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Old 07-16-2014, 02:10 PM   #21
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The argument is that if you prop for WOT...you are only taking into account one point on the power available curves (as there are several that are used) to base your prop.

Without a variable pitch propeller or the more efficient use of torque from an electric motor...the argument is you can extract more of the torque produced at some points of the power curves rather than using a prop that meets the "optimum" at WOT.

All the above is a gross oversimplification of many concepts...but some agree that if you have an old style heavy iron engine (and it's way of producing power), and enough of it in a boat that is run well below WOT for cruising....overpropping may improve efficiency and comfort (less RPM) and possibly actually increase engine life is no harmful parameters re exceeded.

Again... I'm not the guy to ask because I'm not an engine guy much like many others here in this forum.....the point of the post was that a well respected marine "tech" guy while studying hybrid systems found data that suggests overpropping may be a good thing...despite what other may say.
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Old 07-16-2014, 04:37 PM   #22
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Look at those pore curves and draw a horizontal line at constant power then note that that line is also constant fuel. Simply the engine can put out the same power and use the same fuel at many rpms with no advantage but with less rpm and intake cooling
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Old 07-16-2014, 05:36 PM   #23
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This topic was fully debated on this forum last winter. Oh, well so here we go again:

Ski is right but only for displacement hulls.

Most of the displacement hull owners on this forum have engines that have enough power to drive it to hull speed and a little more, but not a significant amount more.

If you have a displacement hull with enough power to get you to hull speed plus maybe 25% you will not get much efficiency improvement by overpropping. Your cruise speed is probably near the optimum point for specific fuel consumption anyway.

So the argument is mostly for semidisplacement hulls that do have significant power to drive the boat well beyond displacement speed. These boats do pretty well follow the 2.5-3.0 exponential prop curve which is related to engine rpm not boat speed.

But if you look at BSFC curves they peak at about 60-70% of wot rpm and only drop 10-15% all the way down to 40% of wot rpm. So you could prop the boat/engine several hundred rpm less than normal so you could cruise at that rpm but the most efficiency you will get is 15%.

On my boat that means burning 2.5 gph vs 3 gph at displacement speed which is about $2.00 per hour. That just isn't worth it because: 1) I am giving up the ability to ever run at rated wot because I can't and 2) I give up the ability to run at a fast cruise of 85% of rated rpm because that will harm the engine.

I am forever limited to running at displacement speed for only a $2.00 per hour benefit.

David
Advice appreciated, David (or others...)

Delfin's 3306 was run in by Cat when she was an empty hull and achieved 2200 rated rpm. Finished, she is a tubby bitch and only able to only hit 1950. I run her at 1250-1450 and heat up the EGT to 850 degrees every 12 hours or so at 1750 for 15 minutes. Exhaust gas at 1750 is clear as a bell. Whaddya think? Good, bad? Immaterial?
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Old 07-16-2014, 06:11 PM   #24
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Look at those pore curves and draw a horizontal line at constant power then note that that line is also constant fuel. Simply the engine can put out the same power and use the same fuel at many rpms with no advantage but with less rpm and intake cooling
The article has "actual performance" graphs with 2 different sized props...one obviously an overprop.

The graphs show the overprop drives the boat faster with less power at lower rpm and thus less fuel burn.

It may not happen for every boat..but it was results like this that probably made him write the quote in post #1..that overpropping in some cases may be beneficial with few or no drawbacks if used intelligently.

I can't veritfy what he wrote...just passing it along to people who might think he has something...and I know many don't.
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Old 07-16-2014, 06:36 PM   #25
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It is estimated that new build trawler such as Selene, Nordhavn etc represent less than 2% of the market for pleasure marine diesels. The 90 +% ers are most definitely in the prop it right camp or trouble is waiting.

In the large vessel commercial market over loading an engine with wrong sized props would be pretty unusual. Having watched large tugs recently moving freight to Alaska, the sounds of a diesel operating at 70% or higher load for days on end is awesome.

