Howzit Stalyoni - welcome to the forum !...
In the first vid - a 3900kv motor powered by 3s is a bit ' high' - you could well have shattered (split) the magnet on the shaft...RPM perhaps way to high. You want to stay within the 30 000 rpm limit (Volts X kv) - remember to consider the voltage on a full charge - 3s =12.6 v's = almost 50 000 RPM..coupled to a 3900kv motor...
The second vid was hard to see the performance.....but it sounded like you had the 2618kv motor fitted.
Karl has pretty much answered your question best - the 2s would be a good start..( 22 000 rpm)..working toward the 3s's ( 33 000 rpm).as you iron out all the creases...
.....adjusting the timing through the ESC parameters allows you to advance / retard the RPM - increase / decrease the torque..
In the video below I am running my new SUP Bro board using a 3650 / 2450 kv motor set at 7.5 degrees timing - 3s lipo - 30 mm plastic prop.set up weighing in at 5lbs......I prefer even more punch from the 4s's ( lost mine in the fire)..set the timing down to zero degrees..keep the 30 mm prop....shipment on route - express mail....
Keep us posted on your sessions !......
Share the STOKE !
Howzit Karl ....Yeah !...these little Bricks can be quite Nasty !... they deserve to be locked up...I've researched someviews on the safety and even transporting them seems best in an old metal ammo case...
TUESDAY SESSION:
Had 'another ' session with SUP Bro rc Surfer to-day. The new board is certainly playing out its design expectations....
...as I become more accustomed to it......riding out the hollows very smoothly .... pulling quick / responsive lip turns.....nice tail slides, due to the wide square tail.....currently powered by 3s lipo's (lost my 4s's in the fire)...looking forward to 'sea - ing' it rip when powered by the 4s's.....
Love the video sound track - Africa - by Toto
https://www.youtube.com/watch?v=sBff71el3ok