Ok, I finally found some more material references wrt nozzle geometry 
wrt. jet length:

http://engr.smu.edu/rcam/research/waterjet/Par4.html

This seems to indicate that going beyond 200 jet diameters is not 
good, and 100 jet diameters is safe (for the jet shape).

For 100:1 these figures result (roughly) (the equivalent Re is 1000 in 
air for the 3 mm nozzle and deteriorates fast with increasing speeds):

nozzle  jet l.  jet speed	pump	pressure
dia.    mm      m/sec		l/min	bar (less nozzle loss)
--------------------------------------------------------------
  3      300     2.4             4.0     0.03
  6      600     3.4            22.0     0.06
12      1200    4.8           130.3     0.09

The low speeds indicate that even a small wind will interfere with 
operation.

Peter
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