[changes in latitudes]

Ok, here's the situation. I just got a 150CC (about a month ago). This is my first outboard powered boat so if this is a dumb question I apologize. I bought it used with only 4 hours on the Yamaha 50 2-stroke. I can get on to plane fine with 1-2 people and the speedo shows 31-32mph top end. So here's the question - at WOT the tach only shows about 2100 rpm. Is that possible? I have to think that the tach isn't working for 2 reasons -1. I would think that the Yam 50 should rev much higher than 2100 rpms and 2. If it really is only turning 2100 rpms how can I get 31mph out of it? Any guesses or suggestions on how I can test it? Thanks!
-Dave