Overpropping

psneeld · Jul 14, 2014

From Aug/Sep edition of Professional Boatbuilder...

Nigel Calder's article...Praise for big props....

Quote on page 58-59....

"I am not a propeller expert, but it seems to me that on many boats, particularly those where WOT is almost never needed, mild oversizing of the propeller would provide instant benefits to the end user, with little or no downside."

Sure it talks about engine damage..and it also talks about being smart enough to know better.

Interesting read and I did skim it as much of the article has been said before by others...and sure there's more to the story but interesting compared to many boater's beliefs.

bayview · Jul 14, 2014

With most engines overpropping results in extra fuel being provided in order to increase revs but the revs are limited by the max hp at that rpm. Now if you don't own the throttle beyond the max rpm I guess you are operating at the max power curve and not over fueling until you open the throttle a bit more. I don't see the benefit. You can get that same power at higher point on the prop curve and same fuel with lower cylinder temps because of freer air flow.

Look at prop and ax power curves and notice that both cross the same power and fuel points Judy at different rpms.

kchace · Jul 14, 2014

No matter what, any boat can only move at a given speed with a certain amount of power being applied. The lower the RPM that power is applied, the higher the load on the engine. Whether that load is too high for a particular engine is a different matter and would take lots and lots of testing to prove one way or another. As for me, I want my engines to last as long as possible and I can't possibly test the various scenarios with different pitch props over the lives of the engines, so I will prop to the mfgr's recommendations.

Ken

Ski in NC · Jul 14, 2014

Most of the trawlers can power to hull speed at a good bit less than 100% power. From hull speed on up, the hp vs speed curve takes a sharp turn upward. It no longer follows the the hull speed propeller law curve.

So when you prop to rated rpm at full power, you are in a strange spot in the hp vs speed curve. There is a good arguement to ignore this region and prop engine to better suit actual cruising conditions.

Each engine has a peak torque rpm on its full load curve. I don't know where it is for the Lehman Ford, but it is probably around 1500 or so. One thing I've found about diesels is if you find out peak torque rpm, and get cruise rpm close to that (not full power of course), can't go wrong.

psneeld · Jul 14, 2014

The article...written by someone that a lot of guys like to hold in reverence...suggests big props and educated engine owners that slightly overprop will get better efficiency with little or no risk.

Charts included to prove his point.

Worth the read....

In my experience engines don't catastrophically fail from something like a slight overprop...they fail from a component defect or egregious abuse like running in an overtemp situation to make port...

DavidM · Jul 14, 2014

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

psneeld · Jul 14, 2014

There's no debate for some here...and assuming the debate last time or anytime was anywhere near accurate.

I posted the quote from a well respected person and where it could be read in full context.

I started the thread to show all those that think they know about overpropping now have either support for their thinking...or a well respected marine type that may disagree with them.

Anyone not sure can take it upon themselves to investigate and have even more data for their final decision..

eyschulman · Jul 14, 2014

psneeld said:
The article...written by someone that a lot of guys like to hold in reverence...suggests big props and educated engine owners that slightly overprop will get better efficiency with little or no risk.

Charts included to prove his point.

Worth the read....

In my experience engines don't catastrophically fail from something like a slight overprop...they fail from a component defect or egregious abuse like running in an overtemp situation to make port...

What you say may well fit old heavy iron. The new lighter higher output tubo motors me thinks are a very different story. These motors run hotter(exhaust temps not coolant) and a lot closer to the edge. Lots of these motors have failed due to small amounts of overloading. If you talk to people who have dealt with many Hinklys you may get a better picture. I lost one of these motors at 272 hrs due to a slight over propping and then too late got educated re. this issue. Yes you can get away with overloading with heavy iron but why? The suggested fuel economy if really present would be relatively small fry in overall boat costs. Also a boat being run at lower rpms for cruise needs to be run out regularly to clean it out and if overloaded that is damage country. A overloaded engine is overloaded at all rpms just less damage at lower rpm curve. All else equal the motor set up with manufacturers power curve will outlast an over propped situation. Then there is the resale issue many smart buyers walk from an over propped boat and you may have to find one who believes as you do. I strongly advise against overloading and would not buy a boat from an owner who has done this over time why take the risk?

Northern Spy · Jul 14, 2014

I may or may not be over propped. Volvo Penta allows a range of 2700 to 3000 RPM for full throttle value. My engine can achieve 2700 rpms at full throttle with a heavy load. But I cannot go slow enough to troll and entering my berth is fun at a fairly high speed of 3+ knots. So the next time that I pull Spy, I will change the pitch downward. Speed at idle is something to consider.

psneeld · Jul 14, 2014

eyschulman said:
What you say may well fit old heavy iron. The new lighter higher output tubo motors me thinks are a very different story. These motors run hotter(exhaust temps not coolant) and a lot closer to the edge. Lots of these motors have failed due to small amounts of overloading. If you talk to people who have dealt with many Hinklys you may get a better picture. I lost one of these motors at 272 hrs due to a slight over propping and then too late got educated re. this issue. Yes you can get away with overloading with heavy iron but why? The suggested fuel economy if really present would be relatively small fry in overall boat costs. Also a boat being run at lower rpms for cruise needs to be run out regularly to clean it out and if overloaded that is damage country. A overloaded engine is overloaded at all rpms just less damage at lower rpm curve. All else equal the motor set up with manufacturers power curve will outlast an over propped situation. Then there is the resale issue many smart buyers walk from an over propped boat and you may have to find one who believes as you do. I strongly advise against overloading and would not buy a boat from an owner who has done this over time why take the risk?

