502 power VS torque

Automotive powered airboat discussion.
nolaboat
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502 power VS torque

Postby nolaboat » Tue Feb 20, 2018 11:56 am

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?

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Re: 502 power VS torque

Postby OneBFC » Tue Feb 20, 2018 12:23 pm

Assuming you run a properly sized propller and a hull that doesn't have problems, the formula below will result in a reasonable performing Airboat across various conditions.

(Total loaded boat weight) / 8 = (required minimum hp)

In terms of thrust to total weight then,

(Maximum static thrust)/(Total loaded boat weight) >= 0.5

How much thrust you get from a given unit of hp will vary greatly from setup to setup due to propeller and rigging choices.

Lastly, exactly how much thrust you need per unit of weight will also vary based on setup due to thrust center line, shape of hull, choice of hull bottom surface, etc.

So, the above two formulas are reasonable minimum targets. The higher the ratio of thrust to weight the better.

You have a large boat. So, don't skimp on power if you have the option not to.

Good luck with your build!
-Russ
-----------------------------------
The only thing stopping you is FEAR
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OneBFC
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Re: 502 power VS torque

Postby OneBFC » Tue Feb 20, 2018 12:37 pm

Don't worry about torque, make hp.
-Russ
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nolaboat
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Re: 502 power VS torque

Postby nolaboat » Tue Feb 20, 2018 1:03 pm

I havent purchased a prop or reduction gears as of yet. So Im open to suggestion on that as well.

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Re: 502 power VS torque

Postby SWAMPHUNTER45 » Tue Feb 20, 2018 5:27 pm

You should focus on a big inch engine that is purpose built by a builder who has a reputation for reliability on an airboat platform.

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.

I would suggest you consider a bigger displacement long stroke engine if running some dry or heavy loads. No disrespect to the 502 but a 540 or 632 may prove a better choice if within your budget. For me with a 542 Cadillac the 2.37 ratio mated to 4 Sensenich S blades has been a great asset but look around at other boats, talk to the engine builder and match the engine, prop and drive. The marriage of the three is critical for a great result.\

If you already are committed to the 502 then you only need to talk with the prop companies because your limited to what that engines output.

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Re: 502 power VS torque

Postby CarMotorBarge » Tue Feb 20, 2018 8:00 pm

nolaboat wrote:I havent purchased a prop or reduction gears as of yet. So Im open to suggestion on that as well.


Here is what to do:

1. Choose the engine and the maximum RPMs you plan to run at WOT. For the sake of the discussion below, let's assume a 502 BBC running at 5400 RPMs.

2. Because you have a heavy boat, you will want a wide or super wide prop such as an R, S, NGR, JR, JX, etc. If you make enough HSP, a 4 blade will push better down low than a 3 blade. You'll want to spin the prop at 2000 to 2100 RPMs. A pitch between 2 and 2.5 is a good place to be.

3. The gear ratio is simply the engine RPM divided by the prop RPM. In this case, 5400 engine RPMs divided by 2000 prop RPM equals 2.7. A 2.68 ratio would work good for this combo.

4. To maximize the amount of prop you can turn, you need to setup the engine (cam, heads, etc.) to make as much HSP as you can at 5400 RPMs. Don't worry about the torque numbers. They are what they are.

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.
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Re: 502 power VS torque

Postby OneBFC » Tue Feb 20, 2018 9:38 pm

ladyblackwater wrote:OneBCF HP. doesn't move a boat. Torque does, horse power sustains the torque. The best motor you can get is one that pretty much goes square around the Rpm you want to run. This meaning the HP and torque are about the same numbers at let's say 4800rpm (if that's the range you are looking for) If you have a motor that builds HP and hardly any torque or vice versa than you will have a useless motor for an Airboat.


Where to start....

Well, quite simply, your wrong here and some math will explain. I'm doing this not to put anyone in their place, etc, but rather to help keep a common belief that is compeltely wrong from spreading through misunderstanding how things actually work.

