Ayuh,..... Did you have Everything in forward gear, when you slid the lower up to the upper,..??
You have to hold tension on the prop shaft to keep the lower from poppin' into neutral,....
Ayuh,.... My guess is,..... It ran without water flow at some point, which burnt up the exhaust shutters, which fell down into the lower exhaust cavity, causin' a blockage, 'n the bellows blew off,....
Ayuh,.... Welcome Aboard,.... In this forum, we don't care what yer boat is,....
But,.... We have to know which motor, 'n drive is in it,.....
Full model, 'n serial number is best,....