What size engine does your boat need to pull a person on a tube?

two stroke wise nothing less than 90hp on 16 ft size boat, It depends on 3 things, the size of the boat, the number of ppl in the boat and the age of the rider. Assuming you have a 16-19ft and 2-4 ppl in the boat to give a teenager a good ride you are going to want no less than 150hp if you have just young kids you dont really need much more than 100hp. If you want to give a terrifying ride you will need 200+ hp.