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Re: 502 power VS torque

Posted: Wed Feb 28, 2018 9:50 am
by Deano
Brother Russ, I am surprised you would post this graph.
I see you got it off their website, so the bullshit is theirs, not yours, but . . . really?

Image

Do you not see a very fundamental flaw here in this graph for this
engine that not only defies the laws of physics, but mathematics as well?

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 9:54 am
by OneBFC
ladyblackwater wrote: I can say I don't know what motor would have more thrust and neither can anyone that hasn't thrust tested that exact motor with the same prop and gear. The Rpm's that these motors need to turn are way to high and that's the reason I stayed away from the 632 motor on my boat.
The bolded part of your quote is what I'm trying to help you learn.

Let's pick this back up once you provide data on your setup so we can use a baseline that you are familiar with.

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 9:58 am
by OneBFC
Deano wrote:Brother Russ, I am surprised you would post this graph.
I see you got it off their website, so the bull**** is theirs, not yours, but . . . really?

Image

Do you not see a very fundamental flaw here in this graph for this
engine that not only defies the laws of physics, but mathematics as well?
Yes Deano your right, this is a bad example. It was late and I'm human. The torque and hP can't cross at that low RPM.

So, indeed, this is a poor example.

I was just looking for an engine with HP output higher than the torque and didn't think to check it.

I'm ashamed to have posted that now and I do appreicate you pointing it out. Let the record stand corrected. I edit the post to clarify the mistake.

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 10:06 am
by loudmouse
There is no denying what I jus wrote. 850hp engine will have more snap(you's words not mine) and turn more prop than the 750hp engine. It will also make more torque and hp through the entire rpm band rpm for rpm. All for the same money spent. Thats what I do!!!

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 10:11 am
by kwanjangnihm
CMB - why don't we use your waterthunder dyno sheet (HP & TQ) & thrust numbers for this thread as an example?

Can you supply slidin gator this data so he can plot & chart so we can all see the relationship. :salute:

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 10:12 am
by loudmouse
You build for torque and tune for hp...
WaterThunder engines will also turn more prop than most BBCs put out by most builders with 100+ less CID. There is a reason for that and its not by accident!

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 10:35 am
by SWAMPHUNTER45
loudmouse wrote:You build for torque and tune for hp...
WaterThunder engines will also turn more prop than most BBCs put out by most builders with 100+ less CID. There is a reason for that and its not by accident!
And he also knows that a (16ft heavy or larger) "big boat" needs big inches and has in the past recommended the BBC platform over the LS

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 10:55 am
by Deano
OneBFC wrote:... I was just looking for an engine with HP output higher than the torque and didn't think to check it...
It aint' a thing brother; I knew that you knew. Just trying to help contain the confusion in this thread. :lol: :lol:

I'll admit to leaning toward the torque side of the debate (and possibly because that is where I started),
but I will also confirm what Jason eluded to earlier (but didn't expand on) and that is that both are relevant.
After all these years of this, I think I could sit on and argue either side of the debate.

Now for this post, I will side with Russ and the HP is king guys and offer you the example Russ didn't think of last night.
For shits and giggles, lets consider a smaller, but quite effective little engine that has, say 135 cubic inches but turns 12,000 rpms.
This little guy by virtue of design and RPM can not produce a lot of torque (comparatively speaking), since it would be decreasing after less than half throttle. Are we in agreement so far ?

Now, I'm going to run that little engine through a 5.8:1 gearbox to turn my 4 blade Super Wides 2069 pRPMS.
If that little engine was producing between 700 and 750 HP, should work reasonably well regardless of the torque, no ?

I don't have a dyno graph because all that information isn't published, afaik. The little engine I referred to here isn't fictitious, it's
the 2.2 litre Honda V6 like they recently ran at Indy for a few years. Surely that would not be perceived as a Torque Monster, but it
ought to be clear that it could turn 4 Super Wides given a 5.8 gear ratio. If I win the PowerBall this weekend, I will show you. :lol:

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 11:22 am
by CarMotorBarge
kwanjangnihm wrote:CMB - why don't we use your waterthunder dyno sheet (HP & TQ) & thrust numbers for this thread as an example?

