502 power VS torque

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Williamalvin
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Re: 502 power VS torque

Post by Williamalvin » Fri Mar 02, 2018 5:14 pm

As I would love to read your attachment and comment why they state what they do it has not openness in multiple attempts.
Again Work=Force X Distance so work is not power.
I hate quoting wiki but look up thrust on wiki may help.
Understand I never said torque does the work I said force(thrust) does and as in jet engine you can have power without thrust until air direction is changed creating it.
It only seemed that people were talking about max hp at 5500 rpms to set a boat up with and contrary option was look at the whole range.
I would think with an Ecotec you completely understand the concepts that are being discusssd as that has good torque curve which will allow to spin the prop up to max rpms faster.
By the way “spring” forces or jerking get the same work done faster than constant forces which is the point.

Lots of these things here are controlled by one another and doesn’t matter here, but the intent is to perform across the board

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Re: 502 power VS torque

Post by Williamalvin » Fri Mar 02, 2018 5:16 pm

Correction not faster with less work

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Re: 502 power VS torque

Post by airduds » Fri Mar 02, 2018 9:05 pm

OneBFC wrote:
All your low rpm torque translates to is.....power available at that rpm.

Torque does ZERO work. Power does work. Power makes thrust.
And..... GA-BOOM!!!!

That's it in a nutshell.
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Re: 502 power VS torque

Post by CarMotorBarge » Sun Mar 04, 2018 7:52 am

Here are 2 EcoTurds running the hill for miles. One even has a single rudder. Remember these are low torque motors. :stirpot: :stirpot: :stirpot: :stirpot: :stirpot:

https://youtu.be/uH4RKCIW1mA

I doubt those motors got 6 MPG doing that. 8)
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Re: 502 power VS torque

Post by airduds » Sun Mar 04, 2018 9:59 am

Abrams M1 tank engine peak torque is 395 lb-ft. At peak hp torque is 275 lb-ft.
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Re: 502 power VS torque

Post by CarMotorBarge » Sun Mar 04, 2018 10:00 am

I can promise you the HSP isn't lower than the torque on those 2 EcoTurds. I would also say that the EcoTurds are complete opposites from a cast iron A/C motor (aka Cadillac). :shock:
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Re: 502 power VS torque

Post by OneBFC » Sun Mar 04, 2018 10:17 am

2200 lbs is light weight? Sure, for a tour boat.

2.38 ratio is a big gear? Though I'll admit my particalur engine could use more gear because it makes best power at 6200 now with the bigger turbo. I just don't want to spend the $ to change to a 2.7. Happily accepting donations from the torque club as gratitude for the HP lessons though! :lol:

Power on both of those boats in the video is higher than torque and they both make best thrust above 5250. At no point do they make as much power as even a mild LS build until above 3000 RPM. Good thing they don't need to, because the prop doesn't need any power below that RPM either.

No harm in building an engine with good low RPM power, but, to spin a prop, you only need so much to get the job done and biasing you bulid for midrange on up will get you better overal performance and "push" for your $$.

BTW, the boat that was behind the boat taking the video burned 18 gallons on that ride. 520 to south end of Lake washington and back. From where the video starts to oak head the boat was on DRY LAND 85% of the time on the way there. I think the little EcoTurds do pretty well as an overall total package. Sure, they don't make LS power, and never will without spending similar $, but for loads below 2500lbs I think they can hold their own.

Anyway, this thread isn't about the little engine that could, it's just about realising that Power is what drives you. Even if you choose to call it torque, in reality it's still Power and the prop doesn't need much of it at low speeds so biasing the build for power in the low end is largely wasted $.
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Re: 502 power VS torque

Post by Andrew McD » Sun Mar 04, 2018 10:45 am

Killer vid Russ
Wonder how them ecoturds stack up on a thrust tester vs a conventional LS.
I’m betting you know that already though. :stirpot:
Food for thought, nowhere in the design files for the Falcon or Maximus Series blades are Torque numbers a notable mention in foil selection. :scratch:
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Re: 502 power VS torque

Post by OneBFC » Sun Mar 04, 2018 1:12 pm

ladyblackwater wrote:Just because a motor can make HP at 6000+rpm doesn't mean it's a good thing to run it that high. I'd like to see you run 6000rpm all day long every time you take it out and let's see how long that motor will last you.
Why would anyone need to run their engine like that?
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Re: 502 power VS torque

Post by OneBFC » Sun Mar 04, 2018 1:51 pm

What does this prove to you in your mind?

