Diesel RPM?

[boatpoker]

Originally Posted by skipperpete

Go straight to the end of the Boatpoker attachment and read the “I’m not a propeller expert” and particularly where he mentions overpropping on boats that are NOT used at WOT. First part of the article is about larger diameter propellers and low shaft speeds more appropriate to power boats and not in contention , few cruising yachts use very large diameter props unless it’s a folding,feathering or CPP and few yacht gearboxes are much above 2.5:1 ( yanmar have a 3:1) while trawlers are able to use deeper reductions, ( 3.5:1 up to a hefty 7:1 Twindisc MG 520) and large propellers.

Sailboat hulls are not much related to planing or semi-planing powerboat hulls and more akin to displacement fishing boat hulls like my great Grandfathers (over my left shoulder) North Sea trawler or my own fantail shown on the trailer. Both essentially sailboat hull forms without significant keels. This makes a difference i.e. I know several displacement power vessels that far exceed theoretical hull speed, some with much less horsepower than the tables show.

[Lee Jerry]

Originally Posted by malbert73

This coming from the “expert” who many posts ago decided that my references to Calder, Dashew, and Harries were “too much homework” despite their expertise suggesting some very valid use cases for overproppng.

I never said I haven't read any of their work. There are at least two of Calder's books on the bookshelf on the boat right now. (I am not currently there.) I've read lots of Calder in books and articles. The other two less so (but they are also less prolific). That doesn't mean I know exactly which bit of writing you are referring to.

Maybe make a case of your own (even if parroting them) rather than just name dropping.

[Lee Jerry]

Originally Posted by boatpoker

Malbert ... I've never tried uploading a pdf. here but if it works, here is the Professional Boatbuilder Calder article ...

Lots could be said about this article, but I'll keep it simple.

As Calder says on the bottom of the first page, bigger props are more efficient, so that kind of limits the usefulness of the whole thing, IMHO. It'd have been more useful if he compared the same diameter at different pitches (but his goal was different than ours).


But more importantly, I particularly like this line on the top of page 54:

Let me just repeat that bit: "...the load they place on the engine at any given engine speed will, at all times, be similar."

Thanks for helping me make my case with malbert73; you're the best boatpoker.


And then there was this:

A couple more like that and it might seem like I was channeling Nigel C the whole time.

[malbert73]

LOL Jerry Lee! At first I thought you were trolling us, then I thought you were underinformed, and now I realize you must not have received and/or read page page 55 of the Calder PDF. Thanks Boat Poker for finding this as I was lucky enough to have this very conversation with him back at the boat show many years ago when I was selecting my Flexofold.

I actually don't know how to insert a picture of the the very easy to understand prop curve which you must have missed but the caption says it all:


Propeller curves overlaid
on the fuel map for
the Volvo Penta D2-75
illustrate that the matched
and undersized props
don’t pass through the
engine’s region of peak
efficiency"

[and the text later on page 55 which helps]

"From my perspective, the key thing
to note is that at no time does the
matched propeller curve pass through
the engine’s region of peak fuel efficiency, which is to say any propeller
that operates along this curve will not
optimally load the engine. If an undersized propeller is put on a boat (in
which case the engine will get to its
full rated speed before the propeller
loads it to its rated power), the situation looks even worse: the propeller
curve passes through chronically inefficient regions of the engine’s “fuel map.” If, on the other hand, an oversized propeller is put on the boat, at
all points the propeller curve passes
through more efficient regions of the
engine’s fuel map than does that of
the matched propeller. In fact, if the
oversizing is pushed far enough, this
particular engine can be loaded to close
to peak efficiency along much of the
propeller curve."

See- there, nice and simply laid out. No homework needed

[Lee Jerry]

Originally Posted by malbert73

LOL Jerry Lee! At first I thought you were trolling us, then I thought you were underinformed, and now I realize you must not have received and/or read page page 55 of the Calder PDF. Thanks Boat Poker for finding this as I was lucky enough to have this very conversation with him back at the boat show many years ago when I was selecting my Flexofold.

I actually don't know how to insert a picture of the the very easy to understand prop curve which you must have missed but the caption says it all:


Propeller curves overlaid
on the fuel map for
the Volvo Penta D2-75
illustrate that the matched
and undersized props
don’t pass through the
engine’s region of peak
efficiency"

Uhhh, yeah, I've said many times that you could move to a more efficient point in the map.

