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.