At peak torque at 4500 rpm that stock engine makes 253hp with a rough thrust potential of 1000 lbs.

At 5600, with 40lbft less torque, it makes 275hp with a rough thrust potential of 1100 lbs.

If you geared it such that you only turned 5200, it would make the same 253hp as it does at 4500 but with less torque and less stress on the motor than running it at 4500 and make the same potential 1000 lbs of thrust.

The cylinder pressure at 4500 is much greater than at 5600 or 5200 and thus the stress on the motor is higher in many regards. The propensity to get into detonation or preiginition is higher the more torque you generate.

Power spins the prop. Prop doesn't need a lot of power at low RPM. Running an engine at its peak torque is not conducive for longevity or best performance. A double lose situation.

Why are you so intent on fighting hard facts? It's bizarre you keep ignoring all the data.

At this point I don't even know what you are really trying to make a point on. If I had to guess you are stuck on perceived loss of acceleration from using an engine with modest low rpm torque ability. But, it's just not an issue in real world use in my experience.

I will give you that if you want to maximize how quickly you can increase the engine rpm from low to mid range, you want as much power down low as you can muster. This usually comes at the cost of being able to make power at the mid to high end however and so you will make less thrust overall.
-Russ
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The only thing stopping you is FEAR
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Life begins at 2 BAR, Just a good ole boy

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Re: 502 power VS torque

Post by CarMotorBarge » Sun Mar 04, 2018 2:28 pm

I like how lady blackwater ignored what Andrew said. Probably makes sense to listen to the prop manufacturer.
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Re: 502 power VS torque

Post by digginfool » Sun Mar 04, 2018 5:51 pm

airduds wrote:Abrams M1 tank engine peak torque is 395 lb-ft. At peak hp torque is 275 lb-ft.
And by the time you gear down the RPM from the gas turbine running, what, 30,000, to a usable value I'll bet the torque is 10 times that value before it hits the tranny.

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Re: 502 power VS torque

Post by diamondback0320 » Sun Mar 04, 2018 7:33 pm

Been awhile since I've followed a soap opera lol

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Re: 502 power VS torque

Post by br,lc,LA » Sun Mar 04, 2018 7:38 pm

I have read every post... this is all hilarious!

Here is how you solve the problem...
1. Get a gearbox that puts the prop at peak efficiency when the motor is at peak torque... measure the thrust (good luck finding that box bc those who made it are after your money not after giving you a boat that can perform)
2. Set up a boat correctly and set the prop to max out and peak HP and measure the thrust...
3. Plot thrust against TQ and HP accordingly... omg its a miracle that the thrust curve is directly proportional to the HP curve and is a first polynomial (linear) function of the TQ curve... that polynomial being rpm.

To add to more... larger wheel turbos like higher rpms... which is why OneBFC says he want to run greater than 6000rpm... the stock turbos are smaller wheels on the 1.4T and the 2.0T so they are happier at lower than 5000rpm...

Yes I am an Ecotec guy... but engine is an engine... doesnt matter if its old school small block, big block, Ls, or Ecotec... HP is the key indication of making a boat move... but the catch is the entire HP curve... quit looking at a single point... TQ is an indication of higher power at lower rpms... but if that TQ falls off rapidly your engine is gonna be shit for a "mover" bc it is going to make crap for peak HP...

Look for flat to rising TQ curves bc it indicates more HP under the curve. HP under the curve is what you care about and set your boat up to run more towards peak HP... if you want a lower cruise RPM then back it down some, but do not lug the motor way down in the peak TQ range.
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Re: 502 power VS torque

Post by OneBFC » Sun Mar 04, 2018 7:56 pm

ladyblackwater wrote:OneBFC you haven't given many facts at all. You have given your opinion.
Brother, I have given absolutely nothing but facts here and provided links to the reference material. Let's just list here again, just so there is no way to mistake it.

ALL of these I have posted in this one thread:
  • 10. Last, but in no way least, all of the actual real world and in person test data I have collected on my own boat over the last 6 years that is in no way biased or modified in any way from actual test measurements
ALL of the above links support my position of POWER is what is important and what produces thrust.

How about you tell us what part of the above information is just "my opinion" and not verifiable fact?

I'll happily stop posting in this thread the moment you stop trying to convince people Torque is what makes Thrust or you provide some type of proof that it does beyond "I've been doing this my whole life."
-Russ
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The only thing stopping you is FEAR
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Re: 502 power VS torque

Post by OneBFC » Sun Mar 04, 2018 9:03 pm

I think there is nothing further to be gained knowledge wise for others in this thread if I keep responding to your posts ladyblackwater.

