Model 149 hydro fan
 

I just replaced the hydro fan on my 149 but when I started the engine the fan slid down the shaft and broke. Now I need another new fan. When I installed the fan it seemed like a loose fit even after I installed the snap ring the fan would spin freely on the shaft. What went wrong? Did I get the wrong fan blade or is there a secret to holding the fan in place. The shaft doesn't appear worn

Without looking at any pictures, I would say that the Drive Shaft has lost some of it's outside diameter due to rust. IIRC the shaft is 5/8" (or .625"). You might want to check the new fan against a new 5/8" rod for the fit.