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Aug 15, 2009, 11:53 AM
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Discussion

Outrunner in a boat


Trying to select an outrunner for a boat build and mostly trying to figure requirements is coming up for airplanes. Where can I figure out motor needs for boats, like amps, watts, prop size etc etc.
Thanks!
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Aug 15, 2009, 06:33 PM
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Thor For this project you are kinda on your own, as outrunners in a boat is still very much a new project. Been mulling one for a tug project for the less maintence point of view. Parameters are 6,000 to 8,000 rpm on the prop, reverse required, been looking at the crawler truck guys because they use the basic numbers we need. A ton of torque, lower rpm's, lower amp draws, weight is not as big a factor but might like to try LiPo 3S just for the Wow factor. Am tending towards the Novak Goat setup but the prop will be determined when I get it in the water. Hope this helps.
Aug 15, 2009, 07:22 PM
NeverAgainVolunteerYourse lf
nick_75au's Avatar
You need to determine what sized prop, then work out the RPM you need to drive the boat at scale speed. From there you can balance voltage and motor Kv to achieve the desired RPM.
Unless its a largish planing hull you wont see the motor pull the rated maximum amps of the motor.

If its a fast electric then everything I've said goes out the window

To work out what I needed for my Springer I knew 2 facts, I had 6 volts and a 40 mm prop, previous Springer gave me a figure for prop speed of around 5000-6000 RPM so 6000 rpm divided by 6 volts gives me a figure of 1000 Kv for the motor.

Generally a motor diameter the same size as the prop in a brushed motor gives a good starting point. But with outrunners a smaller diameter is Ok due to the extra torque.

The length of the motor also has an impact on torque, the longer the bell the more torque but usually lower KV as well so a larger prop can be used

I ended up with a 22-20 motor, 1000 KV motor which draws 3 amps, much less than the 17 amps the motor could handle in a plane.

Here's another example but not tried, my Nexus tide currently has a 24 volt motor spinning a 100mm 4 blade prop at 2200 rpm or so. It has good scale speed.

Say I wanted to keep it on 24 volts so 2200 rpm / 24 = 100 Kv, that ends up to be quite a big motor if I go by the massive range on the Hobbycity site, they have a 130 KV motor.

OK thats not ideal so lets drop the voltage to 12, 2200/12 closer to 200KV
any motor near this level will work up to the 300 KV, the motor will probably run at about 75-95% of its rated KV.

Neither of these motors will get close to the rated power, this is a good thing as water-cooling a outrunner poses difficulties.

Regards
Nick
Aug 15, 2009, 08:34 PM
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Great info, Nick!
Aug 15, 2009, 08:44 PM
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imgumby's Avatar
Been running an outrunner in my springer for the last year or so with good results........6v lead acids and a 1000kv motor with a 3 blade 1 1/2" prop.

imgumby
Aug 15, 2009, 09:27 PM
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Thanks Aerominded
Imgumby what motor and ESC do you use, mine has been running for a around the same time.

Nick
Aug 16, 2009, 06:43 PM
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jaguar75's Avatar

Brushless Motor


The best motor is a Novak #3420
Aug 16, 2009, 07:36 PM
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imgumby's Avatar
Quote:
Originally Posted by nick_75au
Thanks Aerominded
Imgumby what motor and ESC do you use, mine has been running for a around the same time.

Nick
Hi Nick
I am just using a cheap hobby king outrunner of approx 45mm (??) diameter with a 5mm shaft and a turnigy 60 amp marine esc I had laying around .I had the chance to push around a 60 lb barge a few weeks ago which it seemed to handle just fine.

imgumby
Aug 16, 2009, 11:39 PM
NeverAgainVolunteerYourse lf
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Jaguar, thats an Inrunner
Can you qualify your statement.
My opinion is the KV is too high for a Springer tug on direct drive, those motors are designed with a gearbox in mind. Should have good low speed though due to the sensored design.

Imgumby Im using a 22 mm motor, 40 mm prop which has pulled our rescue boat, with passenger all up weight about 300 Kg.
regards
Nick
Aug 17, 2009, 09:05 AM
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Thread OP
Thanks for the post Nick, good stuff, but now I'm wondering......how do I figure out RPMs at the prop? I'll be running a Graupner outdrive w/ 33mm 3 blade prop on a 16" Dumas hull.

