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richb77

Brushless conversion comfusion.

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OK. I'll come clean.

This isnt car or Tamiya related. But i know you lot are bright and helpful.

So i managed to get my hands on a NIB Graupner B24 Nimbus boat. Its the GRP hull model from turn of the century and i have been lusting after one since seeing one in the flesh all those years ago.

Issue is it was designed and fitted with (2) Graupner speed 600 brushed motors. I want to bring it up to date with twin brushless.

I am confusing myself silly with trying to do this and need some guidance.

The 600s have are 1845kv (15500 unloaded max rpm on 8.4v nimh) so i am aiming for circa 1800-2200Kv

Efficiency means the 600s are about 150W so anything above that is a win.

I suspect inrunners are easier (direct swap).

I will run 3s Lipos meaning i will get a good but not ludicrous speed increase (and can up to 4s if i want too).

Now. The sizes are whats confusing me. I can get 3674 2200Kv motors (36mm diameter x 74mm Long) or 2845 (28mm diameter x 45mm long)

now both exceed the wattage of the speed 600, both suit the Kv desire so why choose the heavier, larger and more pricey 36mm over the 28mm? Heat? Torque? How does a brushless measure Torque other than Watts?

Thanks for any help as i am going around in circles with this.

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You'll find it pretty much impossible to find torque figures in Nm or lbft for brushless motors, I spent much time in vain trying to find them when I was choosing the motor for my war rig. I would go with the larger motors for 2 reasons.

1. The bigger motors will be less stressed as they will generate more torque than the smaller motors. They will also be more tolerant of fitting oversize props. My rule of thumb is to fit the largest possible motor into the available space, better to have too much torque than not enough  (This is why I ended up with a 5674 in the war rig, and a 4274 in my TXT-1). ;)

2. The 3674 motors will fit in a lot 1/10 rc models, so if you lose interest in the boat you can always re-use the motors in R/C cars, the 2845 will not be re-usuable.

Hope this helps.

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Silly question, but do you have any brushless systems in any of your cars that you could maybe try in your boat before you buy something else..??

Something like a 21.5 or 17.5 sensored BL motor and ESC that would be around the KV ratings that you are looking at..

Brushless motors do seem to have a lot more torque than brushed motors, so maybe one motor will be enough..

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Thanks chaps. I think I will go with the 36mm motors.

Backlash.

Yes I have brushless set ups (2) but they are not matched and the boat runs twin motors so will probably be worse than guessing a size.

I'll drop some cash on a pair of 3674s and if they dont work, tell the mrs they where only a tenner and try again 😜

Thanks for the sanity check and advise sharing 🍻

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I was suggesting that you try a single car motor, not a dual setup.. If you keep an eye on the motor and ESC temperature, what can it hurt?

You may well find that a single brushless motor is enough, and if you're only running a single motor/ESC setup and a LiPo battery, the boat will be much lighter..

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As you probably know in boats it's not about rpm but torque. 

If I were you I would ask you question here:  www.powerboatforum.de

Serious knowledge base with high quality vendors/manufacturers frequenting the site.

 

 

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