Engine A has 600 lbft of torque between 1500 and 3000 RPM and spins a maximum of 3000 RPM
Engine B has 400 lbft of torque between 2500 and 6000 RPM and spins a maximum of 6000 RPM

Which engine will produce more thrust to move "a boat?"

Given that Hp(power) = (Torque x RPM)/5252

Torque is a Force
To turn a shaft at a given speed requires Power.

(Force x RPM) = Power

To get to the standard unit of "Horsepower" we divide by 5252 as a constant.

If you want to understand how Horsepower is derived see herehttps://en.wikipedia.org/wiki/Horsepower

So, the answer to the above question of "Which engine will produce more thrust to move a boat" is the engine that makes the most POWER, which is engine B. Engine A is roughly 343hp. Engine B is roughly 457hp.

Engine B will move a boat better because it will make more thrust from a propeller.

Torque by itself means nothing. You can generate 10,000 lbft of torque at 10 RPM and it's still less than 20hp.

So, I challenge you, or anyone else for that matter, 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.

I look forward to the response!
-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

Postby SWAMPHUNTER45 » Tue Feb 20, 2018 10:47 pm

Really only 3 blades to consider in my opinion!

21 ft boat is gonna need big push to have any chance on the ground.

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OneBFC
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Re: 502 power VS torque

Postby OneBFC » Wed Feb 21, 2018 8:41 am

Well, a lot of assertions from your side ladyblackwater, but you have only presented opinion and not any facts or physical proof of some kind. Let's step through your last post a bit.

Please explain to me how an electric motor can do more work and carry a heavier load with less HP than an equivalent gas motor?


Well, this one is easy because I don't think you realize that HP is actually work. Therefore, any electric motor is providing the same power as any other type of engine to accomplish the same "work" and "carry a load". If you can point to a specific example of an electric motor requiring less power to perform the exact same task as another engine, I would like to see it.

Also the one with more torque is going to live much longer than the motor that is working it's butt because it has HP but not enough torque. It's called the big HP motor with little torque will be crusing and running at a much higher rpm and will have to run more rpm to stay with the motor that has good HP but equivalent or higher torque to the HP.


Well, again, this is a common fallacy. For any engine a higher torque equates to a higher cylinder pressure. Cylinder pressure is a major contributor to fatigue and component stress as well as overall wear. This is why you don't "lug" engines at low RPM and high load. It's why trucks down shift to go up hills. The cylinder pressure is reduced and the engine turns more RPM to make the required power.

It's better to run an engine outside of the area where it makes peak torque so the cylinder pressure is lower if you want to maximize longevity. If you fully load the engine where it stalls at peak torque the potential for component failure increases.

RPM is not the measure of how much stress or load an engine is under.

Lastly, the amount of power required for most boats is very low compared to the maximum output their engines can produce. "Cruising and running" along at 30 mph will only require 50 to 100 hp in most cases. Any engine, even tiny ones, can produce enough torque at a reasonably low 3000 or so RPM to sustain the amount of required power and have enough in reserve to still accelerate briskly to higher RPM when needed. 100hp is less than 30% of the total available power of most boats. Not really a high load, right?

So, please, give us some examples and perhaps a bit of math to support your opinions. I can't keep typing up detailed examples to counter opinions. If all you can provide is anecdotes and opinions, then we will just have to disagree.
-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

Postby SWAMPHUNTER45 » Wed Feb 21, 2018 9:00 am

We are back to the same circle jerk on whether it is severe duty, percentage of power and HP vs Torque.

The big or heavy boat guys see it one way the go fast or mini's see it another. Let's help this guy it's why I'm on SA.

Glass half empty vs Glass half full

Let's agree this fellow needs a big powerful engine with torque and horse power and a big azz wide prop or are we so far apart we can't do that ?

650 torque with 700 real hp would probably get him his best chance to fulfil his needs.

I'm not sure about a average 502 build but I'm concerned it may come up short. Probably be ok for pushing a in water displacement load but doubt it will move well at all on dry.

On another note can one of you fuzzy math geniuses educate me on the 5250 dynamic?

Why does horsepower "always" eclipse torque at an rpm above 5250 and as as such would torque not then be more relevant to usable power and therefore of more value in an airboat?

