If you spin a 5 blade prop at 30,000 rpm you have as many blades going through the water as a 2 blade prop does at 75,000 rpm, at low speed there just isn't going to be much water passing between each blade strike, I hope I'm wrong but I don't see an easy cure for that, best I can come up with is a scale prop needing scale rpm, fit a super low KV motor or go oldschool with a 2:1 gearbox put a bigger 5 blade prop on and it should behave reasonably.
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