502 power VS torque

Automotive powered airboat discussion.
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Slidin Gator
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Re: 502 power VS torque

Post by Slidin Gator »


I hate to push too much info at once, and this is obviously a topic that should be on the prop forum. But since you brought it up...

The blade area scaling also applies to multiple propellers configured so the props don't interfere (such as a multi engine aircraft as opposed to a contra rotating prop system). You can "roughly" compare a single 100" prop to 2x 71" props as being equivalent(100^2 = 2x 71^2).

Although this is not covered in the thread, Water Walker's Dual Sync concept boat made use of this principle I am sure. I can't comment on how the overlap between the 2 props impacted this relationship.

Referring back to the prop scaling chart again, look at the 72" prop curve. Per the graph, that prop requires 220 Hp to generate 1000 lbs of thrust and 600 Hp to generate 2000 lbs of thrust. So, according to that graph, if I take 2x 72" props and tie them to a single engine, I will get 2,000 lbs of total thrust for an input of 440 Hp.

Now look at the 100 inch prop curve and note that it too produces 2000 lbs of thrust for an input of 440 Hp.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.

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Re: 502 power VS torque

Post by Deano »

Deano wrote:
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.
First of all, I want to clarify I had no intent there other than to find out what you knew that I didn't. :) Evidently it wasn't there.
Since you evidently don't see the problem with that blanket across the board statement, I will point out that said statement would be correct had you stated that "A larger prop CAN make more thrust with less hp" .

The way your above quoted statement reads, it would imply that if a man had a given prop and a given engine that couldn't quite turn it, then if he got a larger prop, he would be able to, and he would generate more thrust by doing so (all without doing anything to the engine)! That would be very cool indeed, but I think we can agree that isn't likely to happen.

Now, had you stated that a larger diameter prop is typically more efficient that a smaller one, not only would I have agreed, but many people that have been here for a long time, would've seen that as a confirmation of what has been promoted here for years. Take a quick glance at this search, and you should be able to tell pretty quick that you are preaching to the choir, so to speak:
search.php?keywords=%2Bdiameter+%2Btrum ... mit=Search

I have no desire to beat a dead horse, but you seem quite unaware that graph you so readily fall back on is based on theory and does not reflect reality in the airboat world. I am not saying it has no value, but it looks to me like you are giving it more credit than it is due. For example, the most thrust recorded on an airboat to my knowledge was 4 lbs Thrust per HP. That was the benchmark for many years, if it isn't still. That graph shows 220 HP generating 1000 lbs thrust, which is nearly 4.5 lbs Thrust per HP. I have no idea if this really occurs elsewhere, but it does not happen in the real air boating world. Consequently, everything that you derive or expand on based on using that impossibility is little more than useless nonsense. In the real world, I think you will find the neighborhood to be more like 2.5 to 3.5 on most geared V8 applications. Typical airboats built, used and operated by the vast majority of people here, do so in the real world and not with Alice In Wonderland or somewhere Over the Rainbow where airboats turn 100" diameter propellers and produce infinite thrust at zero RPMs.

Back on page two, Russ posed the question "to explain, exactly, how any engine with more torque but less power will "move a boat" better than an engine with more power and less torque. You said you would take the bait, and attempted to, but after a few thousand words, still had not. I did see later that you figured he owed you a beer for your answer, and after re-reading your response I still couldn't find it.

I asked you show real world proof of your claim without graphs or theorization and your response was with a graph you made using numbers derived using formulas from a website about Aeronautical Theory. There were a couple other links about "theory" and while they were interesting reading, didn't answer the question factually but with more theory. You also included as proof some unrelated crap from a gearbox vendor that amounted to a commercial for newbies. I can not fathom how that was even applicable to the question, let alone shows proof of a larger prop makes more thrust on less hp.

I suggest you search the board for the thrust testing that was done that includes real world numbers. You will find that those numbers will make that graph you are so fond of look at least a little bit silly. You seem very prone to making things much more complicated than they really are. The length of your answers and what they contain make it seem as though you are trying to convince yourself rather than somebody else.

I will endeavor to stay out of this thread now as I lack the time and/or desire to have this debate with you.
Just for shits and giggles, here is a comment by a guy with mucho credibility among the masses here:
Waterthunder wrote:It's simple physics 101. The bigger the prop the less efficient it will be. Also the wider the prop the more induced drag it has. A small two blade A/C prop by far will be the most efficient. I bet you can take a mini airboat Koller lawn mower motor and win the most pounds of thrust per HP.
A wiser guy than me observed, "It's not necessary to build graphs or swimming pools to determine that a bowling ball will not float."

For the original poster, here is a similar drive line with thrust numbers.
DAGWOOD0520 wrote:You guys made me go dig up an old photo of my first boat, I thrust tested it at least 20 times and the most i ever got out of her was 1580 and that was with a ZZ502 with a 2to1 gear and a 84" 4 blade K turning 4950 on the motor. The only side affect of a 84" prop with a 2 to 1 was that it was a loud sob when you stood in it, but it would run.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

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Re: 502 power VS torque


Just a real world comment for those who do not have field experience and may be a little confused.

A large wide prop will bite and push at a lower blade speed and will require enormous power at the top end to generate speed or accommodate added pitch. While some may debate such a simplified explanation think of the large wide blade such as the EX, S or Maximus as a tractor tire. It will push a big or heavy boat better as a generalized observation. You don't see race boats spinning these style of blade they tend to be a special purpose prop.

The narrow style blades and some of the new generation designs tend to spin up requiring less power down low a favor to an engine with a weaker torque output and make their power at a higher rpm range. They will then load and generate their thrust output with a better ability to produce speed or thrust at a high blade rpm. Think of this design as a sports car tire.

The manufacturers all make a blade design that falls in the middle or is a hybrid, speaking with the sales staff or boat owners with a similar combination as what you are developing will afford you excellent guidance. I would encourage a field observation to actually observe the combination firsthand if that is possible.

The most important thing is to match your engine to your drive ratio with your selected prop rpm range and use as the end game.

The combination is a triad in which a mismatch of the components will cause decreased performance.

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