QSM-11 715hp. How hard can I run these engines?

[fixinbones]

So I’m in the process of purchasing a Tiara 4700 Sovran that will be surveyed next week.  I plan to get fuel burn numbers during the survey to see if it’s overpropped.    The owner says he runs the boat at 2,2oorpms with a fuel burn of less than 25pgh per engine which seems like a reasonable fuel burn compared to the 715hp prop curve.  My question is if the boat is propped right after my survey can I run cruise these engines at 2,200 rpm without exhaust heat issues?  I like a fast cruise and I’d probably run the boat a 2,100-2,200.  

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[fixinbones]

Thanks for the info.   I’m just a little wary about these engines with the dry exhaust/turbos in general.  I don’t really have the patience to start dealing with exhaust leaks and adjusting prop pitch that is significantly over propped. If the boat has any evidence of early exhaust leaks I will pass. Even if the exhaust and turbos look good and the boat is significantly over propped I will likely pass.  The good news is the boat has only 700 hours and is a freshwater boat. In addition the boat is immaculate.    The only information I have thus far is that the owner states he runs it at 2200 RPMs and has a bit under a 25 gph fuel burn. That’s right in line with the 715 hp prop curve shown below.  It is a couple gph over the 535 prop curve but if it’s only a few gallons and I only need to reduce the pitch by 1 inch I’m willing to deal with that.  Obviously i will run the boat at lower rpm and check the fuel burns at those lower RPMs.  

[Steve Lewis]

It’s all a matter of degree

There is no clear cut answer other than you will want to have EGT gauges and monitor those as you run your boat and find your sweet spot for operation.  I would highly recommend that you search the forum the following way:

Search term = EGT

Author = Larry Backman

The results of that search will give you some very practical knowledge about how one man has dealt with his Single QSM over the years.  I call that out because you will have 2 of them.  So think about the costs as double.  The net of it is you should be able to dial in your own boat and operate within a performance envelope that is a little bit of a compromise.  Give a little speed to gain some life.  Your sea trial will be an initial look into how the boat is prop’d.  If you decide to buy then you can load her up and see where thigns fall. 

My wife and I did that with our boat.  We removed pitch, maintained vessel speed but increased engine RPM for the same Fuel Burn.  We spent 2 season dialing in our 480CE’s and we now have new Cylinder Heads.  I found what I consider to be a reasonable compromise.  We can run 2220 – 2300 for long legs and we can run at 1140 until the fuel runs out.  My GPH is a little above the ideal on plane, but my EGT values are well within spec and our vessel speed is what I would consider acceptable.  I am not setting any speed records but we are able to cover distance as needed.  Our real limiting factor is our dog and his bladder…..  We can run 5 hours and then we need to stop and let him off.  We take on fuel while he is stretching his legs.

Here are a couple of easy checks when you are on the boat for the survey.  Before you run the boat, get down in the engine compartment and run your bare hand on underside of the stainless pipes that go into and out of the aftercooler.  Soot will collect there and most owners will not think to wipe that area.  The other place to look for soot is the backside of the after cooler where it meets the intake area.  That is all right above the Exhaust Manifold.  If there is any soot then the leak has started and you can start negotiating the price down…..  Also take note of how cramped it is down there.  Take stock of how you fit and how your trusted mechanic will fit down there.  Those are big engines and they take up space…

My wife and I passed on a 4400 in Detroit a couple of years back because the leaks were bad and there was water intrusion in the laminate around the hullside air intakes.  Your surveyor should be looking in those areas but make sure you ask him/her to focus on those and the areas around the hullside windows.

let us know how things go.

[Author]

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