Need advice on changing Props to protect QSM11s

[Jeremy Bendler]

I have twin QSM11 670s in my 44′ express sportfish. I have owned the boat for one year. I put 400 hours on the engines in the last year, and total hours are now 750. The turbo on the port side cracked in August and I had it replaced. I understand that I need to reduce the heat generated at cruising speed. I have typically been cruising at 25-27 knots at 1850 RPM burning approx 21 GPH on the port side and 19 GPH on the starboard. Port has always burned 10% more fuel than the starboard. Props were tuned and matched in the spring 2019 by Blackdog on Kent Island, MD. Props are 36″ pitch and 28.75″ diameter with 4 blades.

What is the optimal pitch given the data below? Should I look for a different diameter prop or simply a prop with different pitch. I put the RPM/GPH data together this week with smooth seas and full fuel tanks.

Port Starboard
RPM GPH RPM GPH

1350 11.3 1340 10.4
1410 12.1 1380 11.5
1460 13.4 1440 12.5
1510 14.2 1490 13.6
1560 15.0 1540 14
1620 16.3 1610 15.2
1670 17.0 1650 15.8
1720 17.8 1700 16.2
1760 18.7 1750 17.7
1820 19.8 1800 18.3
1850 20.7 1840 19
1880 21.0 1870 19.7
1910 21.6 1890 19.5
1950 22.3 1940 21.1
2000 23.5 1980 22.8
2060 26.7 2040 23.6
2100 27.7 2080 25.3
2140 29.2 2130 26.3

Thanks for your guidance.

Jeremy

Topic

[Larry Backman]

I had it done – easy work for hands on

I had a Pyrometer I bought from Tony installed back in 2013. It was basically a screw into the exhaust elbow port, run a wire and install the gauge. Worst case it could not have been more than 1K including labor.

It’s fun to watch as both sea conditions change or you throttle up and down. It’s also “saved” me in rough conditions/high load as I could quickly visualize the EGT climbing at reduced RPMs and reduce down further before I could damage things.

[Steve Lewis]

Awesome stuff Larry

Larry thank you for sharing your running EGT data. Those are data points that I can add to my running book of knowledge. I have read about EGT as a way to monitor health but have not seen real data for the QSM. I have seen it for QSB/C but not QSM.

Did your boat come from the factory equipped with the Pyrometer or did you have to add it after market? If after market, did you do the work yourself or did you have someone do it? Wondering how easy adding the Pyrometer to the system is? Screww it into the turbo port and run the wires back to a harness? Also do you have standard Cummins display screens, smartcrap or SMX displays? I do not recall seeing you mention that before.

I have my eye on a couple Cummins powered Fleming trawlers that have some serious hours on them, as one would expect, and a Pyro would be an invaluable addition to the instrumentation.

[Larry Backman]

We all agree

If you sedate the engine by removing pitch so your target fuel burn is higher on the RPM curve you push more coolin raw water and air through the engine.

This higher volume helps remove more but obviously not all of the exhaust manifold heat.

I don’t know if we have talked about it on this thread but an EGT gauge is an invaluable tool to see what’s going on with the true engine in whatever conditions you may encounter.

I often adjust upwards or downwards 50 RPMS if I see the EGT jump up 50 degrees in heavy sea conditions. In my case EGT is a pretty steady state 975-1000 degrees for my normal 1875 RPM cruise. If I see it jump over 1000 degrees I can either slow down to 1800 RPMs or push up to 1950 RPMs to get it down. It also shows me quite clearly how bad running juts under plane is for the engine, if I try and slug along at 15 knots at 1600 RPM the EGT will spike up 50 degrees immediately.

EGT, GPH and load are how I run my boat….

[Steve Lewis]

So we agree

Reduce pitch on your props and you will reduce fuel burn AND increase RPM. With the QSM both of those are good. The calculus becomes a decision of how much pitch will yield the correct increase in RPM and resulting decrease in fuel burn. That will all depend on the individual boat.

I am an electrical engineer with conservative tendencies. My recommendations will fall on the end of the scale that yields the highest rpm and lowest fuel burn. So less than 20gph at 2100 rpm. It will be up to you to determine what decrease in pitch you are willing to try.

It’s all good that we are debating the minutiae of inches of pitch in the ultimate goal of boating bliss. Start with shaving off 2 inches and see what that does. If you need to shave another inch then it isn’t the end of the world. My gut says 3.

Cheers!

[Jeremy Bendler]

In the reading I have done here and elsewhere I don’t believe that cruising speed necessarily needs to be sacrificed or reduced. Cruising RPM needs to be raised to allow more coolant and air to flow through the engine at desired speed and fuel burn.