Yeah, the Novak is longer than I want, which is why I'm looking at an outrunner. I have a couple Novak brushless systems, one being the Crawler, and may end up using one of them.
Aug 17, 2009, 09:47 PM
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jaguar75's Avatar

Best Scale Speed Brushless Motor


Quote:
Originally Posted by nick_75au
Jaguar, thats an Inrunner
Can you qualify your statement.
My opinion is the KV is too high for a Springer tug on direct drive, those motors are designed with a gearbox in mind. Should have good low speed though due to the sensored design.

Imgumby Im using a 22 mm motor, 40 mm prop which has pulled our rescue boat, with passenger all up weight about 300 Kg.
regards
Nick
I have researched the heck out of brushless motors and talked to the factory builders and the Novak 3420 is the best for direct drive and low speed and it will go forward/reverse instantly.It is programable directly or on the pc.The Novak rep is telling me that they are going to try and talk with more scale rc sailors out there on these forums about spreading the word.
Aug 17, 2009, 09:57 PM
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jaguar75's Avatar

Gearing


Also the factory rep said that a gear reduction drive will overheat the motors but you dont need it anyway.They will run direct drive and this why i am using 2 of these in a 94.5" submarine.The only thing you need to do is put a water radiator on the esc.They run hot using them for low speed or high speed.The esc has a 6 volt output for an optional fan so you can utilize that output for a 6 volt pump.The escs also come with a built-in BEC,lipo battery level protection switch and thermal cut-off switch.
Aug 18, 2009, 12:10 AM
NeverAgainVolunteerYourse lf
nick_75au's Avatar
Im guessing being an outdrive that its a planing hull, guessing its something like the Short Stuff?
Assumptions made from here on in based on Dumas Short Stuff.

You will want a higher Kv than what I have, say 1500 Kv, will spin at around 10000 RPM on a 7.2 Nixx pack, then if you want more speed just up the pack voltage to 8.4 or 9.6 volts. Lipo is a little less flexible due to the 3.7 volt per cell.

something like this perhaps, has almost the same watts as the crawler motor, 80 watts, 1/10th the weight......
http://www.hobbycity.com/hobbycity/s...trunner_1550kv

Or this, same speed but more powerful, will handle a bigger prop. 115 watts
http://www.hobbycity.com/hobbycity/s...less_Outrunner

If you don't want reverse a 20 amp air ESC suitably cooled will be fine

Nick
Last edited by nick_75au; Aug 18, 2009 at 12:17 AM.
Aug 18, 2009, 07:40 AM
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Jaguar while I agree that it is a good, even excellent system with advantages, it still has some disadvantages,

One of the warnings is don't get it wet, I got literally 1 drop of water on my Novak esc and it fried. Yes, that was my own fault
My setup in the Springer has run inadvertently fully submerged, 6 months later my ESC played up, I pulled off the shrink wrap cleaned off some corrosion and away it went. Never even pulled the motor out of the boat.

Here's one I love, in the spec's, 160 amps current, sounds really good, then they say 225 watts, ok that sounds OK, until you do the maths, 160 amps at 8.4 volts is 1300 watts BUT 225 watts at 8.4 volts is 27 amps WT....run that esc at 160 amps = instant smoke generator.
Marketing hype is what that 160 amps figure is.

When they produce a sub 1000 Kv motor then the tug and displacement hull market will be satisfied, 1800 kv without a reduction gearbox is too high for these applications. What is with them telling you that a gearbox will overheat the motor Reduction gearboxes will reduce current full stop on the same sized prop. Anyway what is the transmission on a crawler buggy, maybe a big fat gearbox .
If you run the Novak motor on the minimum specified voltage of 4.8 volts its still 8500 rpm, that's a lot more than most displacement boats would use.

My esc is rated at 40 amps and 2-5S lipo, 6-16 Nixx.
18.5 volts by 40 amps is ...... 740 watts.
It can be programmed by computer or manually, has lipo protection, comes with a water cooled heatsink, does not have bec but in a boat that's no problem usually, in my Springer I just run the receiver off the main battery anyway. Oh and an adjustable delay for direction change from 0.25 sec to 2.5 sec.

Ok enough of that, what props are in your submarine, is it complete? It sounds like an awesome machine.

Nick
Aug 18, 2009, 08:34 AM
Registered User
Thread OP
Thanks for that Nick, and yes, it's the Short Stuff!

Jag, yes Novak makes a fine brushless system, and I love them for my cars and crawlers, but in the wet, they are too sensative. Also, if the rep told you they are PC programmable, that's wrong, unless they have changed very very recently CC is the only PC programmable ESC's. I have an 18.5 Crawler system, and it's torque and RPM's are very impressive.
Last edited by Thorsteenster; Aug 18, 2009 at 09:18 AM.


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