[and the text later on page 55 which helps]

"From my perspective, the key thing
to note is that at no time does the
matched propeller curve pass through
the engine’s region of peak fuel efficiency, which is to say any propeller
that operates along this curve will not
optimally load the engine. If an undersized propeller is put on a boat (in
which case the engine will get to its
full rated speed before the propeller
loads it to its rated power), the situation looks even worse: the propeller
curve passes through chronically inefficient regions of the engine’s “fuel map.” If, on the other hand, an oversized propeller is put on the boat, at
all points the propeller curve passes
through more efficient regions of the
engine’s fuel map than does that of
the matched propeller. In fact, if the
oversizing is pushed far enough, this
particular engine can be loaded to close
to peak efficiency along much of the
propeller curve."

Why didn't you quote the remainder of that paragraph?

[malbert73]

Originally Posted by Lee Jerry

Uhhh, yeah, I've said many times that you could move to a more efficient point in the map.



Why didn't you quote the remainder of that paragraph?

I wanted to minimize the quote so you would read it. We never disagreed on losing top RPM when overpropping.
You seemed stuck on challenging the well proven benefits of overpropping- better engine loading and better fuel efficiency.

The rest of the paragraph goes into the dangers of overloading which I have covered far upthread (one should overprop gently, you lose top RPM, and an EGT installed). You just didn't want to pick up Dashew's Offshore Cruising encyclopedia and go to page 752, or click the Attainable Adventure Cruising link I left in post 69.

But here's the rest of Nigel Calder's quote:

"In fact, if the
oversizing is pushed far enough, this
particular engine can be loaded to close
to peak efficiency along much of the
propeller curve. However, the most
immediate consequence is the engine
is overloaded at higher rpm. It is
unable to reach its rated maximum
speed and as a consequence cannot
develop its rated maximum power. If
it is operated at wide-open throttle
(WOT) for any length of time, the
engine and/or transmission will quite
likely be damaged."



But don't miss the punch line- he shows 15% fuel consumption gains on page 56 and his prop curve demonstrations are brilliant.
His conclusions:
"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. Are we currently using the
wrong design point? If engine overload
is a potential problem, this can
be resolved through user education
and perhaps by setting limits on the
engine’s governor. If the loss of peak
available power is unacceptable for
some reason, the efficiency gains
could be achieved by installing a nominally
oversized engine, pushing the
propeller curve into the regions of
peak efficiency on its fuel map, and
then setting limits on the governor.
The governed-down limit would then
allow the engine to develop the
desired level of power."


Thank you Boat Poker for finding this open source article to help Jerry Lee see the points that are well proven about benefits of overpropping for some use case scenarios. Let me know how the new prop goes
I'm about to launch, and with over 6000nm under power with a mildly overpropped Beta 50 in coastal and offshore conditions, I can attest to the benefits.

[boatpoker]

Originally Posted by malbert73

Thank you Boat Poker for finding this open source article to help Jerry Lee see the points that are well proven about benefits of overpropping for some use case scenarios. Let me know how the new prop goes
I'm about to launch, and with over 6000nm under power with a mildly overpropped Beta 50 in coastal and offshore conditions, I can attest to the benefits.

Coincidence ! We too are running a Beta 50 in our Benford 38 Fantail ... and are overpropped ... on purpose Our max RPM is 2400 which will take us 1.5nmph over hull speed. Hitting a WOT of 2800RPM would get us nothing but higher fuel usage. I think we'll keep it that way We normally cruise at just below hull speed at 1800RPM using 0.75gph.

[Lee Jerry]

Originally Posted by malbert73

I wanted to minimize the quote so you would read it. We never disagreed on losing top RPM when overpropping.
You seemed stuck on challenging the well proven benefits of overpropping- better engine loading ...

Yes, I challenge this. But I'll ask again: how do you define "engine loading?" Without agreement on this rather important detail we'll most certainly keep talking past each other.

and better fuel efficiency.

I don't see how you could possibly be reading (and understanding) my posts and still make this statement. Maybe it's that parenthetical part...

