For “sail” I’ve always used *{joqwI''a'} – a MUCH bigger version of a {joqwI’} “flab, banner”. 

(Lieven < MO, 1/09/2012):  {tlhIm} is commonly used for a fabric wall hanging (that might, for example, have a picture of the Klingon emblem on it). If a piece of cloth is displayed at the end of a pole, it is considered a {joqwI'} ("flag"); a {tlhIm} covers some sort of surface.

I don’t know if this covers things hanging down from a horizontal pole or out from a diagonal pole.  His point here seems to be that {joqwI’} are not hung on a wall.  Okrand has related {joq} with sails:

 

(HQ 10.4):  when a banner or sail flaps, the correct word to use is {joq} “flap, flutter, wave."

Well sure, but a sail is not doing its job if it's joqing.

Purely as original imagery—that is, I'm not suggesting this is THE word—I think of SuS jonwI' wind catcher. If you want to be more specific, vo'meH SuS jonwI' wind catcher for propulsion.

Ooh! Ooh! How about vo'meH vol sheet for propulsion?

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SuStel
http://trimboli.name