Can you supply slidin gator this data so he can plot & chart so we can all see the relationship. :salute:
This engine was never put on the dyno so I don't have this info. Doesn't matter anyways. Everybody knows that Water Thunder engines are LS motors that don't produce any torque or HSP. They are just stock motors with a Water Thunder emblem thrown on them. :shock:

On a serious note, I would estimate that this motor made 560 foot lbs of torque at about 4800 RPMs and about 620 HSP at about 6200 RPMs. Again no data to back this up, but this is about what I would expect.

What is important to learn from this thread is that this engine does not make its max thrust at 4800 RPMs. I run the motor 5800 to 6200 depending on what I am trying to do with it. A little less RPM gets me better fuel mileage and a little more RPM helps me outrun A/C boats. :stirpot: :stirpot: :stirpot: :stirpot:

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 11:53 am
by OneBFC
Thanks for posting the dyno for you engine ladyblackwater.

The reason your boat accelerates well at 4000 RPM is because the propeller requires considerably less HP at 4000 engine rpm than what you engine is capable of making at 4000. The amount of power you have available from your engine above what you propeller requires is felt as acceleration.

At 4000 RPM you engine is capable of making 542hp. It is NOT delivering that much power while you are running 4000 RPM otherwise it would not have sufficient power to continue reving higher when you apply more throttle. The propeller only needs a tiny fraction of you engine's maximum power output at 4000 RPM, likely in ithe neighborhood of 100 to 200hp depending on what kind of propeller your using. I say this is the likely range of required power because otherwise you would burning a very large amount of fuel to run that 4000 RPM. 100 to 200 hp is in the range of 10 to 20 gallons per hour of fuel, give or take a bit.

Every propeller configuration will require a different amount of power to turn a given RPM. The longer the propeller and the larger the number of blades as well as the amount of pitch in the blades will all impact what the required power will be to turn a given RPM.

Notice i said power here, not torque. People get confused because below 5250, more power also means more torque. Above 5250, that isn't the case. Torque can still be climbing beyond 5250 however on some builds.

If you were to try and use an engine that doesn't make enough power at lower engine RPM to overcome the required power of the propeller at the lower engine RPM, that engine will never reach the higher RPM where it may make more than enough power to turn the same prop at that higher RPM. This is why you have to match propeller and gear reduction properly.

At no point will you ever make less thrust with more power as long as your gearing and propeller choices are matched appropriately.

Your choice to run your engine below where it makes peak power is just that, a choice. If you geared it to be able to run at the RPM where you make max power you would for sure make more thrust.

The reality is, at 4000 RPM and below, almost any reasonably sized engine will produce enough power to turn most props. You would need a very low displacment engine that is designed to deliver all of it's power in a narrow range at high RPM only and combine it with a low gear ratio box of 1.7 to 2.0, etc while also trying to spin a very large diameter prop with wide blades,etc in order to get into trouble of potentially broken setup.

NO ONE is suggesting to do that in this thread. My one and only point is, it's POWER that makes THRUST. Not Torque. Make power at reliable RPM and then select gear and prop to harvest that power.

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 12:04 pm
by CarMotorBarge
And throw the charts/sheets with the torque curves in the trash. They cause more confusion than good. The HSP curve tells you everything you need to know. 8)

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 2:10 pm
by OneBFC
ladyblackwater wrote:OneBFC and Carmotorbarge do either of you know how much horse power is lost in the drive and how much torque is gained or lost in a drive? No one truly knows that I know of because there is no dyno for an Airboat. No matter how you look at it there is definitely a significant amount of HP lost through the gear box/belt box and prop.
Well, to determine amount of power lost via a gear box you can measure the temperature rise of the gearbox at a known load for the engine and convert that temperature to a value in watts and then subtract that from the engines known power to get the power available at the prop shaft.

Essentially you would be establishing the gear box efficiency. Typical gearbox efficiency for low reduction ratios is in the area of 5 to 10% historically.

Factor in 10% and your generally close enough for airboating.

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 3:25 pm
by SWAMPHUNTER45
We use 10% as a rule as well.

First thing Russ and I agree on :drunken:

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 10:19 pm
by Slidin Gator
OneBFC posted earlier:
Well, I will just leave this here and you guys can finish figuring out what's going on from there. This is the Version 3 pt II 502 from the above examples. I mean, you can see that you make more thrust with 545 lbs of torque than you do with 594 lbs of torque. I think it's pretty darn clear what matters?

