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Old 11-26-2013, 09:32 AM   #21
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>I said that an overpropped engine will be asked to put out more torque than it was designed to do CONTINUOUSLY.<

Not if it is done correctly

A simple look at ANY prop graph will show that about a 10% RPM pullback from RPM reached in gear is plenty to not overload the engine.

The prop loading just falls away to quickly.

Marine engines are expected to operate on or near their prop curve.

Sure in commercial service 90 to 105 RPM is expected all the time.

For rec boats , high speed boats may run on the pin to get on the plane , but even the Sport fish pull back a bit , to be sure to get that 1000 hours before a rebuild.

The cruiser owner may use full throttle as an engine check IF the engine was set up like the sport fish , for loads of high speed ,hi RPM operation.

This is not how most cruisers , esp the ones that cant right off their fuel bill operate.

Full rated RPM is great for the IO folks and Sport fish,, PERIOD.

The extra power used by having a cruise prop at low RPM is still minor compared to the engine output ability .

Even on car , pick up or escaped lawn equipment marinization..

25 - 50 HP cont is not that big a deal, for an engine rated over 100 hp.
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Old 11-26-2013, 10:14 AM   #22
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>I said that an overpropped engine will be asked to put out more torque than it was designed to do CONTINUOUSLY.

Not if it is done correctly
How is this engine supposed to "put out more torque" than it has been shown on a dynomometer to be capable of producing?

The dyno test is intended to define the ultimate and maxiumum torque that can be produced ... the manufacturer works hard to obtain that maximum number across the widest rpm range possible ... if they could get more they certainly would.

Have you guys found some magic technique or device that can extract more torque at one end of the shaft than the engine can deliver to the other? Let me know if you have because I want to buy stock.
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Old 11-26-2013, 01:44 PM   #23
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I was thinking the same thing. As if there is some kind of Captain Kirk to Scotty kind of dialog (think di-lithium crystals) between the prop and the engine.

Now if we're talking a Sabb variable pitch, well that is different...
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Old 11-26-2013, 01:52 PM   #24
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Now if we're talking a Sabb variable pitch, well that is different...
Not really ... that might make it much easier to overload the engine but it still can't extract any more torque than the engine can produce at a given rpm.

You could put a feathered prop and/or a shaft brake on the shaft and all they will do is slow the engine down and reduce the power output. No prop can absorb more power or torque than the engine can produce and that amount is already documented as being produced well below maximum rated power output, often well below max continuous power.

Torque is a red herring in this discussion.
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Old 11-26-2013, 02:26 PM   #25
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Agree. Upon review, I think the key word is "asks", which I don't get.

Let's talk pump affinity curves next...
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Old 11-26-2013, 02:28 PM   #26
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A few years ago I was hired to be captain on a 46' sport fish for a sea trial with a prospective buyer. It was a new boat. I brought the engines up to 2300 RPM rated top for the engines but the boat did not get on plane. I ran up over 3000 rpm and the boat ran great. I had this problem once before and remembered that some tachometers have a selector switch for 4 cylinder, 6 cylinder, and 8 cylinder, and the switches were set incorrectly.
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Old 11-26-2013, 02:33 PM   #27
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Let's talk pump affinity curves next...
When on a nuclear sub I can see here understanding pump curves would be helpful
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Old 11-26-2013, 04:07 PM   #28
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I would check the price of the exhaust manifolds. After doing that I would most likely walk away without worrying about the rest of it....... There are lots of boats out there.
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Old 11-26-2013, 04:40 PM   #29
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Thanks for all of the advice, we are actually on a sailing vacation this week so have very limited internet access thus the slow response from me. FYI, Tony at BoatDiesel recommended the following approach:

"Remove all of the seawater components (aftercooler, heat-exchanger, engine oil cooler, gear oil cooler) and service them “top to bottom” on the bench—All will be full of old zinc/calcium built up and other crud. Service the seawater pump ( cam, wear plate, back plate,impeller)

Check for full mechanical throttle. Verify Tach accuracy using a strobe & WOT no-load RPM ( 3685 +/- 25)
Prop down until the engines reach over 3450 RPM easily ( 3425-3525 Yanmar spec).

Coolant- Drain and replace with Yanmar approved coolant.."

I am trying to decide if I walk away now or tell the owner that he needs to do this at his expense and pay for anything else they find. This will probably kill the deal.

Thanks again. I will be much better educated for the next one we find and make an offer on.
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Old 11-26-2013, 05:00 PM   #30
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I am not about to comment anymore on the technical issues, enough has been said. But I will offer some purchase suggestions.

If you really like the boat and the main stumbling block is the engine rpm, antifreeze and overheating, I would offer to split the cost of all of this work with the seller. It won't be cheap, about $5,000 for the pair of engines unless you need new turbos then maybe double that. And there is nothing you can do to check for incipient exhaust manifold leaks (well there I go with the technical stuff) but you can do a cooling system leak test and determine if it is leaking now.

Even if the engines ran up to 3,450, it didn't overheat and it had low silicate antifreeze in it, you still would be doing all of that maintenance eventually. So why not get the seller to pay for half of it. After it is done you will have some nice engines.

Servicing the raw water system will surely fix the overheating. The low rpm could be as easy as a fouled prop or as bad as a stuck turbo. Or maybe you need new props (actually I doubt that).

Get some prices to do all of this work including new turbos and see if the seller will split this with you.

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Old 11-26-2013, 07:10 PM   #31
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Servicing the raw water system will surely fix the overheating.

The OP wrote "The mechanic thought the engines were well maintained, ran fine and he did not find any issues with them. ... the engine showed no sign of overloading"

How and where did that get turned into an overheating issue?
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Old 11-26-2013, 10:14 PM   #32
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[QUOTE=kitelog;194729]
Prop down until the engines reach over 3450 RPM easily ( 3425-3525 Yanmar spec).
QUOTE]

It would seem that the owner would want to do this, with RPM verified by strobe light. Once full RPM are verified and achieved, if any other issues arise they can be negotiated or not - your call.

Tony Athens is a Yanmar specialist as well as Cummins guy, therefore believable as compared to us talking heads. Show Tony's comments to the owner and broker.
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