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Old 01-21-2014, 05:52 AM   #1
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Post Propeller

Good morning.

I ordered a 3 blade 40x27 propeller for a 57 footer wooden fishing boat which is under construction. I arrived at this selection based on the power of the engine which was 164Kw(220hp) @ 1800 rpm. I ordered this engine as well. But when I took delivery of the engine I realized that the hp and rpm is not what was advertised. This engine has hp of 163Kw (218hp) and rpm of 2200. When I complained to the supplier he was told to tell me to ship the engine back to them at my expense.

Since then I have been trying to figure out if the current propeller (40x27) would be efficient. I have used three online calculators to find the correct propeller size and they gave me, 35x25, 35x29 and 34.7x29.6.
Now the problem is, if I decide to keep this engine should I be concerned using the prop with this engine?

Nabb
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Old 01-21-2014, 06:03 AM   #2
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Is there a reduction gear involved in this project?

Do you have purchase documents that specify a certain engine? If so then ask them whose engine you just received as it is not the one you ordered or paid for.

Tell them to send your engine and have the wrong one picked up because storage fees will accrue after 30 days. It might be more effective if your attorney writes that letter.
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Old 01-21-2014, 08:36 AM   #3
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Post the weight of your boat, the lwl, the gear ratio and I will run it through boatdiesel's prop calculator for you. I presume that the 40" diameter prop will fit.

David
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Old 01-21-2014, 12:30 PM   #4
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Is it possible to spec a wheel such that there is no initial tuning required? Wouldn't that be part of the build process, to adjust the wheel to give you the required wot reading? Isn't the process to choose the correct diameter to give the correct distance from the hull and then choose the pitch to give wot? I understand the prop calculators but do they take into account the hull configuration?

I'm not asking these questions to be annoying, but I would expect there to be a tuning process on a new build.

My boat is 52' has 250 hp at 2100 r/m and turns a 36" wheel (damn it I forgot the pitch!).

I'm with Rick, don't take crap from the engine supplier.
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Old 01-21-2014, 03:31 PM   #5
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Nabb, I can imagine you would have some challenges building a boat in Ghana. Where did the engine come from? I'm guessing problems like this in your area are rarely sorted out by attorneys.

I am surprised at the difference in diameter (40" vs 35"). I'm no prop expert but I wouldn't have thought a change of max engine rpm would have affected the required prop diameter; only the pitch. Do you have the required clearance for the 40" prop. If so, the pitch can be changed somewhat, but there is a limit to how much.
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Old 01-21-2014, 09:28 PM   #6
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I hate to admit it I think I am being screwed over. The engine is from China and taking them on legally is a bit dicey as I have learned. This is not going to happen again. Now they have offered me $1500 off my next purchase. They explanation given by the manufacturer was that the one that I had wanted has been phased out in the middle of last year and the one that I was given was the new version of that range of engines. This is bs, why do they have to continue to advertise that model if it has been phased out. The supplier swears by Buhda or Confucius that he was not informed by the manufacturer about any changes being made to the engine.

Djmarchand, the info you requested follows; lwl - 54ft, gear ratio - 3:1 and I estimate the weight to be 45 tonnes (100,000Ib).

I have a clearance for the 40" prop.
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Old 01-21-2014, 09:32 PM   #7
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You need to hold the supplier responsible not the manufacterer. Also you need to have your wife slap you for ordering an engine from china.
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Old 01-21-2014, 10:25 PM   #8
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I plugged your data in to boatdiesel's prop calculator. Usually it gets you in the ball park, but I would always defer to a prop shop or prop manufacturer's recommendation.

So with your original engine spec of 220 hp at 1,800 rpm the calculator says a 40x29 prop would be right. That is pretty close to what you have (40x27) and if that were the engine you received I would go ahead and see how it performs in the water.

But with the new engine spec of 218 hp at 2,200 rpm the calculator gives a prop of 40 x 19. Not close. With that 27" pitch prop you will be badly lugging that engine.

