I'm planning to build a 1/10 or 1/8 scale boat, mostly a V-hull, could be a hydroplane too. I do not want to invest too much into purchasing new equipment, I already have a bunch of outrunners and I'm planning to use one on my boat.

The question is, what are the possible disadvantages of operating a brushless outrunner submerged in water?
I could think of a few obvious factors like corrosion and drag that could potentially cause inefficiency.

Also, for a boat with an AUW of around 800g, what should be the power of the motor (in Watts)?

It's materials are not designed to work inside water. Why not using long shaft ? The windage loss of motor (here due to friction of water instead of air will be high). The neo-magnets may opens from yoke as being fluid,water is incompressible unlike air.

Anjan, there is no real reason to operate a brushless submerged. You can mount it in the boat, use a coupling and a shaft. But the motor should have adequate air circulation, or very high over specification, as these are designed to be cooled by a forced draft of air. The same is true, maybe more so, of the ESC.

800 gms? Aww, use a 380 brushed....cheap! Or use about 170 Watts upward.

You will destroy the bearings if you put the motor in water Since you are into hacking up objects, I suggest you get yourself a defunct motherboard or a pc smps. Mount the motor on the biggest practicable heatsink and you are set. I run brushless outrunners on some of my diy cars, and they get very hot, so cooling is a must. You might also try water-cooling the motor heatsink

KALYANPRODHAN & Sanjai bhai, thanks for the suggestions. My initial idea was to use a shaft drive with an outrunner, but since I don't have a stuffing tube right now, I was just exploring the idea of fitting the entire motor on the outside of the hull- as ridiculous as that sounds.

lastRites, I'm all set to DIY a stuffing tube with flexible shaft. saves me some hassle in precisely aligning rigid shafts.

Go to a auto /bike garage and ask for a defunct speedometer / tachometer cable. Voila! You have a flexi shaft.

VC bhai, that exactly what I'm using.

The metal wire is a bicycle brake cable, the blue tube is a polyurethane pneumatics hose which seems to be nice for a stuffing tube.
The black tube is the sheath that came with the brake cable.

That PU tube is very difficult to get straight. The cable sheath is good. Try to get some lithium grease/Moly grease/graphite grease

Quote from: sanjayrai55 on September 08, 2013, 09:56:23 AM
800 gms? Aww, use a 380 brushed....cheap! Or use about 170 Watts upward.

where do you get brushed motor

Quote from: sanjayrai55 on September 08, 2013, 02:20:39 PM
That PU tube is very difficult to get straight. The cable sheath is good. Try to get some lithium grease/Moly grease/graphite grease

but the cable sheath ID seems to be too big, and stuffing in too much grease might cause drag around the shaft. The cable fits properly in the PU tube,  might need a little bit of grease though. Anyway, I'm going to test them both to see which makes for a better stuffing tube.

Quote from: rcrcnitesh on September 08, 2013, 03:43:51 PM

Quote from: sanjayrai55 on September 08, 2013, 09:56:23 AM
800 gms? Aww, use a 380 brushed....cheap! Or use about 170 Watts upward.

where do you get brushed motor

You can get it locally from electronics stores that sell robotics kits or other project related material. Since you are in Chennai, try in Ritchie Street.
Or you can buy it online from sites like robokits.com -- http://robokits.co.in/shop/index.php?main_page=product_info&cPath=2_3&products_id=208

thanks
how do you know about ritchie street
and do you know the thrust by the motor you told

I read it somewhere on a blog that Ritchie Street in Chennai is equivalent to SP Road in Bangalore and SP road of course is the electronics market of Bangalore.

Stuffing in grease will cause drag? It's a lubricant!

Nitesh, you should have done a search instead of asking inanely again

Hi Anjan,

If you are looking to transfer the torque from the motor to propeller using a cable, it should be a Tachometer cable not a brake cable. Brake cable might fray while tachometer cables dont. also solder the ends for strength.
also search for grinding cable(that is what they call here) which connects the Dremel tool with small grinding stone tools.(See Pic)
Its more Apt for the job.

Quote from: sanjayrai55 on September 08, 2013, 07:08:39 PM
Stuffing in grease will cause drag? It's a lubricant!

Nitesh, you should have done a search instead of asking inanely again

Yes, too much grease can cause insane amounts of drag. Try swishing your finger around in a bucket of grease

Yes, too much grease can cause insane amounts of drag. Try swishing your finger around in a bucket of grease
[/quote]

Last rites:

When you swirl your finger, you are trying to displace a significant mass of semi-solid.

Try rotating your finger about it's central axis.

We call this "boundary layer theory" (Streeter et al, III year First semester, IIT Bombay, circa 1977 )

Check this page out:
http://openrov.com/forum/topics/seawater-vs-brushless-motors

One of the methods to keep brushless motors safe in water.
If others have done an experiment and got a result which works why experiment again.

Sorry I misrepresented
http://www.machinerylubrication.com/Read/28664/dangers-of-overgreasing-

Hardly the same situation.
In one case you have a flexible shaft coated with grease running in an open tube.
In the case you have referenced, we have a sealed unit (bearing with shields) where grease is being injected at pressures up to 15000 psi.

It is better to let the excess grease flow out in our case, than run dry

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In any case,, we were never talking about over-lubing. Just coating the cable with li or graphite grease

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