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Old May 25, 2012, 08:33 PM
TRex 450,500,600, Dragonus II
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Australia, QLD, Brisbane
Joined Nov 2008
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How do you workout what Kv motor for your hull

I have a 45 inch cat hull I want to run twin brushless outboards, i am either looking at running 4s or 6s lipos. How do you guys work out what Kv would suit a hull size or how you want the boat to perform. I just need some advice on what Kv motors to look at for 4s or 6s setup on a twin brushless 45inch cat hull. I know everyone just looks at speed but I would like to also have a reliable setup as well.

Cheers
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Old May 25, 2012, 08:39 PM
Redneck Injuneer
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Bradenton, Florida
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1500 to 1600 kv should give you a good reliable setup for twins. It should let it run decent on 4s and "haul the mail" on 6s.

Edit: running outboards you could get away with running outrunners. That will help keep the tail weight down.
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Last edited by bill34207; May 25, 2012 at 09:14 PM.
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Old May 25, 2012, 10:21 PM
TRex 450,500,600, Dragonus II
hp88's Avatar
Australia, QLD, Brisbane
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Quote:
Originally Posted by bill34207 View Post
1500 to 1600 kv should give you a good reliable setup for twins. It should let it run decent on 4s and "haul the mail" on 6s.

Edit: running outboards you could get away with running outrunners. That will help keep the tail weight down.
Thanks for the reply mate, good point on keeping the weight down on the rear and using out runners .

Cheers
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Old May 25, 2012, 11:18 PM
Thermite + ice = Big boom.
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As i'm assuming you know from heli's kv isn't everything, look at a 550 or 600 size heli motor or equivalent in the 1600 or 1400kv. You want to pair approx. the same cell count to kv as you would in helis without the gearing, ie. 2200-4S 14-1600-6S etc.
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Old May 26, 2012, 12:15 AM
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The right motor for the boat? I thought it must first be worked out how many watts is needed for the hull? Then you decide how many cells you are going to use, then only you decide the KV rating?
My fastest boat is only a BJ26 but I've read through a lot of built threads and I think for a 45'' cat I would look at 1800KV T600 outrunners on 6S. I wont run 4S on a 45 inch boat as it will generate too much heat. You can then always run this combo on 4S if you don't need all the speed for a specific day.
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Old May 26, 2012, 12:22 AM
Thermite + ice = Big boom.
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Sarcfan, the t600 would be a great choice for the hull, but 1800kv is a little high for 6S, the 1600kv would be a better option. You could also look at the fightercat TP power motors (4082's or 4092's) or the 3736 suppo outrunners from fightercat.
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Old May 26, 2012, 12:23 AM
TRex 450,500,600, Dragonus II
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Australia, QLD, Brisbane
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Do you think this is what I would be looking for to power my twin outboards Scorpion HK-3226-1600kv , always liked the Scorpion motors.

Specs,

Stator Diameter 32.0 mm (1.26 in)
Stator Thickness 26.0 mm (1.02 in)
No. of Stator Arms 9
Magnet Poles 6
Motor Wind 7 Turn Delta
Motor Wire 26-Strand 0.23mm
Motor Kv 1600Kv RPM/Volt
No-Load Current(Io/10V) 2.19 Amps
Motor Resistance (Rm) 0.011 Ohms
Max Continuous Current 70 Amps
Max Continuous Power 1550 Watts
Weight 239 Grams (7.77 oz)
Outside Diameter 40.9 mm (1.61 in)
Shaft Diameter 4.98 mm (0.19 in)
Body Length 50.5 mm (1.99 in)
Overall Shaft Length 75.5 mm (2.97 in)
Max Lipo Cell 6s
Max Peak Current 90A (2 seconds)
Max Peak Power 2000 Watts (2 seconds)
Motor Timing 5deg
Drive Frequency 8kHz
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Old May 26, 2012, 12:30 AM
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Quote:
Originally Posted by hp88 View Post
Do you think this is what I would be looking for to power my twin outboards Scorpion HK-3226-1600kv , always liked the Scorpion motors.

