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.
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.
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.