So if you keep the new engine your only choices are a new lower ratio transmission or a new prop since an 8" pitch change is too much. You might talk to a good seat of the pants prop shop. Maybe they can cut it down a few inches and reduce the pitch to get you in the ball park. The boatdiesel calculator says a 36 x 23 prop would work and it just might be possible to rework your prop that much.

All of these numbers assume a 3 blade prop.

David
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Old 01-22-2014, 03:00 AM   #9
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You need to hold the supplier responsible not the manufacterer. Also you need to have your wife slap you for ordering an engine from china.
Bligh, I agree with you, I need to have my wife slap me around a few more times that would make me to remember not to make that stupid mistake again.

Djmarchand, thanks for your advice. I am going to scourt around for a good prop shop and see what they can do.
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Old 01-22-2014, 03:01 AM   #10
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Djmarchand, this maybe a silly question, is it possible run the boat at 82% of its stated rpm which is 2200 at 218hp, 82% would bring it around 1800rpm. Now when I run the boat at this rpm I am wondering what horse power am I going to get out of the engine. Please check the prop sizing on this for me. Thanks
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Old 01-22-2014, 11:36 AM   #11
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I received the following PM from Eddington and have reposted it here with my answer for all to see.:

"I have just received an email from my supplier recommending that If the run the engine at 1858 rpm that would bring the power down 200hp and would be ideal for my prop.
I would appreciate if you could run this through the boatdiesel calculator and see what happens. I have run it through two calculators and the result is: 38 x 29,
38 x 34.

Thanks.
Edington"

Eddington:

What type of engine is it: non turbo, turbo charged, common rail (extremely unlikely from China), etc. And what is the displacement.

I assume that this is a big, heavy- 3,000 lb, non turbo charged engine of about 12 liters displacement. That is the kind of engine that China sells today.

You can do what he recommends, but then you need to cruise it at about 1,500 rpm otherwise you will be overloading it. You will be in a severely overpropped situation, but some trawler owners do it to try to save fuel. But if you do prop it that way you can never run it near wot for long. If you do you will be asking the engine to put out its maximum hp for that rpm and if you do, it won't last very long.

And don't do this with a turbo charged engine.

So with that said, here are some numbers from boatdiesel's calculator for an engine that makes 200 hp at 1,858 rpm:

Recommended prop- 40x26
WOT speed- 9.5 kts
Cruising speed at 1,500 rpm- 7.7 kts

So that is pretty near your current prop's pitch of 27". If you are happy with a cruising speed of 7.7 kts at 1,500 rpm then you should be ok as is.

David
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Old 01-22-2014, 04:19 PM   #12
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One could do a combination of increasing pitch a bit and then cut the prop down to get rated WOT rpm. The loss in efficiency would be extremely small if done skillfully. I'd certainly rather do that than overload a brand new engine.
One could reduce the blade area by trimming the TE and/or LE and retain the same dia. That would take a bit more time and skill but many people think dia is everything.

djmarchand,o
Could you run his numbers w a bit less diameter?
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Old 01-22-2014, 04:39 PM   #13
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Djmarchard, thanks a lot for your help, for the information provided would be very helpful. The engine I have is a turbocharged & aftercooled engine with a displacement of 8.3 liters, weighs about 640 kg. According to the performance curve, peak output torque and power output takes place at 1800 rpm. So cruising at 1800 rpm is really spot on. In the couple of days I would decide what to do. In fact my first option is find a shop that can work on the pitch of the prop. Thanks.
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Old 01-22-2014, 05:00 PM   #14
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Djmarchard, thanks a lot for your help, for the information provided would be very helpful. The engine I have is a turbocharged & aftercooled engine with a displacement of 8.3 liters, weighs about 640 kg. According to the performance curve, peak output torque and power output takes place at 1800 rpm. So cruising at 1800 rpm is really spot on. In the couple of days I would decide what to do. In fact my first option is find a shop that can work on the pitch of the prop. Thanks.
No, no, no and no.