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Re: 502 power VS torque

Postby loudmouse » Wed Feb 21, 2018 9:37 am

If you want torque u need BIG cubic In or boost. Boost is the cheapest way on pump gas and can be dialed in where you want it. A 500+ CID with 10psi on pump gas will make 800+ pounds of torque down low in the rpm band. Just depends on when u set the boost to come in.
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Re: 502 power VS torque

Postby digginfool » Wed Feb 21, 2018 10:22 am

SWAMPHUNTER45 wrote:On another note can one of you fuzzy math geniuses educate me on the 5250 dynamic?

Why does horsepower "always" eclipse torque at an rpm above 5250 and as as such would torque not then be more relevant to usable power and therefore of more value in an airboat?


I'm not going to go into the derivation of the equations but there is a direct relationship between torque and horsepower.

Torque = HP x RPM/5252
HP = Torque x 5252/RPM

If you look at these two equations, you will note that there is only one point that Torque = HP and that will always be 5,252 RPM. That's why the two lines always cross at that point. 5252 is the product of all the constants that describe each of the variables at work. Torque is a measure of rotational force measured in units of distance of application and force applied. Once that force goes into motion, it becomes work, which is measured by the amount of work and the distance. HP is a measure of the rate of work applied. All of these have units such as ft-lbs, feet, seconds, etc. Once you put all the constants into the same set of units and calculate accordingly, you end up with 5,252, leaving just the variables (RPM, HP, Torque). Very simplistic explanation but that is basically how it comes about.
Last edited by digginfool on Wed Feb 21, 2018 10:23 am, edited 2 times in total.
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Re: 502 power VS torque

Postby RJC5778 » Wed Feb 21, 2018 10:22 am

With this size boat, I like to think that he would need a big inch motor with a high gear ratio and as long as a blade as possible DIA wise. this seems to be more of a work application rather than speed. he needs torque and more torque, essentially a large area under the curve, more thrust is to be had from more dia than from pitch. I would not recommend a crate motor especially the 502, the valve geometry is not the greatest from the factor where these engines live rpm wise in an AB application. purpose built (if in budget) is the way to go and focus on big torque. high ratio reduction and big, wide blades will make this boat get up and move. my $.02

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Re: 502 power VS torque

Postby digginfool » Wed Feb 21, 2018 10:25 am

Probably one of the easiest ways to reconcile the difference between HP and Torque is torque accelerates, HP holds.
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Re: 502 power VS torque

Postby OneBFC » Wed Feb 21, 2018 1:08 pm

That explains nothing. You are not willing to learn so I will let it go. No hard feelings.

: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

Postby CarMotorBarge » Wed Feb 21, 2018 1:17 pm

ladyblackwater wrote:OneBFC I have one question for you because I'm done going back and forth with ignorance. How many airboats have you own and built that were 18' or bigger? Maybe his best bet is to build a 800hp Ecotech for his boat LMAO. Explain to me how an AC motor works with HP and Torque. Yelp they are torque motors a 300hp O540 puts out around 700+ torque. This should explain it very well.


An A/C motor makes lots of torque and turns half the prop as a 600 HSP car motor. Should you put an A/C motor on a 21 foot boat?
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Re: 502 power VS torque

Postby CarMotorBarge » Wed Feb 21, 2018 3:00 pm

You can't run an A/C size prop on a car motor using a 1.7 or 2.0 gear ratio? Changing gear ratios doesn't make HSP.
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Re: 502 power VS torque

Postby SWAMPHUNTER45 » Wed Feb 21, 2018 4:24 pm

Thanks Diggin

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Re: 502 power VS torque

Postby Slidin Gator » Wed Feb 21, 2018 4:29 pm

OneBFC wrote:
ladyblackwater wrote:OneBCF HP. doesn't move a boat. Torque does, horse power sustains the torque. The best motor you can get is one that pretty much goes square around the Rpm you want to run. This meaning the HP and torque are about the same numbers at let's say 4800rpm (if that's the range you are looking for) If you have a motor that builds HP and hardly any torque or vice versa than you will have a useless motor for an Airboat.