Did you read the article?

I think you and others are being disagreed with on many levels in the article.

I'm not an engine guy ..just an operator that has seen engines on commercial vessels that would also disagree....they do all sorts of things and keep on going despite the comments here, boat mag experts and hordes of mechanics.

eyschulman · Jul 15, 2014

psneeld said:
Did you read the article?

I think you and others are being disagreed with on many levels in the article.

I'm not an engine guy ..just an operator that has seen engines on commercial vessels that would also disagree....they do all sorts of things and keep on going despite the comments here, boat mag experts and hordes of mechanics.

Love to read the article have been unable to get a copy. I am heading northwest for a couple of months tomorrow but will keep my eyes open for an opportunity to find the article. I like Nigel's thoughts on boats and will read with an open mind.

psneeld · Jul 15, 2014

eyschulman said:
Love to read the article have been unable to get a copy. I am heading northwest for a couple of months tomorrow but will keep my eyes open for an opportunity to find the article. I like Nigel's thoughts on boats and will read with an open mind.

It's definitely worth the read as it discusses marine hybrid propulsion and the ability to use full use of the torque available. In the quest to see what is possible, the data came to light that in his mind overpropping is something worthy of consideration....under certain conditions.

ghost · Jul 15, 2014

Nigel knows what he is talking about. He's not an armchair kind of guy. Not only did he write the best marine electronics book available, he has also written on diesel care.

Now, that said you still won't find me making such recommendations on over propping. It's not so much that I don't believed there are some outside use cases that may have value. It's more my lack of faith in humanity. There are way too many monkey see/monkey do's out there who know how to pretend otherwise. They will take this as a means of justifying what they want to do without inconvenience of a balanced opinion or trade off.

You think we can get the monkey sees to understand something as dynamic as propping? I show you a group who can't understand the meaning if the word trawler.

Thusly, I continue to exclaim more simply that engines must be propped for WOT. Works for all who are in need of an opinion.

Ghost out.

psneeld · Jul 15, 2014

They really don't have to understand overpropping any more than they can repair their own engines...once a mechanical/electronic RPM limit is installed...it becomes just like any other setup...and they can pencil in a new WOT in their manual and a note to the prop size next to it.

I just guess it depends on what you think the benefits are (and probably need to be verified for each and every boat)....some will deny the concept and may even change their minds now about Calder for publishing such blasphemy...I'm just saying the info is out there...and maybe the world isn't flat after all....

Ski in NC · Jul 15, 2014

djmarchand said:
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

Yep, Dave. Your and my boat are capable of running well past hull speed, so we HAVE to prop correctly for full load rpm.

Intentional overpropping should only be considered on boats that barely exceed hull speed at full.

Burn rate per mile will not change much, but noise level could be a good bit lower.

Eric

bayview · Jul 15, 2014

Proper propping on displacement boats allows using the extra power if required to buck head seas and wind.
You may be limited to 8 knots but if you want to do. 8 knots against head seas and wind you will need the extra power

jleonard · Jul 15, 2014

Scott, I think you and I have compared fuel burn a few times in the past. And I believe you are a little overpropped. I am pretty close to dead on.
That particular comparson doesn't look like it yields any bennies to either of us.

MYTraveler · Jul 15, 2014

I don't claim to be any kind of expert, but have given the concept a lot of thought and discussion.

It seems to me that if variable pitch props were readily available, then we would want to adjust the pitch, at any given rpm, to load the engine to equal a consistent percentage (maybe 90%) of max available hp at that rpm. (Some would chose 90%, others maybe only 75%, or less, but too much less and some will be concerned that underloading is just as bad; point is there is a sweetspot, that differs by user, engine and application and could be more consistently attained with a variable pitch prop.)

Engine power curves being what they are, that means that the prop pitch would be significantly increased for most rpms as compared to the pitch that would otherwise result from the conventional wisdom of pitching the prop to make full rpm (even if near or even at 100% load at that full rpm).

So, if you never run your boat near full rpm, a little "overpitching" probably is a good thing. I had this conversation with another highly regarded expert, who didn't disagree; he just observed that for many boaters, the temptation to run at high rpm is unavoidable.

markpierce · Jul 15, 2014

Don't see any need to change a propeller from "perfectly/properly-propped" to "over-propped." Is this a matter of being "over-engined" with excessive/wasteful horsepower? I'm comfortable with achieving hull speed with 200 less RPM than the engine is capable with the current propeller on the full-displacement Coot.

Lollygag · Jul 16, 2014

I just don't understand the argument. Lollygag is 42' loa and has 2 FL120s. She is seriously overpowered. Newer 42' boats have twice the power! With a semi-displacement hull she has a hull speed of about 8.5 knts. More power means more fuel but not so much speed. Overpropping does what? Let's me run at lower RPM? I already run at about 1500 RPM, lower RPM will just load up the engines. Why would I do that?

psneeld · Jul 16, 2014

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.

bayview · Jul 16, 2014

MY
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

Delfin · Jul 16, 2014

djmarchand said:
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?

psneeld · Jul 16, 2014

bayview said:
MY
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.

sunchaser · Jul 16, 2014

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.

O C Diver · Jul 16, 2014

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

Delfin · Jul 16, 2014

O C Diver said:
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.

O C Diver · Jul 16, 2014

Delfin said:
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.

Ted

fryedaze · Jul 16, 2014

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.

markpierce · Jul 16, 2014

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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