Red = hp
Blue = Torque
Yellow = Propeller Thrust
Sir, Thank you for that thrust information, that is actually very useful and I think if you will give me a chance I can explain how useful and where I am trying to get to here. First, I agree with a majority of what you have said and the information posted, as much as I agree with what everyone else has said. I will show where I disagree and why too. My goal is to show that there actually is a set of formulas that can be used to size a variety of boats, perhaps not to the nat’s a**, but enough to provide some baseline sizing figures (for instance, thrust => 2x boat weight and HP=> 4x boat weight). The question “I have motor A and boat B, how to I use this stuff” question comes up all the time on this forum and I hope to compile some spreadsheets that everyone can buy into and understand. I figured that this would be a useful capability for this forum that would save everyone a lot of time so everyone can spend less time arguing on the board and more time arguing in the woods.

I also figured Nola’s initial concept of a 502 on a 21 foot boat was a great example to investigate because making a boat this big run dry with limited power (I did say power) is a perfect example of a point design. Also, running dry is an example of static thrust, which is simpler to model with math. Prop’s running with forward velocity is a much more complex analysis.

As originally defined, this boat will never run the glades grass at 45 mph, then off to the levee, up and over, then through a sticky black mud trail etc… We have already determined that the 502 is a marginal motor for the application so the setup has to be tuned to do one or the other reasonably well and the other poorly, it is not going to be a dual use machine. From my view, Nola can either accept what that boat will do and build it, or make a choice to go for what could be a sweet running boat, but with a bigger budget to match.

HP and Torque are easy to define and measure (ok, Torque is not so easy to measure directly). On the other hand, propeller performance is a very complicated issue to analyze in detail, but there are fairly simple approximations that do work well for roughly sizing the system. Fine tuning on the other hand, is where math, art and seat of the pants come into play.

If I do what I say I am going to do, the result should reasonably match everyone’s intuition and experience. Except that then this experience can be extrapolated to other applications as a means to provide some guidance for all.

So to start:
First, I’m not a dumba** here, I completely understand the main point of this argument, which is make HP not Torque. But I would rephrase the statement as make “HP to Make Torque”, which is where the gear box comes in and will be a key element of all this. In many cases it’s the only variable we can tweak.

Also, I am using 90% gear box efficiency (10% loss) for all calculations, it’s the one thing everyone seems to agree on, it’s good enough for me and I don’t want to kill the good mojo you guys managed to brew up all day!

Referencing the graph showing thrust at max HP, OneBFC posted:
I have measured the thrust my boat makes on a thrust tester. I also know how much power it makes at a given RPM while thrust testing at the same time due to the information the GM ECU provides. This allows you to determine a formula for HP to Thrust (For my boat of course) and I have applied a simplified version of that to the above graph. The actual exact thrust output is not all that important however to this discussion. What's important is the relationship of HP to Thrust. It follows HP, not TQ. Propellers need power to make thrust.
I looked into the graph and spreadsheet posted. I don’t have an issue with the formulas used to create the graph, as stated they are approximations. What is important to me is that OneBFC has run some tests, gathered data and is comfortable that these formulas reasonably match the test data. Since I’m still pushing torque, I do have to point out that the data supplied by the Ecotec (and most other) ECU’s is actually defined as “Torque Demand” values in the software, the HP figures are then course calculated by multiplying by engine speed and the 5252 factor. But no need to go there….

I do have a real issue with the presentation on the chart. The thrust figure appears to be plotted against RPM, but after reviewing the formulas I find that Thrust is plotted directly versus HP. To actually put the curve presented into practice would require a gear change at every data point, or the infamous variable pitch system, which I could use for backing out of a jam.

The formula’s used are propeller scaling formulas that define power vs. thrust for a given propeller diameter. The input variable is propeller diameter and HP and the output is thrust. The Wikipedia link in the spreadsheet did not really cover these scaling formulas directly, but I found a good link (below) that does cover it well and quickly. The RC guys actually have the best data because it is way easier to test the small power stuff. Prop formulas scale reasonably well.

http://www.heli-chair.com/aerodynamics_101.html

What is important is that we have at least one set of data of actual HP that is claimed to match up reasonably well with the estimates, cool, now we are getting somewhere. So using the same formulas, I created a new spreadsheet (attached) and a graph that might make more sense to everyone.

Please double check my work at this point and let me know if you find an issue, otherwise I do have more to come.

