[Quitsa]

What makes this a complicated issue (at least to me) is that with my prior set of props that allowed the engines to turn up to the governor limit of approximately 3350 at WOT, the fuel rate at the speed the engines seemed comfortable cruising was actually quite a lot higher than in the current “overpropped” condition. For example, at 31 kts, the old props allowed the engines to turn up to 2700 rpms but I was burning about 18.7 gph per side. The new props get that same 31 kts burning 15.2 gph per side but turning only about 2450. I have to assume the engines are better off burning less fuel per hour given the very comfortable EGT of only around 700-725.

As I explained in my initial post, I think this startling efficiency difference is because the old props (which have 4″ less pitch) generate too much stern lift and cause the hull drag to go up dramatically with a bow down running attitude.

I am going to take some cup out of the new props and see if I can pick up 100 rpms or so and get more boost, which I think won’t produce same the hull drag effect I was seeing before.

[Quitsa]

Thanks Tony, actually you are framing the issue that defines exactly what I am trying to figure out. I had learned from you previously to ignore “Load” on the instrument display and instead focus on the fuel rates as a percentage of maximum rated fuel consumption as the indicator of how much work the engine is doing.

To answer the question, at my “happy” cruise, the engines are turning 2500 rpms and burning 16.2 gph, which based on the rated 29.1 gph at WOT is about 56% load. At my “fast really happy” cruise, the engines are turning 2720 and burning 19.6 gph, which would be 67%.

Those numbers are heavy with the fuel and water tanks full. One other factor that is relevant to how hard the engines will be working is that the boat has very big fuel and water tanks in relation to the displacement. We did that so that I would have lots of range and water if I ever took her to the Bahamas or someplace with poor fuel quality. In normal circumstances, I start my offshore canyon fishing trips with 5/8 tank which gives me over 300 miles range with a reserve. Since mounting the new props, I have not run down the fuel enough yet to see how much impact it will have on the fuel rate with my normal “full” load, which will take about 15% off the total displacement of the boat. I had filled the tanks to do seatrials under fully laden conditions.

The new props have a lot of cup in part so that we could increase WOT rpms easily by taking some out rather than having to bend the blades to reduce pitch. I think that I will probably go ahead and do that, which should get me pretty close to 3300 at WOT. The main reason I have that in mind is because of the high EGT at lower speeds, where I think more rpms and boost would be beneficial.

[Quitsa]

No but it is about 10-15% over the published prop exponent curve at lower and mid range rpms and then converges at WOT. As noted, the EGTs at cruise speeds very close to what is was with the lower pitch larger diameter props that could turn up to 3360. At lower rpms where the engines ran hotter with both sets of props, they are also very similar and never get above about 875, which is 100 degrees below the Cummins published EGT at WOT.

[Quitsa]

Thanks Tony.  I did map the actual fuel curve against the published Cummins curve back when testing props.  I am within 5% from planing speed on up to WOT, being slightly above at the lower end and slightly below from 2600 approaching WOT of 3360, where I see about 95%. 

By the way, the Cummins folks are still unable to diagnose the source of the knocking noise I asked about a few weeks ago.  They tried plugging the injectors one at a time and also disconnected the fuel rail solenoid and there was no real change in the noise.  What is odd is that the noise goes away when the gear is engaged.  That might indicate the coupling was the source, however, the frequency of the noise is about 300 rpms, not the 600 rpm idle.  Since there is nothing turning in the reverse gear at 300 when it is in neutral, that would seem to rule out the coupler, which would be rotating at the engine idle speed of 600 rpms.

[Quitsa]

Thanks.  I asked the mechanic and apparently one of the things they tried was to unplug the fuel pressure sensor, which puts the pump to default mode of 12,000lbs. He also overrode the actuator raised the pressure to 24,000lbs.  Neither of those changed the noise.  Of course that is not the same as replacing those components but I wonder if what he did would have demonstrated a fault?

[Quitsa]

Thanks.  I should add that the engines have about 15 hours now and the noise has been present in the one engine from the beginning.  The Cummins tech who did the installation and sea trial checks did not even notice it until he came back after I had pointed it out. It has not gotten any louder or changed at all.  We have done several sea trials testing props that included short WOT periods and I took a small trip that involved a total of about 6 hours running at a 2600 rpm cruise. As I mentioned, they both run fine and temperatures, boost, and fuel rates are very close for both.

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