If you want a cheap alternator you have to go for high speed and that means a high tsr.

If you want a quiet peaceful life you have to settle for lower speed and pay the price with a bigger and more expensive alternator.

Low tsr blades need a higher solidity so you need wider blades or more narrow ones.

To achieve the highest possible tsr you could use 1 blade but 2 is a lot simpler.

2. blades suffer from vibration during yaw because the momentum about the yaw axis changes as the blade changes position.
3. blades are free of this problem and can be used reasonably up to a tsr of 8 but beyond that there is a lot of interference between one blade and the next.

In your case you can perfectly well use a 2 blade prop in place of the 3 but you must expect increased vibration during yaw. If you have a good clear site with little turbulence it shouldn't cause you much trouble but if it is a poor site it is not such a good idea.

If you are using Hugh's design of alternator, it loads up quite quickly with speed and the type of prop you intend to try will work ok but I would aim to make that blade with a nominal tsr of no more than 6. The highly tapered and twisted blades tend to be slower than a simple constant angle blade.

You should easily manage well over 300W from a 2 blade 8 ft prop, but go for a bit coarser setting angle than the old Wincharger which was meant for a higher speed.

You could also make a 3 blade by your simple means and again I would go for a coarser setting angle.

For instance if you have an alternator that makes 12v at 100 rpm and you'd like your blade to start charging at say 6 mph and your working with an 8ft blade... the circumference of the 8ft blade is 25.13ft so the formula:

Rpm/(mph*88/circumference)= TSR

100/(6*88/25.13)=4.75

so you could design the blade to run at around a TSR of between 4.5 and 5 and it would work well.  At 4.5 it might cut in at just above 6 mph and 5 would cut in just below 6mph which would be just fine.

The "88" in the formula is simply a quick conversion to convert mph to feet per minute so the output of the formula is in rpm.

The area i have where i run my genny is somewhat turbulent.  I had usually run a 3 blade with most of my gennys and never with any real problems.

     I had made a 9 foot 2 blade tsr 7 out of ash wood.  Often wanted to try it just to satisfy my own curiosity.  Within approx 1 1/2 months it had completely torn apart the steel base that was welded to my 3 phase conversion and allowed it to tilt forward and smack the tower.

Here is the end result:

LOL, not much left eh

Its unbelievable the vibration that goes on during yaw (speaking from experience). Unless you have a great wind area without much turbulance, i really suggest you go with a 3 blader.

Just my opinions  :)