Where to start....

Well, quite simply, your wrong here and some math will explain. I'm doing this not to put anyone in their place, etc, but rather to help keep a common belief that is compeltely wrong from spreading through misunderstanding how things actually work.

Engine A has 600 lbft of torque between 1500 and 3000 RPM and spins a maximum of 3000 RPM
Engine B has 400 lbft of torque between 2500 and 6000 RPM and spins a maximum of 6000 RPM

Which engine will produce more thrust to move "a boat?"

Given that Hp(power) = (Torque x RPM)/5252

Torque is a Force
To turn a shaft at a given speed requires Power.

(Force x RPM) = Power

To get to the standard unit of "Horsepower" we divide by 5252 as a constant.

If you want to understand how Horsepower is derived see herehttps://en.wikipedia.org/wiki/Horsepower

So, the answer to the above question of "Which engine will produce more thrust to move a boat" is the engine that makes the most POWER, which is engine B. Engine A is roughly 343hp. Engine B is roughly 457hp.

Engine B will move a boat better because it will make more thrust from a propeller.

Torque by itself means nothing. You can generate 10,000 lbft of torque at 10 RPM and it's still less than 20hp.

So, I challenge you, or anyone else for that matter, 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.

I look forward to the response!


I will take the bait here. I tried to keep it simple, but this is a bit long. If you want math to back this all up, I can do plenty of math, but I don’t want to complicate this discussion. I have been paid good money to apply the following information in practice, so I don’t know why I’m giving it away for free…

Torque = Acceleration = Thrust. It does not matter if we are talking about a car on the road, an outboard engine or an Airboat propeller. Power = Torque x RPM.

A propeller makes thrust, by accelerating air, F=M*A (Force = Mass x Acceleration). Torque is nothing more than force over a distance (moment arm). I will point out that it is not just a force as OneBFC stated. Torque is applied to the propeller and it in turn applies force to a mass of air. The result is acceleration of the air in one direction (to the rear of the boat) and a thrust reaction, pushing the boat forward.

A key relationship that everyone should note here is that force (thrust) is directly proportional to propeller torque, F=Y*Torque, where Y is a function of a particular prop, pitch, air density etc. This is a simplification since prop efficiency varies over the range of speed, but it is close enough for the range of normal operation.

Now, here is where the head/nut scratching, beer drinking and arguing comes in. Acceleration (of air) occurs in both the steady state (constant RPM) and dynamic (varying RPM) conditions! Think about that for a few and then read on.


Consider the situation of a propeller at constant speed, let’s say 2,000 RPM with the boat stationary on the trailer. In this condition, the propeller is accelerating a constant flow of air (mass/second) from 0 MPH in front of the propeller to some velocity out the back (100 MPH for round numbers). The prop is also consuming power in this condition because it has to run at some speed to maintain a constant acceleration of a mass flow of air over time. This is where power comes in. It is similar to accelerating a car at speed, since the wheels are turning, it takes power to keep applying torque (and accelerating). Performance curves for props show a certain amount of torque required to maintain XX speed, as well as the output thrust. Any engine/gearbox etc. combination that produces that amount of torque at the prop and at the speed will drive the propeller at that steady state speed.

Now consider the dynamic situation where the operator mashes the accelerator (to accelerate that is why they named it that after all). In this situation the engine now produces MORE torque than what is required to maintain steady state thrust and prop speed. A portion of that additional torque is applied to accelerating the engine, gear and propeller mass, but a majority is immediately applied at the prop to the air with the result being a higher acceleration rate (of air). With more air acceleration comes more thrust, and that thrust is immediate, before the prop even increases speed. Eventually the propeller speed reaches the point that the steady state air acceleration matches the new thrust and the propeller spins at a new, constant speed.

Pick any speed point on a propeller curve and overlay your engine/gear box output torque curve. The difference between the steady state torque requirement (for the prop) and the engine maximum torque (at that speed) represents the reserve thrust available on tap, instantaneously. That means a flat torque curve engine can produce (roughly) maximum thrust instantly, even if the speed is much lower that the matching steady state point on the prop curve. Prop charts represent the steady state condition only. They also provide insight to, but do not fully define the dynamic situation.

