11-08-2013, 10:43 AM   #44
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Ranger I thought you really had something for a moment but what ever happened to 9,10, 11 knots ect ect?

But your numbers sure dramatically showed one thing. Slower = better (less) fuel burn and more mpg. Graphic evidence literally.
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Old 11-08-2013, 11:08 AM   #45
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Ranger I thought you really had something for a moment but what ever happened to 9,10, 11 knots ect ect?

But your numbers sure dramatically showed one thing. Slower = better (less) fuel burn and more mpg. Graphic evidence literally.

Those were the only RPM/speed combinations I could remember off-hand, my latest RPM/speed test table is on the boat, and I'm not (yet, today).

There's also a place in there around 1400-1600 RPMs (I think) where the boat is simply plowing, so it's neither fuel- nor speed-efficient in that range anyway.

In retrospect, I should have mentioned the engine setup is twin Cummins 6CTAs (450s), and speed/fuel consumption in the table is the total with both engines running.

-Chris
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