Think phillip_stevens is right. I had a similar problem with my Yam 15 when the rubber bush went after fouling some fool's floating dinghy kedge line. The rubber bush separates from the prop. This saves the gears from being stripped and is a effective get you home as you can run slowly. As you speed up then the prop and hub start spinning so you lose power. You can usually get the hub replaced - not a complicated job but you need a shop with the right kit.