Formula For Calculating Speed (Revisited)

[10gauge]

GEORGE'S SPEED EQUATION:

Here is a mathematical equation I derived 3 years ago to calculate speed for FE setups. I derived my formula on quantitative analysis and it is a fairly accurate statistical approximation of actual setups. My formula was verified by Jay Turner on RRR and it's within +/- 3% of his calculations.

knpc was asking how I was calculating my speeds, so I decided to repost this old thread to share with everyone. I find it very useful for approximating performance without having to take the boat on the water, or rely on the GPS all the time, and even save money on wrong props, or finding the props that I need for my speed goal. It takes the guess work out of the equation (no pun intended). You can plug-in various combination of props, kV's, cell voltages and accurately approximate your speed.

mph (cat) = [Prop Pitch] x [Prop Dia.] x [kV] x [LiPo V] / 36970

mph (Hydro) = [Prop Pitch] x [Prop Dia.] x [kV] x [LiPo V] / 33300

(Prop Dia in mm). Multiply Pitch, Dia, kV, V, then divide by 36970 for catamarans, and divide by 33610 for Hydros.

Here are the original threads from Feb 2008:
http://forums.offshoreelectrics.com/...ead.php?t=2695

http://www.rumrunnerracing.com/fefor...culating+speed

The above formula is in mm - more useful to plug in the numbers. But, here's my original formula from 2008 that had Prop dia in inches - it's basically the same formula:

mph = [Prop Factor] x [Prop dia] x [kv motor] x [LiPo Volts] x 0.000687
(Prop factor or pitch is 1.4, 1.5, 1.6, etc.; and Prop dia. is in inches).

mph = [Prop Distance per Revolution] x [kv motor] x [LiPo Volts] x 0.000687


EXAMPLES. Comparison of GPS and calculated speeds:
Setup 1: Lehner 2250/11, 8S, x447, claimed speed 60mph
(1.4 x 47 x 1076 x 29.6) / 36970 = 57mph calculated speed

Setup 2: (Jay Turner) Neu 1521/1D, 4S2P/3300/25C, m440, claimed speed 62mph
(1.4 x 40 x 2800 x 14.8) / 36970 = 63mph calculated speed

Setup 3: Feigao 12XL, 6S2P/8000, Y537/3, claimed speed 48.6mph
(1.5 x 37 x 1390 x 22.2) / 36970 = 46mph calculated speed

Setup 4: Feigao 9XL, 4S2P/10000, m545, claimed speed 50mph
(1.5 x 45 x 1853 x 14.8) / 36970 = 50 mph calculated speed



Martin asked about the formula for monos, I haven't tested it for monos, but based on the hull slip factor, it should be close to:

mph (mono) = [Prop Pitch] x [Prop Dia.] x [kV] x [LiPo V] / 39600

-George

[martin]

I use a very simular system & my speeds come out very close indeed to your calcs on the examples you list, Do you have a final division number for monos as you list one for hydros & cats ie 33610 & 36970. I suspect it would be around 40000+. I find these calcs quite accurate & usefull, As you mention you can get an idea of what to expect from different props or kv, voltages etc before you even decide what boat or equipment your going to buy. Martin.

[martin]

Something i often do is when someone post details of their runs where they have a sat nav in their boat & give the speed along with all the details of their set up ie kv,prop,voltage, type of hull etc. I calculate & get very close to speeds they have done. Martin.

[jcald2000]

Or you can go here

http://www.offshoreelectrics.com/conversion.htm

[martin]

I already have my own calcs for mono,cats & hydros but wondered if you had a calc for monos so i could compare them all. As i said my calcs on cats & hydros are very much the same as the examples you give. Martin.

