Need help for a brushless conversion

[kff]

Hello,

I would like to convert my boat (robbe marlin 600 : ) with a brushless combo without changing anything instead of motor and controller.

you can see the boat here :
http://www.rchobby.co.uk/robbe_marli...e_racing_.html


boat spec : V deep monohull with immerged propeller
Length: 625 mm
Width: 220 mm
Height: 170 mm
Total displacement: 1150 g


Actually I've got :
7 cell RC2000 batteries pack
brushed graupner SPEED 600 7.2V (kv = 2500 rpm)
direct drive stock propeller (30 mm) I don't know the pitch

with that setup I've used a power meter to have some informations :
peak current at top speed : 27,15 amp
batterie voltage : 8,15 volts

So I'm looking for any idea about the brushless motor (KV ???? 2500 - 3000 rpm ????).

I would like to stay in the 30-35 amp current range

What do you suggest for the motor : if I have to change the propeller : what do you suggest ????

thanks for your help

[ED66677]

big issue Sir!

this boat has a submerged prop, so the motor has to have torque and low RPM, and will never get hight speed,

if you start with a motor that you will use at 80% of his max Amp, I would suggest a Feigao 14L or 16XL, but you will then use a 70Euro motor that you might never use on any other fast boat because they all use surface piercing prop (hélice de surface ou semi-immergée en français!)

stay with brushed motor, get a 700SC or 700Turbo C 9,6V, witch is a really good cheap beast!!!... for 30Euro

if you choose the 700Turbo C 9,6, you may have to change the ESC (variateur)! is there any in this boat???

[kff]

Hello ED,
why Big issue ????
I don't understand why you tell me to get a speed 700 or feigao L or XL with low RPM but high torque. I would have to put a bigger propeller ??????

Yes the boat will never get high speed with a submerged prop. But can I find a brushless than can drive my prop on 7 cell with better efficiency than my actual
brushed motor : more efficiency is more power for the prop so more prop rpm !!!! no ???

if I can stay in the 30-35 amp draw range : it would be fine.

ANY IDEA ?????

[ED66677]

because submerged prop can't push you boat as fast as surface piercing prop, if you want to keep using a submerged prop, it can't spin fast, if it does then its efficiency just drop down!

when people want to increase speed with sub prop they increase RPM, and they reach a point where the prop start to cavitate, at this point, the thrust of the prop simply decrease considerably, surface piercing prop are design to be very efficient at hight/very high RPM, plus, sub prop are just behind the drive shaft, the shaft prevent a perfect water flow to the prop, reducing again the efficiency, and the faster you try to go the worst is the effect of the shaft/cavitation

you have a 30mm prop, choose the 700 brushed motor, buy a Graupner K Carbon 36mm (I would say up to 45mm) and your toy is gonna fly!

the brushless motors we use in FE boat are design to run at relatively high RPM, 30000 to 60000 in race condition, try to compare RC plane BL motor to FE boat can style motor the diameter is much bigger for plane because they need torque, when we get to the point that we need more torque, we add gear (réducteur)

if you don't use a low RPM/high torque motor you will only waiste power, time and energy to get faster, it might get much faster but become unstable or unsteerable!!!

Question :
how fast do you want to go?
how much do you want to spend?
is it worth spending any money on this hull?

[ED66677]

j'ai retrouvé ça!!!

"Note that cavitation can also be associated with sudden loss of thrust and high propeller slip"

sur http://www.well.com/user/pk/SPAprofboat.html

et ça http://www.icboats.org/boat/document...nformation.pdf

[ED66677]

kff,
acceptes les PM!!!

[ReddyWatts]

Ed what about the SV motor /esc combo? Start out with 8 cells on a small prop (X432 -X435) and see how many amps it uses submerged. Then add batteries or larger props, if it will handle more. The 6-pole brushless should have the most torque at low amps and be the best light brushless for this application and be in his 30-35 amp range???

[kck741]

I converted my friends Hammer EP to the SV setup it flys!!!It has a submerged prop setup

[ReddyWatts]

How many cells and which prop did you use? amps?