I have made my case clearly and an impartial reader has plenty of reference material to explore to decide for themselves. The water is right there, here is a glass if you care to drink it.

I wish you well with your boat building business.

:salute:
-Russ
-----------------------------------
The only thing stopping you is FEAR
400+hp Ecotec, 12x7.6 DBDO, 80" 3B Maximus, 2.3 OX,85+mph, water = purely optional
Life begins at 2 BAR, Just a good ole boy

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Re: 502 power VS torque

Post by Deano » Mon Mar 05, 2018 10:31 am

Intentional or not, there is a good, realistic overview of this entire thread in LBW's second to last post:
ladyblackwater wrote: That's right it's on the internet so it has to be True. I'll leave it at that. . .
This problem is compounded exponentially when someone starts with a theory (that may or not be correct to start with) and then starts using fictitious/abstract premises and graphs to prove fictitious/abstract presumptions, that ultimately lead uninformed people to fictitious/abstract conclusions about concepts that are in no way fictitious or abstract. This is not to say or imply that any given information was/is wrong in every case, but may have been be presented in such a way so as to foster rather than eliminate confusion. This is a realistic drawback of the internet that leaves the reader with the responsibility of having to discern who and/or what to believe. Welcome to the world of unlimited information.
ladyblackwater wrote: . . . I've said from the beginning and have said it many times in this Thread you need BOTH, one is no good with out the other.
To this, I would add that not only do you need both, but you CAN NOT have any power without first having some torque.
Horse Power is defined as Torque x Time (rpms)/5252, this is a fact. Horse Power is also a derived number, even on a dyno; which measures torque output and then derives the resulting HP using that formula. This is why at ANY GIVEN RPM, adding torque will always result in added horse power.

This whole debate has been little more than mental masturbation without a happy ending. My hat is off to whoever really read ALL of this mess, I tried multiple times and even though I understand the subject, it was confusing me just trying to visualize what some people were trying to convey.

While IT IS true that torque is a measurement and not an action, it IS a measurement of ROTATIONAL FORCE which is ultimately what you are measuring and what the propeller needs to generate thrust. This is why the prop doesn't care about what gear is used (and Russ' insistence that power is what matters). Everybody is right !

With all that having been said, I will point out a repeatedly re-occurring misconception in this thread, pertaining to thrust.
Every blade design in existence will produce a DIFFERENT thrust curve even on the same engine, let alone different engines.
The presumption that All thrust thrust curves produced are the same is analogous to all camshafts producing the same torque curve.
Pure folly . . and of less than no use for anything constructive or comparative.

Any given thrust curve was produced by a specific prop (blade set) which is based on the power applied to those blades at a given Prpm.
(Note that this has NOTHING TO DO with the engine supplying the power or any gearing involved.)

You can NOT take one prop's thrust curve and apply it to whatever engine you happen to have numbers for UNLESS you are comparing engines and not propellers (read blade sets). To do so, only promotes more confusion and moves everybody away from a happy ending.
Last edited by Deano on Mon Mar 05, 2018 1:18 pm, edited 4 times in total.
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Re: 502 power VS torque

Post by CarMotorBarge » Mon Mar 05, 2018 1:12 pm

ladyblackwater wrote:13060.png. Here is my actual Dyno sheet. This is not a build one motor and use that Dyno sheet for everyone I build. I'll have to dig through my files to find my graph and when I do I'll post it. Now around 4000 rpm my boat will throw you to the seat, but wait how is that when the HP is much lower than the torque :scratch:
Ladyblackwater,

I was looking at your dyno sheet. I think you run your boat at 4800 at WOT. Why didn't you have Mike move the torque curve to the left so the max torque was at 4200 instead of 4800?
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Re: 502 power VS torque

Post by br,lc,LA » Mon Mar 05, 2018 6:32 pm

according to the dyno sheet referenced you are still climbing in HP even out to 6300 so you are leaving some power and therefore thrust on the table... but the difference since the sheet stops at 6300 is only 25HP+/- there may be more past that... cant tell unless it was dyno'd out there.

Should there ever come a time you do get in a sticky spot (at 750HP I hope you dont find it) you can always back a degree or so out of the prop to spin it up a bit more if you have room to spare in the max prop limits.
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Re: 502 power VS torque

Post by Slidin Gator » Mon Mar 05, 2018 8:47 pm

Man, a lot of stuff happens around here in one weekend!