[10gauge]

This formula is not based on theoretical conditions. The data to derive my constant for cats (36970) and hydros (33300) was from a pool of 30 actual and running setups. So the results yield an average speed for a particular type of hull and the data pool doesn't represent "best case scenarios or runs." If anything, the formula doesn't overshoot speeds. This way, it keeps the numbers modest. You may be pleasantly surprised if your actual setup goes a few mph faster than calculated speed. I've also found that any differences between manufacturers may vary between 1-2 mph in most cases when testing.

jcald2000, that's a great reference. The Offshore calculation is within 3% of my measured results. My formula undershoots as it is an average of various setups, the Offshore formula overshoots as it is a best case scenario. Both have their uses, but to use the Offshore formula you have to be online - can't just compute on a handy calculator or phone.

Martin, I didn't post the mono formula because I've only tested the formula for cats and hydros. The essential difference between the various types of boats is their hull slip factor. Martin, for monos, based on the hull slip factor monos vs cats, I would place the mono factor around 39600. Again, I would want to test that number to verify it and average it for different manufacturers in the water under normal conditions so that it gives a result within 1-2 mph of the GPS readings. For monos, the formula should be close to:

mph (mono) = [Prop Pitch] x [Prop Dia.] x [kV] x [LiPo V] / 39600

[SweetAccord]

George,

Thank you for this formula. I like it! Accurate and easy to use. Since you are a numbers person and with quantitative analysis, can you come up or know of a formula to determine timing settings on a motor? There is one for PWM but not one for timing. There seems to be a formula for everything else for FE but that.

Thank you.

[10gauge]

Thanks SweetAccord. I haven't delved deeply into the electromechanics to calculate motor timing settings. However, I am curious, is there a formula to calculate amp draw for a particular prop and motor combination?... If there isn't, it would be nice to correlate hull type and size, prop, motor, kV, LiPo, and current draw - all in one neat formula!

[SweetAccord]

George,

I don't recall but didn't the FE calculator give an amp draw value per prop pitch and size? I recall it had something in there. If not the boataholic site I think had something like it but the site seems down. I may be wrong.

[sailr]

Some enterprising user out that should write a simple little program for this! PWEASE? Although I guess a calculator would work OK. Haha.

[10gauge]

I've heard FE Calc and FE Calculator mentioned, but have never found it on any site. It would be a great adjunct to knowing what the amp draw would be for a particular prop, as well as the speed and run times before ever taking the boat in the water.

I forgot that I bought a V3 Logger - still new in the box. I guess I may have to start using it to derive a new equation. If I have time in my busy schedule, I'll see if I can get back into RC boats this year...

[SweetAccord]

I've heard FE Calc and FE Calculator mentioned, but have never found it on any site. It would be a great adjunct to knowing what the amp draw would be for a particular prop, as well as the speed and run times before ever taking the boat in the water.

I forgot that I bought a V3 Logger - still new in the box. I guess I may have to start using it to derive a new equation. If I have time in my busy schedule, I'll see if I can get back into RC boats this year...

The FE calculator has been on OSE as far as I know:

http://forums.offshoreelectrics.com/...ad.php?t=15117

[ksm2001]

Welcome back George, nice easy to use formula thankyou.

I used the values on my current build I'm working on a PT SS45 running a Neu 1527 1.5D, 6S2P 45-90C, m447, according to the calculations I should see 65.8mph. Ok now I want to get this boat finished to see it go.

I tried the cat formula for my DF29 that is gps'd at 57 mph and according to your calculations it came out to 55mph,cool.
Edit:Miscalculation in the prop size input was a x642 instead of a m445 which brings the speed to 65mph so the veehull doesn't add-up.


Ken

[H2OCamel]

Nice formulas! Thanks so much!

[10gauge]

Thanks SweetAccord for the link on FE Calculator. For some reason I can't enter new data for Neu motors and Castle ESC... I'll see if I can come up with something.

Thanks Ken, it's great to be back. That PT SS45 is a fantastic hull. Raydee had a fantastic build on it too. That 1527/1.5D is a beautiful motor for large hydros! I have not had the opportunity to put mine together yet as I've got the same setup - Neu 1527/1.5D, 6S2P/25C/10000mAh, CC-240HV.

Anytime guys - Sweetaccord, Ken, H2OCamel.... I wanted to make a small contribution in return for all the great advice I have received here.