Power and Speed Mathmatics

[Crumri4]

I am wanting to get feedback from people on different mathematic formulas that I have found over the years that help determine different aspects of Boats. Please chime in if you agree with the equations or if you think the formulas are somehow flawed.
#1 I once saw a formula that said an electric boat needs 150 watts of power per lb. I was corrected on that through one of these forums to say that number should be more like 230 watts per lb. A bit more research showed that a full scale (30', 6800lbs, 3000 hp) Unlimited hydroplane has a ratio of about 329 watts per lb and they achieve speeds of around 200 mph.
#2 To figure out the wattage my motor is putting out, I found the maximum wattage and the maximum voltage. I figured the percentage of voltage I was running based on battery cell count and resulting voltage. I divided the voltage being used by the maximum voltage to find what percentage of the maximum I was utilizing. Then I multiplied the decimal equivalent of the provided voltage by the max wattage of the motor to find out the amount of power I should expect the motor to put out.
#3 Speed is a formula of (Volts x kv of motor = rpm)(rpm x pitch of prop=inches per minute)( inches/min x 60= inches per hour)(Inches/hour %12 = feet per hour) (Feet/hour / 5280 = miles/hour) ( miles/hour x .8 (20% prop slip) =actual speed of boat.
#4 Amp draw (Watts / volts = amp draw)
Like I mentioned, I am actually looking for feedback on how reliable or accurate these formulas are. I am hoping that if you have feedback, that it is more than "no that is wrong", please feel free to add a different formula and explanation as to why one is right and the other wrong.

[NativePaul]

#1, The more the merrier for speed, and the fewer the better for runtime. I like the compromise of 230W/lb and most of my boats are around that figure, but that won't be ideal for everyone, I also have some SAWs boats for example that are about 1000W/lb.

#2, There is no way to figure out watts used from that data, the load is such a huge variable that you really need to measure it. Pulling numbers out of my arse here so don't quote them, but the same motor/ESC/Battery combo may pull 500W spinning an x427 and pull 10,000W spinning a v950.

#3, Basically correct, the 20% prop slip is a big variable, and neither the battery or motor is 100% efficient so you wont get 4.2v/cell or the quoted KV in the real world, but it is a good place to start.

#4, spot on.

[sciesielka]

#1, The more the merrier for speed, and the fewer the better for runtime. I like the compromise of 230W/lb and most of my boats are around that figure, but that won't be ideal for everyone, I also have some SAWs boats for example that are about 1000W/lb.

Paul, out of curiosity, I took my latest build which is a 37" long mono that weighs 7.75 lbs (3.5kg) with 2, 3-cell LiPos installed. If I use the 230W/lb, I would have needed to be able to supply 1,783 watts continuous correct?

I finished the build over the winter and did a bunch of test runs and have been running it a lot this summer so far. I have a 150A ESC, a 4082 1250KV Leopard motor (3500 watt MAX rated) and a sharpened and balanced Octura x447 prop.
When I logged the data from some test runs, I saw peak wattage around 2,400 watts and the average at about 900 watts. I'm just curious because if the calculated wattage was 1,783 and actual averages 900, then it validates that my system is setup pretty well and I can use that calculation for another build and expect similar results?
To be honest, I sized my motor based on Steve from OSE's recommendation and the KV based on this being a "sport boat" and the prop size was based on testing lots of Graupner props in different sizes. So next time I can hopefully save some time and money using the W/lb estimate right?

[Speed3]

Those formulas are useless.

Only formula 4 is correct.

In my experience you need a datalogging esc to know how much amps your setup is drawing.

I am getting 55mph at 467w per pound.

And 69mph at around 950w per pound.

175w per pound gets me around 31mph.

As for the voltage being used divided by the maximum voltage.
The wattage is the voltage being used by the ampage being pulled.

The amps being pulled does change at difference points when full throttle is applied.

Example a prop that hooks up quick will pull more amps on start of acceleration and the amps graduately decreases as the rpm increase.

A prop that cavitates will pull less amps during cavitation and then the amps would increase when the boats gets up to speed.

As for prop slipage.

Prop slipage is high at lower rpm 10000 to 17000rpm. 43 percents or greater prop slipage at those rpm.

When the rpm gets up to 19500 to 20000rpm the slipage may be around 30 to 35 percent slipage.

As the rpm increase the prop gets more efficient atleast to a certain point.

So basically many things changes so using formulas to predict speed is useless.

Even trying to predict motor loaded rpm based kv and voltage is useless. The design of a motor and the can size does change the loaded rpm.

[plinse]

What benefit do you expect from these formulas?

Formular #3 has the problem that I have boats having ~4% Prop-Slip and I have some showing 30% slip.
This is "real slip" as I don't calculate rpm, I measure rpm with a logger and I measure speed so the adapting of expected rpm to real rpm does not have to take place.
The problem here is the first principle of calculation: in leads to out and the worst data you can get is data on RC-components.

Propeller diameter is something everyone can measure quite easily and that's it with accurate data. Pitch is something where you can have indications. The pitch of Octuras is quite nominal, Propshop don't have the pitch that they are rated, chinese props, well we have one popular type of aluminium propellers here that are rated 1,9 pitch but most people agree that it is more like 1,2. Have fun calculating with that.

kV from the motors - also something "funny".
Most chinese ratings are "no load", you can expect an rpm-drop of 10-15% between "no load" and "loaded kV", this 10-15% applies to most motors but the given data and what you can expect is different.
Lehner gives loaded kV so without load the motors turn faster. Most chinese manufacturers give unloaded kV and some just don't really tell.
You get to know once you have measured it.