For our toy boats running a few hundred lightly loaded hours per year, the issues lurking are many years out there. Other things will wear the engine out first, namely benign neglect.
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Old 07-16-2014, 09:33 PM   #26
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I don't think a little over propping is bad ( 200 or 300 rpm below rated WOT).

I don't think more air for the same fuel burn and HP generation is bad.

Often there are several HP options for the same engine block. For a Cummins 6BT the range was from 210 to 375 HP. Over propping the 210 would probably be a none issue, where as over propping the 375 IMO is just begging for trouble.

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Old 07-16-2014, 10:11 PM   #27
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I don't think a little over propping is bad ( 200 or 300 rpm below rated WOT).

I don't think more air for the same fuel burn and HP generation is bad.

Often there are several HP options for the same engine block. For a Cummins 6BT the range was from 210 to 375 HP. Over propping the 210 would probably be a none issue, where as over propping the 375 IMO is just begging for trouble.

Ted
That's a very good point.
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Old 07-16-2014, 10:30 PM   #28
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Advice appreciated, David (or others...)

Delfin's 3306 was run in by Cat when she was an empty hull and achieved 2200 rated rpm. Finished, she is a tubby bitch and only able to only hit 1950. I run her at 1250-1450 and heat up the EGT to 850 degrees every 12 hours or so at 1750 for 15 minutes. Exhaust gas at 1750 is clear as a bell. Whaddya think? Good, bad? Immaterial?
Probably immaterial if you see no change in the exhaust gas. At 1450 I'm guessing you are pretty close to peak torque. Most diesels are operating at there best efficiency at peak torque provided they have reasonable load. Something to consider: many large generators run there entire life at reduced rpm. My 9 KW Onan has a 2400 rpm Kabota Diesel running at 1800 rpm. Clearly it doesn't have a problem as long as it's load falls somewhere between minimum and maximum. That's kind of like running a slightly over propped propulsion engine at reduced rpm.

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Old 07-16-2014, 10:42 PM   #29
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OK, I have followed this thread. My OPINION, you don't get something for nothing. Bigger prop, more speed, more power needed for the speed, more torque at given RPM, more fuel needed. I just went from a 26X26 to a 25X27 prop. I get more speed at lower RPM but I think I am going to find that I don't get any improvement in fuel economy. The designers of these boats know a little bit about what they are doing. Given all of that rent, I don't know sh%! about boat design and just a little about physics and engineering.
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Old 07-16-2014, 11:03 PM   #30
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Idle (one-third WOT) for me equals 3 knots. Normal cruise (one knot less than hull speed) three-fourths WOT. Maximum (hull) speed a 92% WOT. See no need for overpropping here. Especially don't want a higher minimum speed.
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Old 07-16-2014, 11:25 PM   #31
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Idle (one-third WOT) for me equals 3 knots. Normal cruise (one knot less than hull speed) three-fourths WOT. Maximum (hull) speed a 92% WOT. See no need for overpropping here. Especially don't want a higher minimum speed.
With my 26X26 I did 3.8 ktns at idle. The 25X27 now does 4.2 at Idle. At 1550 RPM the 26X26 di 7.5 knots. The 25X27 does 8 Knots. I am now over propped and can not reach my 2650 design WOT for the 6BT 5.9 Cummins. WOT is 2590. I am pretty sure I am ok. BUT, I don't believe I gain anything based on the laws of physics. It still cost the same to move the 30,000 lbs of boat through the water.
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Old 07-16-2014, 11:30 PM   #32
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With my 26X26 I did 3.8 ktns at idle. The 25X27 now does 4.2 at Idle. At 1550 RPM the 26X26 di 7.5 knots. The 25X27 does 8 Knots. I am now over propped and can not reach my 2650 design WOT for the 6BT 5.9 Cummins. WOT is 2590. I am pretty sure I am ok. BUT, I don't believe I gain anything based on the laws of physics. It still cost the same to move the 30,000 lbs of boat through the water.
BTW, I did change prop sizes on purpose. I tore up my props running aground. My spare props just happened to be a different size. I assume they were the original design and the props on the boat were from a previous engagement with the bottom by the previous owner.
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Old 07-17-2014, 06:58 AM   #33
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With my 26X26 I did 3.8 ktns at idle. The 25X27 now does 4.2 at Idle. At 1550 RPM the 26X26 di 7.5 knots. The 25X27 does 8 Knots. I am now over propped and can not reach my 2650 design WOT for the 6BT 5.9 Cummins. WOT is 2590. I am pretty sure I am ok. BUT, I don't believe I gain anything based on the laws of physics. It still cost the same to move the 30,000 lbs of boat through the water.
You are correct about that it takes the same amount of force to move the boat through the water. However, there are things you can change on your boat to generate that force with less fuel. Engines have a range where they are most efficient (generate most HP per gallon of fuel). This is generally near peake torque. There is a range of optimum propeller sizes for a given boat at a given speed relative to the drive train. As an example, if your propeller was optimal for your boat, and you swapped it with one that was 2"smaller in diameter, it would yield less MPG. Not suggesting you start swapping props, it's just that the object of the exercise is to get the most MPG out of your boat and maybe reduce engine noise without any detrimental effect.

