If you ever snorkel around the Keys or Biscayne Bay you get real
sensitized to 'blade count' only in this case it's more of a scream as
some cowboy comes ripping past at 50+ in his Cigarette. When you hear
'up doppler' it's time to sink to the bottom until you hear the 'up'
become 'down'. Those props can mess you up real bad! Now, big boats go
'thrum thrum thrum thrum . . . ." if they're just poking along. Still,
250 rpm is only about 16 a second on a 4 -blade prop so you can hear
and recognize the beat on say a freighter or tanker. Walt BJ (now
land-locked - Colorado is amazingly short of reefs and surf))