http://www.hobieclass.com…ook/2014/RBook140401.pdf
HOBIE 21 CLASS RULES
5.5
e) A spinnaker pole no longer than the distance from the center line of the front crossbar to either bow may be used to fly the open sails. Rigging is of open design.
i have had 3 poles of different length and diam and prefer the tornado sized spin and pole (13'2) on my 5.5
i have had tapered, 1.5" and 2" OD poles and the 2" is probably the most standardized pole if you plan on getting a snu (snuffer) etc
fyi
The Tornado pole is 13.125ft (4000mm)
the F18 pole can be no longer than 800mm from the bows,
f16 the poles must be 3500 or shorter; irrespectly of the where the mainbeam or bows are