Rich,

I only know a *little* about this. On my boat the flapper is just
a rubber part. The exhaust pipe sticks out the back of the transom
by about an inch or so. The flapper just goes over that part of the
exhaust pipe (on the outside) and is held on with a simple clamp.

If yours are setup that way, it would be pretty easy to change. Maybe
a little harder for you than for me since your swim platform it probably
over the exhaust pipes. But very easy to do it was out of the water.

Now, from what I read here, it sounds like some models have the
flapper in the pipe (not attached outside). Then you'ld be into
taking part of the exhaust pipe apart.

Can you see the exhaust pipes coming out of the transom outside?
Do they have rubber flappers over them?

Gary