How much water flies forward to the drivetrain Due to the rainy weather and my daily commute, I've decided to invest in some SKS Raceblade fenders. The fenders extend only just past vertical of the axle, and so I wonder about how wet I/my drivetrain can expect to get from the remaining exposed tire. 

Has anyone looked at this carefully/experimented with the amount of water coming off the tire at different angles/speeds? I would assume that most of the water stays near the ground, and incrementally less water goes around the frame. 

What I'm trying to understand is whether I need to buy another fender to block water that will hit my drive train, or if so little will be present by the time it gets past the fender that it won't really matter. 

I've uploaded a photo to illustrate my question.
That rear mudguard/fender is definitely better than nothing, but its not a good full-coverage guard.  As you note, water will fly off the tyre and hit the frame and underside of your legs while riding.  

A good rear guard will start low at the back, perhaps even with a mudflap. Then will go over the whole outside circumference, with its lowest point below the BB/chainstays.

If your roads have any dirt on them it will be picked up an ddeposited on the rear of your seat tube, and possibly on the top of the kickstand bracket if your bike has one between the chainstays.

Most-all full guards have stays that support and brace the rear of the guard.  I can't see any in your pic, but I bet the guard's a bit floppy and perhaps even buzzes on the tyre while riding.



Related - the tread on your tyre has a lot to do with how much water is carried around the wheel.  A blocky MTB knobbly will throw road water like a rooster tail, but a road bike in the same conditions and at the same speed will throw far less water.



Separately, that front mudguard is okay but not great.  It could go down further, again with a mudflap to protect the BB and chainrings and your feet from road water.