That engine sounds like a Cummins 6CTA. That engine produces 4-500 hp. How come this engine only produces half of that. Something doesn't hang together. I suspect that the distributor is telling you what you want to hear and not the full story behind this engine. The Chinese make Cummins knock offs, so it may be based on a Cummins design but with Chinese sourced components.

For a turbocharged, aftercooled engine you definitely don't want to overprop them. I said it was ok for a non turbocharged engine, but definitely not for this engine. You will get sky high exhaust gas temperatures which will lead to piston scuffing and dropped valves.

Since you have the power and torque curve, how about posting them.

David
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Old 01-22-2014, 06:01 PM   #15
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They are making 6CTAs anywhere from 200 to 400 plus hp. I have a 6CTA8.3 M1 at 250/2100 r/m and my buddy has a pair of 6CTA8.3 M3s at 420 hp/2600 r/m. Just Google 6CTA 8.3 and all the Chinese manufacturers pop up - look at Ali Baba... The 6CTAs came in all horsepowers depending on what you want. Mine is rated continuous, his are rated one-hour.

Go and look at the fuel pump on a Dodge truck with a cummins, it's likely to be Chinese.

Sad to say as I am still pissed that there was such a run to make things offshore and prices are still high, but the Chinese are making some quality stuff and I have heard their wages will reach North American heights is a very few years. For example, the guys that made Dewalt drills got laid off when production moved to China and now those workers can't afford to buy a Dewalt drill. The prices never fell, the companies made windfall profits and now the skilled workers in North America flip burgers.

They say most CEOs are psychopaths and I have never doubted it.
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Old 01-22-2014, 06:59 PM   #16
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OK, Xsbank is correct.

Cummins even makes an 1,800 rpm continuous duty 6CTA that is rated at 250 hp at 1,800. So if this really is a Cummins design with all of the beef that a "real" Cummins 6CTA has, then I do accept that this engine can be propped to reach 1,858 rpm making 200 hp.

And that being the case, you can cruise at any speed that you want, no problem.

I apologize for over reacting, Edington.

David
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Old 01-23-2014, 12:43 AM   #17
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I have attempted to post the performance curve but I have en counted problems. Anyway thanks to all who contributed to this thread. Just before I came on line I had then decided to end the chase and order a new prop. But from what i have just read I will give a try and see what happens. Money is difficult to come by these days.
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Old 01-23-2014, 09:00 AM   #18
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A proper sea trial is in order.

To keep from overloading the engine I would , after 10 hours at various RPM up to 1600 or so open the throttle full and note the engine RPM and GPS boat speed.

I would then pull back in 100 RPM increments and create a graph from full bore to idle.

Long term overloading is harmful to any engine , 15 min of a power test is meaningless.

Till you decide IF the prop is almost suitable , run 200RPM below what ever RPM you find at full bore , 300 RPM if the engine is blowing black smoke at -200. IT shouldnt be.

IF you decide its close , and perhaps can keep the big prop install a EGT gauge , where the engine MFG requests and follow the eng mfg instructions as to max EGT.Paint a red line on the boats Tach.

IF the engine is fairly rated , an industrial engine ,my GUESS is you will be OK with the existing setup.

Only a test will tell.
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Old 01-24-2014, 03:33 PM   #19
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I am very concerned about overloading the engine. Anyway I have been speaking to a prop manufacturer and just as David earlier suggested, he said I should change the gearbox ratio or the propeller. He recommended 36"x24" prop for that engine which is close to what boatdiesel calculator gave David, which was 36"x23". I have therefore decided to buy a new prop.
Thanks to all those who contributed to this forum.
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Old 01-24-2014, 03:43 PM   #20
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How do you confuse an engineer? Stick him in a round room and ask him to sit in the corner.
The confusion would come from trying to understand the thought process of someone who would ask something so obviously absurd.
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