From experience towing a wakeboard with an
equipped with a BF50, I'd suggest the Solas Amita 11.8 dia. x 9" pitch x 4-blade. The original prop typically supplied for this engine on this boat was the Honda 11.75" dia. x 10" pitch x 3-blade, which was also made by Solas, but the blades were supercavitating shape, where the trailing edges were sort of cut away (sickle shaped blades). I can tow a wakeboarder (pushing 200 lb) with the original prop, but the 4-blade is better for that tractor-like pulling, at lower speeds (naturally) since it's dragging something through the water.
I use the dock line cleats on the rail. A fixed bridle makes the pivot point for the tow rope far behind the transom, so steering at low speed can become difficult, or even impossible. Like when trying to turn to face away from the shore with the skier in the water, before hitting the throttle. That's a common problem for me with the prevailing winds, as the boat turns broadside to the wind, and that can aim me at the shore. From a force vector POV, it's equivalent to connecting the tow line to a rigid pole sticking out well past the OB - a long lever arm resisting or preventing you from turning with a skier in the water creating a lot of drag.
A solution is to use a single line between the cleats (I use small 3-strand, with eye splices in each end), with a small block that connects to the tow line, so you can turn the boat and the block will roll across that bridle toward the inside of the turn. The effective pivot is then actually somewhere forward of the transom, and how far forward depends on how long the bridle line is. But it makes turning much easier.
To clear the rudders, you have to lay them out almost flat. They don't touch the water, so no danger there. BUT, I found out the hard way that if you get off the throttle fast, then start turning before the stern wave catches up to you, those rudders out back will grab that stern wave and turn you much faster than you expected, heeling the boat at the same time. The solution is to get off the throttle, wait a couple of seconds for the stern wave to catch up and calm down, then go into the turn. That's at low speed, of course - at high speed, it doesn't matter.
And make sure your CB is all the way up. The boat needs to slide sideways to make high speed turns, and having some CB in the water may roll you the wrong way.
As to the best prop for the OP's OB, no lo sé. From my experience (alone), I think you can tow with whatever prop gets you top speed at max engine rpm now, but of course, the boat won't go as fast since you're dragging someone through the water. But I've found that the 4-blade (same diameter, but an inch less pitch) gets the skier up on top faster, which seems to be a good thing. I don't know if that's important for a tube, though, since it's already on top of the water. And with younger kids, you probably want to go slower anyway, at least until they get used to it, so I don't think hole-shot is that important.
she was reading the paper and was only half heartedly listening to me. She thought I said I was researching how to pull a TOOTH with our Mac. 