[kck741]

8 cells-the minimum for the SV esc -stock prop

[ReddyWatts]

WOW, I bet it did move on out. Thats good to know.

There ya go, kff. If the hull will handle it?

[kck741]

yea it will handle it tempted to put it on 12 cells

[ED66677]

ReddyWatts, what type of motor (KV?) is in the SV combo?
you are probably right, chances are that if he starts with 6 or 7 cell he's not gonna damage anything, but once again is it really worth doing it?
If it's an ABS hull it would be so easy to remove the existing drive, even if it requires to remove a part of the hull and put some ABS sheet instead, and go for surface piercing!!!

[kff]

Hello guys,
thanks for the reply.
Ed : I understand the prop cavitation : It is the same in aircraft spinner.

Let say a simple idea :
my brushed speed 600 has a 2500 KV : nearly 30 amp at full speed.
at that current efficiency is about 60%

if I take a BL feigao 10L : 2458 KV and try to set up the prop to stay in the 30 amp range at full speed. at that current efficiency may be between 75-80% , so
15 to 20% more power to the propeller.

Now If I use my aircraft motor calculator :
I know that a speed 600 7.2V at 30 amp and 8V has a 13500 rpm.
If a take a FG 540 10L , RPM will be about 16000.

Now I've heard that a submerged prop start to cavitate over 15000 rpm : OK !!!!

If a setup my transmitter gaz curve to stay around 15000 rpm to the prop, I still have 1500 rpm more with the BL motor and current at full speed is the same .

I will have a motor that can have a very long life (instead of a brushed motor) and I'm happy because i've got a little bit more power and speed.

What do you think about that ????

[ED66677]

what is sure is that the 600 motor will not last @ 30amp while a brushless (any of L or XL that has a max amp above 40) will do it better

stay away from limitating motor rpm with transmitter gaz curve!!! if you have a certain power available, use it!!! so if you want your prop to stay below 15k rpm,
it could be very interresting is to use and 8XL (yeah I know it way too much but you can use it on another hull later on, and there's a small price difference with a L serie motor), with 6 cell, under load, it'll be just below 15K rpm, try to get a cheap Mystery80Amp brushless ESC on Ebay, and it'll cost you less than 100Euro (motor, prop and ESC), if you get to the point that the prop cavitate, you can always use a gearbox to lower prop rpm and use bigger prop

once your boat is under control (after few hundreds of run!), you can try to put 1 or 2 extra cell and it'll fly!!!

[kck741]

I thought if you limit the amount of current to a brushless motor,it hold back current in the ESC therefor leading to heat,and then fail.thats why brussless motors perform best at there peak!

[ED66677]

I'm not sure it is really possible to limit current easily, in DC current system limitating current means reducing voltage or increasing resistance, and I'm not talking of current limitation but reducing amp draw, once again, a 8XL is given for 88amp max (under 40volt max as far as I remember, more than 3000Watt!!!), that doesn't means you can't run it at 30 or 40 amp, such amp draw represent a quite nice power under 6 cell, think about it, a 600Race motor is given for 110 watt max power, with any brushless motor that can drain that much amp under 6cell you'll have twice this power, the big difference with the 600 class is the efficiency, around 70% for brushed motor, close to or more then 90% for brushless, and you are right on that point, because your 600 at 30amp under 6 cell is probably around 50%! choice is made!

I made a simulation with FECalc, 600 7,2V, 6 cell, K43,5 carbon prop (surface piercing situation, so only 22mm more or less sub prop theorycaly), 32amp, 17,6mph, 10500rpm and 55% efficiency!!! you'r already out of range with the 600!
Same for the 8XL with a K40,5, FECalc says 32amp,21,6mph, 13799rpm and 85,8% efficiency, a 9L for example will have a higher kv that, in surface piercing scheme, gives better speed and equivalent amp draw and efficiency but at 18000rpm!!!

Remember that double power doesn't means double speed!!! it's some kind of exponential!!!