Deano,
Thanks for laying down some rules! It is not my intention to confuse; although I admit what I am suggesting can be difficult to grasp. Now stirring the pot every now and then, count me guilty. What I have been trying to show is why a wide torque band is a key element of driving the prop and why one should pay attention to the whole curve, not just the place it meets a prop thrust curve. As you stated, the important point is what is applied to the prop, so the actual engine/gear platform does not matter, as long as the output to the prop meets the needs. Now, which platform is the best choice to do that is a whole other discussion.

I can assure you that the dynamics I describe do occur and I think DigginFool summarized it as well as anyone:
digginfool wrote:Probably one of the easiest ways to reconcile the difference between HP and Torque is torque accelerates, HP holds.
Mr. Williamalvin summarized why I find dynamic torque/thrust important, more so for smaller boats. He is also correct that I am long winded… I am using portions of both of his posts:
Williamalvin wrote:I know there has to be one engineer in this thread and it sounds like explanations have been poorly conveyed due to excessive explanation.

Second let me put this in simple perspective. If you slowly lean your weight onto a car it will more than likely not move, but if you fall into a car with same person it will shake because you have accelerated the mass.

By the way “spring” forces or jerking get the same work done faster than constant forces which is the point.

Lots of these things here are controlled by one another and doesn’t matter here, but the intent is to perform across the board
I described it as linear to simplify, but you are absolutely correct that this is very prop dependent and will vary by application. Honestly, I would like to see dynamic testing added to some thrust test runs, not simply to demonstrate it, but as a measure. I don’t believe I said on here you “should” set your prop at maximum torque. If my writing came out that way it is wrong, quite the contrary. By moving maximum speed/thrust out further to the higher Hp area of the curve there will be more available (peak) torque in the working range of the engine, so it is a win win, up to the point that one wants to run the engine and assuming normal operations are not miles away from the extreme.

But I’m not going to beat that dead horse any further. I think we got a great summary yesterday on what the goals are in matching engine to gear to prop
br,lc,LA wrote:I have read every post... this is all hilarious!

…Yes I am an Ecotec guy... but engine is an engine... doesnt matter if its old school small block, big block, Ls, or Ecotec... HP is the key indication of making a boat move... but the catch is the entire HP curve... quit looking at a single point... TQ is an indication of higher power at lower rpms... but if that TQ falls off rapidly your engine is gonna be **** for a "mover" bc it is going to make crap for peak HP...

Look for flat to rising TQ curves bc it indicates more HP under the curve. HP under the curve is what you care about and set your boat up to run more towards peak HP... if you want a lower cruise RPM then back it down some, but do not lug the motor way down in the peak TQ range.
Swamphunter45 also provided a good summary early on of what to look for in a motor:
SWAMPHUNTER45 wrote:Don't buy in to a big horsepower number on a dyno but look for a balanced hp/torque curve that gives you big torque and big horsepower together.

To buy an engine that makes only a big horsepower number up top will be a pig on an airboat when your trying to launch or cruise in an efficient range. Your not going to want to run it at 5,500 rpm all day.
CMB added the following, which actually applies to any target RPM range:
CarMotorBarge wrote:If you want more snap, you will probably have to give up some HSP at 5400 to make more torque down low. Reducing HSP at 5400 means you will turn less prop. Ultimately this trade off between HSP (bigger prop) and torque (snap) has to be made by every engine combo. On a bigger boat I personally would want a bigger prop instead of the snap. Hope this helps.
I agree with Mr. CMB that a bigger boat is probably better off with more maximum thrust vs. low end “Snap” or acceleration thrust. This is because the large boat is not going to react as quickly to any condition changes (thrust or steering) anyway, so prop dynamics have less of an impact either way. It is a balance, torque at the prop is most important over the typical power band, roughly the top 1/3rd of the RPM range.

The example I posted previously regarding CMB’s system was simply intended to show how one can “give up something for nothing". If they are not careful in matching everything. The purpose was to show how one change, not properly considered can have the opposite effect of the goal. In that case, without Mr. CMB making the decision to start operating his engine well above 5,350 RPM, and changing gearing and/or prop to match, then a modification that is sold or envisioned as increasing Hp actually results in money wasted and poorer overall performance.


