QSM11 prop size

[Jack Blaisdell]

Hello all, New to this site. Like many others I have experienced exhaust leak issues on my QSM’s. They are 2003 builds, commissioned in 2004. The data plate hp is stamped 650. Now have 2500 hours. Within the first 500 hours my starboard had leaks and since then there have been three services on the motor, the last being with new manifold and turbo. In the last year my port has succumbed to the same issue, but for some time exhibited very little exhaust soot. This last year was different, and while in the yard for service I took up the salon deck and removed the insulation from the turbo.

I found that the turbo flange was cracked and the mating surface on the manifold was quite badly affected by heat. Moving further we removed the manifold and found two bad leaks and about 4 of the port mating surfaces on the manifold to be quite pitted. New turbo and manifold are now ordered.

In the last 5 years I have had Cummins reps on the boat for sea trial and was advised to reduce pitch in my props. Initially, I was turning 2350 at 100% throttle, and now make 2360 at 98% throttle with a bottom less than sparkling new.

Using another website’s prop calculator I find that given my parameters my performance is quite closely aligned with the calculators predicted fuel curve. What I observe to be an issue, however, is that my fuel burn is still above what Tony has posted in his tips, and what Cummins predicts from their fuel curve.

Now a lot of reading later I truly believe that the parameters I have been asked to meet are bogus and I need to concentrate on the fuel burn rate. To that end I have asked Cummins for recommendations-no answer after two weeks, and attempted some “reverse engineering” of my own to get to a prop size recommendation that will put me in the correct fuel burn zone.

Using that other site prop calculator, I kept reducing hp on a “fictional” engine until I got down to the recommended fuel rate. At 540 hp I am about on it and get a prop recommendation that removes 2 more inches of pitch down to a 26 x 26.5. Builders original was 26 x 30. The parameters I used are boat weight 36500, hull factor of 5.9 (based on website calculation) water line of 44, twin disc gear reduction of 1.48.

So my basic question is am I on the correct track of trying to zero in on the correct prop size using my method, or should I be looking at this in a completely different way?

Thanks

Topic

[Rob Schepis]

The Boat & Hull Design

No problem Jack.  Finest Kind is gorgeous.  She just has to raise some fish lookin’ that good!

My sportfish also has a sharp entry and flat stern section (even flatter than Finest Kind) making for a very easy planning boat and a super stable platform at anchor or drift..

[Jack Blaisdell]

Larry, Good to hear of your success. I’ll have to see how I can run cables up to my bridge. Certainly would help understand the relationship to heat buildup.

[Jack Blaisdell]

Rob, Thanks. I’ve got a good set of scans from my prop guy, so I’m confident of the current condition. Your calcs come pretty close to what I was getting using the method described above as I focus on cruise rpm and fuel burn. I definitely want to increase 200 rpm based on the recommendation. I can probably overshoot a bit. The speed at WOT is not something that has ever been a huge concern.

Thanks again

[Rob Schepis]

I played with the prop calculators a bit BUT the calculators are a great tool that surely have their place, especially in a new build but you already have a real working model in your actual boat so that is the absolute prop calculator in your case.. Your boat, your operating parameters and your conditions are what really matter..

Next WOT load/RPM and GPH at WOT is really not of any use at this point as you have already proved without any doubt with your wallet.  So forget WOT as your issue is happening at “cruise loads”…Agreed?

It really now depends on what your current props “scan at” by a good prop guy.. If they are  26″ x 28.5″ ‘s, then 26 x 26 or 26 x 25 would do the job.. Regardless of all else, it’s  your cruise speed that matters (NOT WOT LOAD) and you need to bring that cruise speed load (GPH) up not less than 200 RPM regardless of losing some WOT vessel speed..in other words bring the RPM up to burn the same fuel as you did at 200 RPM less.

[Larry Backman]

I did basically the same thing with a single engine downeast/QSM 11 -670.  I acquired the boat with a 26 X 26 prop and 550 hours on the engine.  The boat was severely overpropped and I cooked the exhaust manifold within 100 hours, most likely by running at 1700 RPM for an extended 5 hour period.  WOT I barely made 2300 RPM and at 1850 RPM (cruise) I was burning 19-21GPH.

 

I had the manifold replaced that year and since then have 2100 hours.  I cut one inch out of the prop to 26 X 25, made full 2350 RPM at 100% load.  My cruise 1850 RPM burn rate dropped to 19 GPH.

 

Last year with Tony’s advice I did just what you are doing, propped for the 580 HP variant and cut another inch out, going down to 26 X 24.  Now I explode from 2000 RPM to 2300, then quickly hit 2350, settling in at 96% load.  My fuel burn at 1850 is down to 17.5->18 knots.

 

One piece of advice from the not so cheap seats as its at least 1K/side – install a pyrometer and run your boat by using it and GPH to keep the heat in your exhaust manifold down.

 

My old QSM was running 1050 degrees at cruise, would bump to 1075 in hot 75 degree water.  I found the “hot spot” on the engine is ~1600 RPM to 1800 RPM, EGT up at least 25 degrees in that range over normal cruise RPM.    Surprisingly, if you bump up to 1950 RPM EGT drops which Tony and others explained to me is because your pushing more cooling air and water through the engine.

[Jack Blaisdell]

I’ll attach a couple of photos, but it’s a planing hull with a sharp entry, but low deadrise. Beam is 15.5 feet, draft is 3.5 feet. Top speed wot is 30kts, cruise at 22 kts 1870 rpm

[Rob Schepis]

Jack,

Boathull type?  Planing hull sportfish?

Beam?

Draft?

What speeds are you seeing now?

[Author]

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