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Old 09-12-2010, 17:44   #16
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I have a 25 year old Yanmar 2GM, and it would only do about 2200rpms with a 15x10" 3 blade fixed prop. Many people thought I would only get about 200-300 rpms, for every inch of pitch reduced. I had 1" of diameter and 1" of pitch taken off and now it revs to about 3200 comfortably. Likely still a bit overpropped, but I am OK with that for now.

Point is- in theory I should have only gained maybe 500-600rpms, and I ended up gaining 1000 or so.
I feel that I was only utilizing about 8 of the available 15hp, at the lower rpms. Once the pitch was reduced, the engine was able to rev higher, now having more usable rpms, which adds to the gain.
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Old 09-12-2010, 18:15   #17
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... I'm negotiating with Yanmar to not reduce the pitch 2". I agree with four winds that I'd rather cruise at a lower rpm. I'm not sure Yanmar will go along with me.
Yanmar is probably correct. Your new engine will not like operating overloaded. If you want a slower prop RPM you need to change your transmission ratio, not the prop.
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Old 10-12-2010, 08:30   #18
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Yanmar is probably correct. Your new engine will not like operating overloaded.
I still don't understand how the engine will be overloaded if you operate at 80% of WOT regardless of what that WOT number is. The only problem I see is if you're unable to achieve the design rpm at WOT then you're not having all the horsepower available. If operating temperatures are achieved at this lower rpm then how is this damaging the engine and causing glazing of the cylinder walls?
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Old 10-12-2010, 17:57   #19
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I still don't understand how the engine will be overloaded if you operate at 80% of WOT regardless of what that WOT number is. The only problem I see is if you're ...
There is no such thing as WOT on a diesel. The FUEL control selects a RPM.

The designers expected a certain torque and power output for any given RPM. That is what is on the charts in the manual. When the engine is overloaded by the prop, or transmission ratio, the selected RPM can only be achieved by supplying more fuel. This is operating outside of the curve on the chart. The engine will not be happy about this. The excessive torque will wear out cylinders and crank bearings.

The RPM lever should be able to select any RPM from idle to the specified maximum. Trust the design engineers.
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Old 11-12-2010, 07:13   #20
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There is no such thing as WOT on a diesel. The FUEL control selects a RPM.

The designers expected a certain torque and power output for any given RPM. That is what is on the charts in the manual. When the engine is overloaded by the prop, or transmission ratio, the selected RPM can only be achieved by supplying more fuel. This is operating outside of the curve on the chart. The engine will not be happy about this. The excessive torque will wear out cylinders and crank bearings.

The RPM lever should be able to select any RPM from idle to the specified maximum. Trust the design engineers.
If the prop is overpitched then the fuel control can only achieve a given rpm. This is what I'm referring to as WOT. Take this number and don't operate the engine above 80% of this rpm for any prolonged period. How is this operating outside the charts??
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Old 11-12-2010, 07:29   #21
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Originally Posted by Northeaster View Post
I have a 25 year old Yanmar 2GM, and it would only do about 2200rpms with a 15x10" 3 blade fixed prop. Many people thought I would only get about 200-300 rpms, for every inch of pitch reduced. I had 1" of diameter and 1" of pitch taken off and now it revs to about 3200 comfortably. Likely still a bit overpropped, but I am OK with that for now.

Point is- in theory I should have only gained maybe 500-600rpms, and I ended up gaining 1000 or so.
I feel that I was only utilizing about 8 of the available 15hp, at the lower rpms. Once the pitch was reduced, the engine was able to rev higher, now having more usable rpms, which adds to the gain.
You state in your post that you changed both pitch AND diameter. That probably explains the large increase in rpm. The 200-300 rpm per inch applies to pitch only. Not sure what the rule of thumb for diameter change is.
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Old 12-12-2010, 20:50   #22
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If the prop is overpitched then the fuel control can only achieve a given rpm. This is what I'm referring to as WOT. Take this number and don't operate the engine above 80% of this rpm for any prolonged period. How is this operating outside the charts??
Because RPM is not a direct indication of torque or horsepower. The chart is not depicting what the engine will do. It does not indicate that at a given RPM the engine is producing the indicated torque, or the indicated horsepower, or using fuel at the indicated rate. It is depicting what it should be configured to do. You may be at 80% of some RPM, but your torque or horsepower may not be as indicated on the chart. Since there is no direct way to know the torque or horsepower the rule is that the loaded engine should be able to achieve some specified RPM. When that is achieved the lower settings on the chart will also be relatively well met. If that RPM cannot be achieved the engine will be overloaded at all RPM settings. Even in gear at idle RPM.

This test does not give any indication of underloading. I suppose that can only be determined by increasing the prop pitch until the RPM cannot be achieved and then backing it off until the RPM is just reached.

