I couldn't find a test with a 50 but with a 90 Mercury the top speed was 24 and 26 with the 90 Etec but those boats had slightly longer pontoons. This actually can help speed in a non planing setup but I still think it would equate. A 20ft Bentley with a 60 EFI goes right around 20. With a 50 on that boat you are getting I would expect to see mid to high teens at best. You won't have any problem seeing max speed for a tube on that setup, and honestly it won't be a lot of fun for the bigger kids. Even though you lake is smaller, I second the above poster saying you need to get something in the 90-115 hp range to make it a decent tubing setup.
