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Since it was sand you probably didn't hurt anything. The hub inside the propeller is designed to help protect the lower unit. I would keep an eye on the water indicator, you don't want any blockage from sand or mud.
It is common and normal to stall an engine running aground. As long as the engine starts, develops full rpm, you are probably fine.
If you do run into any problems, particularly with the lower unit, post back and let me know. But for now don't worry about it.
You may want to pick up a depth finder though!
Actually a very good motor, I like the 90/115. Follow the owners manual and use ring free in your fuel. Try to use non ethanol fuel if possible. Use what Yamaha recommends not what someone on a message board on the Internet. Even the best of us go aground at times. Good luck with your new boat. Please don't forget to rate my service.