or in other words,the motors might be ok,but bcos there rated at 12000 rpm,,,it takes serious skill to fit,,,or at there rated 12000 rpm they just suck,for vibration,,
i would not be surprised if one of you quality boat building guys could get these to purr like a kitten,,,,but i would not be surprised if you couldnt,what ever skill level,,,,
the only last option i can see is ,,that rubber couplings,,,and single red hinged couplins just wont do at that rpm,,,
i think there desighned for small speed boats,on 12v,,and as such not to botherd about the un avoidable noise on standerd setups,,
two red joints on each motor might help,but its more bloomin money