Re: How Fast
Well depends upon the tube. Some tubes offer more drag. The other issue is if you can get him up on plane. I think you might. If you are wondering if you can get him air born as he crosses the wake as you make a hard turn so that all you see are arms and legs flying-well probably not.
Power is what you need pulling a tube. BTW, I learned to ski behind our Johnson 25hp on a fiberglass boat.