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  #21  
Old 12-15-2012
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Re: overpropped vs underpropped

Quote:
Originally Posted by srah1953 View Post
I have a Yanmar 3JH5CE with SD50 saildrive. How do I know what RPM should be at WOT. The only figures I can find is that the manufacturer quotes a maximum output figure at the crankshaft which is 39hp @ 3,000 RPM. With the original fixed prop, I can get around 3,200 RPM. What does this mean in terms of being over- or under-propped?
Many thanks
In neutral, push throttle all the way forward, record rpm. Then while in gear (out and about) do the same. IMHO (same with Beneteau warranty yard) if you hit the same rpm under load as you hit in neutral you might be under-proped. Just to make your self comfortable, ask the yard who does warranty work for your model/engine.
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  #22  
Old 12-16-2012
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Re: overpropped vs underpropped

Quote:
Originally Posted by groundhog View Post
well, the engine seems to effortlessly move the boat as set up.
It reaches hull speed well before full throttle.

I just dont understand what being 250 or so rpm off makes any difference.
If I was well over propped, I could see re-adjusting. But they are going to force me to re-lift the boat now, well after they signed off on the boat when we could have easily made an adjustment.
Sizing the prop is like picking the correct gear on your 10-speed bicycle. I'm sure you can reach maximum speed in 10th gear on a flat road, but you'll need to put more force into it and you could stretch the chain and deform the bearings.

If you are in 8th gear on a flat road, the force required is less, though you need to pedal more RPMs to do it. You still reach your maximum speed but you haven't stretched the chain, deformed the bearings, or worn out anything else in doing so.

Your engine manufacturer believes their engine will last longer if it isn't working so hard and the only way to know how hard it is working is if you are ABLE to reach max RPM at WOT.

I hope this makes sense.

MedSailor
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  #23  
Old 12-16-2012
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Re: overpropped vs underpropped

You are driving along at 65 mph in a car with a six speed gearbox.

Do you

A drive in 3rd
B drive in 4th
C drive in 5th
D drive in 6th

I hope you all came up with D [ No Ferrari owners need answer ]

Now ask yourself can I get to max rpm in 6th gear? No of course.You are pulling to high a gear but you WILL get more MPG at 65 mph than any other gear.

That is why on a cruising boat you select a prop that is overpitched and you should NOT be able to reach max rpm as quoted by the manufacturer.

This thread contains some inaccurate information. A diesel engine will be at it's most fuel efficient when run at it's peak torque RPM. This is basic mechanical engineering.

Assuming that bearings are designed to cope with the BMEP at that speed then the engine life will not suffer EVEN if it is running at full fuel setting to achieve that speed.

Accurate figures for diesel engine life are notoriously difficult to arrive at but there is a considerable body of empirical evidence that suggests that engines that are run below their max power rpm last longer.

So in practice know what RPM your engine generates max torque and prop it so at full throttle with NO WIND and FLAT WATER your RPM is a bit higher, say 20%.

Now if you know you are going to spend your time running the ICW then go 10%.

Or if you know you will be bashing to windward in heavy chop maybe 30%.

BTW Cruising sailboat diesel engines rarely see enough use to wear out.
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Last edited by TQA; 12-16-2012 at 08:43 PM.
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  #24  
Old 12-16-2012
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Re: overpropped vs underpropped

Hi...
Remember me?
I am not a mechanical engineer, but i am an engineer...

My situation seems like... lets go with the ten speed bike..

The bike rider should select gear 3 for overall optimum match to his legs over all terain.

Me being 240 rpm below target seems like I am in gear 3.1 on the ten speed bike.
Not in gear 10 and way overpropped.

Am I correct mechanical engineers?
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  #25  
Old 12-16-2012
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Re: overpropped vs underpropped

TQA- your analogy is incorrect. You can't compare a car that generally has way more maximum horsepower that it will ever need to a sailboat that generally only has a bit more hp than needed to achieve hull speed. You also can't compare a care with a lot of horsepower leisurely coasting along at 65 mph to a sailboat that is just below it's maximum speed due to hull design and high drag through the water at that speed. To make your comparison somewhat more realistic you should have the car going up a mountain pass at 65 mph, which gear are you going to have to be in to get the rpm up high enough for the engine to develop the hp required to maintain the 65? It isn't going to 6th gear, probably not even 5th if it's a steep hill. And it still isn't a good comparison because of the hp difference. They don't make cars that barely have enough hp to get to 65 (anymore) but that would be a fairer comparison. You're not even close to comparing apples to apples. If you want to run your engine at a higher load than is recommended by every engine manufacturer on earth and every marine mechanic that knows what he's talking about fine, but don't encourage others to follow your advice. Some of the older heavy duty engines might do OK running over propped but the modern, lighter, higher revving, turbo engines are a different story. How much fuel do you figure you have to save to even pay for a minor engine problem like a blown head gasket? Is the small amount you save by running a few hundred rpms lower even measurable? Seems penny wise, pound foolish to me.

Ground- not knowing what your recommended WOT is I don't know if 240 rpm is a big deal or not. I do know that on a Volvo that has a WOT of 3800 the factory mechanic told me that I was asking for trouble only being able to hit 3600. I had the prop re-pitched. I would call a couple of marine shops that work on your engine and run it by them for opinions.
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Last edited by jrd22; 12-16-2012 at 10:58 PM.
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  #26  
Old 12-16-2012
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Re: overpropped vs underpropped

TQA,

Not trying to pile-on here, but I also disagree with your analogy. The OP's problem is NOT being able to reach max rated RPMs whereas in your analogy any gear other than D would result in more RPM's.