A larger prop makes more thrust with less Hp but it will run out of steam at speed quicker. I believe the prop scaling chart that has been posted previously showed that relationship well. But we are working within limitations and for the purposes of focusing further review I intend to compare engine platforms against the model I built for CMB’s prop and engine/gear. This is probably as big of a prop (diameter) as the Nolaboat can fit, so it should give a reasonable estimate of potential thrust. In any case it provides a fixed variable to compare the different engine/gear combinations against.

So let’s get to meat of the original question posed by Mr. Nolaboat 7 pages of BS ago:
nolaboat wrote:Question for the group. I have a 21ft dixie airboat. I m planning on putting a chevy 502 in it. what kind of power do I need to push this size boat?
The contenders I have to work with are:
• 427 LS7 (Per Mast Dyno Curves)
• “Torque” ZZ502 build
• “Hp” 502 V3A build
• Ladyblackwater’s stroked 555 dyno data

Image

Image

I think these curves speak for themselves and it should be pretty obvious which motor can swing the most prop. I think it is also pretty obvious that the ZZ502 Torque build lags way behind all the rest and I will eliminate it from further consideration. But, I do find it interesting how close the LS7 data matches up with the Hot Rodded Hp 502.

I am going to use Ladyblackwater’s engine/gear figures as an example of a great running, big boat engine/gear for comparison against the contenders. LB stated that he runs the engine up to 5,400 RPM, which is 750 Hp from the Dyno figures. I do want to confirm from LB that the boat is running a 2.37 ratio gear box, which would put maximum prop RPM at 2,280, which is right around the maximum for an 82” prop. Just for the record I calculate Mach at 74% at 2,300 RPM.

I have prop torque figures to post next, just a bit more work to do. Suffice it to say that either the LS7 or 502 V3A are going to have to turn some rev’s to play in the same ball game.
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Re: 502 power VS torque

Post by Deano » Tue Mar 06, 2018 12:38 pm

OK. Timeout, back up the bus . . .
Before we go down the rabbit hole next to the one we just climbed out of and end up 5 generations of graphs and two pages later,
I will point out that this statement (particularly the first half) is just plain wrong.
Slidin Gator wrote:... A larger prop makes more thrust with less Hp but it will run out of steam at speed quicker ...
I challenge you to show proof of this with facts that need not be graphed or theorized about, but can be found at credible sources.
If you can, I will readily admit that I am wrong and I will have learned something I didn't know, which to me is always a good thing.

If you misstated your intent (as I suspect you did), clarifying what you did mean will help aid our understanding.
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Re: 502 power VS torque

Post by Slidin Gator » Tue Mar 06, 2018 1:03 pm

Which part are you questioning? The less Hp for more thrust, or the running out of steam portion. Or both.
I grew up thinking I-10 was the Mason Dixon line.
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Re: 502 power VS torque

Post by CarMotorBarge » Tue Mar 06, 2018 2:00 pm

What do you mean by run out of steam quicker at speed?
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Re: 502 power VS torque

Post by br,lc,LA » Tue Mar 06, 2018 4:01 pm

What I have found through many discussions is that it isn't really the large prop vs small prop... its actually air flow to the prop.

Larger props move the location of grabbed air further way from all the rigging and crap that can restrict flow from getting to the prop and essentially loading the blade and motor, therefore absorbing more power and making more thrust.

If you've ever had nearly two identical set ups (motor, box, and prop) and one boat can spin a several degrees more pitch than the other then you've witnessed this also... the boat that can spin more pitch likely has a ton of boxes and seats that block airflow so more pitch is needed to make up for the flow restrictions on the low pressure side of the prop ... pitch is required to grab more air and create the same equivalent compression across the blade... tight wire on the cage is the #1 restriction most experience as it has a two fold affect of both drag and restricting airflow.

i've always been curious what the pressure rise across the prop actually is before it becomes free air behind the boat. measure thrust and you can know what the pressure is pushing against the theoretical blade disc area created by the prop spinning and calculate the pressure built behind the boat. but that is about as close as you'll get unless you are in a lab!
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Slidin Gator
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Re: 502 power VS torque

Post by Slidin Gator » Tue Mar 06, 2018 5:31 pm

Sorry guys, I re-read Deano's question a 3rd time and better understand the question.
br,lc,LA’s information above is all correct and certainly a bonus to the following. No charts for now, but some typing.

