Brushless Motor - Size?

[Rob_P]

Coming from a model aircraft background there is plenty of data to help match a motor and prop to a plane. Its all a related to watts/oz with published tables for what wattage/oz equates to a type of performance - Park flyer, sport or 3D etc.
Data is readily available to help determine the output (watts) produced by a particular motor / prop / battery.

Does anything similar exist for boats?
Maybe you could treat the KV rating as similar to a gearbox.

I'm currently looking to replace a Speed500 1:1 drive 6cell Nimh with a direct drive brushless outrunner again on 6cell.
The boat is my plywood construction 24" Kingfisher speedboat.

Has anyone had cooling issues with a brushless outrunner because you can't eaxctly wrap some copper tube around them.
Rob

[Rex R]

for 6 cell Nimh's and a 24" boat I'd suggest a motor with a kv in the 3000-3500kv range. note 3500kv has a similar speed to a brushed 27t 05 motor, 4500kv ~ a 19t rpm wise.

[Rex R]

about the only practical way to cool an outrunner(for most people) is to have a cooling plate/motor mount like this http://www.offshoreelectrics.com/proddetail.php?prod=tfl-509b14 hth
rex

[pompebled]

Has anyone had cooling issues with a brushless outrunner because you can't eaxctly wrap some copper tube around them.
Rob

Hi Rob, next to the watercooled motormount, getting the right size motor is equally important; the outrunner has to be able to do the job without breaking a sweat, if you think you've got the right size, go one bigger...

Regards, Jan.

[Shaun Hendricks]

Rob:

I too have posed the same question a few times but there is no "Motocalc" for boats. The closest thing I've found is FECalc but it's not overly accurate, especially for non FE uses.

I've often said there is opportunity here for some company to step in and create a standard and then push it across the electric boating industry. It might be built in such a way to include the IC side of the hobby as well but you can only correlate IC and Electric without introducing some innacuracy points.

Personally, I'd push for a triple standard to be created:

1. Electric & IC standards
2. Drive train standard
3. Prop standard

Those three in place would create a mathematical model of a theoretical application for design purposes. It would eliminate a lot of guesswork.

[jshander]

Rob,
I would have to agree with Shaun.
One major problem is prop RPM
unless running surface drive props the ideal rpm may be in 3000-5000 rpm range and the torque is more critical. Brushless tend to have higher rpms
I am trying an astro 20 geared in my Italeri PT as it is close to a Graupner
400. I have not had a chance to run it yet, but will submit photos soon.
My hunch is that geared reduction is the way to go for submerged props.

Jim

[Rob_P]

How about these - Water cooled brushless outrunners?
To me the trouble is we know the theoretical RPM but no torque info.

http://www.horizonhobby.com/Products/Default.aspx?ProdID=CYLM2000
http://www.horizonhobby.com/Products/Default.aspx?ProdID=CYLM2500
http://www.horizonhobby.com/Products/Default.aspx?ProdID=CYLM3700
http://www.horizonhobby.com/Products/TechnicalSpecs.aspx?ProdID=CYLM5000


If our target RPM is 5,000 then with a (6 cell) 7.2V that would equate to a 690KV set up and I don't see many that low.

Rob

[nick_75au]

There are not many small brushed motors with very low KV either and most require gearboxes as well.
1000 KV at 6 volts is 6000 rpm, will be around 5000 in water and be very efficient as well. KA22-20L motor 1050 Kv outrunner, similar size to speed 400, will spin a 40 mm prop with ease and would spin up to 55 mm (2 inch) with no cooling direct drive.
Ive run a 3200 kv 3:1 gearbox also with no cooling of motor (I cooled esc on that one though.
Nick