Why can (big) outrunner boats not be very..very fast.

[Nevs]

The motors do exist.
https://www.kdedirect.com/products/kde700xf-505-g3
Half the Weight but the same watts as a TP 5850 .. At half the kv. ??

[NativePaul]

They can be, look an Manuel's big outrunner CT03, it is very fast on outrunners.

However in boats they do have a big disadvantage, a water jacket on an inrunner canwick away a hell of a lot of heat from the can which is thermally vert well connected to the windings, and it dumps that heat outside of the boat to the lake. Whereas an outrunner has to make do with air cooling which is not as effective and there is no airflow at all in a boat so what air it moves with a fan at the expense of efficiency remains inside the boat raising the ambient temperature, and making itself, the ESC and the LiPos run hotter. the only RC situations where Outrunners have an advantage are in helicopters and 3d aircraft were they have to be air cooled, yet have low speed airflow and hence a motor that is short and fat for its weight can have and make use of a bigger diameter fan for better cooling.

While both are types capable of similar efficiencies, capitalism's love of profit, the ability to hand wind outrunner windings but not inrunners, and the cheapness of unskilled labour in developing countries, means that it is cheaper to make an outrunner than an inrunner, and when building a cheap motor it is tempting to use cheaper material too, so the average outrunner is less efficient than the average inrunner.

While the shaft has a 2 bearings, the can only has a bearing on one end of it and at high RPMs they tend to fall apart so they are generally rated for Lower RPM than inrunners, and their (generally) higher pole count makes them harder for the ESC to spin at high RPMs too. You can get back speed by running bigger higher pitch props, but those high pitch props have more lift and more prop walk.

They are not some sort of physics defying black magic, whether it is an inrunner or an outrunner a motor of X efficiency, making Y power, needs to get rid of Z waste heat, and they do that though their thermal mass, as motors are made almost totally of thermally conductive metal, I trust the weight of a motor to signify the power it is capable of a lot more than I do the manufacturer's rating.

[Fluid]

When the very high quality Scorpion 4035/800 outrunners were still made, many of us ran them in 10S boats to good effect. That motor could swing a gas-sized prop and pushed my F-41 Stryker cat into the 60s easily. I run my 10S boats at about 160 amps draw - within the safe zone for the big Scorpion.

Heat was never a problem in hulls with large interior volumes like monos and cats. Quality outrunners like the Scorpions can handle much higher temps than inrunners and using a watercooled mount helped control the heat. For the length of oval heats there was no problem, and I won a few heats with mine too boot.

Today I run a smaller Scorpion 4025/1100 in my SprintCat 60/90 on 6S as a sport boat. BTW these Scorpion motors have three bearings and are very stout. Running in the 50s it is fun and powerful. The only motor cooling I use is a computer fan blowing onto the end of the motor. Heat is not a problem running 6500 mAh packs down on a typical Texas summer day.


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[Nevs]

I have a 45 mono with 2x helidrive 4956 /1350 kv./6s (4030 stator). its also running about 60 mph with 5514 props. it has front water cooling..But I also added air inlet/outlet connected to tubes, (so the water does not get in when the boat should turn). That had great great cooling effect. In an outrunner you can know how hot the magnets are. in an inrunner you will never know. The rotor has very little surface and gets very little direct cooling..But yes the high watts at low rpm. would take "insane" props to make "insane" speeds. Imagine a 50-60" cat with 500 kv/ 12s attempting 130 mph..

[Fluid]

The biggest issue with high speeds (130 mph) and outrunners is rpm. Due to their design they cannot achieve the rpm of inrunners. To go 130 mph with props having a 4" pitch would require around 40,000 rpm. No large outrunner (which would be required in the PO's scenario) would survive long at that speed. An Most ESCs could not control a big outrunner with all those poles (usually at least ten)....


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[Nevs]

Yes I agree. Think "The outrunner speed limet" would be around 100 mph. (so that someone can disagree). In the example with the KDE a 7016 prop, (4,5" pitch) should be possible at 14s. Especially i a twinn setup cat/rigger. I once ran a 6214, (pitch 3,5") with a "small" Helidrive 4962, (4035 stator) 560kv /12s. it was a 14 pound mono. Not running perfect at all...But that prop was close to the limet I think.

[Nevs]

Hmm found this one close to 30.000 rpm and 665 props.(+4" pitch)..
https://www.youtube.com/watch?v=7omD_HrM8DM
( no idea how long it lastet ).