The first part of the statement, but more clarification.
A larger prop makes more thrust with less Hp
This statement applies to propeller scaling rules. Take propeller A of a given design (generic 2 blade, 72" prop for example). Scale that design up in all dimensions to make propeller B (now a 2 blade, 82" prop). For a given Hp input, propeller B will operate at a lower RPM, higher torque operating point. But, the 2 numbers do not scale the same, such that total static (boat not moving) thrust, for the same Hp will be greater with the larger propeller. This is the basis of a statement I made originally in this thread:
Infinitely large prop, with infinitely large torque will producing infinite thrust at 0 power.
Now I know you don't want me to go into the math and much of this makes my head spin too. I will also fully submit that there are many more variables involved in propeller design. But these relationships do hold up as long as every other variable is constant.

First two examples. Helicopter propellers have a much different job to do than an airplane propeller. A Helo needs to hold station, which is the rough equivalent of an airboat stationary on ground. An airplane prop needs to produce enough thrust to take off, and then cruise efficiently. A helicopter blade is “almost” always a long (large diameter) 2 blade propeller because it converts Hp to thrust/lift more efficiently. But an airplane has limits on diameter (like landing) and operates at high advance rates (flying speed). Providing thrust at 130 mph airspeed requires a smaller prop (more on this below).

As another comparison, a tug boat needs to make maximum thrust at low speed, but a race boat needs to produce the thrust at high advance speeds. Even if they are both 3,000 Hp machines, the tug boat will have a huge prop while the speed boat prop is orders of magnitude smaller.

The first reference I will cite is the following statement on Hoverhawk’s website:
http://www.hoverhawk.com/rotatorgearboxes.html
An engine with a geared reduction drive unit will swing a larger diameter propeller, a propeller with more blades, and with a greater angle of attack (more pitch). All of which generates more thrust per HP, than going "direct drive" from the crank shaft with a propeller.
Now to be clear, adding blades actually decreases efficiency, but it is the only way to keep adding power once the design hits some limit on prop diameter (boat width in our case).

This reference gets technical real quick. Don’t bother reading past page 1, unless you want to take a nap. But he does have a “smart” hat on!
http://www.nar-associates.com/technical ... screen.pdf

Now the following link has already been posted previously and is the basis of the propeller scaling chart that I posted previously. It would be in my last post on page 6 of this thread. I’m not going to claim this particular formula is more or less accurate than any other set of formulas. But it does result in the graph posted, is on the "Internet" and was cited originally by OneBFC as a reasonably accurate formule, which is why I used it for further discussion. I will note that this graph applies to 2 blade props and will over spec thrust as blade count increases. As I noted previously, I am happy to share the spreadsheet for that chart to anyone, but the SA forum won’t accept Excel file types and I have less than 0 interest in joining Google! So PM me with an e-mail address and I will send it. Referring to that chart, look at the 2000 lbf thrust line. This shows a 72” prop requiring 600 Hp at the prop, while a 100” prop needs only 440 Hp (it's cool, I can buy my own beer).
http://www.heli-chair.com/aerodynamics_101.html

The final reference is way too much math, but please scroll down to the bottom where they show a graph of thrust vs. diameter for a fixed 400 BHP input. I included this link specifically to back up the prop scaling graph relationship.
http://www.jefflewis.net/aviation_theor ... p_eff.html

I am happy to discuss/debate this point further if needed.

Now to the second part of the statement:
but it will run out of steam at speed quicker
This is covered to some degree in the links, but this is actually the simpler part to understand. With Hp fixed, as prop diameter increases the prop is grabbing and accelerating more air. Since power is fixed, the air is accelerated less (more mass, less acceleration), so the exit velocity of the air is lower, or more specifically, the change in velocity is lower. As the boat (or plane) starts to move forward, the air feeding into the prop hits at higher velocity so the propeller has to spin faster just to keep up with the incoming air, and make output thrust. For a given amount of thrust, the prop needs “approximately” a constant torque, but since the prop needs to spin faster just to keep up with the incoming air speed it consumes more power. Drag on the larger propeller also increases with speed more rapidly, adding to the torque load. It's the same thing as swimming upstream.

So my point about running out of steam is comparing thrust vs. fwd speed (input power fixed), the larger diameter propeller has a steeper slope and hits 0 lbs thrust at a slower boat speed than a smaller diameter propeller.

This means that we need different props for different purposes, which is exactly what the prop vendors are selling. A high speed, glades grass runner needs a different prop than a ground running bulldozer.
I grew up thinking I-10 was the Mason Dixon line.
1986 Airboat Engineering Inc., 14' Marsh Master. Refreshed narrow deck, SV O-540, 72” NGQ. A Bob Stossel original.

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