A garage door opener of 20 foot or more would definitely be at least half horse. For retrieving targets any horse power would work the trick is to find the proper length for your application. You might want to check with your local overhead door repair man they take in alot of old openers and you may be able to find extra rails to extend your length.
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A standard garage door opener would not work the limits would time out before 21' could be reached. The most travel you can get out of a residential garage door operator is 12'. Plus anything that is made after 2001 has to have a safety system that only allows for so many seconds of run. You would have to use a commercial operator and a heavy duty one at that to facilitate the 21' of reach. Just adding rail will and chain will not work. Pm me of you have any other questions.
Gads, for 20 whatever feet, why not use a pulley type Clothesline and do it by hand?
Going further, any Motor with a small pulley will work fwd and reverse by tilting the motor against the upper or lower line. Keep it simple.