I also suppose loading should be determined with the boat at speed, or perhaps accelerating to speed, rather than when tied to the dock.
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Old 13-12-2010, 07:24   #23
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. You may be at 80% of some RPM, but your torque or horsepower may not be as indicated on the chart. .
Point well taken...thanks.. even though I'm not sure if I agree.
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Old 02-02-2011, 17:37   #24
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These newer smaller more powerful engines need to run at proper temps. If your engine is rated at 3000 rpm, first check it can reach this in neutral.If it does then, on a calm day without tide if possible in gear bring boat to 3000 rpm. If you can hit that you are good, if you are under or over you need to be repitched. By good I believe 1 to 2 hundred rpms either way is close, and probably ok. I did the above, and bought my shaft and propeller at General in FLA, I could only acchieve 2600 RPms on 3000. Joe from Sound Marine in Conn sold it to me and I had 1 free repitch coming to me. Its done and back home I will let you know in the spring how it workred out. Red
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Old 06-02-2011, 16:37   #25
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Perchance- Yes, I did reduce both pitch and and diameter, and yes, they would have both had some effect on reducing the load, and therefore increasing max possible rpms.
However, my prop guy, who has worked on props for 25 + years (mostly for much larger vessels, as large a propeller as will fit in the shop door!!) really didn't think the diameter change should make that much difference, as my prop does not have alot of surface area, with somewhat pointy tips.
Anyway, I just wanted to let others know that it is possible to see greater increases than 200-300 rpms per inch of pitch. Even if you allowed 200-300 more for the diameter change, I should have only went up 400-600 rpm. In reality was able to rev about 1000 rpm more than before, and I think some of that is due to the extra hp that is now available at the highre rpm, which in turn, helps to rev higher than what would have been possible with less hp!
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Old 10-02-2011, 10:58   #26
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Daddle, I disagree with you re. an engine being underloaded or overloaded at a given rpm. The WOT setting would be when the governor is limiting the fuel supply thus rpm. Since the prop is not directly connected to the water as a car wheel is connected to the ground the engine rpm would settle at that rpm to produce the power absorbed by the prop. When one is powering then raises the main sail the speed picks up. To maintain the same speed as before then one lowers the throttle and the rpm's drop thus the engine is supplying less power. If over proped at full throttle where the governor is not limiting the fuel supply because the rpm is lower than rated then the engine is being 'overfueled' and damage is occurring. But if the the rpm is at 80% of the governor limit but less than max achievable rpm then the engine is supplying the power necessary for that speed through the water. The lower the rpm the higher efficiency is being achieved and less 'slip' or wasted power.
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Old 10-02-2011, 13:12   #27
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Hi Capbillh

There is a point I don't understand. I realise you are talking about diesel engines here but you mentioned that lower RPM was better at WOT.

I have two V8's petrol and I can only get 3000 RPM from them at WOT where as they should rev out to about 4200 RPM.

The problem is as I apply the power the temp on both engines start to rise. I have been told that my props are the wrong size cause at low revs they will run all day at the right temp.

The cooling system has been serviced and or replaced.

What do you think.

Thanks
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Old 10-02-2011, 14:36   #28
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I know several quite knowledgeable folks who believe Yanmar is overly conservative on over propping. I can see Yanmar's position. They have no way to know if someone is routinely running a badly over-propped engine at full throttle (not to mention many other bad-for-your-engine practices such as always running WOT). Remember that the majority of marine Yanmar diesels are in fast powerboats. Sailboats are a bit of an afterthought. Bad fuel economy and slow speed from conservative propping on your boat might make you grumpy but sure won't cost Yanmar anything in warranty repairs.

An engine running with good fuel economy, no smoke, no overheat (but running nice and hot), with most time spent at 60%-80% RPM's is a happy engine. One of the above fellows believes that the temperature gauge is your best friend for long engine life (e.g. not too hot, not too cold).

Of course, if you choose to slightly overprop and require warranty repair for some reason - you need to consider Yanmar's possible response.

Carl
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Old 10-02-2011, 15:57   #29
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Flybridge, one can not directly compare a gas engine to a diesel since a diesel runs with 'wide open throttle' all the time meaning that there is no restriction in the air intake circuit. A gas engine has a torque peak about mid rpm range while a diesel has its peak about fast idle and the dropes from there.
The efficiency that I mentioned involves propeller not engine.
I will expand more later. My granddaughter is hungry.
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Old 10-02-2011, 21:15   #30
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Ok, I looked at the curves as Yanmar puts out for the 3JH4E that I have. The torque curves starts at 1625 and goes down from there.
Since horsepower is the product of torque X RPM then the horsepower increases up to the top governed RPM since the RPMs increase at a faster rate than the torque falls off. The fuel consumption goes up from idle in a shallow curve. A set of curves for a gas engine for torque and fuel consumption different from this diesel pattern.
I often thought that the most efficient RPM for a marine gas engine would be at the torque peak provided that it came out on plan. I never had a gas powered boat so I am just speculating here based on my experience with car engines.
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