Upon looking at the site you bought the PMA from, you will notice that they show curves of open circuit voltage and short circuit current.  You will not get the 87 V output at the 11A short circuit current.  You will not be able to get more (and will get less) than the 10 or 11 amps into a battery at any reasonable RPM you can get out of your 5 ft prop. - except possibly if you leave it running in very high winds.  The correct way to calculate the power would be to multiply the charging battery voltage (say 14V) times the charging current of 10 amps to get 140 Watts.  I suspect you may only get up to 125W max out of that PMA after all the other losses are factored in.  In order to get 12V out at 150 RPM they had to rewind the stator with fairly small wire (AWG 22 maybe) to get enough turns (around 300 turns per phase)to get that output.  And the output current is limited by the resistance of the small diameter wire and the reactance of the core.  There is nothing wrong with this, just that it is not a high power PMA.  Ive rewound and rebuilt the armatures with NEOS on many of these alternators and get results similar to what the alternators on that site gets.  If all you are looking for with a direct drive prop is around 100W on that PMA it will probably do it.  The other models they advertise have fewer turns of larger diameter wire and will get more power, but require some gearing up (belt drive) to achieve 12V in reasonable low startup (7mph) wind speeds.

I am curious, how much does the alternator cog when trying to start it?  At what wind speed (approximately) does it start turning over?