Lehner 1950:

You see the blue graph, no load it goes to 110% static, loaded it goes to 100% static, with Lehner you know what you get as they tell you in detail. The measurement confirms this.

Some chinese 3660 3180kV - such a blue one...

I have something like ~93% of the given 3180kV unter load, calculating with that rpm and the nominal pitch this boat has 4% slip.
Are these 4% worth to tust them or is it just a propeller with higher pitch then nominal?

I had other chinese motors that showed the nominal kV under load and some that were 15% away from the nominal data with the best fit (25% away no load, 15% away under load - new motors, both actually behaved the same and worked fine but the given data was useless).


Next topic: Do you have good or not so good batteries? Are they warm? Do you run really high load or not so high?
Do you expect 3,5V/cell or 3,7, perhaps 3,75V/cell?
If you measure you can tell and you can do quite precise predictions on changes that you do on your setup.
Otherwise it is just guessing.

kV: 10-20% unclear
Voltage of batterie ~10% unclear
pitch of propeller: easily 30% mistake on that one
boat trim - slip of propellers: Different on a mono vs. a hydroplane for sure...

If you don't know one input you can easily mix adaption factor rpm with slip and other uncertainties - and you just don't know.

I have gone into details on some of my boats, on other ones I am just satisfied that they run well and I don't care about details.
With a datalogger you can reduce the unknown details quite a bit but just starting with the nominal values from some manufacturers you can be close to what you get when using a Lehner motor for example but most manufacturers don't even tell you whether the nominal kV is under load or unloaded so you have the first 10% of uncertainty that can be defined to be slip of the propeller or just not knowing the properties of your components.

If you want to have some fun just ask the dealers whether the kV values of the motors they sell are loaded or unloaded. The good dealers might know, at least they understand your question, some will just don't understand but whether they give you this as an honest answer is a different topic. If you want to have even more fun just ask some more dealers on the same type of motor and compare the answers

Regards, Eike

[sciesielka]

What does everyone use for a data logger? I know a few people looking for one. I know that Eagletree went out of business in mid-2020. I know some of the high end ESCs have data logging built in, but what else is available? I personally found a new-in-the-box old Hyperion Emeter V2 that I use but they were discontinued a while ago and what inventory there was of new ones seems to be gone. And with multiple boats, having something like the Emeter or an Eagletree makes it easy to move between boats.

[Speed3]

The swordfish esc's.

Swordfish 220x datalogs up to 255Amps. The 300x datalogs even higher. The give esc temperature, battery voltage, amps and motor rpm. You need a laptop to access the datalogs.

With the log open you can move the mouse to see the specific data at certain points in the run.

These is no other way of knowing motor amps and motor rpm without a datalog.

Propslip vary depending on motor rpm, how loose the boat moves across the water, the amount of prop in the water while the boat moves.

The more efficient the prop the greater the loading on the motor and higher the ampdraw will be.

As for the loaded kv. It varies under load with a very light load you may get 96% of the rated kv. And I have seen the kv fall off to 45 percent of the rated kv.

The better the design of the motor and bigger the motor the better it holds its kv.

Higher quality motors like lehner and neu will hold the rpm better at higher loads than the cheaper motors.

[NativePaul]

1783 for a 7.75lb boat is indeed 230W/lb.
I use average watts for W/lb, as initial acceleration will always have a big amp spike that would skew data if using peak power, over a 6 minute oval race that 2 second holeshot spike is pretty meaningless to me.

Driving style and the way you measure it changes things dramatically though, in my SAW boats each run is basically 2 big peaks so I want to analise those 2 peaks in detail, and an average doesn't tell me much. I have not used a Hyperion Emeter does it start recording as soon as you plug it it or does it start when it sees over a certain current being pulled? If it starts as soon as you plug it in it will be skewing your averages pretty badly as it may be a minute or more with 0 amps at the start and end of the run, if so using the data for energy used and manually timing the power on part of your run to work out the average will be a lot more accurate.

[plinse]

I don't like ESC with data-logging actually.
I want to know my measurement-equipment and I don't want different software, different formats, different this and that from boat to boat just as I might be using ESC brand A, B or C.
That is crap to me.

I run the Unilog2 from SM-Modellbau: https://www.sm-modellbau.de/UniLog-2
You can combine it with the GPS-Logger: https://www.sm-modellbau.de/GPS-Logger-3
For Speed-Boats you actually need the 400A sensor only: https://www.sm-modellbau.de/UniLog-S...ker-am-Pluspol
Also useful information is the rpm-sensor for brushless: https://www.sm-modellbau.de/Brushles...t-2-UniLog-1-2

Let's say it this way: I don't need more information on my boats but I don't want less either.
Average values are crap, I can reduce data myself. My Unilog2 is set to 20Hz sampling, this way I can see the current peaks in adequate resolution.

This actually is the engineers-approach - know what data you want and get the right equipment for it.

I am not willing to pay for the logging function at each ESC. With the Unilog2 I pay the equipment once, the data I get is always the same and what I really like is getting current, speed, voltage and rpm in one plot - not just having some averages and the max. speed from the GPS as a photo from some display

[LibertyMKiii]

Just throwing out another idea for data logging. Having rpms, amps volts, ripple voltage, and temps really help you analyze what changes to make.
Castle creations MMX8s and XLX2 have lots of power and excellent data logging abilities. (the original XLX also, but the new model is better)

This is an example from an RC car run at 119 mph using 8s power with a 40x74 1915kv motor.

Typhon 119mph 8s Capture.jpg

With the XLX2 I have seen other guys logs over 600amps and 19kW using a TP 5680 motor.