The rest of the paragraph goes into the dangers of overloading which I have covered far upthread (one should overprop gently, you lose top RPM, and an EGT installed). You just didn't want to pick up Dashew's Offshore Cruising encyclopedia and go to page 752, or click the Attainable Adventure Cruising link I left in post 69.

But here's the rest of Nigel Calder's quote:

"In fact, if the
oversizing is pushed far enough, this
particular engine can be loaded to close
to peak efficiency along much of the
propeller curve. However, the most
immediate consequence is the engine
is overloaded at higher rpm. It is
unable to reach its rated maximum
speed and as a consequence cannot
develop its rated maximum power. If
it is operated at wide-open throttle
(WOT) for any length of time, the
engine and/or transmission will quite
likely be damaged."

Originally Posted by Lee Jerry

This is too much of a simplification. It's more than that. It's: you do it (not overprop) for the health of the engine and to maximize the performance of the prop/engine system across the widest range of conditions.

But don't miss the punch line- he shows 15% fuel consumption gains ...

NOT from over-propping. The majority of the gains (~2/3) are from the increased diameter. If that diameter fits the boat, then why wouldn't you use it with "proper" matching? I've said before, maybe not in this thread but in at least two others, that the process of prop matching is to determine the max diameter that will fit then determine the pitch that allows rated RPM at WOT. Or maybe a better question is to reverse it: if your goal is efficiency and a 26" diameter prop fits (properly) on the boat, why would you install a 22" diameter prop? Of course, you wouldn't. And yet that is what he did.

Calder says on the first page: "It has long been known that when it comes to propellers in displacement hulls, size counts. The larger the propeller, the more efficient it will be." So yeah, he demonstrated what we've known for a hundred years.

That leaves ~1/3 of the fuel consumption gains (~5%) to over-propping. Where have I seen that number before?

... on page 56 and his prop curve demonstrations are brilliant.

Not so much brilliant as flawed. A quick visual inspection can see this. The claim is for a service speed "of 7.3 knots in 15 knots of wind speed with associated waves." Why don't the points (of both props) fall on the "rough" water curves (red)? Do you see those little numbers on each of the curves? Those are supposed to correspond to the various boat speeds. In rough water, 7.3 kt would be at/near the continuous duty line. So why is it shown around 6 kt? The power / fuel consumption he uses are for those wrong points. Clearly something is amiss.

His conclusions:
"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. Are we currently using the
wrong design point? If engine overload
is a potential problem, this can
be resolved through user education
and perhaps by setting limits on the
engine’s governor. If the loss of peak
available power is unacceptable for
some reason, the efficiency gains
could be achieved by installing a nominally
oversized engine, pushing the
propeller curve into the regions of
peak efficiency on its fuel map, and
then setting limits on the governor.
The governed-down limit would then
allow the engine to develop the
desired level of power."

Originally Posted by malbert73

This coming from the “expert” who many posts ago decided that my references to Calder, Dashew, and Harries were “too much homework” despite their expertise suggesting some very valid use cases for overproppng.

So which is it, is NC not an expert like he said or he is an expert like you claimed? (I know who I tend to believe.)

"Almost never" is not the same as never. As others have said above, when you need that extra thrust you're glad you have it.

Define what "mild oversizing" is. 600 RPM reduction from a 2800 RPM rating, as used by NC in his analysis, does not seem mild to me. I'll also point out that he uses the term "oversize" rather than over-prop; diameters should be the same.

I've already addressed this. I don't think there is "no downside."

I've already addressed why I think this is a bad idea: higher cost, heavier, larger...

Thank you Boat Poker for finding this open source article to help Jerry Lee see the points that are well proven about benefits of overpropping for some use case scenarios. Let me know how the new prop goes
I'm about to launch, and with over 6000nm under power with a mildly overpropped Beta 50 in coastal and offshore conditions, I can attest to the benefits.

Let's not overlook this little tidbit from NC, page 51:

Someone claimed that over-propping increases maneuverability. NC disagrees, as he said: "There are two problems. The first is the boat may become harder to maneuver in close quarters." IOW, he says maybe the same, maybe worse, does not suggest better.

Yeah, thanks again boatpoker.

[Lee Jerry]

Originally Posted by malbert73

You seemed stuck on challenging the well proven benefits of overpropping- better engine loading and better fuel efficiency.

I can't believe I forgot one of the most important questions.