Specs,

Stator Diameter 32.0 mm (1.26 in)
Stator Thickness 26.0 mm (1.02 in)
No. of Stator Arms 9
Magnet Poles 6
Motor Wind 7 Turn Delta
Motor Wire 26-Strand 0.23mm
Motor Kv 1600Kv RPM/Volt
No-Load Current(Io/10V) 2.19 Amps
Motor Resistance (Rm) 0.011 Ohms
Max Continuous Current 70 Amps
Max Continuous Power 1550 Watts
Weight 239 Grams (7.77 oz)
Outside Diameter 40.9 mm (1.61 in)
Shaft Diameter 4.98 mm (0.19 in)
Body Length 50.5 mm (1.99 in)
Overall Shaft Length 75.5 mm (2.97 in)
Max Lipo Cell 6s
Max Peak Current 90A (2 seconds)
Max Peak Power 2000 Watts (2 seconds)
Motor Timing 5deg
Drive Frequency 8kHz
They seem just perfect.
What hull do you have?
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Old May 26, 2012, 12:31 AM
Thermite + ice = Big boom.
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Look for something around the 3000 watt range, those are a little small.
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Old May 26, 2012, 01:59 AM
TRex 450,500,600, Dragonus II
hp88's Avatar
Australia, QLD, Brisbane
Joined Nov 2008
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Quote:
Originally Posted by SARCFAN View Post
They seem just perfect.
What hull do you have?
The hull is a custom designed cat hull from Evolution Rc in Australia, he designs and builds high quality 1/5 scale hulls that are original. This is a pic of the test boat,

Evo C-45


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Last edited by hp88; May 26, 2012 at 02:54 AM.
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Old May 26, 2012, 02:57 AM
TRex 450,500,600, Dragonus II
hp88's Avatar
Australia, QLD, Brisbane
Joined Nov 2008
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I am using two 3.5 cc lower outboard legs with a brushless adaptor ,which uses a 540 motor with 5 mm shaft .
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Old May 26, 2012, 03:04 AM
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What about these? http://www.hobbyking.com/hobbyking/s...dProduct=11980. Over 3000 watts. I think that boat will fly with a pair of these
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Old May 26, 2012, 07:11 AM
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Quote:
Originally Posted by SARCFAN View Post
What about these? http://www.hobbyking.com/hobbyking/s...dProduct=11980. Over 3000 watts. I think that boat will fly with a pair of these
"If" he can get a 6mm coupler and the hole pattern will line up with his adapter those would be a better choice power level wise than the Scorpions.

Most of us that have been boating for a while have already learned it's a whole different game than flying requirement needs. I'm not sure about heli's but with planes most of your flying is done at a power level well below nominal watts for the motor. With boats (especially racing boats) you are hitting the edge of peak power output about 95% of the time.
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Old May 26, 2012, 09:01 AM
TRex 450,500,600, Dragonus II
hp88's Avatar
Australia, QLD, Brisbane
Joined Nov 2008
104 Posts
Ok all good info, I will run 6s but as I said i need a 5mm shaft for the brushless motor adaptor to fit. This is the OS outboard leg ,



This is the adaptor to run brushless

This is the Electrics Conversion Kit for the OS 3.5cc Outboard Legs.

Includes:
Motor mounting plate for Motors with 25mm mounting screw spacing. For example most 540 size brushless motors.

5mm coupler to fit square drive 3.5 Flex Cables.

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Old May 26, 2012, 10:01 AM
Redneck Injuneer
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Bradenton, Florida
Joined Aug 2007
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It would put more weight on the rear but you might consider running something like a pair of these. http://shop.turbofanelectrics.com/pr...&categoryId=33 Not sure if you have a HET distributor in OZ but would think so.

Watt rating is similar to the Scorpion but with the addition of water cooling they can be pushed to near peak wattage for much longer than the 2 second burst shown for the Scorpion.
Specs: http://www.highendrc.com/member/3170...8motorspec.pdf
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