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Jun 23, 2017, 05:27 AM
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Need motor advice for 850 mm boat. Electric Graupner Key Biscayne


I have an old RC boat that I think used to have a 700 motor and NiXX batteries. After a failed conversion to glow engine it's time for electric again.
The boat is a "deep V" design with about 850 mm length.

I want it to be supermegapsycho fast. I'm already in possession of two 6s batteries that I would like to use.

I'm looking for a water cooled brushless motor. I'm absolutely not updated on this stuff so any advice would be greatly appreciated.
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Jun 23, 2017, 05:37 AM
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Johnnysplits's Avatar
Two 6s batts are a lot of juice that will put you in supermegapsycho speeds. Might want to get some 3s lipo's for this project. Maybe a 36mm motor about 1600Kv.
Jun 23, 2017, 05:41 AM
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Is this one too fast?

graupner key biscayne 3s 45 C 4200kv (0 min 47 sec)
Jun 23, 2017, 05:59 AM
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That one looks good. Maybe I could go a little faster? What would I look for , just 4200kv or is the dimensions of the motor important?
Jun 23, 2017, 06:35 AM
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Well, the can diameter and length coincide with the amount of torque a motor will have. Being that you're not looking for crazy speed, and the boat is small and light, you could do a 2960 3300Kv motor on 3s with a 36-38mm prop. 4200Kv on 3s is too much for a fun, run around boat.
Jun 23, 2017, 07:16 AM
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The boat in the video looks fun though a bit slow on the straight runs.

Thinking of using the 6s batteries with a voltage stepdown so that I can get maximum speed by turning up the V if it disappoints.
Jun 23, 2017, 07:30 AM
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I found one on eBay, 3680 max 34 volts. For 800-1000 mm boat. 2000w 50 000 rpm.
My calculations tells me that it would deliver around 1400 watts with 6s. Could it do the trick?
Jun 23, 2017, 07:40 AM
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A ESC with active freewheeling would be a good bet but Im not sure if they make them for boats or not.
Jun 23, 2017, 08:49 AM
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Quote:
Originally Posted by rccp
I found one on eBay, 3680 max 34 volts. For 800-1000 mm boat. 2000w 50 000 rpm.
My calculations tells me that it would deliver around 1400 watts with 6s. Could it do the trick?
Sounds right.
Jun 24, 2017, 11:10 AM
Boaters are nice people.
Hi RCCP,

If you're planning on keeping the steering outdrive, I wouldn'd go beyond something like this:
Key Biscayne de Yoggi (1 min 12 sec)


If you replace the steering outdrive, this is another option:
https://youtu.be/2TlW4GshLXU
I don't say it's a wise option, because it's a very light, thin ABS hull and a set-up like this is already way past it's design limits...

The 1500Kv GOOLRC motor is a nice 1500Kv option:
http://www.ebay.com/itm/GoolRC-3680-...-/222323137242
On 6S you'll have 30.000 rpm under load, start with a 40 - 42 mm two blade prop and work your way up (propwalk and torque issues will grow with propsize), use (at least) one turnfin on the right side to avoid spinouts at speed (turning right offcourse).
Left hand turns with a single prop boat will always be wider.

Regards, Jan.
Last edited by pompebled; Jun 24, 2017 at 11:16 AM.
Jun 27, 2017, 08:12 AM
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The seller who has those goolRC motors isn't "authorized" to sell to Europe so I ordered a 3670 2150kv with 1600 watts power. That means 50 000 rpm.
Will the original flexshaft handle that force?
Jun 27, 2017, 10:59 AM
Boaters are nice people.
Hi RCCP,

You didn't fill out your profile completely, so we don't know where you live, the seller from my link does ship (free) to the Netherlands.

A 2150Kv motor is 4S territory in my book, 5S at most.
Do you have a link to it?

On 6S you'll have about 38000 rpm under load, which is 'rather a lot' for the stock drivetrain, to put it mildly...

The stock steering outdrive with the flexible section and the bronze bearings will be reduced to rubble in no time.

If you want to use this motor, either go down in cellcount, or change the hardware to a strut, or stinger with a flexshaft and seperate rudder.

Something else to keep in mind is the fact this ABS hulls were not designed for the speed a brushless set-up can generate, the thin ABS will flex under load and is prone to breaking when you hit something, reducing the entire hull to a jigsaw.

If you read up on the subject, you'll see that most of these ABS hulls have been beefed up with glasscloth and epoxy before there's roughly ten times the stock power installed.

Regards, Jan.
Jun 28, 2017, 12:08 AM
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Yeah, I dont understand why the guys with abs hulls dont reinforce them anymore.

I prefer 3/16th plywood and epoxy resin over the entire transom to reduce flexing even if its just for the rudder.

With an od its almost manditory.
Jun 28, 2017, 03:21 PM
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I'm sitting here in Sweden. Thought the motor didn't have a CE certificate but could be another reason.
http://www.ebay.com/itm/172232069973?redirect=mobile

That's the one I'm waiting for. Might as well reinforce the hull in the meantime and maybe step down a cell or two ��
Jun 28, 2017, 03:34 PM
Boaters are nice people.
Hi rccp,

I hope you also ordered a fitting cooling jacket (a 55 or 60 mm long one, car motors have the wired coming out of the side...), or the fun will be short lived, as these motors have lots of airflow when used in a car, but will run very hot when not watercooled in a closed boat hull.
Something like this: http://www.ebay.com/itm/GoolRC-Alumi...3D130606097733

Going down in cellcount is a good idea.

Regards, Jan.


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