Image

I tried to upload the spreadsheet as an attachment, but I see that is not an option on this forum. I understand, files with Macros is one way the hacker and spammers get in. I am attaching a PDF of the actual spreadsheet. Anyone that wants a copy can PM me with an e-mail address and I will send the spreadsheet on along.

Re: 502 power VS torque

Posted: Wed Feb 28, 2018 11:00 pm
by Slidin Gator
OneBFC wrote:
Deano wrote:Brother Russ, I am surprised you would post this graph.
I see you got it off their website, so the bull**** is theirs, not yours, but . . . really?

Image

Do you not see a very fundamental flaw here in this graph for this
engine that not only defies the laws of physics, but mathematics as well?
Yes Deano your right, this is a bad example. It was late and I'm human. The torque and hP can't cross at that low RPM.

So, indeed, this is a poor example.

I was just looking for an engine with HP output higher than the torque and didn't think to check it.

I'm ashamed to have posted that now and I do appreicate you pointing it out. Let the record stand corrected. I edit the post to clarify the mistake.
I have to bust both of you guys on this discussion. Everyone knows that CM guys will lie and cheat when needed. But Deano, your an AV motor guy! You should get it. :violent1:

The fundamental flaw here is that the units are all screwed up, at least as far as you are used to looking at them, and I get it. On the UL260i curve, power is given in HP on the left and torque is specified in Nm on the right, with a different scaling. 1 Nm = 0.738 Ft-Lbf. Here is the re-scaled graph in the form you guys are used to seeing them.


Image

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 6:50 am
by SWAMPHUNTER45
It all comes down to this!

Big inch power mated to a 2.37 or 2.55 with big wide blades.

SlideGator I commend you on the graphs very nice product

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 8:05 am
by OneBFC
Thanks for noticing that slidin gator. Completely missed the units on right side.

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 10:28 am
by Slidin Gator
SWAMPHUNTER45 wrote:We use 10% as a rule as well.

First thing Russ and I agree on :drunken:
For reference, here is a quote from Waterthunder regarding gear box efficiency:
by Waterthunder » Thu Dec 21, 2006 12:37 pm

I looked into that and wow what a can of worm's I opened. The efficiency of a gearbox can change 10% in 15 minutes alone due to heat the hotter the box get's the less efficient it becomes. I spoke to a gear reduction engineer about testing my gearbox he told me it's a complex process and they cost around 30K per test. These company test's all the gearbox's for machinery and all the auto manufactures. He knew his stuff and defiantly educated me on reduction efficiency and parasitic drag coefficients changing with heat and horsepower changes. When he was done educating me I realized there can not be a good average number for gearbox efficiency percentages
THE PROOF IS IN THE PROP!

"If you copy someone you will only achieve what has already been done."

http://waterthunder.com/
321-508-5316
viewtopic.php?f=2&t=4304

So, the next time you are stuck, lay off the throttle for a few minutes, let the gear box cool down. Then try it again!

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 9:11 pm
by Slidin Gator
Ladyblackwater,
That is too easy on a car motor boat :cheers:. Just use a power steering pump, bracket etc. that fits your particular motor. You know, more of that crap that I don't have on my boat! :stirpot:

Actually, those are not ideal pumps, they would need some tuning to pump gear lube. The flow requirement for cooling is really small, a small gear pump would work great and I would make it electric driven so you can turn it on as needed. That way you don't have another belt etc. to fail on the motor, you know you can get home without the cooler.

Go to the local hydraulic supply house and get a 1/2 gpm or less (as low flow as you can find) gear pump and 12 Volt motor assembly rated for as low of a pressure and Hp as you can find. Maybe even a trim tab pump?? Mount the pump as low as possible so the high viscosity gear lube will precharge the pump. Put the cooler anywhere above the pump but below the gear box. Make sure the gear box is vented and make sure the pump is not cavitating, otherwise it is flowing too much.

Put the electric on a thermostat so it does not turn on until the oil heats up and viscosity is in operating range.

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 11:33 pm
by Slidin Gator
It is time to wrap this discussion up on my end for a bit. The gobblers are getting anxious and Turkey season opens Saturday morning, bright and early. It is time for me to go get my yearly schooling, by an old Osceola gobbler. That’s my kind of schooling!

I have some paying work to do and this little hobby is taking some time I won’t have for a little while. So I’m going to lay out what I have got and leave it for everyone to absorb for a while.

This next post is a long one, so if you want to quote this discussion, please use this post instead of the next so everyone can keep from wearing out the mouse scroll wheel!