My advice is to take a powerfull motor that you'll be able to use on a better hull, and use it in you given condition, 30-40amp 6-8cell, at 40 amp the 8XL will not require water cooling!!!

[kff]

Hello ED,
thanks
There is a motor calculator for boat ???? FEcalc ????? I don't know it : where can I find it please ???

For my electric aircraft I use Motocalc for years and it a great tool.

[kff]

I've just download FECalc, will see how it works.

[ED66677]

you can take any data from motor, prop, cell and add them to the txt file, but do so only one after the other, if you make any mistake FeCalc won't start!!!

Attention FeCalc is destined for surface piercing prop and not sub, so for aproximation divide prop diameter per 2

[kff]

devide by 2 ????? for sub prop !!!!

No, x2 I think because sub prop need more power than surface prop !!!!!

I've done a simulation and for a speed 600 7.2 (I have created a new line in the database with my motor specs) , I've found with a octura x438 the same current than reality (27 amp with 7 cells).

the x438 is a 38 mm diam and my prop is a 30 mm prop !!!!!!!!!!!

[ED66677]

nope! divided a surface piercing prop only has half of it in water, so one blade must be equivalent to the 2 blades of a sub prop!!!

If you make a simulation with a K45 in FeCalc then you have to consider that the appropriate sub prop must be around 25mm!!!

[kff]

Yes you're right
but in my case to find the reality measurement of 27Amp at full speed I have to take a X438 octura witch is a 38 mm prop and in my case I've got a 30 mm prop and not a 17 mm : there is a big difference.

Never mind. in my case (je fais un calcul relatif et non absolu : le but est de trouver le moteur qui me fera rester dans un ampérage résonnable : sorry english readers it 's sometimes difficult for me to speak technical english)

If a take a FG 540 12L instead of my brushed motor , with the same parameter , I am in the 27 amp range but with 30W more power to the prop and 2 mph more.

If a take a FG 540 10L : 38 amp and 90w more power to the prop and 5 mph more.

(I won't take a XL because it is too heavy et je ne veux pas plomber le bateau)

that's what i'm looking for !!!!

what do you think about that.

have a nice weekend
see you on monday

[ED66677]

somthing you have to take to account is the fact that if you don't have a tool like the EagleTreeMicroPower E-Logger or DPR, you can't really be sure of what's going on in the boat, you originaly say that your setup is pulling 27amp, how did you do that? with a multimeter and static boat in a pond? that's is certainly a bit far away from reality, if my guesses are correct, you then did measure while the boat was not moving on water and then create more resistance to the prop, when running your setup probably pull only 20amp!!!

the XL is only 100g heavier than the L, won't be a big issue!!!

[kff]

Hello ED,
thanks for our help,

I'm sure about my setup because I made few runs with a wattmeter inboard.
I fly electric aircraft and heli for many year and I have every tools (wattmeter, DPR II ... ) to check a setup.

with 7 cells
27,15 amp max : 8.15V min at full speed with slow accéleration
29,54 amp max : 7,95V min fast acceleration

with 6 cells
20,42 amp max : 6.66V min at full speed with slow accéleration
23,47 amp max : 6.50V min fast acceleration

best regards

[ED66677]

OOPS!!!
cool then!

[kff]

hello ED,
You could'nt know !!!!!
Many thanks for your help, I've got now a idea on how it's work.
time to find the right motor, and I'm not in a hurry.
I'll tell you my choice and some DPR infos if the first runs.

regards

[kff]

Hello,

just happy to tell you that my mono hull brushless conversion works great.

I bought a feigao 540 12L (2050 kv) at : offshoreelectrics.com
(thanks to Steven for quick postage)

I put a aircraft controller (Xpower 60 bec : same as hextronik) : converted with home-made cooling jacket.

I use my stock propeller (immerged) and my stock 7-cells RC2000 nicd.

The boat is more powerfull with better speed for 5 minutes.

motor and controller run cold :

I use my DPR : power is 220W and amp is 27A , so it is about the same than my old speed 600 but with a better efficiency.

I am happy about this conversion for few dollars

REGARDS
Olivier