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Author Topic: Brushless motor replacement for Johnson 750 motors  (Read 1431 times)

mudway

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Brushless motor replacement for Johnson 750 motors
« on: December 06, 2016, 02:30:59 AM »

Iíve just picked up a couple of boats from a friend who is no longer able to run them, a 32Ē Riva with a 35mm plastic 2 blade prop running on 7.2 volts Nimh and a 34Ē Fairy Huntsman with a 45mm 2 blade plastic prop running on 10.8 volts Nimh. They have some sort of Johnson 750 in them i.e. no name or anything marked on the casing.   As one boat pulls 6 amps no load with the 10.8 AH Nimh it came with, I figure it is time for them both to have an engine change.
Brushless seems to be the way to go. My local shop has a couple of 3648 size motors, one a DYS at 1,100kv and the other a Turnigy at 1,450kv. I was thinking of the 1,450 in the Riva on 2S and the 1,100 in the Huntsman on 3S. I want fast, not ballistic.
Has anyone used these motors?
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canabus

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #1 on: December 06, 2016, 05:14:59 AM »

Hi mudway

The 3648-1100kv on 3S with a 40-45mm 2 blade will work OK in the Fairy Huntsman.
I am using this setup in my Sea Commander(60 amp ESC).
The 1450kv if it's the 3648-1450kv is a bit of a beast, but, on 2S maybe OK.
I would use the same 1100kv same prop size, but, if you require the Riva to be quicker change to a 4S Lipo.
The Huntsman will handle a 5800amh, but, with the Riva a 3700mah would be better.
As for an ESC, I use the 60amp car one's(HK-60A SL with the program card).
Same setup for both boats on 3S, but, only the cutoff voltage change for 4S.
I use the same setup for al my boats(nice and simple).
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mudway

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #2 on: December 06, 2016, 08:19:01 AM »

Canabus,
Thanks. The Riva used to carry a pair or 2AH Nimh batteries in it either side of the motor. Luckily, these battery boxes are the right size as some 3AH 2S lipos I have. Hopefully, 6 amp hours ought to keep me happy for a while. The Fairy is in a sorry state and need some TLC before it appears in public but I'll get it running again first.
Then again, a 60 amp esc.  :((  How many amps are you pulling?
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canabus

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #3 on: December 06, 2016, 10:40:55 AM »

Hi mudway
The 60amp is what Hobbyking what I had at the time.
We tested a few props on a Spearfish boat from 38 to 50mm and around 40amps at top speed (40km).
The 38mm was the lowest amp, highest speed of 43km.
The 3639-1100kv motor would be an easier fit into your setup as it is front mount not rear mount.
I use solid coupling from motor to drive line, had one red universal fail, so my mate made me a few solid one's !!!
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mudway

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #4 on: December 06, 2016, 11:43:05 PM »

 
 
 
Canabus,
Thanks. Oh good, 60amp had me worried.
Iíve been using these on 24 > 28 inch models without any problem. https://hobbyking.com/en_us/hobbykingr-tm-brushless-car-esc-30a-w-reverse.html
I might start the Huntsman on a 35mm prop and work up to 45 if Iím not happy. First I need some test tank hours to check the in water rpm and amps pulled on various props for these motors.
Both these motors are front mounted and fit quite happily on some aluminium 500 series motor mounts I picked up last time I was in the UK. Also, the motor mounts in the boats were bolted down to 1/8th inch thick aluminium plate to help act as a heat sink.
I use the black couplings instead of the red ones for high power motors courtesy of John Darke. Talking of him, if all else fails, I have a couple of his non geared 700 specials I could use in the hulls. They weigh in at 390g compared to 350g for the ones in the boats and 180g for the brushless ones. I know the Darke pulls 8 amps flat out on a 35mm 2 blade prop using 3S lipos.
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mudway

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #5 on: December 07, 2016, 04:11:01 AM »

Both these motors are front mounted


Oops no they are not, only the 1450 is.
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canabus

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #6 on: December 07, 2016, 08:52:32 AM »

Hi mud way

A MICK STEAK !!!

The 3648-1100 kv will work if you turn the old mount around.
The 3648-1450 kv would wasted in the small Riva on 2S, this motor is for a larger fast boat.
My Sea Hornet has a B28-57-15L (28mm) on a 3S 2650mah with a 32mm 2 blade prop and that's fast.
The 1450 has drop off the Hobbyking site.
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mudway

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Re: Brushless motor replacement for Johnson 750 motors
« Reply #7 on: December 07, 2016, 09:40:37 AM »

Hi Canabus,
I picked up three 3648s today. The shop guy went to put the DYS 1100 into its box and discovered that it was actually a full box with a 900 in it. Got all 3 for $60 so canít complain. Basically he wanted to get rid of them. I can understand it for the 1450.
I tested all 3 this afternoon on 2S or 3S and a 35mm 2 blade prop. Now thinking not to bother with anything bigger and may be dropping to a 30mm.
The 1450 ďshudderedĒ (for want of a description) on 2S turning less rpm than the John Darke and more amps in the process. It howled around on 3S but no idea of the amps pulled as my ammeter only goes up to 20 amps.  There must be a use for it but no idea what.
For the time being the Riva is going to have a quick transplant to a John Darke special as it is the same size as the current motor so drops straight into its motor mount but gives over 2,000 more rpm in the water pulling less amps. If Iím not happy with how it runs, one of the DYS on 3S will end up in it.
Iíll use one of the DYS motors in the Huntsman which is over 1kg heavier than the Riva but may be on a 30mm prop. The DYS are quite impressive motors.
The test results are attached.
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