So, here is the key point to understand, the optimum engine/gear has a flat torque curve over the entire operating range. Obviously, the higher the prop torque, the better. Interestingly, this exists in the real world as an electric motor. This is why electric cars out accelerate internal combustion cars off the line. In a prop drive application, a flat torque curve results in the ability to produce maximum thrust over the entire speed range of the propeller (from 0 to maximum RPM). Call it stirring the pot, but aviation engines are specifically designed to produce a relatively flat torque profile over a wide speed range, which is one reason that they make good airboat engines. On the other hand, many airboats are outfitted with auto engines that were built with too much focus on maximum HP and little to no attention given to torque over the full range of performance. These engines typically have a rather steep torque curve (hump) that peaks out close to the maximum HP peak. They may produce a lot of thrust at wide open, but they do not have immediate thrust on tap over the full operating range. You can hear the difference in the woods, particularly when running dry. An engine with a flat torque curve requires limited accelerator input to account for changing ground conditions (slippery then sticky etc.) because the thrust is immediate and proportional to the accelerator position. As the engine and prop speeds up, thrust remains relatively constant without change to the accelerator position. This makes it easy to drive these set ups over ground since they are predictable and responsive.

On the other hand, an engine with a steep torque hump requires major accelerator inputs to account for changing conditions. For instance, the boat is running along at XX RPM, but the ground gets sticky. The operator mashes the gas to WOT to get all the torque that the engine can produce. But at lower speed this is limited, so the additional thrust is limited. The boat slows down (because of the sticky marsh grass) while the engine slowly builds speed. As the engine speed increases, the engine torque and thrust increases proportionally, even though the operator did not change the accelerator position. Now, all the sudden the boat breaks free, the engine starts to scream at maximum RPM, producing too much torque/thrust, the boat lurches forward and the operator has to lift off the gas. The operator then usually lifts too much, to the point of too little accelerator, so the thrust drops, the boat starts to stick again and the process repeats.

The name for these kinds of boats is “WaWa” because that is what they sound like in the woods! These are the boats whose drivers (you know who you are) are constantly on and off the throttle just to run a constant speed, particularly on ground. I often wonder how often the accelerator pump has to be replaced on these set ups since the constant movement has to wear these things out.

Everyone keeps referring to this as “Snap”, as in "my engine really snaps the prop." This is really an engine that makes good torque at lower speed, so it can accelerate the air and propeller. Now keep in mind that high HP, humped torque curve engines have their place, racing for instance, where they operate in the high torque area. This is why automotive drag cars use a high stall torque converter, so the engine can spin up into the high torque band before coming on line. The so called “Super Snapper” props are exactly the same thing as a high stall torque converter, they are designed to produce low thrust, and therefore require low torque at lower speeds to let humped torque curve engines spin up quickly into the high torque range of the engine. But for general use, riding, running ground etc., a flat torque curve produces flat thrust and throttle response and the thrust is instantaneous. If your engine can “Snap” a wide blade prop to speed quickly, it is producing good torque down low and probably responds well in operation.

Now, to wrap this up and finish winning the bar bet, take engine/boat A that produces 500 ft-lbs at 0-2800 RPM (humor me on the 0 RPM) peaking at 300 Hp and compare that to engine B. Engine/boat B produces 1500 Hp and 1000 ft-lbs at 8 billion screaming RPM, but it only produces 200 ft-lbs at 0 RPM. Put them side by side and mash the gas to hit peak torque right off the line. Assuming these engines could start right at 0 RPM, for a split second, the flat torque curve of engine A produces more thrust right off the line while the humped torque curve set up has a ramped thrust profile. Sure it eventually produces more thrust than boat A, but boat A already broke ground and started moving while boat B had to spin up the prop to break free. Now boat B is in run away mode and out of control, operator B lifts the gas and gets stuck again. The aviation boat just cruises right along while the humper riders get whiplash.

