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Old Jul 12, 2013, 12:39 PM
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Putting together an outboard drive

Talk about diving in the deep end. I am not a complete novice when it come to RC boats but this new project I have embarked on has taken me well out of my comfort zone with a high end scratch build.

To keep field repairs simple for this project I have opted for and outboard. The boat will not be built for speed but for towing power. Any feed back in addition to helping me with specific question I have will be greatly appreciated.

First point; matching a motor to an outboard leg. After tolling through rafts of web searches and various conversations with people I also took outside of their comfort zone, I settled on the Brushless outboard leg Large manufactured by TFL. That seems straight forward enough but now I have to match it to a motor.

In searching for a slow spinning motor for the larges prop I can attach is found the NTM Prop Drive 35-48 Series 900KV / 815W which seems like a reasonable starting point.

Here is my problem. Reading through all the info I can find on the Outboard leg I know that the motor shaft is the correct size but its casing is 1mm smaller in diameter than the acceptable range specified for the outboard leg.

Can anyone confirm if these two components will bolt together without any issues? Also, can anyone suggest a better combination? Remember I am going for pulling power not speed.
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Old Jul 14, 2013, 08:45 AM
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WOW! Glad I'm not in Australia! Lotta money for that lower unit!
No experience with what you are attempting, I go fast.
As long as the bolt pattern matches up you can pretty much mount any motor.
My new rescue boat project will be running a brushed high turn rock crawler motor turning a large diameter low pitch prop. Inboard though, and solid shaft so I have no issues with running a flex cable the wrong direction when/if I need reverse.
Just a basic hull to tow a line that will snag dead race boats and bring them back.
New territory for me but all the guys I talked to suggested a brushed setup over brushless.
The hull I'm using is one of my old retired 30" inboard deep vee race boats just to give a bit of perspective.
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Old Jul 14, 2013, 10:16 AM
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Quote:
Originally Posted by Sibe View Post
..........................As long as the bolt pattern matches up you can pretty much mount any motor.............
Hi Marcus,

I think Sibe is correct, being 1mm smaller should be ok. I would buy the lower leg first and do some measurement tests before buying the HK brushless. Then check here for adapters if you do need one. Also, maybe post this on the Fast Electrics forum to see if anyone there has any experience with this combo.
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Old Jul 14, 2013, 04:00 PM
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Diameter doesn't really make much difference, the motor sits on a flat plate, as long as the lower unit has a standard 25mm bolt spacing, that motor WILL fit as it has a 25mm bolt pattern. I'd assume since they say 36-40mm motors it has a standard spacing.
I just have no idea if it's a good match for the intended use. My outboards are set up for all out speed on a course, and I know nothing about motor selection for going slow!
The other thing I would look into is replacement flex shafts and the stub shafts the prop fits onto. The bronze bushing the steel propshaft rides in wears out on all outboards, it's just considered a consumable and replaced when the prop starts to wobble.
The flex shaft will rust unless it's removed, dried and lubed after a days running. Most standard outboards you unscrew the propshaft (reverse thread) and the flex shaft slides out.
This goes for an inboard as well, just part of the maintenance before it gets put away.
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Old Jul 15, 2013, 02:50 AM
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MarcusAdler,
The outboard unit looks like an OS clone, dear me. These units are built for speed, not towing.

Not saying you can't use it for your application, but you'll have to remove the skeg for any prop bigger than say, 47mm. Case size irrelevant, just worry about the motor mount hole spacing, as mentioned, mount, coupler size etc.

Where are you? I might be able to help. PM if you don't feel comfortable posting details here.
Pete
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Old Jul 16, 2013, 11:49 AM
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@Sibe You are the second person who has suggested going with brushed motors. What is the rational behind that? Is it simply a cost thing?
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Old Jul 16, 2013, 11:57 AM
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@10x8 once I get my head around a standard setup with motor, esc and battery selection to go with the leg, I was planning to play with larger props by removing the skeg. The only issue I can see with removing the leg is the what to do about the water intake for cooling. Given this is not a racer I won't be running the motor flat out and I am hoping that selecting an esc with a higher current rating than necessary will avoid the need for water cooling.

Any thoughts anyone?
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Old Jul 16, 2013, 05:00 PM
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I'll be honest I have no idea why it was recommended to me! I was ready (still am) to go with a relatively large brushless outrunner, more efficient motor, can be run both directions with equal power.
Kind of wondering if the tugboat guys just don't know that much about brushless stuff, they all seem to love the antique Dumas setups.
I'm considering using some kind of 12v pump for cooling, not sure I'll have the speed to use a simple water pressure setup.
Going with water cooling because it's going to be towing stranded race boats, fan cooling like on a car esc may very well be enough for your application.
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Old Jul 16, 2013, 06:43 PM
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Most likely because brushed is much easier to calculate and get going, but brushless is the way of the future. Right now low KV high torque brushless motors (what you want in a tug) are few and far between, but the landscape is improving. The current crop of low KV high torque brushless motors are outrunners, which means you have to deal with a spinning motor case in your tug. For an outboard, you would get higher performance and reduced weight, especially with lithium battery packs. And I think you would be better off using an inrunner, not an outrunner.
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Old Jul 18, 2013, 07:49 AM
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I have found a 900kv inrunner which doesn't look too bad.

For water cooling, given that I am very likely to remove the skeg to accommodate a larger prop I cannot plan on using the simple water pressure or prop wash water intake either. I have not research them yet, but my first thoughts turned to fish tank pumps.or medial fluid pumps.

Before I go down this path I am going to run some load tests in the bath and monitor the temperature of the ESC and motor. I can play with fans but if normal convection doesn't keep them cool enough with the low power drain then I don't really like the idea of fans in my boat.
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Old Jul 18, 2013, 08:44 AM
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http://www.hobbyking.com/hobbyking/s...li_880kv_.html

This is what I'm thinking, I have one that's 2000kv, another buddy runs one that is about 1600kv on 6s with his outboard tunnel.
Tons of torque with these things and a reasonable price.
I'd be running the 880kv one on 4s, should put me in a good rpm range for "scale" towing speeds depending on the prop.
I have no worries about using an outrunner in a boat, the spinning motor shouldn't make any difference at all, not like you have your face or anything stuck in there when it's running! And well if you do you get what you deserve! LOL probably be the person that tries to lick a spinning prop!
I haven't looked into pumps yet either, I'm projecting a very low amp draw so I really don't know if I'll need it, but am keeping the plan in reserve.
I have a ton of data collected from the fast boats, be interesting to see what the logger will say about this setup when I get it on the water.
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Old Jul 18, 2013, 02:07 PM
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@Sibe - Well that is a bit embarrassing. I was just going over my design specs. again and realised the 900kv motor I have called an inrunning is actually an outrunner. Like you though, I am not planning to stick my tongue on the running motor. Stopped doing things like that after I my tongue got stuck to a lamp post for 10 hours last winter. I have since bought a temperature gun.
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Old Jul 22, 2013, 08:24 AM
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That should run good on 2s lipo for pulling Marcus
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Old Jul 28, 2013, 12:41 AM
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http://www.rcgroups.com/forums/showthread.php?t=1948131 something like this?
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Old Jul 28, 2013, 11:28 AM
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Can I ask a silly question? In your first post, simple and cheap was key, as it's only used for towing. If so, a fixed shaft is most likely the simplest and cheapest drive line around. And it gives you the most freedom in swapping out motors later. If you want one step up, how about a Kort nozzle? Then one step up again to a vertical shaft with directional thrust like the commercial units from Graupner.

Dave
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