The end of Tony’s article says: “You can burn 21-22 GPH, bur you have to do it at 2100 RPM to control the heat”.

This changing the pitch of the props would reduce the top speed, but really just shift the cruising speed up the RPM curve controlling the heat.

[Steve Lewis]

Some Theory to Consider

Jeremy Bendler wrote:

Fully loaded on my way out to the canyons I generally cruise at 25-26 knots at 1850rpm. On the way back in I cruise at 26-27. I’m fine with those cruise speeds. Certainly wouldn’t hurt my feelings to go faster though. I’m not too worried about the top speed since I don’t race around. I believe current top speed is 36knots. I’d like to maintain something north of 30knots I suppose.

I have had someone suggest that taking the diameter down from 28 to 27″ would increase speed and increase RPM by 150 at a given fuel burn.

Would it “hurt your feelings” to slow down a bit? The reason I ask is the health of your particular engine is predicated on a particular max sustained fuel burn rate. That rate is ~20gph per engine. Based on Tony’s articles, that burn rate should be achieved at or slightly below the 2100 RPM engine speed. The way that the folks on the forum are doing this is by modifying/reducing the pitch of their propellers.

I guess it comes down to a balance. that balance is engine life vs. performance of your boat. If you want longer life out of your engines, manifolds and turbos then you reduce the pitch on your props so that you can get as close as you possibly can to that 20gph @ 2100 rpm. The trade off is going to be some speed. The amount of speed you trade for engine life I guess comes down to your own calculus.

You pay “The Piper” one way or the other. You pay money to replace your manifolds and possibly turbos if you want Speed. You get that speed by running the engines hard and burning more fuel than you “should”. Alternately, you pay in more time spent getting to your destination by extending the life span of your engines by reducing pitch and as a result, reducing speed a bit.

Larry seems to have found a balance for his boat. My recommendation is theoretical only, but it is informed by reading a lot about this engine in different boats. You might consider reducing pitch by 2-3 inches to get the fuel burn down close to where it should be. I am sure Tony will have some words of wisdom. He has written quite a bit on the forum about these engines.

Good Luck and let us know what you end up deciding to do and the results of your decision.

[Jeremy Bendler]

Best average fuel burn I can give you at 1850 RPM is 21gph port and 19gph starboard.

[Tony Athens]

Next——-What is your average fuel burn in GPH at 1850 RPM? Be as accurate as you can in varied conditions..

Tony

[Jeremy Bendler]

Fully loaded on my way out to the canyons I generally cruise at 25-26 knots at 1850rpm. On the way back in I cruise at 26-27. I’m fine with those cruise speeds. Certainly wouldn’t hurt my feelings to go faster though. I’m not too worried about the top speed since I don’t race around. I believe current top speed is 36knots. I’d like to maintain something north of 30knots I suppose.

I have had someone suggest that taking the diameter down from 28 to 27″ would increase speed and increase RPM by 150 at a given fuel burn.

[Tony Athens]

1) What is the happy vessel speed you need to go from point A to Point B–Loaded as you use the boat, weather permitting.

2) Do you have a minimum top speed you need to reach (for maybe 5 minutes) above your cruise speed? …..

Tony

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Jeremy Bendler

[Jeremy Bendler]

Thanks for the response. I was barely hitting 2350 this spring. Don’t think I’ve tried since. I guess the question is how much does each inch of pitch effect the fuel burn. Reducing the pitch on a 36″ pitch prop by 2 inches would be a reduction of approximately 5.5%

If that 5.5% reduction in pitch equates to a 5.5% reduction in speed at 1850 RPM, and also equated to a more significant reduction in fuel burned that would be great.

I’d love to be able to spend a few days trying a bunch of different props at the beginning of the 2020 season, but I think I am stuck trying to get this right on the first shot.

Thanks for any help or suggestions.

[Larry Backman]

My 2 cents as a QSM user

I’m not a mechanic but I have 4500 hours of QSM usage.

Two things to focus on :

1. Can you make WOT of 2350 RPMs? If not you need to cut at least an inch of pitch out of your prop to make 2350 RPM, then redo your numbers.
2. I cruise in the 1850 range also. I want to see 16.something GPH in ideal FAC solo test conditions and want to see 17.something in more typical 2-3’ chop with a couple people on board. Under heavy load, 5 guys/1000# of ice I want to see 18.something for an extended run.

I’m no prop expert but given your 19-20 GPH at 1850 cruise, I’d take at least at least 2” of pitch out of your prop to achieve these 2 things.

1 user thanked author for this post.

Jeremy Bendler

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