10.8 International - cruise speed & economy

prowlersfish · Post by **prowlersfish** » Fri Feb 26, 2010 9:31 pm

captainmaniac wrote:RWS - I hope you have a problem with your chart... if you are going through 60 gph at 1800 RPM / 10ish knots its time to look for a hole in the tank!

Something is not right with the chart for sure

RWS · Post by **RWS** » Sat Feb 27, 2010 7:54 am

Hey guys, thanks for noticing.

The old chart had STACKED DATA

I updated the chart and corrected the problem.

The new chart is strictly RELATIONAL.

I show how the data has been modified in the legend.

I had to modify the data to get all the numbers to track together. I'm sure there is a better, more efficient method to do this, but I'm not a Climate Engineer.

Modifying the data allows you to see how it interacts together, otherwise the gph scale of 2 - 20 and the horsepower & prop curve data at around 300 would not allow the graph to show the relationships very well.

Finally, the prop curve, GPH and torque come from Yanmar. Based on my own usage and unscientific fuel burn usage it appears that my NMPG is significantly better than the published data and it appears that the actual sweet spot is 3300 RPM, slightly higher than the published data would indicate.

This may be for a variety of reasons, including the prop pitch which we left lower as to not overload these engines.

The only real way to determint the actual burn rate would be to install a flowscan device, however I'm looking at well over $1,000 for that upgrade. (I think it's about 1800 with the 4 sensors)

Meanwhile, I've reposted the data as it is.

RWS

ready123 · Post by **ready123** » Sat Feb 27, 2010 2:08 pm

RWS wrote:Finally, the prop curve, GPH and torque come from Yanmar. Based on my own usage and unscientific fuel burn usage it appears that my NMPG is significantly better than the published data and it appears that the actual sweet spot is 3300 RPM, slightly higher than the published data would indicate.RWS

Why do you say sweet spot is 3300... the way I read your chart that point should be 2800 just before the NMPGx10 starts to drop off.... 17 knots.
Or am I reading the curves incorrectly? Or maybe I don't understand your sweet spot.

prowlersfish · Post by **prowlersfish** » Sat Feb 27, 2010 6:27 pm

Sometimes the sweet spot is not were the MPG is , Its were the boat just feels good The engines are at just the right rpm or the boat is at the right speed , My sweet spot is 2400 RPM ,Why ? becuse thats where the boat and engines feel and sound the best .

RWS · Post by **RWS** » Sat Feb 27, 2010 9:44 pm

ready123 wrote:
RWS wrote:Finally, the prop curve, GPH and torque come from Yanmar. Based on my own usage and unscientific fuel burn usage it appears that my NMPG is significantly better than the published data and it appears that the actual sweet spot is 3300 RPM, slightly higher than the published data would indicate.RWS
Why do you say sweet spot is 3300... the way I read your chart that point should be 2800 just before the NMPGx10 starts to drop off.... 17 knots.
Or am I reading the curves incorrectly? Or maybe I don't understand your sweet spot.

Like Paul said, that's where she sounds & feels the best.

Absent flowsacns, I can only rely on the Yanmar published fuel data. Since I am a bit underpropped (intentionally) and I keep decent fuel burn records, it appears that my burn is less than the published data.

That near fact combined with the sound & feel that Paul described leads me to conclude that right about 3300 RPM (82.5% of WOT) is my sweet spot with this setup.

Keep in mind these are low displacement high RPM mechanically injected diesels.

RWS

prowlersfish · Post by **prowlersfish** » Sun Feb 28, 2010 8:18 am

Mine are lower rpm diesels ( still not low speed) But I also am under propped my engines are rated at 2800 rpm but I can turn 2950 . and like you this is on purpose and I bet its for the same reason , we like our engines and want them to last a very long time .

My sweet spot is right at 81.5% of WOT rpm the 80% rule for HP Diesels seems to hold true for both of us

RWS · Post by **RWS** » Sun Feb 28, 2010 9:05 am

Makes sense Paul, but I wish I had the Floescans to determine this for sure, better than my ear and the "feel" of the boat.

By the way, with the side exhausts and the counter rotating 454's synchronizing the engines was simple.

Can't do that with th ediesels as the turbo muffles the sound to an extent and the gear ratios on the gearboxes are very slightly different. We made up for this with a bit more cup on one prop and we sync the PROPS by means of balancing the turbo boost pressure gauges.

RWS

foofer b · Post by **foofer b** » Sun Feb 28, 2010 9:29 am

Sweetspot! Don't get me started, we gotta keep it clean.

prowlersfish · Post by **prowlersfish** » Sun Feb 28, 2010 10:45 am

Rws I am lucky as my boxes are the same in both A and B position . My diesels are high flow so the floscans are $$$ ( just under 2 grand for a set) so I pass for now but even if I had them and found out better MPG at say 2200 rpm I may still run at 2400 as it just has a better feel to it at that rpm .It even fells better at 2600 But I think that is getting to close to the edge