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Old 07-17-2014, 08:02 AM   #34
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. Not suggesting you start swapping props, it's just that the object of the exercise is to get the most MPG out of your boat and maybe reduce engine noise without any detrimental effect. Ted
Or prop it by the book and ignore internet wives tales. Unless a problem exists, chasing the mythical most MPG by swapping out props is a money losing proposition.

Today, the quest on larger high hour yachts is prop selection to minimize noise and vibration while not damaging the engine (by operating it out of its sweet spot) and associated systems.

But as said already on this thread, the few hours per year most TF vessels see invites over propping discussion with little measurable difference one way or the other. The rub for many, me included, is when sea trialing a vessel if rated full RPM cannot be reached, wondering what is wrong?
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Old 07-17-2014, 08:31 AM   #35
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Or prop it by the book and ignore internet wives tales. Unless a problem exists, chasing the mythical most MPG by swapping out props is a money losing proposition.

Today, the quest on larger high hour yachts is prop selection to minimize noise and vibration while not damaging the engine (by operating it out of its sweet spot) and associated systems.

But as said already on this thread, the few hours per year most TF vessels see invites over propping discussion with little measurable difference one way or the other. The rub for many, me included, is when sea trialing a vessel if rated full RPM cannot be reached, wondering what is wrong?
Tom, to OC Diver's point, diesels run best when loaded a bit. If you run a diesel at 1/5th of its rated horsepower - which is all it takes to move Delfin at 7.5 knots, yet she takes 80% of that hp to move her at hull speed in a bit of seaway - then doesn't a bit of overprop at minimal rpm but peak torque load the engine a mite? If so, isn't this a benefit regardless of its impact on fuel consumption? I suppose it could be harmful if you needed to run at WOT for more than a few minutes, but that never seems to happen, to us at least.
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Old 07-17-2014, 08:33 AM   #36
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Probably immaterial if you see no change in the exhaust gas. At 1450 I'm guessing you are pretty close to peak torque. Most diesels are operating at there best efficiency at peak torque provided they have reasonable load. Something to consider: many large generators run there entire life at reduced rpm. My 9 KW Onan has a 2400 rpm Kabota Diesel running at 1800 rpm. Clearly it doesn't have a problem as long as it's load falls somewhere between minimum and maximum. That's kind of like running a slightly over propped propulsion engine at reduced rpm.