So here is NC's little summary box:

So, the questions are:
- Which engine has higher loading*, the one producing more power and burning more fuel, or the one producing less power and burning less fuel?
- Which engine will have higher EGT, the one producing more power and burning more fuel, or the one producing less power and burning less fuel?
- Which engine is more susceptible to damage from cylinder crazing, etc.?



* Notwithstanding that you haven't defined it yet. Maybe this would help me figure out what you mean.




For no particular reason, I feel I should include this:

Originally Posted by Lee Jerry

So at any given boat speed, say cruise speed, a vessel has a given resistance. This can be converted to a power value, call it effective power. The prop thrust/power then has to match this value of resistance/power to maintain that speed. It doesn't do this at 100% efficiency, let's call that efficiency propulsive coefficient (PC). (PC can be broken down into multiple different components, but that’s not necessary here.) So divide effective power by PC to get the power needed by the prop, call that thrust power or delivered power. There are then mechanical losses between the prop and engine, and you get to brake or shaft power provided by the engine. (I've simplified / combined some things here, but you get the idea.)

So consider two identical boats except for the props. Boat A is “properly” propped and Boat B has a similar prop except for the higher pitch (i.e. over-prop). They are moving at the same cruise speed, so resistance and the associated effective power are the same. The mechanicals on the upstream side of the prop are the same, so the losses there are the same. That leaves the PC to account for any differences going from A to B. I see three options: the PC increases, the PC decreases, or the PC stays the same.

If the PC increases, then the required brake power will decrease. This would not help (increase) with engine loading (but would be good for fuel consumption). If the PC decreases, then the required brake power will increase. This would increase engine loading (but would not help with fuel consumption). If the PC stays the same, then the required brake power will stays the same.

I contend it is that last one - the PC and therefore power (and loading) are the same (at least to the fidelity of this discussion). The only way loading increases is if PC goes down - why would you want that?

[Wandering1]

Lots of talk about engine hp but what about torque? The maximum torque in my 4.4L Perkins is at 1200rpm. We are over propped on purpose. Maximum engine revs is 2400rpm but we can only rev to 2000rpm. This gives us 8 knots in flat water. 41,000lb sailboat.
We never run WOT. Cruise speed is 6kts at 1200rpm. Top of the torque curve. Super fuel efficient and a happy engine.
Mathematically, horsepower equals torque multiplied by rpm.

[donradcliffe]

I also have to disagree with NC, when he cautions against the potential for damaging engines run at full throttle when moderately overcrowded. The real result is some black smoke caused by unburned fuel, and a higher potential for overheating in marginally sized cooling systems. Both of these potential issues can be managed by a careful operator, and wouldn't stop me from WOT when conditions warrant. For example, when the pirates were chasing me in the Red Sea, both our boats were putting out black smoke.

[malbert73]

Jerry Lee-

Thanks for pointing out my lack of clarity. I define loading as more closely matching HP produced by engine (at given RPM) to HP needed to drive the boat (or car/truck)

So, it’s inefficient and underloaded for an engine to be producing way more HP than needed at a given RPM, which is always the case and sub WOT when a prop is sized to reach max RPM.
So I assume the well proven fuel efficiency gains are due to reduced frictional losses at lower rpm

[dannc]

Per the article,

Figure 3 includes fuel consumption data derived from laboratory testing. The various contour lines delineate the amount of fuel burned to generate each kilowatt-hour (kWh) of energy created at the fly wheel at that particular engine speed and load. This is known as the specific fuel consumption (SFC). For example, if the engine is operating at 2,200 rpm and the load is 17 kW (the load imposed by a matched propeller at this engine speed), the SFC is between 260 and 270 grams per kWh (g/kWh). Let’s call it 265 g/kWh.

Can't get the graph to post so one will have to look at the previously posted PDF to see the graph.

For the over propped prop, the load is higher but using less fuel.

[malbert73]

Originally Posted by boatpoker

Coincidence ! We too are running a Beta 50 in our Benford 38 Fantail ... and are overpropped ... on purpose Our max RPM is 2400 which will take us 1.5nmph over hull speed. Hitting a WOT of 2800RPM would get us nothing but higher fuel usage. I think we'll keep it that way We normally cruise at just below hull speed at 1800RPM using 0.75gph.