Further, at 0 RPM, applying torque to the propeller does produce thrust and it does so with no power. The power comes into play as the prop speed increases. A very smart propulsion engineer (now gone) once told me that he could produce infinite thrust with 0 power, all he needed was an infinitely long propeller, pushed by infinite torque. The 0 power would come in because the prop speed is infinitely close to 0 and without speed, there is no power consumed.

Swamp, Mouse, Ladyblackwater etc. are correct, a good airboat engine needs to produce torque over a wide performance range. OneBFC, the ECO has good lower end torque because of the boost available, but there is always some lag involved (argue if you want). Humper engines just cry - WaWaWaWaWa.

So, feel free to go back to arguing 4 cylinder ECO, vs. LS vs. big bore/stroke V8, vs. boost vs. my johnson is bigger than yours vs. blah blah blah. But do it while considering the above information. In the meantime, my O-540 will just cruise along fine, no issues, no argument.
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Re: 502 power VS torque

Postby OneBFC » Wed Feb 21, 2018 5:41 pm

So, feel free to go back to arguing 4 cylinder ECO, vs. LS vs. big bore/stroke V8, vs. boost vs. my johnson is bigger than yours vs. blah blah blah. But do it while considering the above information. In the meantime, my O-540 will just cruise along fine, no issues, no argument.


Ok first off, show me once where I recomended any platform anywhere in this thread? Get the bias out of your minds, just because I currently use platform A doesn't mean I am trying to say everyone should use it? What the heck? Maybe go back and read what I have said in this thread again?

Here's a couple of other old threads that I posted in the past that are very similar to this one. I see some of the same names in this thread actually...so, it's clear no lessons learned from the past yet.

viewtopic.php?f=2&t=68946#p669099

viewtopic.php?f=48&t=68970&start=25#p669387

Sliding gator, you haven't won anything unfortunately as my bet was to show an example of a lower hp engine that has more torque producing more thrust than a higher power engine with lower torque. ALL you have said is that the power curve dictates acceleration characteristics (of course?) which is not what we have been discussing here and I've covered previously quite some time ago.

The only point I am making here, in this thread, is that TORQUE is not the trait you need to spin a prop a given RPM. POWER is, as torque does no work. Take two engines of the same displacement. One engine with 400lbft of torque at 6000rpm will produce the same thrust with the same prop as another engine with 600lbft of torque at 4000rpm (same displacement mind you). And, the engine that produes LESS torque wil last LONGER and be more reliable. Let me put on my fire suit now as I'm sure someone will disagree with THAT statement too....

The reason the lower torque engine will last longer is because it will have a lower BMEP (Brake Mean Effective Pressure). You can read up on BMEP here:
http://www.epi-eng.com/piston_engine_te ... dstick.htm

The original poster asked:
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?


I gave him an answer that he can use to make a working boat. I did not say use this engine or that engine or anything else. He wants to know how much power he needs and he got an answer of "you need lots of torque." If he just builds an engine with good POWER he will get plenty of torque as a by product. If he builds an engine with good TORQUE, he very well may end up with less POWER than he could have for the same money and ultimately end up with LESS usable thrust because of it and an engine that will not be as reliable!

This is why people need to stop preaching Torque is king because it's not. Torque will give you lots of "snap" that isn't used most of the time by most people that are not racing. Power is the determining factor in how much thrust you will get and is what most people need most of the time.

Put some energy into actually looking at what I am typing out and what I am providing for supporting information and documents instead of thinking I am some kind of fan boi preaching one platform is better than others at all things. If you think I'm "ignorant" or whatever after doing that, well, I would ask if you have shown any better information than I have to counter my points? Because, seriously, I've provided a LOT of information since I've been on this forum and I rarely see similar contirbutions from others, but have on occasion.

