QSB Propping

[Quitsa]

We have been having a real challenge attempting to prop a custom sportfishing 37′ boat with QSB 6.7 550s. The boat is very light at only 17,000 wet with full fuel and water. The problem is that the prop load appears to rise very sharply above about 28 kts, which we now think may be due to excessive stern lift generated by the props increasing hull drag. In order to get 3300+ rpms at WOT, it is necessary to carry no more than about 32 inches of pitch. While the boat runs very efficiently below 28 kts, she rapidly becomes horribly inefficient at higher speeds. For example, at 27 kts we see 1.05 nmph but at 32 kts (about 80% load), it drops to 0.72 nmpg.

As an experiment, we tried a radical departure and tested a set of smaller diameter props with more pitch (22.5 x36 versus 24×32). They are dramatically more efficient at higher speeds — about 0.94 nmpg at 32 kts versus the 0.72 nmpg noted above.

The problem is that the engines only turn up to 3200 with them. Running at a comfortable cruise speed of 32 kts, the engines are showing about 70-72% load and 35 lbs of boost with an EGT of about 700 degrees. The EGT is about the same at cruise as with the props that will turn 3320 at WOT even though the boat is going 4 kts faster.

Just how bad is it to be running around cruising at 60-70% of the rated power of the engines being technically overpropped if the EGT is in a very safe range suggesting that the engines are not really so overloaded? These electronic common rail engines really complicate matters compared to the mechanicals given their self-regulating aspects.

Topic

[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.

[Steve Lewis]

Over Propp'd and a little more color

Based on the 2 data points you have shared you are definitely over propped. Ignore % overage as a concept and just look at the attached fuel burn curve(same as the one Tony attached) and compare it to yours.

As you see, Cummins indicates 2900rpm for 19.0 GPH and you are burning 19.6gph at ~2700 RPM.

Using linear interpolation on the Cummins chart, I arrive at 16GPH at 2700 rpm. You are burning 16.2gph at 2500rpm.

You are burning TOO MUCH fuel at both 2500 and 2700rpm. You need to decrease pitch or otherwise tune your props to get those burn rates up to 2700+ and 2900+ rpm respectively. That will give your engine longer life, you will still be able to maintain your happy cruise and high cruise speeds just at higher engine RPM’s.

Tony and the team indicate that when you burn more fuel than Cummins publishes at cruise RPMs, that is stressing the engine beyond design parameters. Take that same fuel burn and move it up into a higher rpm range and that unloads the engine. As long as you stay below the max recommended “Reduced Power” RPM, in your case 300 rpm off the rated RPM, then you will be ok. That means running your engine at less than 3000rpm. Also you should be doing all of the burn rate measuring when your boat is at its heaviest.

So if you can tune your props to get your happy cruise at 2700 and high cruise at 2900 while still keeping fuel burn below what Cummins indicates then you should be good to go. Again ignore load % and look only at Your Fuel Burn at RPM and compare that to the Cummins chart. Stay at or below what Cummins recommends.

Cheers!

[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.

[Tony Athens]

Just How Bad?

Your question would be like me asking you “how long is a string”………….. A definitive answer does not exist..

There really is only one possible response———–YOU have to balance “your need for speed vs. engine life”……….If you run 300 hours a year and 20% of that time is at your pre-conceived “technically over propped condition” , IMO maybe you are good for 2000 hours before you see issues..

35 lbs of boost at “cruise” tells me you are at well over the load your instruments are telling you or you think–

WHAT IS THE GPH at your “happy” cruise speed and at what RPM is that?

That’s in the real load on your engine. “Load %” is totally meaningless—————–GPH vs. RPM is the only thing that matters if you are concerned about engine life..

Tony

[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.

[Rob Schepis]

Do you have any data charts of RPM vs GPH ?

[Author]

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