Cteany

Hi all, new here. I have a yacht but the kids wakeboard and I am wondering whether to buy a speed boat/rib/dory for their entertainment. Kids are 24, 22 ands 18 so with some proper instruction I will be able to leave them to it. How fast does a boat have to go in order to water ski and which is best a speed boat, rib or dory. I do not want an inboard engine and would prefer a dory if it will do the job. Thanks.