Also, your analogy seems to point to fuel efficiency as the goal. No mechanic, or holder or a warranty gives a rats ass about MPG. I think they are worried about wearing out (or breaking) the engine prematurely, which is also what I'm worried that the OP will do.

On the topic of MPG, for any given hull/boat, there is an optimum engine HP rating. In a perfect world you would choose an engine that is exactly sized and propped to reach hull speed (or your desired cruising speed) at the engine's maximum torque and efficiency curve.

Now we enter the real world of the boats, props, and information that we have. The OP has an engine of a certain HP installed in his boat, and presumably can't/won't change that. Now he has a choice; he can either choose a prop pitch/size that will optimize MPG or engine life.

I'm with jrd22. I think he should ask SEVERAL mechanics that are preferably familiar with his engine and ideally his boat. He should ask them exactly how close to rated rpm at WOT is close enough, and then prop to suit.

From what he said earlier, he can reach hull speed without being anywhere near full throttle, so he likely has an engine that is well sized to his boat. Once he de-props it a couple inches he'll still likely be hitting hull speed easily and hopefully it'll be closer to where it should be on the mileage curve.

On my boat I solicited the opinions of several mechanics, and the one I trust the most told me that the rating on my engine (4000rpm) is actually bogus and could back this up with some good reasoning. He recommended that I prop mine to hit 36-3800rpm at WOT and I've done that. My circumstance is a bit different as I have a screwy, older engine (Perkins 4.108) and this mechanic had messed around with my particular engine on my particular boat before. My engine is also undersized for my boat, but old, sturdy, and heard to break, so I've pitched for a slightly shorter lifespan in exchange for being ABLE to reach hull speed at 75% throttle. I mention this because I've given the topic a LOT of thought and time over the years.

Speaking to the physics again, here is another thought experiment. Imagine the OP fitting a tugboat prop to his boat. He can hit hull speed at 500RPM and WOT gets him 550RPM. He looks at his fuel curve in his manual and shouts with joy because he's only supposed to be burning 0.5gal/hr at 500RPM. The only problem is that that consumption curve is assuming a regular load. What he will actually find is that he's burning more diesel than an appropriately size prop (due to inefficiencies in overly-rich combustion) and he will be putting so much stress on his engine as it labors with all that torque that he will wear out his engine in no time.

TQA, in the interests of continuing our dialog, can you give us specifics on which information you feel is inaccurate?

Medsailor

PS To the OP: all of these discussions about reaching WOT are intended for a boat with a CLEAN BOTTOM AND PROP. If your bottom and prop are foul don't re-pitch based on your experiments. That is, unless you expect to always have a foul bottom and prop.
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Last edited by MedSailor; 12-16-2012 at 11:24 PM.
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  #27  
Old 12-17-2012
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Re: overpropped vs underpropped

I don't want to get involved in a war over what is the right RPM to achieve with a boat diesel.

The OP asked a specific question related to satisfying the warrenty conditions on a new engine. Rather than tell him which way to go I was trying to give him some advice BASED on his expected boat and engine use.

But there are some basic facts about diesels that people who decide to change things need to know.

The throttle on almost all modern diesels is connected to the governor and sets the RPM, the governor in turn adjusts the fuel supply to achieve that rpm. This means that at half throttle the engine can easily be in a full fuel situation and producing all the power it can AT THAT RPM.

Coolant flow from a water pump with an impeller is linked to rpm in a linear manner over it's normal rpm range. IE double the rpm and you double the flow. RAW WATE PUMP

However flow from a centrifugal pump is NOT remotely linear. This type of pump is likely to be used to circulate coolant around the engine. At very low engine RPMs coolant circulation can be impaired. The only time I have seen this being a problem was an engine set up to drive a large refrigeration pump and a double alternator set up, when run just above tickover the rear cylinders ran hot. FORD 6D.

In my professional opinion as a engineer an engine installation should allow full fuel operation at ANY REASONABLE RPM without overheating. I would be happy to go to court on this.

Cooling an engine gets more complex if it has a turbo fitted. This is because the cooling is usually done by the engine oil. [ I would NOT have a turbo diesel in a sailboat although I am out of touch with some of the latest developments in the field where they claim that no cool downs are required]

An engine operated at it's max torque RPM will be running at is most fuel efficient RPM. Generally this would also be the most miles to the gallon in a boat. Look up Brake Specific Fuel Consumption.

Just to complicate things engines are more efficient when run hot.

My background is vehicle engineering but I am aware of the large body of research on maximising fuel economy and engine life on ships diesels.
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Last edited by TQA; 12-17-2012 at 10:12 AM.
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  #28  
Old 12-17-2012
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Re: overpropped vs underpropped

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Originally Posted by TQA View Post
....I would NOT have a turbo diesel in a sailboat although I am out of touch with some of the latest developments in the field where they claim that no cool downs are required]...
I have one and curse the bloody thing all the time. Utterly useless for a sailboat with our relatively small horsepower needs. In fact, my engine is a 150hp model, de-rated to 100hp, so any weight savings for powering up a smaller lighter engine is lost. Don't get me started.
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  #29  
Old 12-17-2012
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Re: overpropped vs underpropped

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Originally Posted by Minnewaska View Post
I have one and curse the bloody thing all the time. Utterly useless for a sailboat with our relatively small horsepower needs. In fact, my engine is a 150hp model, de-rated to 100hp, so any weight savings for powering up a smaller lighter engine is lost. Don't get me started.
Why don't you see if you can get it re-tuned to be 150HP???





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Old 12-17-2012
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Re: overpropped vs underpropped

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Originally Posted by MedSailor View Post
Why don't you see if you can get it re-tuned to be 150HP???
The bloody displacement hull keeps getting in the way.
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