Nevermind. I figure it out. If you rotate too much speed, it going crazy, but not give much thrust.

I tried the big propellor and havent it get working yet.
Drive into water, turn on the prop but wont budge a bit. 0 thrust.
Has it some RPM limit, below it wont work at all?

I'm unsure but adding more propellers on the same shaft might increase boat speed.
Or Adding more props on a separate bearing powered by the same engine.?

are you talking about for water or flight? I just attached 2 propellers to an electric engine and they worked fine for propelling a boat at about 30 mph.

Probably "Cavitation".
Due to the Prop spinning so fast the water is replaced by bubbles of air therefore triggering the "Is in Water" part of the mod script to be false therefore the propeller is not providing thrust.

or a separate part of the script limiting RPM of the propeller.

After all, who'd want a propeller attached to a bunch of stacked bearings each having increased RPM from a level 5 engine to allow the speed of sound to be reached on a water transport mainly made of bubble block and plastic.? Lol

Thinking about it, I wonder if I can get my Evil Kinevil costume glitched in and tweak some code. Hmm. Incoming YouTube video.? Ha ha.

2 answers there also addresses the "Flight" question as the propeller doesn't work in air.