Whatever happens from here, I'm sitting the rest out. You guys can have at it. Ultimately it's my own fault and failing to provide a convincing argument. Guess I suck at that occasionally then. :dontknow: GOOD LUCK. :usa:
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airduds
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Re: 502 power VS torque

Postby airduds » Wed Feb 21, 2018 6:10 pm

I'm staying out of this one. I'd rather argue religion. I'll say only the prop has no idea if the prime mover is a chainsaw engine or a steam locomotive.
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Slidin Gator
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Re: 502 power VS torque

Postby Slidin Gator » Wed Feb 21, 2018 8:00 pm

I tried to remain platform agnostic, specifically using Engine/gear as a combination. All discussion of torque is at the prop. I agree with Airduds that the prop does not care what the prime mover is, but I will qualify that as the prop does care what the torque characteristics are, as seen at the prop.

OneBFC, this quote from you really does summarize my point well:
my bet was to show an example of a lower hp engine that has more torque producing more thrust than a higher power engine with lower torque. ALL you have said is that the power curve dictates acceleration characteristics (of course?)


We are on the same page here, the power curve does dictate acceleration characteristics. In fact the example given did show that a lower hp system (engine and gear) which produces higher (prop) torque at all speeds can produce more thrust during the acceleration phase. In my opinion this is a key element of a well performing boat.

This is the statement that I disagree with:
Torque will give you lots of "snap" that isn't used most of the time by most people that are not racing. Power is the determining factor in how much thrust you will get and is what most people need most of the time.


My argument is that the acceleration (snap) characteristics of the set up does dictate performance of the boat for all around use, not just racing. These characteristics are seen when getting on a plane, turning, flipping around to stop and particularly when running ground and especially for my sore back. It improves the operators ability to control the boat. As several people have noted, most airboat engines do not operate at a steady state, these are dynamic applications.

No one has ever said "I wish my boat had less power" and there is no question that more maximum thrust comes in handy plenty of times. But, when it comes to comparing less power with a flat torque curve vs. higher power and a steep torque curve, there is a trade off to consider. The trade off is a "Snappy" boat that responds and controls well vs. a sluggish boat that eventually gets rolling and then will blow over trees, but is hard to control.
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Re: 502 power VS torque

Postby CarMotorBarge » Thu Feb 22, 2018 7:28 am

Below is the thrust data for my LS3 with a 2.68 Rotator turning a 4 blade R on Water Thunder's thrust tester:

https://southernairboat.com/members/carmotorbarge/media/468/

Image

The X axis (bottom line) is the engine RPM. The Y axis (left line) is the thrust in foot lbs divided by 1000. So 1.0 represents 1000 foot lbs, 1.1 represents 1100 foot lbs, and so. This engine made 1860 foot lbs of thrust at 5350 engine RPMs.

So why did I post this? The shape of the thrust curve has a direct impact on how the torque/HSP curve on an engine should be setup. The shape of this thrust curve is what is referred to as a quadratic equation in algebra. The concept behind a quadratic equation is that as the X axis (engine RPM) increases, the Y axis (thrust) increases at a faster compounding rate.

If you look at the thrust curve, you will notice that 50% of the thrust is made at 4100 RPMs and 100% of the thrust is made at 5350 RPMs. So it took 4100 RPMs to build up to 50% of the thrust and only 1250 RPMs to generate the other half. Also as the engine RPM increases above 5350, the thrust curve will keep compounding and the prop will get significantly harder to turn.

The shape of this thrust curve applies to all props. This includes DD and reduction props. The only item that changes is the amount of thrust that is made at each RPM.

So here is the question. Given the shape of this thrust curve, do you want torque below 4000 RPMs or torque above 4000 RPMs on a car motor? Where does the prop put the biggest load on the engine? It is nice to have snap below 4000 RPMs, but isn't nice to have snap higher up also? Also what is more important on a 21 foot boat? Is it better to have snap down low or make more thrust so you don't get stuck. :stirpot:
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Re: 502 power VS torque

Postby loudmouse » Thu Feb 22, 2018 10:04 am

Carmotor, I would be interested seeing ur same thrust test repeated with a 4 blade JX to see if it requires more or less hp to make same or more thrust. As you approached MBT, you Gained 200 lbs in several hundred (300) rpms. This is typical of a engine that's built for performance not economy.
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Re: 502 power VS torque

Postby kwanjangnihm » Thu Feb 22, 2018 11:16 am

CMB I fixed the graph so it now shows in post :thumbleft:
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