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_marlin_off-shore_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!!!