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 11:42 pm
by Slidin Gator
I am going to say that I have given some looks at the 502 Nola boat and I think Nola should hold off on pouring any money into a stock 502 rebuild. If the budget allows for a big motor, 540 and up, then that should be the plan. Otherwise, I’m a run what you got kind of guy. Personally, I would tune that Lycoming 540 motor up, put a gear box on it and throw an 84” 2 blade prop or bigger if possible. By the time you put 1000 lbs (maybe more) of cast iron big block motor, radiator, gear box etc., vs. the IO540 450 lb weight, a reasonable 502 build that makes 550 Hp at 5,350 RPM is going to make enough extra thrust to net maybe 300 lbs of extra dry ground running capacity. It’s not going to burn up the hill with a load, but reasonably empty, that is about the difference of getting your big a** brother in law to get out and walk up the hill. Take the $10-$20K that a motor build, gear and prop is going to cost and use it to buy 5,000 gallons of AV gas now. Then that boat will run for years while saving money for what this boat really needs. Then the whole kit can be sold for a premium later to offset the next build. That or sell the boat and put that motor on a smaller boat.

Either way, let’s call that part 3 for later, turkey time and all.

Here goes:

I assume no one found any major issues with the prop scaling stuff. I’m now going to try and convert you guys into being torque guys instead of Hp guys. I’m not saying less Hp is better, I saying you need to think in terms of Torque instead of Hp. Hp is useful as a basic measurement for very crude sizing. The prop curve scaling diagram shows this. But Hp figures are not very useful for truly tuning in a setup, that is where torque figures come in. I noted that the ECOTech ECU calculates torque demand as a value, it does not really give a crap about Hp. The ECU can’t use Hp to figure out the correct fuel, timing etc., it maps torque demand vs. RPM!

The problem with Hp is that it already includes RPM (HP = Torque x RPM x 5252) in the measurement unit. When you graph, try to envision or even control acceleration of the boat using Hp as the variable? There are two units of RPM in play with Hp parameters and it dilutes the true relationship. If we strip the RPM figure out of the engine performance parameter (HP vs. Torque) and graph torque directly against RPM and all the other factors, we end up with a direct measurement of acceleration and therefore thrust, at anywhere along the RPM scale. Just remember, a Torque curve plotted against RPM shows how the system will perform, jump, accelerate, cruise etc. across the whole operating range. Hp tells you what it does WOT, assuming it is tuned correctly.

If you can grasp the concepts I am putting forward in the next few posts, then you my friend understand Calculus, you just didn't know it.

As a refresher, I am starting with this curve from CMB and the prop scaling data from OneBFC:

Image

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 11:43 pm
by Slidin Gator
On the following chart, I took the prop scaling formulas from my previous post and did some algebra on it to calculate HP required versus a given thrust point. I then matched that math up with the Thrust vs. RPM chart that CMB posted way earlier in this thread. CMB has reviewed an earlier version of all of this and agrees that it is a reasonable match to what he thinks he has. The result is a set of numbers I have been after here, a torque and thrust curve for a specific prop set up! The best part is that this prop is a damn good example of the minimum prop that Nola is going to need to swing to push that monster. So it translates well.

Image

Re: 502 power VS torque

Posted: Thu Mar 01, 2018 11:50 pm
by Slidin Gator
One item of note, to make this whole discussion match up well, I used an 81% gear box efficiency figure. In my experience, this is well within reason (I have seen plenty of 75% efficiency figures for all kinds of equipment). But if you can buy into that, this whole story is a much easier read. Just remember, gear box losses are all torque, if you are losing RPM in the gear box you have some real issues to deal with. :slap:

This chart shows my model of CMB’s prop as pitched at test, with a 2.68 ratio gear box. The torque and Hp lines are the prop steady state requirements. Note that the torque and hp curves do cross at 5252 RPM. As noted previously by others, this is just a function of the 5252 constant used to calculate Hp from Torque and RPM. I want to point out that this is nothing more than a feature of the units used and the graphs scaling, something I pointed out last night on an airplane engine curve. Otherwise, there is nothing unique about this RPM with regards to performance. If I change the torque units to bananas per finger length, this crossing point would occur at a different RPM, but nothing else changes. Forget about that anomaly, it is not an important or magic point on the RPM curve. The only real important limit is the Mach number of the prop. As this model grows beyond the CMB boat here, Mach number will be a key discussion element.

