Alright, so the title basically says it all, my port side exhaust keeps loosing the external flapper..noticed it was missing about a month ago and at that point it was less then a year old. No big deal, replaced it and first day out with the new flapper and same deal, flap gone..torn from the top. Any ideas??

How’s your water pressure at idle? How is the temperature of that header vs the other?

May be getting too hot.
Is it getting same water flow as right side?

Its rained every day since Saturday up here, next decent day I will hook it up to water and see if I can find something. I didn't not notice any water pressure issues or temp issues on Saturday when I was running the boat..I would really like to figure this out before I have to winterize!

Definitely check out the water pressure. You may not find much out running it on the hose either. You may have to back it in the water and rev it up a bit to get an accurate picture.

You could also upgrade to these:
https://www.hardin-marine.com/p-1257...-flappers.aspx

I have 4 on the shelf if ya need some.............