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Ok everyone i need some help Reduction query

Well my little airboat is almost finished but could use some good advice on a pully system . My motor throws about 6000rpm i am going to use a ultralight 52 prop with changable pich and can not go any bigger the motor i am using You wont Belive it :oops: its a citroen 3cv - somthin i am sure has dissapeared in the us for quite a while
http://www.cocheargentino.com.ar/c/citroen.htm just so you no what im talking about
any way . whoud be rely fantastic if somone could tell me
1. 6000 rpms to
2. what pich to apply
3 and what reduction 2.1 i havent a clu
Just want to add That this web site is fantastic and has rely helped me resolve a lot of loose ends in putting together my Booat and i am looking foward to posting photos and a small vid in homage to the great people on this web site for now just thanks to you all : :D
The goal is to manipulate the prop speed, and let the motor run wherever it makes
the most efficient power. Most airprops start to lose efficiency beyond 3000 rpm
give or take, and the hottest thing going now is bigger prop slower rpm for maximum
push with minimum noise. Check with your prop manufacture if possible, they will
tell you the rpm range the prop was designed for, and you can calculate the reduction
ratio needed. A 2 to 1 reduction will roughly double your engine torque ( less the
mechanical loss from your reduction unit ) and cut your output shaft speed in half.
This will put you in the high end range for prop speed, making me think a bit more
reduction may be optimal. maybe 2.5 to 1, which should make your prop turn
around 2500 rpm. or 2.25 to 1 , which would put you at 2750 rpm, I believe that
2750 is a common recommended prop speed. Hope this rambling has been useful.