I think this model is reasonably accurate in the higher power range. But the prop scaling formulas are for a 2 blade prop, compared against OneBFC’s 3 blade prop and then applied here to a 4 blade prop. I don’t think that graph is very accurate down low, so this next graph zooms into the power range of interest. Keep in mind this is all a model of the prop, I have not added in engine parameters yet. :scratch:

Image

Re: 502 power VS torque

Posted: Fri Mar 02, 2018 12:07 am
by Slidin Gator
What we are missing is an idea of what the motor performance curve really looks like. I found a good set of dyno curves for a 427 LS7 engine to use at this link:
http://www.powerperformancenews.com/tec ... sx-intake/

Image

This curve shows side by Hp and Torque for a stock LS7 and with the addition of a Mast Superflow Intake manifold. Going back in CMB’s posts, he made the following statement about his engine:
On a serious note, I would estimate that this motor made 560 foot lbs of torque at about 4800 RPMs and about 620 HSP at about 6200 RPMs. Again no data to back this up, but this is about what I would expect.
Please note that the Red graph HP and Torque lines on this dyno chart match up with the 2 performance figures that CMB is claiming. Basically, the Stock 427 LS7 performance curves appear to be a reasonable match to CMB’s 419 engine. For the purpose of this discussion, I am going to call the red line the “CMB Torque” set up, he just did not know he was a torque guy!

The blue lines show a Mast aftermarket intake manifold upgrade. You can read all about it at the link provided. As these curves show, this motor makes more Hp (maybe 30 Hp more at 6,500 RPM). It also makes up to 10 Ft-Lbf more torque after 4,750 RPM. But look at that torque and Hp take a nose dive below 4,750 RPM! Don’t forget that the prop is set to limit max RPM at 5,350 RPM. So the hot rod intake manifold has a narrow window where it out performs. I’m going to call this the “Hp” set up

Re: 502 power VS torque

Posted: Fri Mar 02, 2018 12:27 am
by Slidin Gator
Now, if I have not yet sold you on the HP vs. torque thinking yet, here is CMB’s set up matched up versus these two engine configurations. This set of graphs is why I changed gear box efficiency to 81%. Doing that made the propulsion torque curve (Orange Line) cross the stock LS7 torque curve (Red) at 1,860 lbf of thrust. Otherwise it would not make sense to show the thrust limiting out with torque remaining. As I said, it makes the story easier to tell and is well within the error range here:

Image

Zoomed in for detail:

Image

So yes, the hot rod Mast intake does make a whole extra 10-20 lbs of thrust at top end, standing on it, stuck on the green grass. But Look at how flat the “CMB Torque” (red) curve is from 4,500 to 5,350 RPM at or above 565 Ft-Lbf. Look how torque remains above 500 Ft-lbf at 3,500 RPM where the Mast intake “HP” set up makes 380 Ft-lbf of torque. Now go back and re-read my first round of posts on matching instantaneous acceleration torque against the prop curve to get instantaneous acceleration/thrust.

Which set up do you want in your boat? If you say the “Hp” set up, you need to quit reading this thread, I can’t help you! Just go back to what you know works the best. The ”CMB Torque” set up is going to jump out of a hole, jump the river bank, and get going on dry ground MUCH better than the “HP” set up. In short, the Torque set up is more enjoyable to run.

Consider that static friction (boat not moving) is always higher than dynamic friction (boat moving). To get the boat moving, you need all the thrust that you can get, but only for a brief moment, just to get the boat moving, then you can back off the throttle since friction has gone down.

Also consider this, the acceleration or instantaneous thrust off the line may actually be higher than the curves. At low RPM, there is less airflow through the cage and over the prop, so there are fewer torque losses. More torque can go into making instantaneous thrust. Inertia of the prop and rotating equipment does subtract from the acceleration, but I am proposing that this is a small amount. I could be wrong here, one of these days I will model this aspect, but not now. :fishing

Have you ever noticed that a lot of times, the best way to get the boat moving is to stomp the accelerator and turn the rudder hard to make the boat jump on a turn? It will jump initially but then might stall out, standing on the throttle after that does nothing more, unless you rock the rudder. This may not be the case for you, but that is because you are using a double rudder set up and you have never seen how a big airfoil, single rudder can move a boat. But that is a whole other thread and discussion for another time! :stirpot: :stirpot: :stirpot: :stirpot: