Duncan, as Bob has demonstrated, inrunner motors can be used in "slower" models 😀 but usually they are used in models where higher revs are required e.g. fast electrics/raceboats and those with water jets.

Outrunners offer more than enough performance for sports cruisers where you don't really need more than 10k. rpm on the water (12k. out) and the prop shaft can be the limiting factor whereas raceboats usually use flexible drives.

I usually recommend outrunners for heavier sports cruisers and speed boats as for a given motor diameter they offer more torque but in reality brushless motors are so powerful it doesn't really matter!

The rpm of brushless motors (sorry if you know this) is governed by the kV stated which equates to 1000 rpm per battery volt which can lead to some seriously high rpm with inrunners as they can have very high kV.

I was late to the party as regards model boats and use brushless motors in all my builds, even the very slow ones, as I just love their engineering. Most of mine are outrunners with the one exception being my future water-jet jet project where high revs are beneficial and so I'm using an inrunner. Also inrunners can be more effectively cooled by using a water-jacket as the outer casing doesn't spin.

First photo shows an outrunner in one of my Fairey sports cruisers and the second is an inrunner as used in my water-jet project fitted with a cooling jacket.

Chris

Nicely explained Alessandro - well done 👍

Bob.

Hi Duncan, I'll answer your question.

Both inrunner and outrunner motors are brushless.
In brushless motors, the winding is on the fixed part, unlike brushed motors, which have the winding on the rotor (in fact, to transmit current to the moving windings, they require brushes that, rubbing on the contacts, close the circuit).
The fixed part of brushed motors can have permanent magnets or (if more power is needed) electromagnets.
In brushless motors, the windings are always on the fixed part (stator).
The stator can be external, meaning the rotor (which has permanent magnets) will be internal; in this case, we're talking about an inrunner brushless motor.
If, however, the stator is internal, the rotor (which has permanent magnets) is external; in this case, we're talking about an outrunner brushless motor.
A few images can clarify the concept better.

Perhaps a general explanation of brushless motors would be helpful, but to avoid clogging up this thread and going off-topic, it's better to write a separate article on how brushless motors work, like the one I wrote about brushed motors (see attached link). What do you think, Duncan?

wiki/147457