Ted
Yes, peak torque looks to be right at 1375 or so on this engine. As per my question to Sunchaser, isn't a bit of load from minor overpropping potentially beneficial in that it provides a more "reasonable load"?
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Old 07-17-2014, 09:10 AM   #37
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Tom, to OC Diver's point, diesels run best when loaded a bit. If you run a diesel at 1/5th of its rated horsepower - which is all it takes to move Delfin at 7.5 knots, yet she takes 80% of that hp to move her at hull speed in a bit of seaway - then doesn't a bit of overprop at minimal rpm but peak torque load the engine a mite? If so, isn't this a benefit regardless of its impact on fuel consumption? I suppose it could be harmful if you needed to run at WOT for more than a few minutes, but that never seems to happen, to us at least.
Like most with larger trawlers, I am over powered, but in in my case not over propped. In our cases our existing engine and prop set ups will do the engines no harm so long as oil temperatures are in the 185 to 200 F range and no excessive coolant heat exists.

I just read today on boatdiesel the old axiom, "under propping creates no issues, whereas over propping does." I'm too lazy to refute these statements and ignore my half century of large to small diesel use.

My doubts on the over prop question stem from no verifiable data is available but lots of arm waving takes place. In today's diesels precise fuel mapping exists with new vessels, whether a slow Nordhavn or a fast Grand Banks, using the fuel mapping precise GPM and load to assist in prop selection.
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Old 07-17-2014, 09:13 AM   #38
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Or prop it by the book and ignore internet wives tales. Unless a problem exists, chasing the mythical most MPG by swapping out props is a money losing proposition.

Today, the quest on larger high hour yachts is prop selection to minimize noise and vibration while not damaging the engine (by operating it out of its sweet spot) and associated systems.

But as said already on this thread, the few hours per year most TF vessels see invites over propping discussion with little measurable difference one way or the other. The rub for many, me included, is when sea trialing a vessel if rated full RPM cannot be reached, wondering what is wrong?
Thus the point of the post.

Maybe it's not really internet wives tales....maybe there is data out there that proves something...maybe not everything...but something.

A very knowledgeable poster and I have traded PMs over the subject and yes the broad stroke comments may be as flawed as the arguments against overpropping. Each tidbit and data point has to be put in perspective. Comments that allude to "science" and that it takes the same energy to drive the boat at a given speed are often correct...but fail to address the key point of efficiency or as you point out the trouble that it's worth.

The quote I picked addressed that there were dangers involved but even a published "expert" didn't see much danger in saying it...just that those risks could be "mitigated" to a degree that in some instances...overpropping might be worth the effort.

Thus again the point of the OP.

I knew I wouldn't change many or any minds...but totally unfamiliar people might deserve all the info out there....as it trickles in.
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Old 07-17-2014, 10:47 AM   #39
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Scott, are you referring to our comparing fuel use, etc?
The way I look at it is if propped "reasonably" one would be ok unless the enigne is run "on the edge" and then it would be necessary to have it dialed in.

By the way I think your boat rocks more than mine...... LOL
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Old 07-17-2014, 11:26 AM   #40
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Scott, are you referring to our comparing fuel use, etc?
The way I look at it is if propped "reasonably" one would be ok unless the enigne is run "on the edge" and then it would be necessary to have it dialed in.

By the way I think your boat rocks more than mine...... LOL
Have you been following the 1988 Albin interior thread?

Another couple dollars and there's a few marine stores that could lay claim to her any day....there's not enough left to prove it's the boat I bought except for the HIN...

No Jay....I have no idea whether my bigger prop than yours helps or not...the only way to really know for sure on any boat is testing props on that single boat. Sure there are charts that can point us in the right direction but a whole lot of other factors can affect speed and fuel consumption on 2 different boats so coming close only can make us guess why.

Based on the generic whole of all boats reporting in...I feel that my fuel numbers are close to what they should be as are yours. Some reports of 8 knots and 1.5 gal/hr on a Lehman 120 in an old Taiwan trawler make me cringe when it comes to reality....unless it's per engine on a twin.
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