Ha ha pretty much exactly my setup. Usually I cruise at 1850 and 7 knots, with EGT in low-mid 400s. Quiet and smooth, less than 1GPH
Efficient cruise setting is 1650 RPM, around 6.5 knots. EGT is in low 300s so on the underloaded side.
Motorsailing is a whisperquiet 1400 (which gets me to 5.5 knots without sails) and barely moves the EGT, so need to run it a bit harder after hours of this.
I can hit 2500 WOT.

Here's the amazing thing about the data you get from an EGT. Even overpropped- it takes until 2000RPM to get the EGT into the 600s (midrange load: 450-900 is the best operating EGT range I could find from Kubota). And, at that speed I am pushing a bigger wake than I want and I am nearing 7.5 knots. That wake costs too much fuel to generate. 2000RPM also gets me 6-6.5 motoring into 20-25 knots and choppy. No need for more. 2200 RPM got me off a midchannel sandbar easily (westport MA for those of you who have braved that).

I was truly surprised how underloaded an engine can be at slower RPMs even when overpropped (which for me is a combination of extra diameter and pitch). 50hp is pretty common in 40 foot sailboats these days and if they are all pitched to achieve full RPM it would take a lot of fast RPM, fuel burning speeds, to get them to load up.

[Lee Jerry]

Originally Posted by malbert73

Jerry Lee-

Thanks for pointing out my lack of clarity. I define loading as more closely matching HP produced by engine (at given RPM) to HP needed to drive the boat (or car/truck)

That's an unusual definition for loading. It seems more like the definition of "efficiency." The former would generally entail a reference to the maximum capability of the engine. Common metrics are power, torque and RPM. A quick internet search shows that one type of load gauge measures air flow and compares to max flow to indicate the load %.

Originally Posted by malbert73

So, it’s inefficient and underloaded for an engine to be producing way more HP than needed at a given RPM, which is always the case and sub WOT when a prop is sized to reach max RPM.

Inefficient yes; underloaded no (or not necessarily). Those two are generally (always?) opposites - the less efficient the more stuff (power, torque, whatever...) needs to be produced which will be higher loading; more efficient means less power and loading.

Originally Posted by malbert73

So I assume the well proven fuel efficiency gains are due to reduced frictional losses at lower rpm

I don't think we are ready or qualified to discuss the intricacies of engine design.

Originally Posted by malbert73

{snip}

Here's the amazing thing about the data you get from an EGT. Even overpropped- it takes until 2000RPM to get the EGT into the 600s (midrange load: 450-900 is the best operating EGT range I could find from Kubota). And, at that speed I am pushing a bigger wake than I want and I am nearing 7.5 knots. That wake costs too much fuel to generate. 2000RPM also gets me 6-6.5 motoring into 20-25 knots and choppy. No need for more. 2200 RPM got me off a midchannel sandbar easily (westport MA for those of you who have braved that).

I was truly surprised how underloaded an engine can be at slower RPMs even when overpropped {snip}

Yeah, it's almost as if over-propping doesn't really change the engine loading at all, which is determined by the resistance.



So I looked at the Dashew reference you gave above. It consists of ~1/2 page where he states the same as you (or more likely, you as him) with no real discussion or justification (which is probably why you don't either).

Here are some extracts of what he says:

"The key is to be sure to always run at no more than 80 percent of the engine’s available power."

"Here’s how this works. You want to momentarily find the point at which the engine begins to “lug,” or becomes overloaded. {snip} Once this point is established, back off 20 percent on rpm, and you’ll be at about the 80 percent point on available power."

So, starting with the gross error in the latter: for a displacement boat, a 20% reduction in RPM is NOT a 20% reduction in power. A 20% reduction in RPM from max would be ~45% reduction in power. You can see this in N Calder's "brilliant" prop curve demonstrations. A 20% reduction in power would be 7-8% reduction in RPM.

Now, following his advice, even the 20% power reduction (and not 20% RPM reduction), that seems like a pretty big hit to me. That would mean your Beta 50 instead of losing just a couple HP by over-propping, you are now losing ~10 hp.

That doesn't seem like "no downside" to me. I'd say your "experts" are 0 for 2.

And with that I am done. You give little indication that you have even read my posts let alone understood them, so there's no point in continuing further.